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Microelectronics : Systems and Devices

1. A sensor that gives a logic 0 or 1 output depending on whether the temperature is below or above a set level An actuator that is switched on or off by a logical 0 or 1 signal and something appropriate to temperature regulation As an example a cooling system is built up from a temperature sensor a microcontroller and a motor driven fan Fig 10 2 is the circuit for a temperature sensor based on a thermistor The set point of the switching circuit depends on the setting of the variable resistor If the 159 Input and output Figure 10 2 This sensor circuit gives a logic high output when the temperature is above the set point The supply voltage must not exceed the supply voltage of the processor Figure 10 3 A transistor switch can be used to drive a wide range of DC actuators The actuator may be an LED a lamp a solid state buzzer a relay a solenoid or a low voltage DC motor Include the diode when the actuator is inductive 160 Microelectronics Systems and Devices temperature is below the set point the output of the buffer gate is low logic 0 If the temperature is above the set point the output is logic 1 The actuator is a small low voltage DC motor connected as in Fig 10 3 The transistor must be rated to pass the current required by the motor The motor is an inductive load so a protective diode must be connected across the motor as shown by the dashed lines The temperature sensor is connected to pin
2. Designers have analysed the programs written for CISC machines and noted that in practice a relatively small number 20 of instructions are used for the majority 80 of operations Other instructions are less often used These instructions can be omitted and also the internal hardware associated with them leaving the MPU with a reduced instruction set RISC of fewer than a hundred instructions It is also possible to eliminate the internal software that many CISC chips have for performing certain operations This simplifies the structure and the operation of the MPU and makes it faster It also has other advantages for example the instructions can be all the same length making the processing faster In many of the newer CPUs all instructions can be performed in a single cycle of the system clock Examples of RISC processors are the Digital Alpha and the Intel 80960 microprocessors and the PIC17CXX microcontrollers If a RISC computer has to perform certain tasks that have been eliminated from the instruction set it will have to be programmed to perform the operation in several steps instead of just one It may take longer than a CISC processor Because such operations are needed only rarely there is an overall gain of speed by using the RISC processor Suppose the CPU is reading from a DRAM chip The states of the eight transistors reading from D7 to D0 are OFF OFF OFF OFF OFF ON ON OFF This is what one byte of a pro
3. 80 7C 23 0173 0 77 7C 23 0173 7C 76 7C 23 0173 7C Inside the CPU 60 Microelectronics Systems and Devices in response to pulses arriving from the system clock It repeats this cycle indefinitely Control lines not shown in the figure run from the control unit to all other parts of the microprocessor Through these it exerts its control over the whole MPU It also has connections to external parts of the system memory and ports for example through the control bus Internal bus This has similar structure and performs similar functions to the address and data busses of the external microelectronic system The internal bus may not have the same width as the address and data busses For example the Pentium has a 32 bit internal bus but has a 64 bit data bus The 68000 family has a 32 bit internal bus but the address bus has 24 bits and the data bus has 16 bits Data bus register The register is the equivalent of an I O port through which the CPU receives data from the rest of the system or outputs data to it Incoming data can be held in the register until the CPU is ready to receive it Exchange of data between the internal bus and the data bus is under the control of the control unit Figure 4 1 The internal structure of a typical microprocessor includes complex logic circuits control unit and ALU with numerous registers all linked by the internal bus 61 Inside the CPU Address bus register It is
4. Security system V 2 input 0 output 1 out1 0 ensure LED off onalert if in0 0 then onalert pause 500 if in0 0 then onalert out1 1 switch on siren The debug statements have been deleted as they should no longer be necessary Notice how the beam has to be broken for at least half a second to trigger the siren The program can be reset to silence the siren by pressing the restart button on the Stamp but it may be more convenient to install a pushbutton Connect a button to pin P2 as in Fig 2 6 This sequence of versions shows how a program may be 165 Input and output The cooling system and security system that have just been described show that it is possible to build useful systems with only one or two sensors and one or two actuators There are dozens of similar systems that can be built from 1 bit sensors and 1 bit actuators 1 To begin with build the cooler or security system adapting the design and programming to suit the microcontroller microprocessor or computer that you are using Build it on a breadboard to start with though you could transfer it to stripboard or make a PCD for it if it is successful Try to think of improvements and additional facilities One practical point if you are working on the Activity 10 5 Projects with 1 bit I O developed stage by stage testing each stage as it is added This is not the Top down approach to program writing so often recommended but i
5. Systems and Devices Figure 1 5 This simplified diagram of a PC shows that it has much in common with the typical control systems of Figs 1 1 and 1 3 a pair of loudspeakers a joystick a scanner and a digital camera One of the distinctive features of a PC and other computer like systems is the bus To assist the rapid transfer of data between the CPU and the other parts of the system the units are linked by a set of parallel conductors shown for simplicity as a single conductor in Fig 1 5 In practice the bus consists of three separate busses each with its own task as will be explained in Chapter 2 A PC is programmed from various sources First of all it has a block of permanent memory in which the operating system is stored It has numerous programs stored on its disk drives and the user can purchase other programs on compact discs or download them from the Web These programs are temporarily transferred to the computer s memory when they are to be run Programs include word processors spreadsheets accounts programs games educational and training programs and information programs such as dictionaries encyclopaedias telephone directories catalogues and atlases A wide range of specialised programs is obtainable for use by travel agents theatre booking agents medical centres libraries and other medium 11 Systems in action sized organsations Major businesses and organisations such as banks and oil companies emplo
6. 1 11011 Ignoring the carry digit in brackets this is the two s complement of 5 3 Adding a positive number to a negative number to get a positive result for example adding 4 to 3 Two s complement of 3 11101 Binary equivalent of 4 00100 Add 1 00001 The result is 1 4 Adding positive to negative to get a negative result for example adding 2 to 6 Two s complement of 6 11010 Binary equivalent 2 00010 Add 11100 The result is 4 The technique also works with two positive numbers for 207 Programming projects example adding 3 to 2 Binary equivalent of 3 00011 Binary equivalent of 2 00010 Add 00101 The result is 5 Most assemblers have a mnemonic for forming two s complement Write a routine for taking two values negating one or the other or both and calculating their sum Microlectronics Systems and Devices 208 Answers to questions Chapter 1 Test your knowledge 1 1 Digital integrates circuits CPU programmable 1 2 To free the CPU for less routine tasks 1 3 The system clock 1 4 The CPU 1 5 A bus Multiple choice questions 1 C 2 B 3 D 4 A Test your knowledge 2 1 Requires only one line 2 2 Faster than serial transfer 2 3 Disable all other devices with outputs connected to the data bus 2 4 8192 2 5 To enable a device when its address is on the address bus 2 6 Logic high 2 7 500 ns 2 8 0 3 V Multiple choice questions 1 B 2
7. logic 0 A suitable value is 10k Note that V is the processor supply voltage 25 The CPU Figure 2 7 Simple one bit control of a filament lamp by a microcontroller The resistor limits the current drawn from the output pin to a safe level V may be higher than the processor supply voltage if required Other input devices such as photocells or Hall effect magnetic detectors need a simple circuit to interface them to the microcontroller Similarly the pins of typical microcontrollers are able to source a few tens of milliamps so indicator devices such as LEDs can be driven directly from the pins Devices that will accept logic level inputs can also be driven directly Other output devices such as solenoids and motors require heavier currents and so need special interface circuits such as a switching transistor Fig 2 7 The 1200 is the simplest member of the AVR family of microcontrollers Others have larger on board memory and additional features such as a serial port a pulse width modulator capture compare circuitry and an extra timer At one time the Z80 was used as the CPU in several of the popular hobby computers It still has many applications in microelectronic systems particularly those that require more computing power than a microcontroller is able to provide However as the CPU of hobby computers and other desktop computers it has been superseded by much more powerful processors such as the Intel
8. register variable number Using labels makes it much easier to understand what a program is doing and how the different parts of it link together Labels must be defined at the beginning of the program Example Program example 1 introducing assembler def counter r17 def trial r5 equ body 37 ldi counter body loads r17 with body temp inc counter increment temp mov trial counter store in trial 118 Microelectronics Systems and devices After the title but before the actual program listing are two definitions def which tell the assembler the names we are giving to two of the CPUs registers In the program we always refer to the register by its name instead of its number There is also a statement to say that the label body is equal equ to 37 This is actually an extra step that was not included in the previous program There we directly loaded counter with 37 Now we are loading 37 into a constant called body After that we copy the value of body into counter The name body can be used anywhere later in the program where we need to use the value of body temperature In addition if for any reason we want to change the value throughout the program we can simply change it in the equ definition Rarely does a program run straight through from beginning to end More often the CPU is required to jump from one part of the program to another often skipping back to repeat a part of th
9. 26 Data bus register 60 Data direction register 70 Data logger 13 Data sheets 194 Data transfer operations 137 Debouncing 103 203 Debugging 129 Decoder 29 44 47 Decoupling 101 Definition 118 Delays 154 Design structure diagram see DSD Digital 1 2 Digital Alpha processor 57 Direct interrupt 190 Direct memory access 32 Distributed processing 14 DRAM dynamic RAM 43 Dry run 119 DSD 172 DTMF 4 Dual processing 64 ECL emitter coupled logic 39 99 EEROM electrically erasable ROM 46 EMI electromagnetic interference 6 97 EPROM erasable PROM 46 212 Microelectronics Systems and devices Errors programming 115 Fanout 100 Fetch execute cycle 108 Flags 61 139 Flag register 62 Flash converter 86 Flip flop 74 Floating point unit 65 Flowchart 169 Function 180 Ground plane 101 Handshaking 33 High impedance output 28 Indirect interrupt 190 Indirection 138 Initialisation 178 Instruction register 62 Instruction set 56 65 107 Integrated circuit 1 Intel 8086 family 36 Intel 80960 processor 57 Interface 6 Internal bus 60 Interpreter 123 Interrupts 32 187 I O port input output 19 24 26 Iteration 175 Label 117 Ladder logic 130 Latch 74 Logical instructions 136 Lookup table 204 Loops 142 155 Machine code 58 110 Masking 147 Mask programed ROM 43 Memory 10 30 46 Memory map 49 Microcode 64 Microcontroller 2 19 22 24 55
10. A microprocessor does not have input and output ports Instead it connects with the other parts of the system through the data bus However the data pins of a microprocessor are not necessarily capable of sourcing or sinking sufficient current to drive the large number of Microelectronics Systems and Devices 72 devices that may be attached to the bus Additionally a system such as a microcomputer may be designed so that the CPU can drive the internal peripherals keyboard and disk drives for example but could not be capable of driving external devices printer and scanner for example that may be added to the system Ports must be attached to the data bus to interface it to external devices Ports may be built from ICs belonging to the 74XX and CMOS families or we may use special ICs intended to interface with particu lar CPUs Ports are divided into two types Parallel ports Serial ports The difference between parallel and serial transfer is explained in Figs 2 2 and 2 3 Parallel ports are used where there is a large amount of data to be transferred and when the peripheral is not far from the CPU For example a printer is connected to the system by a parallel port A serial port would be too slow for this application Serial ports are used for long distance communication such as sending data by telephone They may also be used where speed is not of prime importance and the complexities of having 8 parallel lines
11. Compiling is a complicated process that takes the CPU an appreciable time Unfortunately the program can not be tested and debugged until after it has been compiled If any errors are found the programmer has 125 High level languages to go back to the source program correct the errors recompile the program and test again A compiled program will usually take much longer to write and test than an interpreted program but once it is done it runs much faster Compiled programs make use of a collection of ready made standard machine code routines that are held in library files Some of the routines from the library files are requested by the programmer when writing the source code When the program is compiled these routines are added to the compiled program by a program known as a linker The RAM addresses are added and checked by a loader program The final version of the program is then ready to run We will now look in more detail at a some of the high level languages that are useful for programming microelectronic systems C is one of the most popular languages for programming microprocessors and microcontrollers Since the C compiler has to be matched to the features of the particular type of CPU it instructs there are many versions of it However the American National Standards Institute has drawn up a set of rules for the way in which C programs are written and most compilers follow these rules These compilers are
12. Note the label again beginning the line to which the CPU is to return later Later in the program the CPU could be sent back to this line by the statement GOTO again 129 High level languages As in the previous program it probably would be better not to use GOTO A properly constructed loop should be used instead PBASIC has a useful feature for debugging programs To find the values of all the variables after the program has been executed we append these debugging statements to the program DEBUG DEC body DEBUG DEC counter DEBUG DEC trial This instructs the CPU computer to obtain the decimal values of the variables and the constant and send them back to the computer along the programming lead The computer displays them on the screen in decimal form As soon as the program has run a panel appears on the computer monitor with these numbers on it 37 38 38 Debug commands can be inserted anywhere in a program to show the values of variables at any stage The Stamp is primarily intended for use in control or measurement systems It has a number of features that fit it for this and therefore PBASIC includes a number of commands for using these features These include BUTTON debounce a button and jump to a different part of the program PULSOUT output a timed pulse DTMF generate DTMF dialling codes POT read the value of a potentiometer thermistor or any other sensor with variable resistance These and o
13. Some CPUs such as the Pentium can operate in either CISC or RISC mode depending on the requirements of the application Fast processors 64 Microelectronics Systems and Devices Microcode A CISC processor has several hundred instructions some of them very complex It is not possible for these instructions to be executed directly Instead when an instruction is waiting to be executed the processor looks in a special ROM that is on its chip and finds there a short program that tells the processor how to carry out the instruction In this way the instruction is replaced by a microprogram of a special machine code known as microcode The microcode takes over the operation of the control unit the ALU and other units of the processor until the microprogram is completed Calling up decoding and executing the microprogram takes time The decoding may taken even longer that the actual execution RISC processors not only have fewer different instructions but all of these instructions can be executed directly as they reach the control unit There is no microcode ROM on a RISC chip This saves time so nearly all instructions are executed in just one clock cycle As a result a RISC pro cessor executes instructions about four times faster than a CISC processor Wide busses Parallel transfer of data both inside the CPU and in the system outside allows more data to be transferred at each cycle The faster processors have busses that
14. The reader should not take these examples as something to be copied exactly They are intended as models that the reader will adapt to a different processor and in doing so obtain a close understanding of their particular programming system 143 Mnemonics 1 If you are using an assembler look through the list of mnemonics used by your assembler and find the mnemonics corresponding to those used in our examples Write short programs to demonstrate them Use the examples given here as a guide 2 If you are using machine code or a high level language write short programs based on our examples to demonstrate the corresponding actions of your processor 3 Take each of the programs you have written in 1 or 2 above and rewrite it to do something a little different For example write programs to add two different numbers to add three or four numbers all less than 64 to subtract different numbers including an example that produces a negative result Instead of the assembler programs based on breq experiment with the IF THEN statement in BASIC or the IF statement in C 4 Investigate three more mnemonics opcodes or statements in the language you are using Write short programs to demonstrate their action In some of these questions you are asked to write a short segment of a program Base your answer on an asssembler and processor with which you are familiar Work out the program on paper testing out your program with a dry
15. and turns the fan motor on for cooling In the cold routine decimal 254 is binary 11111110 which puts a 1 in the output register turning the motor off Programming in BASIC The sensor is connected to D0 in the 4 bit I O port pin 1 The fan motor is connected through a transistor see Fig 10 2 to D0 pin 2 in the 8 bit output register 10 REM Cooling system control 20 OUT amp H0378 0 30 OUT amp H037A amp H04 40 state INP amp H037A 50 IF state amp H04 THEN OUT amp H0378 amp H01 ELSE OUT amp H0378 amp H01 60 GOTO 30 Detailed notes Line 20 makes all outputs low at the start of the routine In a full length program this routine could be preceded by a routine which left the register with high levels on some of its lines Line 20 sets all the outputs to low ready for this routine Line 30 sets up the 4 bit I O port in the usual way with all high outputs Line 40 reads the port and assigns the state of the port to the variable state Line 50 tests state and turns the motor on or off accordingly If state amp H04 it means that the temperature is high the equivalent input to button not pressed in the previous programs The fan is turned on by sending a 1 to the output port Otherwise temperature must be low so a 0 is sent to turn the fan off The use of IF THEN ELSE makes the program shorter and simpler Line 60 sends the CPU back to line 30 to re
16. input The waiting routine begins with a label for the CPU to jump back to as it cycles round the next three lines It first reads the port 147 Input and output values into register r16 Then the program uses a technique known as bit masking When the port is read we are interested only in the LSB If we AND r16 with 1 or more specifically with 00000001 the result will be 0 if the LSB is 0 and 1 if the LSB is 1 The values of the other bits will not appear in the result as zero ANDed with 0 or with 1 always gives a zero result The zero flag is set if the result is 0 Then we use BRNE to test the state of the zero flag If the result is 1 Z 0 it shows that the switch has not been closed so the CPU jumps back to the start label and continues from there It cycles round the three lines jumping back repeatedly until the switch is closed and the bit changes to 0 Then Z 1 and the program can continue The last line gives the CPU something definite to do when the switch is closed so that when we are debugging the program we can tell when it has reached the end Masking This is a technique for picking out one or more bits of a byte to see whether they are 0 or 1 It uses the logical AND operation Example The input at a port is 01011001 and we want to know whether bit 4 is 0 or 1 We do this by ANDing it with a byte in which bit 4 is 1 and the rest 0 Input is 01011001 AND with 00010000 Result 0001000
17. perhaps by different programmers Yet the same basic principles apply to modules that apply to short programs Each module has input processing and output The difference is that the input may take the form of data made available by other modules and the output may be data required by other modules The programmer has to ensure that all the necessary data is available and to know in what variables or in what addresses in RAM it is stored Conversely when the module has been run it must leave the required data in a form suitable for passing to another module either as the value of a variable or as a number of bytes in RAM Some languages are particularly suitable for modular programming In C a self contained program module is known as a function Using C forces the programmer to think in modular style and so produce well structured programs A function begins with the function name and in brackets the names of variables the values of which are being passed to it Similarly if a function calls another function values held in its variables can be passed to a variable in the called function This is one way of the ways that a function can have input and output There are other ways of passing values in C such as by addresses in RAM Also some function will have input and output through ports in the way we have already studied The functions in C are of two kinds Some are so often needed in so many different programs that they are avail
18. 0 of port B of the microcontroller The motor is connected to pin 1 of Port B and turns a small fan The program switches the motor on when the temperature is higher than the set point and switches it off again when the temperature falls below the set point Cooling system control equ ddrb 17 the first 5 lines are equ portb 18 the same as in the equ bit0 1 input output program ldi r20 254 out ddrb r20 sample in r16 portb reading bit B0 andi r16 bit0 masking breq cold cold or hot ldi r17 252 hot so turn out portb r17 fan on rjump sample to sample again cold ldi r17 254 cold so turn out portb r17 fan off rjmp sample to sample again The main points about this program are The initial stages are the same as for the input output routine to set up B0 as an input and B1 and the rest of Port B as an output r20 is not output at the beginning as we do not want to turn on the fan until the temperature has been sampled The sampling routine uses bit masking as in the input output program If the sample is 0 low temperature the bit is equal zero Z 1 and the program jumps on to cold to make the output low If the sample is 1 high temperature the program runs on to the next line 161 Input and output Decimal 252 is binary 11111100 which puts a 0 in the output register In the 1200 a 0 in the register produces a high output
19. 00000002 2E51 MOV R5 R17 This display reveals that the program occupies the first three 16 bit memory locations which contain E215 9513 2E51 in that order We shall not be looking at the machine code after this On the screen there is a further entry on the extreme right of the first line 0x25 0b00100101 37 This is a comment or remark put there by the assembler It explains that the hex value 25 corresponds to the binary value 00100101 and to the decimal value 37 When the assembler lists a program or makes comments a hex value begins with 0x A binary value begins with 0b and a decimal value is written as we would normally write it Other assemblers may set out values differently Note the semicolon preceding the comment This is a conventional way of indicating that what follows on that line is not part of the program We shall return to this point in a moment Whether or not the assembler that you are using has exactly the same features as the one described here the point is that an assembler allows you to type in a program and assemble it It searches for and reports errors It allows you to view the program in both assembler and in machine code What it does not do is inform you if what you intended the CPU to do is not the same as what you have actually told it to do This is where debugging is important The bare program as listed earlier is all that is essential but can be made more understandable to the p
20. 31 4 0103 30 5 0104 31 6 0105 30 7 0106 31 8 0107 31 9 0108 30 10 0109 31 11 010A 30 12 010B 31 205 Programming projects Negative numbers We usually represent a negative decimal number by writing the negation sign in front of it This does not work in a computer because the registers contain 0 s or 1 s and there is no symbol There are several ways of representing negative numbers in binary without using the negation sign The most useful of these ways is the two s complement We must first decide the number of digits in which we are working This includes the sign digit on the left We will work in four digits plus the sign digit To form the twos complement of a number write the Addressing modes Microprocessors have several different ways or modes of specifying addresses though they differ in which modes they can use The four modes most often found are Direct addressing The mnemonic is followed by the actual memory address or register where the target data is stored Indirect addressing The mnemonic is followed by an address or register at which the address or register of the target data is stored Indexed addressing There is an index register in the microprocessor The mnemonic is followed by a value that is added to the contents of the index register to obtain the address at which the data is stored Relative addressing The target data is stored at an addre
21. 48 3B29 0E Stack pointer 3B2B Accumulator 65 Address Stored data 3B2C 0 3B2B 65 3B2A 48 3B29 0E Stack pointer 3B2C Accumulator 65 The stack pointer holds 3B2B The accumulator holds 65 The top of the stack is currently at 3B2A but the stack pointer points to the address after this for this is where the next item of data will be stored The most recently stored data is 48 Now the PSH opcode occurs in the program and is executed The data stored is The stack pointer holds 3B2C The accumulator still holds 65 The microprocessor may now be engaged in other operations not involving the stack which remains unchanged Eventually with say Microelectronics Systems and Devices 186 Address Stored data 3B2C 0 3B2B 65 3B2A 48 3B29 0E Stack pointer 3B2B Accumulator 65 Address Stored data 3B2C 0 3B2B 65 3B2A 48 3B29 0E Stack pointer 3B2A Accumulator B7 B7 in the accumulator The stack pointer holds 3B2C The accumulator holds B7 Now the POP opcode occurs in the program and is executed The stack pointer holds 3B2A The accumulator holds 65 again The stack pointer has been decremented to point to the address one above the new top of the stack Note that the value 65 is still in 3B2B but this is now above the top of the stack The 65 is no longer needed and will be overwritten the next time PSH occurs The stack is often used for temporarily storing an intermediate result in a
22. 71 Microprocessor 2 25 34 55 Microswitch 23 MMX multimedia extensions 65 Mnemonic 112 Modem 86 Modular programming 179 Multimeter 12 Negative numbers 205 Noise 97 100 Noise immunity 38 98 Non maskable interrupt 189 Octal devices 72 One bit I O 24 145 Opcode 111 Open collector output 79 Operand 111 Operating system 5 Org directive 184 OTP ROM 44 Parallel data 20 Parallel port 72 77 92 Parity 82 PBASIC see also BASIC 128 149 152 157 162 164 PC personal computer 9 92 Pentium microprocessor 36 PIC microcontroller 22 57 127 Pinout 19 195 Pipelining 66 PLC programmable logic controller 5 71 130 Polling 190 Port 70 Power station 11 Prefetch buffer 64 Program 1 56 109 168 178 Program counter 61 Program structures 175 PROM programmable ROM 43 Protocol 80 RAM random access memory 41 Read operation 33 109 199 Real time clock 51 193 Remark 116 Reset 10 Resistance of track 101 RISC processor 22 57 63 ROM read only memory 43 RS 232 port 78 Security system 162 Selection 175 Sequence 175 Serial data 20 23 Serial port 78 Shift register 81 Source code 112 123 Square wave generator 201 SRAM static RAM 42 Stack 184 Stack pointer 61 184 Stamp 127 199 Status register 71 Subroutine 181 Successive approximation 88 Synertek 6502 microprocessor 34 System clock 5 19 35 55 96 98 Telephone cordless 3 Tempera
23. 768 kHz crystal To start with the periodic interrupt rate and square wave output are not being used so the RS bits are set to 0000 The write program is run with location 10 and value 32 working in decimal for convenience Register B holds flags for functions which are of no concern at this stage so the write program is run again with location 11 and value 0 Timing The clock is now ready for investigation by writing values into the various locations and reading them back To check that the clock is operating read the seconds register location 0 at frequent intervals and check that it is incrementing Similarly the value in location 2 is found to increment once every minute Other registers are tested very quickly by setting up the time and date as 11 59 pm on Sunday 31 December 2005 for example and reading the data back again a few minutes later 201 Programming projects Square wave output The IC has several other functions all described in the data sheet and we look at one of these as an example of how they can be investigated The square wave output pin 23 can be configured to provide output at one of a number of frequencies by setting the RS bits in Register A For example to obtain a 256 Hz wave R3 is set to 1 and the other RS bits are 0 If we also need 1 in DV1 as explained above the bits in Register A are 00101000 The values for the write program are location 10 and value 40 In addition the S
24. Bit 7 6 5 4 3 2 1 0 S Z HC P V N C between B3 and B4 This is not used in ordinary addition but is important if the program is adding in BCD The table below shows the positions of these flags and others in the flag register F of the Z80 microprocessor Instruction register When an instruction code is read from memory it is placed in the instruction register The code is then passed on to the control unit which than carries out the instruction Accumulator and other registers In many CPUs a special register the accumulator register A is set aside as the main register used in processing A high proportion of the instructions refers to operations involving the accumulator We will refer to the accumulator as A from now on Examples of operations involving A include loading data from memory into A storing data from A into memory incrementing and decrementing A adding another stored value to one held in A and manipulating data in A by shifting the bits in various ways These things are done according to the instruction that has been stored in the instruction register The result of the operation is placed on the internal bus and is often circulated back to the accumulator to replace the value that was there before At the same time one or more of the flags in the status register are set or reset depending on the result of the operation Some operations involve two values one stored in the accumulator and one in
25. IC is running on a 5 V supply Provided the design keeps to these limits and that the total current sunk by the port is not more than 80 mA it is safe to drive external circuits directly from the port Some simple examples of this are shown in Figs 2 6 and 2 7 In Fig 2 7 we have not driven the filament lamp directly because such a lamp may require 60 mA or more Instead we have used a transistor as a switch with resistor to limit the base current to a safe level Figure 5 1 A DC input interface of a PLC has an opto isolator to allow the microcontroller to operate on an entirely separate DC supply usually at lower voltage and free from noise Here the sensor is a switch which might for example be a physical limit switch a pressure switch a tilt switch or attached to a float A PLC input card usually has 8 or 16 channels identical to the one shown here As already mentioned the sensors and actuators used in industrial applications often operate at voltages greater than that used for power ing the microcontroller or may require current greater than a microcon troller output port can provide It may be necessary to interface the microcontroller to AC circuits and to circuits carrying an excessive amount of noise see Chapter 6 PLCs have input and output interfaces that isolate the microcontroller Fig 5 1 is the circuit of a DC input interface Similar precautions are taken in the circuits for DC output and for AC input and output
26. Section B The Software 7 Instructions 8 High level languages 9 Mnemonics 10 Input and output 11 Structured programs 12 Programming projects Answers to questions Index vi 1 18 41 55 70 96 107 122 133 145 168 193 208 211 Preface This book is written for a wide range of pre degree courses in Microelectronic Systems The contents have been carefully matched to current UK syllabuses at Level 3 but the topics covered depth of coverage and student activities have been designed so that the resulting book will be a student focused text suitable for the majority of courses at pre degree level around the world The only prior knowledge assumed is basic maths and science The UK courses covered by this text are Advanced GNVQ Units in Microelectronic Systems from Edexcel BTEC National units in Microprocessor systems Micro Electronic Systems and Software Design Methods Essential theory is provided here but the book is strongly practical in its approach encouraging students to assemble and test real microelectronic systems in the laboratory The examination syllabuses do not specify which processors and which programming languages the student should cover The suppliers catalogues list several hundred microprocessors and microcontrollers and any one or more of these could be selected as a subject for study There is likewise a variety of languages or versions of languages that may be use
27. a general register Fig 4 1 shows only one general register but most microprocessors have several of these to make operations more flexible The Z80 for example has two banks of eight registers The main register set is A F B C D E H L A is the accumulator and F is the flag register The remaining six registers can be used for storing data temporarily for example the intermediate results in a sequence of calculations All registers are 8 bits wide but the six general registers can be combined in pairs BC DE and HL to hold 16 bit values The machine code program included 63 Inside the CPU an example of this for the HL pair where as a 16 bit register they were used to hold a RAM address There is an alternate set of 8 registers called A F B and so on which can be switched over to when a second line of processing is required The Z80 also has Register I which points to a table of interrupt service routines stored in memory Register R which counts the number of executed program steps Registers IX and IY which are two index registers used in indexed addressing Chapter 12 Fig 4 1 and the description above apply generally to many microprocessors and to many microcontrollers too The main difference is that microcontrollers may also have memory and other devices such as timers built in to them There are also CPUs such as the 6502 which rely solely on the accumulator for processing In
28. and trial when it needs them The program also defines a constant BODY This will hold its value for the whole program We have typed its name in capitals to indicate to people looking at the program that this is a constant not a variable As in several other languages calculations are performed by assigning a value to a variable In C this is done by using the equals symbol In the first line of the main program variable counter is assigned the value of the constant BODY We have already assigned the value 37 to BODY so BODY holds 37 The first line of the main program then assigns the value of BODY to counter Then both BODY and counter hold 37 Perhaps the only item that needs an explanation is the double plus on the last line of the sample This line means increment counter and assign its new value to trial The operation could also be programmed as counter counter 1 trial counter The sample listed above does not have any label indicating the program line that the CPU is to jump back to later This is because C does not have this feature There are several other ways of making a program section repeat but we will leave descriptions of these until later Most C compiler programs comprise several sections for handling the different stages of production of the final machine code file The first section to use is the Editor which is rather like a word processor When the program above or a sufficientl
29. are 32 or 64 bits wide Dual processing Some processors have two ALUs working in parallel so that two instructions may be processed at once This speeds up execution but this technique can not be used to full effect if the two instructions take different lengths of time to execute Prefetch buffer In a conventional computer the instruction is fetched from memory then executed This is referred to as the fetch execute cycle The time required for fetching and executing which may involve fetching data to work on sets the speed of operation of the processor A processor with a prefetch buffer such as the 8086 family including the Pentium does not wait for the cycle to begin before fetching the instruction Instead it takes any opportunity when it is not busy to fetch the next and subsequent instructions It stores them in the buffer without executing them The instructions are then there stored on the chip ready to send on to the control unit as soon as the time comes to process them 65 Inside the CPU Cache memory This is a special type of RAM with short access time It may be located on the processor chip so giving the fastest access or there may be a special cache memory chip as part of the system s RAM It is used for storing addresses and data that might be useful to the CPU in the near future For example it can hold data that has been read in ahead of the time it is required When the CPU needs this data it looks in the
30. are high but when any one output is addressed by the inputs at A to C it goes low When all three inputs are low for example output 0 goes low When A and C are high and B is low output 5 goes low As an output goes low it enables the selected 6116 which then can be written to or read from Example Given the address 1E6D how is this decoded Write the address in binary 0001 1110 0110 1011 The lower 11 bits go directly to the 6116s The lower 11 bits are 110 0110 1011 in binary or 66D so byte 66D is addressed in all eight chips Which of the eight chips will function at this stage depends on the top 3 bits of the address which are 011 In the decoder input A is 1 input B is 1 and input C is 0 This selects output 6 so the seventh chip in the set is selected Data is read from or written to this chip only The data at the corresponding address in the other seven chips is ignored In this way the eight chips cover these addresses Chip no Chip no Chip no AAAddress range in hex ddress range hex 0 0000 to 07FF 1 0800 to 0FFF 2 1000 to 17FF 3 1800 to 1FFF 4 2000 to 27FF 5 2800 to 2FFF 6 3000 to 37FF 7 3800 to 3FFF The highest address is 3FFF In decimal this is 16383 so the set of eight chips stores 16K Just to confirm the calculation the address bus has 14 11 3 bits so the number of addresses is 2 14 16384 16K Many systems have a 16 bit address bus but the circuit of Fig 3 5 is unaf
31. at the START input use a 10 k resistor to pull the pin up to 5 V and connect a push button between the pin and 0 V Use an LED indicator on the EOC output The data sheet may describe other connections that should be made Make conversions at a number of input voltages between 0 V and the supply voltage Plot a graph of the digital out put reading against the analogue input voltage Comment on the curve you obtain Activity 5 2 DACs Study the data sheet of a digital to analogue converter and note its main operating conditions Set up the DAC on a breadboard and provide it with A suitable power supply A set of 8 switches to provide each data input with 0 V or the positive supply voltage R 2R DACs convert in the short time it takes for the logic to set the switches and for the op amp to settle DACs are available for inputs up to 16 bits wide and a settling time often less than 1 s Microelectronics Systems and Devices 92 A multimeter to measure the output voltage The data sheet may describe other connections that should be made Make conversions at a number of digital inputs between 0 and 255 assuming an 8 bit input Plot a graph of the ana logue output reading against the digital input Comment on the curve you obtain Interfacing to a PC When we interface to a computer we are not connecting directly to the CPU as described in other parts of this chapter The computer is a complete
32. at the motor than it is for all the ailerons and other control surfaces to be controlled from the central computer In addition this approach requires less cabling and is less subject to electromagnetic interference Distributed processing is part of the new fly by wire principle adopted by Lucas Aerospace as used in the A320 Airbus aeroplane 15 Systems in action Summing up Microelectronic systems may be divided into Control systems examples resin production flight controls automotive applications Instrumentation systems examples testmeters data acquisition and data logging Communications systems examples cordless telephone facsimile machine Commercial systems example personal computer automatic teller machine EFTPOS station stock logger Find out as much as you can about a machine or equipment that is based on a microelectronic system Examples include Washing machine Fax machine Telephone answering machine Dot matrix printer Stock logger Smart room heater Compact disc player Garden or greenhouse reticulation watering system Car park entry and exit control EFTPOS machine Electronic fund transfer at point of sale ATM Automatic teller machine also known by other names such as cash dispensers Global positioning system device Radar systems including radar speed traps Traffic control systems Car engine control Robotics Sources of info
33. better for this purpose The multimeter makes up for this with a bargraph display which can be seen running along the lower margin of the display in Fig 1 7 The meter can also process the measurements If the meter is run for a few minutes or more the user can view the values as they change or the microcontroller can be programmed to pick out the maximum value and the minimum value and to calculate and display the difference between maximum and minimum and the average value It can also measure the voltage produced by a thermocouple and calculate the equivalent temperature in Celsius or Fahrenheit The next grade of microelectronic instruments above the multimeter is the data logger In practice these instruments perform two related but distinctive tasks Data acquisition receiving data voltages counts from sensors Data logging recording data and processing it The Datataker from Data Electronics upon which this descripton is based acquires measurements from a number of sensors connected to its array of input terminals The measurements may be displayed on the Datataker s own screen or on the monitor of an attached computer Subsequently the data can be stored logged in its own memory or on removable memory cards The data can then be processed For example the device can calculate maxima minima and other functions It can also convert voltage for example into temperature in Celsius or Fahrenheit The
34. bus produces a corresponding combination of logic levels on the data bus Making special masks is very expensive so this type of ROM is used only when it is known in advance that thousands of them will be required For example it might be suitable to mask program a ROM for use in a popular model of a washing machine PROM This is programmable ROM which is programmed after it is manufactured but the program can not then be altered It is sometimes Read only memory 44 Microelectronics Systems and Devices referred to as one time programmable OTP ROM Fig 3 2 shows how the programming is done In the simplified circuit in the diagram there are only three address inputs for the memory locations which can be decoded to one of eight addresses There are four data output lines which run across the lines of the memory locations At each place where the two sets of lines cross there is a transistor connected as in Fig 3 3 The emitter of this has a fusible link connected to it This thin connection can be blown by passing a relatively large current through it when the ROM is being programmed To blow it we connect the data output line to 0 V and then apply a high level to the memory location line If V is made higher than usual a large current flows through the transistor and melts or fuses the link All of this can be done automatically with the PROM plugged into a socket on a special PROM programmer This is loaded with the
35. chip To address 2K locations requires 11 address lines 2 11 2048 Since each location stores a byte the IC has eight data lines for input output Typically the time needed to read or write a byte of data the access time is 100 ns compared with 20 ns for a hard disk and 200 ns for a floppy disk Figure 3 1 The 6116 SRAM holds 2 KB bytes of data It therefore needs 11 address inputs and 8 data inputs outputs It has OUTPUT ENABLE and WRITE ENABLE inputs which must be made low for reading and writing respectively but the chip will take no action un less the CHIP ENABLE input is made low at the same time 43 Memory Dynamic RAM DRAM A DRAM IC consists of an array of MOS FET transistors Each of these can be switched on by storing a charge on its gate Writing to DRAM consists of charging or discharging the gate Reading consists of finding out if the transistor is on or off by registering the level 0 or 1 it produces on the data line The single transistor of a DRAM location is much smaller than the flip flop of a SRAM location so many more locations can be accommodated on a DRAM chip For example a typical large SRAM IC stores 4 Mb but the equivalent large DRAM stores 16 Mb This makes large memories easier to build In addition DRAM access time is only 70 ns which is an advantage in computers for high speed operation The problem with storing a charge is that it eventually leaks away This means that the data stored in the
36. circuit and build a fresh one on a new circuit board The flexibility of microelectronic systems is one of the main reasons that they are so widely used High and low The two values that a digital quantity can take are often referred to as high and low A high value represents logical high which is equivalent to the binary digit 1 A low value represents logical low and is equivalent to the binary digit 0 Some systems operate with the values the other way round negative logic but this is very unusual In microelectronic circuits these values are represented by voltages Low is often represented by 0 V or a value fairly close to 0 V High is often represented by 5 V but other values may be used in other types of microelectronic system Note that to a power engineer high voltage means something more than mains voltage for example 450 V or even 132 kV To a microelectronics engineer high usually means a mere 5 V It is slightly higher in certain kinds of system However high is just 3 3 V in the low voltage systems that are intended for portable battery powered equipment 3 Systems in action A survey A simple system Before we look in more detail at what what goes on inside a CPU we will take a few examples of the way microelectronic systems are used in everyday life and at work The cordless telephone is a typical example see over As in any other microelectronic system the circuit centres on a CPU I
37. clock is required this can be obtained as an IC such as the 6872 With a suitable crystal this can operate at frequen cies up to 10 MHz Another source of clock pulses is a crystal oscillator module which includes the crystal and logic circuitry required to give a TTL output These can be obtained to run at frequencies up to 50 MHz with suitable short rise time and fall time In many systems a frequency of 1MHz is fast enough in which case it is feasible to assemble a clock based on standard logic gates Fig 2 11 In PCs and other computing systems processors are clocked to operate very rapidly so as to process as much data as possible in a given time This becomes very important when computers are running graphics and multimedia programs It is also important when clocking telecom munications systems so that data may be transferred as rapidly as possible Clock frequencies of several hundred megahertz are common and frequencies of 1 GHz or more are attainable The frequency quoted for a CPU is usually the maximum frequency at which it can reliably be run It can be run at frequencies less than the maximum because the clock pulses co ordinate all parts of the system to operate as one The system clock must not be confused with the real time clock which is described in Chapter 4 System clock Figure 2 11 A system clock built from two CMOS INVERT gates The gate on the right is a buffer to avoid over loading the oscillator and
38. counters are particularly sensitive to noise Noise may cause them to change state or mis count Noise on the 0V ground line in minimised if the line has low impedance Any suddenly increased current flowing into the line is immediately carried away to the 0 V terminal of the power supply To reduce this noise we can Make power tracks wider particularly the 0 V track If possible widen the 0V track into a ground plane occupying all vacant areas of the board Make power tracks as short as possible This advice applies to tracks of all kinds for the shorter the track the less likely it is to pick up EMI and the less likely it is to emit EMI This is particularly necessary when designing the system clock a potent source of noise and at the same time easily affected by noise Decouple the supply This consists of placing capacitors for example 100 nF multilayer or disk capacitors across the supply lines close to the IC which is to be decoupled The distance between the capacitor and the terminal pins must be kept as short as possible and so must the leads of the capacitor Otherwise the Track resistance The resistance of a typical circuit board track is 4 m per cm If a section of track is 10 cm long its resistance is 40 m If one end is connected to the 0 V terminal of the power supply and a current of 100 mA is applied at the other the voltage difference betrween the track ends is 100 mA 40 m 4 mV
39. discussed To interface a digital system to an external analogue system there are analogue to digital converters for input to the digital system and digital to analogue converters for output ADCs include flash converters and successive approximation converters The DACs are represented by an R 2R converter A microelectronic system needs some way of communicating with the outside world It needs input by which it receives data and is told what to do It needs output so that the processing of the data can have some useful result Microelectronic systems communicate with the outside world through ports We have already seen that a microcontroller such as the 1200 Fig 2 1 has two ports on its chip known as Port B 8 bits and Port D 7 bits The bits of each port are individually programmable as inputs or outputs Each port has a data direction register DDR where a series of 1s and 0s determine whether a bit is an input or an output The DDR has an address in the memory space of the microcontroller The program writes values into the DDR to set the pins as inputs or outputs For example if the value 11000000 is written into the DDR for Port B 71 Interfacing the top two lines B6 and B7 are set as outputs and the remaining lines as inputs As outputs each line of the 1200 ports is able to sink 20 mA when the output is logic level low When the output is high it is able to source up to 4 mA These figures assume that the
40. initialise the I O port If this is not done the output at D1 stays low indefinitely and the system latches with the fan off 162 Microelectronics Systems and Devices Programming in PBASIC The PBASIC version of the cooling system program is as follows Cooling system control input 0 first two lines are output 1 the same as the I O program sample if in0 0 then cold debug hot out1 1 make P1 high to turn goto sample on the fan cold debug cold out1 0 make P1 low to turn goto sample off the fan Security system To keep the programming simple this system has been limited to two inputs and one output The light sensor is a photodiode A beam from a lamp shines on this If an intruder comes between the lamp and the photodiode the beam is broken and a siren is sounded There is a reset button which resets the system silencing the siren and putting the system on the alert for the next intruder In real life it would be best to hide the reset button but in this demonstration system the button is simply connected to pin B2 It is wired as in Fig 2 6 to give an active low output The light sensor Fig 10 4 gives a low output when the beam is broken and is connected to pin B0 The siren is a low voltage solid state type driven by a circuit such as Fig 10 3 omitting the diode and connected to B1 163 Input and output The listing for the 1200 mic
41. intervals In a data logger for example the clock could be programmed to interrupt every minute On receiving the interrupt the CPU jumps to the ISR which instructs it to sample and record input data In between interrupts is is occupied with general routines such as updating the display and accepting commands from the keypad Peripherals The keyboard buffer may be holding key presses which the CPU needs to know about so the buffer sends an interrupt signal usually a low voltage level to the interrupt input pin of the CPU The ISR of the CPU then enables the buffer allowing it to place the keypresses on the data bus one at a time The result is further action by the CPU perhaps storing the ASCII codes in memory or displaying characters on the monitor screen A printer is another device that frequently interrupts to tell the microprocessor that it is waiting to receive data for printing It can be seen that some interrupts are the result of programming or operating errors but others are a useful way of regulating the system When a CPU receives an interrupt it Finishes its current instruction first Pushes the contents of its registers on the stack The content of the program counter is important so that it can return to the same place in the program after the interrupt routine has been completed Jumps to a fixed address in memory where it finds another jump instruction giving the address of the start of
42. is present One essential point about a data bus is that only one unit can be allowed to place data on it at any one time It is the same situation as a discussion group If one person is talking the others must keep quiet so that person can be heard by all However in all the units in Fig 2 9 the data output pins are permanently soldered to the bus It is inevitable given the way in which microelectronic systems are constructed The difficulty is that even if the outputs of the quiet units are at zero volts they will pull the bus voltage levels down The signal from the talking unit will be swamped by the 0 V levels from the quiet units The solution to this problem is for all units connected to the data bus to have three state outputs The three states of such an output are logic 0 the pin is at logic low voltage logic 1 the pin is at logic high voltage the pin is in the high impedance state In the first two states the pin is electrically connected to the bus so that the output voltage high or low of the internal logic circuit appears on the bus In the third state a very high resistance is switched in between the pin and the internal circuit The resistance or impedance between the pin and the circuit is very high In effect the pin becomes 29 The CPU Bus width A typical microprocessor such as the Z80 has eight lines in its data bus We say the bus is eight bits wide and w
43. language you need the appropriate language program This is the program that accepts the statements you type into your computer and displays them on the screen It allows you to edit your program to check it for errors and to save it The form a program is saved in varies with the language Most versions of BASIC for example use an interpreter program The BASIC language program saves your source program not as machine code but as a sequence of bytes Some of the bytes represent BASIC commands while others represent numbers and text The numbers and text are represented by ASCII code In this form the program is meaningless to a CPU which understands only opcodes and operands To be understood by the CPU the BASIC source program must first be interpreted When you run the source program the interpreter takes over It goes through the source program line by line and command by Microelectronics Systems and Devices 124 ASCII code This is the American Standard Code for Information Interchange and is the standard way of coding letters numerals and punctuation and also a number of printer control commands It is an 8 bit binary code with a range of values from 0 to 255 The first 32 values are the control characters such as carriage return bell produces a beep in modern computers but used to ring an actual bell in the old Teletype machines and end of file Code 32 is a space Codes 48 to 57 are the nume
44. lines are numbered the example shown is line 46 and the symbols are each identified with a number or code which indicates memory locations corresponding to terminals on the input or output cards The system used on Mitsubishi Figure 8 1 One line of a program for a PLC which controls a pump to pump liquid out of a heating tank This happens when 1 there is a receptacle ready to receive it and 2 when the liquid has reached the required temperature or the pump out switch is closed manually Symbols marked X are inputs and that marked Y is an output The numbers refer to the I O cards that are wired to the corresponding sensors and actuator additional programs which may be downloaded from the World Wide Web but there is no guarantee that these will always be available Programmable logic controllers play an important role in industrial and other control systems Many of them are programmed by keying a statement list into a PC Fig 1 4 The statement list is much like a program written in BASIC though it has its own keywords and syntax Most PLCs can also be programmed in ladder logic This is not so much a high level language as a visual way of representing a sequence of control instructions It was designed to be programmed by electrical engineers rather than by software experts 131 High level languages Figure 8 2 A program that controls the raising and lowering of the barrier of the car park It has t
45. lines if necessary as the program is developed In line 70 if Phase A is operative and the barrier is up this marks the end of Phase A and the beginning of Phase B barrier raised car drives in M42 is set to indicate this In line 75 the first symbol inverts the action of the signal from the sensor by using a the symbol of a relay with normally closed contacts These are open if the car is there If the car is not waiting at the barrier and the barrier is up and it is Phase B then the flag is set to allow the barrier to be lowered In line 80 if the flag to raise the barrier is 1 this flag was set or not set in line 65 and the flag to lower the barrier is 0 set or not set in line 75 and the barrier is not up the barrier lifting motor is started a Y output As in machine code and assembler the program proceeds in many small steps 1 What are the main differences between programming in assembler and programming in a named high level language 2 Explain the difference between a compiler and an interpreter 3 Describe the main features of a named high level language 4 What is ASCII code Give some examples 5 Explain what is meant by assignment 6 What is ladder logic and in what applications is it used Problems on high level languages 133 Mnemonics Mnemonics Short assembler programs are used to demonstrate how a CPU performs elementary tasks The mnemonics investigated are those for arithmetic and logical ope
46. maskable interrupt is always attended to immediately there is no need for the CPU to acknowledge that it has been received No acknowledging signal is sent to the interrupting device The NMI is generally reserved for an event which threatens to crash the system For instance an NMI is generated if the power supply fails On receiving this signal there is a brief time while the capacitors of the power supply contain enough charge to keep the CPU running In that short time essential data in RAM can be saved to disk When programming a non maskable ISR for the Z80 it should always end with a special RETN instruction This not only returns the CPU to the program but also resets the latch on the NMI input The maskable interrupt pin of the Z80 INT has three separate modes of operation which are set by software instructions These take second priority to the NMI so it is essential for the CPU to acknowledge receiving the INT signal Otherwise the interrupting device may have ceased sending its signal by the time a current NMI signal has been Microelectronics Systems and Devices 190 dealt with and the INT signal will be missed The Z80 responds to an INT signal by making its IORQ and M1 control outputs low The Z80 has three types of maskable interrupt Vectored interrupts If the Z80 has been programmed to operate in Mode 0 it acknowledges the interrupt as described above and this causes the interrupting device to put a restart i
47. microelectronic system and it has ports to which we can connect external circuits of our own design The PC usually has one parallel port and one serial port though some models may have more The most appropriate port for the interfacing projects described in this book is the parallel port usually known as LPT1 This is the port to which a printer is normally attached It may have other devices daisy chained to it such as an external disk drive The standard parallel port has a D type 25 pin socket on the rear of the computer The pins of this socket are allocated to three ports as shown in Fig 5 14 There are eight ground lines of which one must always be connected to the 0 V line of any device attached to the computer As might be expected the ports are allocated addresses in the PC s memory space To output and input data we send it to the address of one of the ports Using a high level language we do not have to deal with data direction registers or with handshaking All this is taken care of by the machine code generated by the running of the program In Chapter 10 there are some programs which demonstrate how this is done using BASIC Figure 5 14 The parallel port of a PC is seen as a 25 pin socket It has pins for three ports and eight ground lines In the figure the socket is viewed from the rear as you see it when you are about to plug in the printer cable 93 Interfacing This table shows the addresses used for sending or
48. must be re formatted into parallel data so that it can be processed further Here we need a serial to parallel converter such as the 74LS164 Fig 5 9 This has a serial input and eight data outputs The way it works is the converse of the previous shift register Data is clocked in starting with D0 and ending with D7 D0 is first stored in register A As clocking proceeds D0 is right shifted along the registers until it reaches register H with all the other bits in order along the chain Then the whole byte is unloaded through the eight parallel outputs The data can be fed directly to a device such as a decoder that controls a 7 segment LED display If it is to go on to a data bus it must first be stored in a set of latches with three state outputs Figure 5 9 Received serial is converted back to parallel form by a serial in parallel out SIPO shift register Shift registers perform the essential conversion between serial and parallel data but in most systems it is necessary to add start stop and parity bits before transmission On reception it is necessary to remove the start and stop bits to check parity and finally remove the parity bit Although parity checking ICs are available the logic circuit required for this degree of processing is very complicated and it is easier to use a ready made IC Most CPU families include a serial output interface IC that accepts parallel data and produces serial data complete with the additional bits
49. need for an arrow The stages of the program are represented by boxes that branch off along the main stem Figure 11 4 Some of the symbols used in drawing design structure diagrams The sequence of the program runs from the START to the END 173 Structured programs Figure 11 5 DSD symbols for processes that depend on a condition being true or not In the upper diagram the process occurs only if the condition is true In the lower diagram a process occurs if the condition is true and a different process occurs if the condition is not true An example makes the DSD system clear Fig 11 6 shows the delay routine as a DSD Instead of following arrows as in a flowchart different rules apply Begin at the top and follow down the stem until you reach a branch Follow the line that enters the branch until you reach a box Text in the box describes either a process a definition a subroutine or function more about these later a loop or a decision In the figure the line goes to a box labelled FOR first number 255 to 0 This is a loop as indicated by the line beneath it looping back to the box The text in the box states how many times you have to run the loop In this case you loop until the first number becomes zero Like the main stem of the program the loop line has branches to describe what the CPU has to do as it runs the loop The first thing is to run another loop in which the second number is reduced from 255 to
50. one or more signal LEDs that indicate when a call is in progress It has output to a separate IC which includes an oscillator to generate the ringing tone 5 Systems in action communicates with the public telephone system through the subscriber s ordinary telephone line The circuit of the base unit is similar to that of the handset in many ways The operating system is stored permanently in a part of memory when the telephone is manufactured There is also a section of memory to hold useful data such as the number currently being dialled and a list of frequently used telephone numbers This data is changed from time to time by the user A necessary part of any microelectronic system is the squarewave generator known as the system clock This provides the regular series of pulses that drive the CPU It is not shown in Fig 1 1 because it is usually included on the same chip as the CPU The timing of the clock usually depends on a quartz crystal just as in a digital watch There is no room for the crystal itself on the CPU chip so this is connected across a pair of terminal pins of the IC The frequency of the crystal may be several hundred kilohertz or a few megahertz One of the essential outputs of a telephone is the ringing tone It would be possible for the CPU to be programmed to generate this tone itself but generating the tone would occupy the CPU at times when it could more usefully be doing something else It is common in mi
51. other processors there is no special accumulator register set aside for the majority of the processing Instead the CPU has a number of registers any of which can be subjected to the whole range of arithmetical and logical operations For instance the 1200 microcontroller has no accumulator but instead has 32 registers any of which can be used in the same way as an accumulator On the whole the modest processing speeds of microcontrollers are more than adequate for the tasks they have to perform A washing machine that spends about 10 minutes at each washing stage and 5 minutes in rinsing and spinning does not need a processor capable of processing data at 10 million operations per second In contrast a flight control computer may have a mass of data to analyse as fast as possible before passing instructions to the control surfaces of the aeroplane Similarly a PC running a complex animated graphics program in over 16 million colours and with high quality sound requires a high speed processor These applications have led processor designers to increase the processing speed of microprocessors in various ways An increase in the frequency of the system clock is an obvious solution provided that the processor can be re designed to operate at the increased speed Other measures to improve operating speed include RISC processors These have been described earlier in the chapter In most applications they are faster than CISC processors
52. other features of DSDs First there is a sequence of three processes which set up the system ready for action Then there is a loop but this is one in which there is no test to see if the task is completed Consequently the CPU circles this loop indefinitely Each time round the loop it samples the sensor and a decision is taken on whether to switch the fan off or to switch it on When the decision has been taken and acted upon the CPU runs back along the way it has come until it reaches the loop Then it continues around the loop back to the DO box and on around the loop again It samples the temperature and switches the fan accordingly every time round the room It never reaches the End terminator 175 Structured programs Program structures The examples we have studied have illustrated the three basic program structures Sequences a series of processes take place one after the other Figs 11 2 11 3 and 11 7 all begin with a sequence of three processes Some programs consist of nothing but a single sequence and run straight through from Start to End Iteration the program has a loop The loop may continue indefinitely Figs 11 3 and 11 7 or until some condition is true Figs 11 2 and 11 6 Selection there is a choice between alternative paths depending on whether a condition is true or not In some programs only one path the condition is true leads to specific action In other progra
53. parts of the system memory of various kinds an assortment of input and output circuits and devices mainly depending upon the application The controlling centre of a microelectronic system is the CPU Its function is to read data from certain parts of the system to act on it 19 The CPU process it and to output the results As explained in Chapter 1 microcontrollers differ in several ways from microprocessors so we consider them separately There are several hundreds of different microcontrollers and microprocessors In this chapter we consider a few typical examples As our first example of a CPU Fig 2 1 shows a commonly used microcontroller the Atmel AT90S1200 which we shall refer to as the 1200 for short Remember that this is a complete system on a single chip so its use of terminal pins its pinout is very different from that of the microprocessors that we describe later This simplicity is what makes systems based on microcontrollers so much easier to design and build The terminal pins fall into four main groups power lines the positive VCC and 0 V GND short for ground lines connect to these pins The voltage between them should be in the range 2 7 V to 6 V crystal the system clock circuit is inside except for the crystal which must be connected across these two terminals XTL1 and XTL2 The maximum crystal frequency is 16 MHz I O ports there are two of these port B
54. previous operation is zero If Z 1 the CPU jumps to the branch address 140 Microelectronics Systems and devices This program demonstrates the action of BREQ Demonstrating BREQ ldi r16 5 repeat dec r16 breq done rjmp repeat done ldi r17 100 The program introduces another arithmetic opcode DEC This DECrements a register reduces it by 1 Here is the beginning of a dry run of this program Column Z shows the zero flag and PC is the program counter program lines are numbered from 0 Program R16 R17 Z PC Program R16 R17 Z PC The zero flag may be set or not to begin with depending on the result of the previous calculation Register r16 is loaded with 5 and the first decrementing results in 4 The CPU runs round lines 1 2 and 3 repeatedly with r16 being decremented each time round Eventually the CPU is sent back to repeat at line 1 with r16 holding 1 and with Z still zero This segment begins with the zero flag being set when R16 is eventually decremented to zero 141 Mnemonics C Register 0 0 1 1 0 1 0 0 1 C Register 0 1 1 0 1 0 0 1 0 C Register 1 1 0 1 0 0 1 0 0 C Register 1 0 1 0 0 1 0 0 1 The routine ends with r16 0 and r17 100 The PC shows that the CPU has gone to line 4 The zero flag is still set because there have been no subsequent operations that involve it Bit and bit test instructions
55. receiving data from the three registers The correct address to use can be found by looking in System Information Referring to Fig 5 14 the pins of the 8 bit register are as follows After data has been loaded into this register it can be read back to check it The 4 bit output register is more complicated It is an 8 bit register but the lower 4 bits are not used for data output Bit 7 is inverted If its input line is low D7 equals 1 This bit causes an interrupt when it is made high that is its input line is made low However it does not do this if it is disabled by making bit 4 of the 4 bit I O register low The 4 bit I O register has the following pin allocations Bits 0 1 and 3 are inverted Bit 2 is normal To use this register as input all bits D0 to D3 must first be made high Then input data either pulls the bit register low or leaves it high It is then read 8 bit output 4 bit input 4 bit I O 03BC 03BD 03BE 0378 0379 037A 0278 0279 037A Pin 9 8 7 6 5 4 3 2 Bit D7 D6 D5 D4 D3 D2 D1 D0 Pin 11 10 12 13 Bit D7 D6 D5 D4 D3 D2 D1 D0 Pin IE 17 16 14 1 Bit D7 D6 D5 D4 D3 D2 D1 D0 Microelectronics Systems and Devices 94 Problems on interfacing 1 Describe and compare two different output interfaces 2 Describe and compare two different input interfaces 3 List and describe the characterist
56. run When your answer is complete test it by using an assembler program and a real or simulated processor 1 Describe the action of two arithmetic mnemonics in an assembler program with which you are familiar 2 Describe the addition of two values 197 and 88 by a named CPU 3 Show how a CPU calculates the bitwise AND of B1 and AB What happens to the carry flag 4 Outline the sequence of steps needed to output a byte of data at an 8 bit port Write an assembler program to do this based on an assembler and CPU of your choice 5 The opposite of the BREQ nmemonic is BRNE branch if not equal to zero Write a short program to demonstrate its action 6 Write an assembler program to multiply an integer number by 4 The maximum value that the number will have is 64 7 Write an assembler program to multiply an integer number by 6 The maximum value that the number will have is 42 Hint This program Problems on mnemonics 144 Microelectronics Systems and devices involves both rotation and addition 8 Select two mnemonics other than those described in this chapter and write an assembler program that uses them both Add full comments to your program 9 Rewrite any two of the assembler programs above using BASIC or C Discuss the differences between the assembler and the high level language 145 Input and output Input and output A single bit input or output is all that is needed in many control systems Assembler a
57. so alter its frequency This clock has a 1 MHz quartz crystal but crystals of other frequencies can be used Microelectronics Systems and Devices 36 Other processors the 8086 family When we say that the Intel 8086 is a 16 bit processor we mean that its data bus is 16 bits wide Instead of fetching data a byte at a time like the Z80 6502 and many other processors do it fetches a double byte or word This gives it a big increase in speed of operation Another innovation that increases speed is the prefetch buffer in which instructions are queued ready for execution The address bus is 20 bits wide so the 8086 addresses up to 1 Mb of memory To increase the width of the buses while still keeping the processor in the standard 40 pin package as used by the Z80 and 6502 some of the pins have been multiplexed They perform different functions at different stages of the operating cycle Fifteen pins AD0 to AD14 are used both for the data bus and for the lower fifteen lines of the address bus An example of this kind of multiplexing appears in Chapter 8 The four upper pins A16 to A19 of the address bus are multiplexed as status output pins One of the control pins is the MN MX which selects between two modes of operation When this is high the processor is in min mode it operates much like the earlier 8085 processor When the pin is low it operates in max mode it is able to work in conjunction with other processing ICs such as
58. so rapidly Do not try to remember these figures Just remember the bottom line and you can always work out the rest on a calculator if you ever need them Bits Addresses In K 8 256 10 1024 1K 12 4096 4K 14 16384 16K 16 65536 64K 18 262144 256K 20 1048576 1024K 1M 24 16777216 16M 32 4294967296 4G n 2n address is on the bus Fig 2 10 illustrates the principle of address decoding with a practical circuit The circuit is practical in the sense that it uses obtainable types of logic gate An address consists of a set of 0 s and 1 s lows and highs The decoder must respond only when all the address lines that are supposed to be high are high and all the address lines that are supposed to be low are low In Fig 2 10 the highs are taken care of by the 13 input NAND gate the 74133 For the address in the example the address lines that are 31 The CPU supposed to be high are connected to this gate There are four spare inputs and these are wired to the positive supply line When all inputs to the gate are high its output goes low We require a high output to send to the final gate the 7410 and we use an INVERT gate to produce this The 7400 family has no NOR gates with a large number of inputs so we use two 4 input NOR gates The 7425 has two such gates on a single chip The two gates handle the seven low lines in the target address The eighth input is held low
59. the ISR Jumps to the address of the ISR and executes it Pops the return data from the stack Resumes processing as it was before the interrupt Interrupts by different devices are dealt with according to their priority because the microprocessor can not handle two interrupts at the same time An interrupt from the keyboard for example may have a higher priority than one from the printer If the printer interrupts while the microprocessor is dealing with an interrupt from the keyboard the printer is ignored until the keyboard has been dealt with This raises the problem of how the CPU finds out which device is interrupting With some CPUs there are several interrupt input pins ranked in order of priority Devices are connected to these pins according to their priority Priority is fixed by the wiring Other CPUs may have only one interrupt line which is part of the control bus The CPU knows there is an interrupt but has to find out its source In some systems there is a line in the control bus called interrupt query When 189 Structured programs the CPU puts a low signal on this line the signal automatically goes to all the devices that might possibly be interrupting The one that is interrupting puts its own device code on the data bus so that the CPU knows what action to take Another though slower way of obtaining the same result is for the CPU to interrogate each device in order of priority reading a flag re
60. the serial output and goes to the output interface The other data bits are shifted one step to the right The process repeats for the next six clock edges the bits appearing at the serial output in order D2 D3 D4 D5 D6 and D7 83 Interfacing More on parity Other more complicated systems of parity checking have been devised In the system outlined below 16 bits of data are arranged in four groups of four and a parity bit is added to each row and column Five groups of five bits are transmitted instead of four groups of four This means that the rate of data tranfer is reduced but there are advan tages Example This example is worked with even parity Data P D 1 1 0 1 1 a 0 1 1 1 1 t 1 0 0 1 1 a 0 1 1 0 0 Parity 0 0 0 1 1 In the table above it can be seen that the parity is wrong for the second column and the third row This locates the incorrect bit which is in the shaded box The analysis locates the incorrect bit which can then be changed auto matically to a 1 in this case There is no need to repeat the transmission This technique also checks that the parity bits have been received correctly Data P D 1 0 0 0 1 a 1 1 1 0 0 t 0 0 0 1 1 a 1 0 1 0 0 Parity 1 1 0 1 1 Parity is wrong in the 2 nd row and 5 th parity column Microelectronics Systems and Devices 84 At the receiving end of the transmission the serial data
61. the two links where shown leaving the other links intact When the PROM is in use a high level on the location line produces high levels on D1 and D3 but D0 and D2 remain at logic low Blowing the links destroys them permanently so a PROM can be programmed only once If a mistake is made in programming or the data needs to be altered another PROM must be programmed with the corrected data In the early stages of development of a system PROMs are programmed singly until the program and the device in which it is to be used have been fully tested Once a correct version of the program has been developed the mass programming process is automatic and reasonably inexpensive 46 Microelectronics Systems and Devices EPROM This is erasable programmable ROM The ROM consists of an array of MOSFETs one for each bit During programming a charge is pulsed on to the gates of those MOSFETs that are to be set to 1 The principle is similar to that of DRAM except that the charge remains for very much longer usually for several years EPROMs have a quartz window though which the chip can be exposed to ultraviolet light if the data is to be erased Ultraviolet is an ionising radiation and the ions produced in the chip discharge the gates and erase the data An EPROM can be reprogrammed after erasing which makes it very suitable for use when a system is at the development stage and frequent revision of the program and data are needed EERO
62. this 255 times Decrementing r19 takes 3 cycles so the outer loop takes 759 cycles Running it 255 times takes 255 759 193545 cycles At 1 MHz this takes 0 193545 or approximately 0 2 s This is a reasonable delay for flashing an LED though some times we may want a delay that is longer 156 Microelectronics Systems and Devices There are several other ways of producing delays Some microcontrollers have built in timers for this purpose Timers are sometimes included on the same chip as another device such as an I O port The 68230 PIT is an example of this see p 77 Programming in BASIC BASIC provides two ways of creating delays One of these is to make the processor run a loop many times With interpreted BASIC which has to interpret the program each time round the loop quite long delays can be obtained For example 100 FOR j 1 TO 1E7 NEXT 110 PRINT Finished This takes about 18 s on a PC with a 300 MHz clock The delay is ten times longer if we increase the 7 to 8 and even longer if we make it larger The other technique is to make use of time This is a string variable with the format hh mm ss It automatically registers the time since it was reset in houre minutes and seconds At any time in a program time can be set to any chosen value Conversely we can read its value at any time This routine gives a delay of approximately 12 s 100 time 00 00 00 REM Resetting 110
63. to pin 1 of Port B The program If you have not already done so read the introduction to Activity 9 1 1 Adapt the one bit input program to the system you are using 2 Write new versions of the input program to make it do something slightly different For example revise the program so that it responds to a switch connected to bit 3 of Port B instead of to bit 0 3 Write a program that responds only after the switch has been closed and then opened again Hint it has to read a 0 then a 1 4 Write a program that responds only after the switch has been closed twice 5 A very simple security system has two input buttons To gain access the user must press button 2 then button 1 Pressing them in the order 1 2 or pressing both at the same time is not allowed The user has only one chance to press them in the right order Write a program that functions in this way 6 Expand the previous system to three buttons Decide on the order of keying and write the program 7 A security system has three buttons Write a simple combination lock program that responds only when a selected pair of the buttons are pressed at the same time Activity 10 1 One bit input 151 Input and output Figure 10 1 An LED is a useful device for indicating the state of an output port or pin In a the LED is driven directly by the output from a microcontroller The circuits in b and c take far less current and are therefore more suita
64. transistor must be refreshed at regular intervals Under the control of a clock running at about 10 kHz the remaining charge on a transistor is passed on from one transistor to another and topped up during the transfer In this way the data is kept on the move giving this type of memory the description dynamic The need to refresh memory means that a certain amount of the computer s operating time must be set aside for this The DRAM can not be used for reading or writing during this time This makes the operating system more complicated to run ROM is a permanent or semi permanent form of memory Unlike RAM it does not lose its stored data when the power is switched off This is why it used for storage of the booting routines in microprocessor systems In microcontroller systems such as the cordless telephone and the multimeter the program is unalterable and is stored in ROM during manufacture In the larger more intelligent microcontroller systems such as a data logger there is ROM to store the basic routines with a medium sized RAM as a temporary store for programs that are currently in use There are several kinds of ROM including Mask programmed ROM Special masks are used when the chip is made so that the content of every memory location is fixed from the beginning It can never be altered In other words the ROM is a special kind of logic device in which each different combination of logic levels on the address
65. voltage and hold it while it is being converted Digital to analogue converter A system that has processed data in digital form may need to output it in analogue form For example the circuit of a CD player has to output an audio signal which is an analogue quantity This requires a digital to analogue converter or DAC There are several types of DAC but the commonest is based on the R 2R ladder Fig 5 13 shows a 4 bit example The switches are actually CMOS switches under logical control The most frequently used DACs depend on a resistor network usually known as an R 2R ladder Typically R equals 10 k There are CMOS switches S0 to S3 in each rung of the ladder which can be switched either to the 0 V line or to the inverting input of the operational amplifier Whichever way a switch is set the same current flows out from the rung so the currents flowing in the network are not affected by which switches are open and which are closed A short calculation Microelectronics Systems and Devices 90 shows that the current flowing out of a rung is equal to half of the current flowing out of the rung to its left For example if the reference voltage is 5 V and 2R is 20 k the current flowing through S3 the MSB switch is 250 A The current flowing through the other 3 switches is 125 A 62 5 A and 31 25 A The currents through switches S3 to S0 are proportional to the values of the corresponding b
66. 0 The result is positive so the zero flag is 0 meaning that bit 4 is 1 This is tested and the program continues accordingly If bit 4 is 0 masking produces a zero result Input is 01001001 AND with 00010000 Result 00000000 The zero flag is 1 meaning that bit 4 is 0 Masking can be used to pick out more than one bit at a time 148 Microelectronics Systems and Devices Using a simulator and single stepping through the program we can see the CPU repeatedly jumping back to start The closing of the switch is simulated as follows Step as far as breq start and edit the I O register to make Port B equal to 1 switch closed Resume single stepping The CPU jumps back to start once more then jumps out of the loop at breq the next time round Programming in BASIC This program is for running on a PC It requires a switch or button Fig 2 6 connected to line D0 of the 4 BIT I O pin 1 of the connector see Fig 5 14 Connect also to one of the ground pins Make sure that the voltage of the power supply is 5 V DC Assuming the base address of the parallel port is 0378 see p 95 the program is 10 REM 1 bit input routine 20 OUT amp H037A amp H04 30 state INP amp H037A 40 IF state amp H04 THEN 20 50 PRINT Switch closed Lines are numbered in tens as is conventional On the PC port input is more complicated than output First we have to make all outputs high then rely on the input data to pull on
67. 0 This box too has a loop line below it There is only one branch on this loop This describes a process which is Decrement the second number As you come to this line enter it obey the instruction in the box and leave by the same line So the CPU runs round the loop line decrementing the second number each time until it becomes zero At this point the action of the second loop is completed So you leave the second loop by the way you entered it This returns you to the first Microelectronics Systems and Devices 174 loop and you are still on your way round it for the first time The next branch you enter holds the instruction to decrement the first number After that return to the loop box and begin the next time round the first loop Every time round the first loop you enter and run the second loop then decrement the first number The diagram does not show the nested loop structure in the same way as a flowchart but it is clear that the CPU has to decrement the second number all the way from 255 to 0 every time it runs round the first loop When the first number is eventually reduced to 0 the task is complete and the CPU returns to the main stem There is only one process after this do nothing and then the program ends Figure 11 6 A DSD version of the delay program The second inner loop is reached by way of the first outer loop and runs completely for each single run of the outer loop Fig 11 7 illustrates some
68. 0 Hz or 60 Hz in USA may appear as ripple on the power lines of a microelectronic system This can be eliminated with good shielding and a properly designed power supply Natural sources of EMI include thunderstorms and the effects of cosmic radiation from outer space As a result of one of these factors a logic gate may become set to the wrong state and the system will fail Fig 2 12 shows the input and output levels of LSTTL and CMOS gates The voltage at a gate producing a high output is at least 2 7 V A gate receiving that output will accept it as a high input if it is at least 2 0 V This means that the gate will still give the correct result if there is a spike of say 0 5 V on the output signal taking it down to 2 2 V It still could just work if the spike is 0 7 V but might fail occasionally These values show that the noise immunity of LSTTL is 0 7 V for logic high For logic low the corresponding voltages are 0 5 V and 0 8V giving a noise immunity of 0 3 V Spikes as high as this Noise immunity 99 Planning the system may occur quite often in a badly designed circuit The noise immunity of CMOS operating at the same voltage 5 V is 1 45 V at both high and low logic levels This is better than LSTTL Increasing the supply voltage to 15 V increases the noise immunity still further to 3 95 V at high and low levels For a system working in a noisy environment it may pay to use CMOS operating at 15 V As mentioned
69. 270 resistor The base resistor of each transistor should be about 10 k Try to store a value in one of the locations of the IC Make the control inputs high so that they are inactive Set up the address bus and the data bus with suitable values Then make the write enable line low Finally make the chip en able line low then high again to latch the data Note that these instructions may need to be modified to suit the type of RAM used Now try to read the data you have written with LEDs connected to the data outputs Set up the address but to the same address as before Make the output enable line low then the chip enable line Activity 3 2 Addressing memory Extend the circuit used in Activity 2 1 by adding a decoder as in Fig 3 5 Decide on one or two addresses set the lower bits by connections to the RAM Set the upper bits by connections to the decoder Connect the chip enable input of the RAM to the appropriate output pin of the decoder Try to write and read to various addresses in the RAM Problems on memory 53 Memory Multiple choice questions 6 Design a SRAM to store 128 Kb kilobytes of data in four blocks of 32 Kb each Use a supplier s catalogue and or manufacturer s data sheets to select a suitable type of RAM and decoder Draw a diagram to show all the connections 7 Repeat question 6 for a SRAM to store 48 Kbits of data in three blocks of 16 Kbits each 1 Which type of memory must be refresh
70. 373 74LS574 programmable interface ICs Investigate the action of these ICs using them as input or output ports for the processor Connect them to input data from one or two sensors these might be simple switches as in Fig 2 6 and to output data to one or more actuators The system could include address decoders to enable the port IC Connections to the data bus must use three state outputs Program the system as a control system using an appropriate sensor s and actuator s Contrast the action of these ICs suggesting to what kinds of application they are best suited Investigate the use of handshaking and interrupt signals as detailed in the data sheet ADCs and DACs Program an ADC to sample the input from a light sensitive circuit convert it into digital form and display it If your system lacks a disply program it to switch on an LED when the input voltage exceeds a given level Program a DAC to accept a digital input This is converted by the DAC into a varying voltage that can be used to control the speed of a small electric motor or the brightness of a lamp Programming projects This chapter concludes with a selection of programming topics In each case read the explanation and then write a program to demonstrate the topic Debouncing Several of the programs have used the conventional routine of waiting for a keypress An improvement is to debounce the key by using software The routine should wait for the first conta
71. 6818 is a real time clock calendar that is intended for use in a microprocessor system It is driven by a crystal oscillator The output from the oscillator goes to a 20 stage divider to produce a frequency suitable for driving the timing circuits As might be expected the logic of the IC is complicated and we shall not describe it here The data sheet includes tables of Electrical Characteristics The most important of these is the supply voltage which in this case is 5V 0 5 V It is essential to check on this before beginning a project because it may sometimes happen that the operating voltage is not compatible with that of other devices in the system The Stamp system includes a 5 V regulator so this can be used to power the clock It is intended to operate it at 32 768 kHz and the table in the data sheet shows that the current required is only 500 A which is well within the resources of the system A data sheet provides a vast amount of detailed information Coming direct from the manufacturer it is nearly always complete and accurate However it is not necessary to read every page of the data sheet The information that is needed for a given project is often scattered in different parts of the sheet Although the presentation of information is systematic it is not necessarily in the order in which it is needed for developing a particular project The experienced data sheet reader soon learns to skim quickly through the data sheet to as
72. C in Fig 1 4 was in the process of being programmed by the laptop PC on the right The program is tested on the microcomputer and when it is free from bugs downloaded into the memory of the PLC Once the program is running correctly the PC is disconnnected from the system and the PLC runs independently The program is not normally altered except when there is to be a change in the operating procedure of the plant PCs and similar computers There is much to be said about computer systems in later chapters so for the present we will simply state the main ways in which they differ from the typical microcontroller systems described above In essence all computers have the same main features and we may take the typical personal computer PC as our example Fig 1 5 The basic features are the same as in any microelectronic system CPU memory input and output Because the PC is intended to perform a wide range of often complex operations at high speed a microprocessor is chosen as its CPU Usually the system clock is a separate unit as shown in Fig 1 5 In contrast to systems such as the cordless telephone and PLCs the PC has a full sized keyboard with over a hundred keys It normally has a colour monitor The PC has several other input and output units either built into it or connected by special sockets These include disk drives of various kinds a mouse and a printer There may also be other devices such as 10 Microelectronics
73. C might contain this line area length width Two variables labelled length and width are to be multiplied together to obtain the value of a third variable labelled area This command sends the CPU into a long sequence of fetch execute cycles in which it loads the variables length and width multiplies them together a fairly complicated operation for a CPU that can only add or subtract and finally store the result as area A high level language program includes an extensive range of ready made machine code routines for programming complicated sequences such as the multiplication routine mentioned above so that the programmer does not have to work them out every time they are needed Although programming in assembler may be complex and tedious at times it does have the advantage that the user is always fully aware of what the CPU is doing This can lead to efficient programming carefully tailored to the project in hand The ready made routines of a high level language save much work but are not necessarily efficient In order to make them usable in a wide range of programming situations they may be longer and therefore slower than an equivalent routine specially written to fit into one particular program If maximum speed is essential a programmer may use a high level language for most of the program but use assembler when writing routines that need to run at maximum speed To be able to create a source program in a high level
74. D 3 D 4 C 5 A 6 A 7 C Chapter 2 209 Answers to questions Test your knowledge 1 a 101 1010 0100 b 010 1001 1111 2 The gate is insulated from the body of the MOSFET by a layer of silicon oxide 3 16384 4 18 5 A0 A2 A3 A7 A10 A12 all low the rest high Multiple choice questions 1 C 2 A 3 B 4 A 5 C 6 A 7 C 8 B 9 A Test your knowledge 1 255 2 There is an overflow and the carry flag is set to 1 3 Z 0 S 1 C 0 Multiple choice questions 1 B 2 C 3 A 4 C 5 C 6 A 7 B 8 B 9 A 10 D Test your knowledge 1 1110 1010 2 The small circle on the end of the enable line indicates active low 3 That there is a family of devices with type numbers ranging from 74LS00 upward 4 1111 0010 5 F 6 a 0 b 0 c 1 7 If output is 010 the outputs of the opamps must be 0000011 from top downward and the voltage input is 2 V between 1 5 V and 2 5 V 8 1 97 V Multiple choice questions 1 B 2 D 3 A 4 A 5 B 6 C 7 A 8 B Test your knowledge 1 So that address voltages have time to settle 2 E6 28 3 1110 1110 Chapter 3 Chapter 4 Chapter 5 Chapter 7 Microlectronics Systems and Devices 210 Test your knowledge 1 To act like brackets enclosing titles or remarks that the CPU is to ignore 2 This defines counter as an integer 3 Defining a constant Test your knowledge 1 The CPU would add all three and put the result in r17 leaving r16 unchanged 2 Becau
75. D off pause 200 0 2 s delay out1 1 LED on pause 200 0 2 s delay goto again to repeat the flash The LED is connected to pin P1 This routine is easily expanded to flash at different rates and to flash more than one LED 158 Microelectronics Systems and Devices Activity 10 4 Delays 1 Adapt the delay program to the system you are using 2 Write new versions of the delay program to make it do something slightly different For example revise the program so that it produces delay of 0 1 s or 0 15 s 3 Use the delay routine to turn an LED on for 0 2s then off 4 Write a program to turn an LED on for 0 1 s then off for 0 2 s repeating Try varying the on and off periods 5 Install four LEDs Write a program to light them for 0 2 s one at a time in order 6 Install two switches or push buttons Start flashing an LED when one switch is briefly closed or a button is pressed and stop the flashing when the other switch is closed 7 Install two switches and an LED Take the two inputs to be coded in binary equivalent to 00 01 10 and 11 Depending on the input flash an LED 1 2 3 or 4 times 8 Install two switches and four LEDs Depending on the input flash the selected one of the LEDs 9 Write a delay routine that uses INC instead of DEC Temperature control system The elementary input output programs in this chapter can easily be adapted for simple control functions For input and output we need
76. Datataker is able to perform its calculations at a higher level than the multimeter For example it has selectable routines for different types of thermocouple instead of being restricted to the one type supplied with the instrument The scope of processing is increased by including statistical operations such as calculating standard deviations and plotting histograms There are many other refinements in data presentation For instance each measurement is time and date stamped with the time and date at which it was taken 14 Microelectronics Systems and Devices Another major bonus of the data logger is that it is programmable It can be set to take periodic readings from a number of different sensors and store the results for display later Or it may be set to produce an alarm output when values fall in a specified range The data logger is programmed by using word processing software running on a PC It has its own specialised programming language The finished program is downloaded from the PC into the Datataker This can then run the program on its own when it is no longer attached to the computer Distributed processing In a conventional microelectronic system the CPU has direct control over all the input and output devices in the system Figs 1 1 1 3 and 1 5 show examples of this Now that a wide range of microcontrollers is available cheaply a new approach to control has become more widespread Each sensor or actuator in th
77. Flags I T H S V N Z C 0 0 1 1 0 1 0 0 R16 0 1 1 0 0 1 1 0 R17 0 0 1 0 1 1 1 1 To prepare the program for demonstrating ADC the carry flag is set and the two numbers are loaded The registers hold this After the final line of the program ADC the registers hold this The addition has not produced a carry that is there is no ninth bit so C has been reset Some other flags have been set but these are not relevant to this calculation Now r16 holds the sum 197 and r17 still holds the value with which it was loaded SUBtract without carry SUB ignores the state of C A short program to test this is Demonstrating SUB subtracting 47 from 149 ldi r16 149 load r16 with 149 ldi r17 47 load r17 with 47 sub r16 r17 subtract r17 from r16 and put the result in r16 The carry flag may be 0 or 1 depending on the result of a previous operation After the numbers have been loaded the registers hold this After the final line of the program the registers hold this 136 Microelectronics Systems and devices No carry is produced so C has been reset R16 now holds the difference 102 and r17 still holds the value with which it was loaded Logical mnemonics Logical operations are bitwise between corresponding pairs of bits as these examples show There are no carries with logic so the C flag never gets set AND is demonstrated by a short program using the
78. IF VAL RIGHT TIME 2 lt 12 THEN 110 120 PRINT Finished Line 110 evaluates the last two characters in time and sends the CPU back to the beginning of the line until the value exceeds 12 Very long delays can be obtained by using time and the length of time does not depend on the frequency of the system clock There are two disadvantages of time One is that being a string variable it is more difficult to extract the information you need as demonstrated by line 110 above The other disadvantage is that resetting time in a BASIC program resets the clock in the Windows Date and Time routine There are ways of using time without making this happen 157 Input and output Programming in PBASIC Timing in PBASIC has a useful PAUSE instruction For example PAUSE 1000 This gives a delay of 1000 ms equal to 1 s The pause can be up to 65535 ms long or just over 65 s If a longer delay is required PAUSE may be put in a loop FOR j 1 to 60 PAUSE 1000 NEXT This repeats a delay of 1 second sixty times giving a total delay of approximately 1 minute The precision of the PAUSE routine is not high The ceramic resonator in the timing oscillator has a precision of only 1 and there are slight delays in reading the interpreted PBASIC Programming in PBASIC A PBASIC version of the delay program uses the PAUSE command to flash an LED once every 0 4 s delay output 1 pin P1 as output again out1 0 LE
79. M This is electrically erasable ROM which can be programmed and reprogrammed while still connected in its circuit The ROM of many microcontrollers is of this kind Although this type of ROM could be used for temporary storage in a way similar to RAM writing new data into EEROM is too slow for this to be practicable Some microcontrollers are made in two versions with an EEROM program memory or with a PROM program memory The EEROM version is used at the development stage then the PROM version is used for mass production of the finalised program Addressing memory The example given in Fig 2 10 explained how to decode a single device located at one particular address in the computer s address space In practice devices such as disk drives I O ports and the keyboard have several registers in them and each of these has its own address Fig 2 10 shows several gates being used to decode just one address However it is possible to decode a number of adjacent addresses with only a small amount of additional logic Devices which occupy a range of addresses often have a decoder included on the same chip so there are fewer problems for the system designer A memory chip is a good example of a device which occupies a block of adjacent addresses and which contain its own decoder In Fig 3 5 the 6116 RAM chip has 2K 2048 memory locations within it Each location has 8 memory cells so each location holds a byte of data The chip has 8 data in
80. Microelectronics Systems and Devices This Page Intentionally Left Blank Microelectronics Systems and Devices Owen Bishop OXFORD AUCKLAND BOSTON JOHANNESBURG MELBOURNE NEW DELHI Newnes An imprint of Butterworth Heinemann Linacre House Jordan Hill Oxford OX2 8DP 225 Wildwood Avenue Woburn MA 01801 2041 A division of Reed Educacational and Professonal Publishing Ltd A member of the Reed Elsevier plc group First published 2000 Owen Bishop 2000 All rights reserved No part of this publication may be reproduced in any material form including photocopying and storing in any medium by electronic means and whether or not transiently or incidentally some other use of this publication without the written permission of the copyright holder except in accordance with the provisions of the Copyright Designs and Patents Act 1998 or under the terms of a licence issued by the Copyright Licensing Agency Ltd 90 Tottenham Court Road London England W1P 9HE Applications for the copyright holder s written permission to reproduce any part of this publication should be addressed to the publishers British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0 7506 4723 X Printed and bound in Great Britain Contents Preface Section A The Hardware 1 Systems in action 2 The CPU 3 Memory 4 Inside the CPU 5 Interfacing 6 Planning the system
81. O 74LS244 octal tristate buffer 74LS373 octal tristate latch 74LS574 octal D type flip flop 6116 or similar SRAM RS232 output and input ICs Programmable parallel interface 8255 PPI 6522 VIA M68230 PIT or the Z80 PIO Programmable serial interface 8250 UART 16550 UART M68661 DUART M6850 ACIA Z80 SIO Analogue to digital converter CA3304E ADC0804 but see Topic XX Digital to analogue converter Or any other processor support IC of interest Data sheet Power supply regulated This may be provided by an on board regulator or taken from an external PSU Microcontroller Test bed Depending on the system you could build this on a breadboard or on stripboard Your prototyping system may have one already Study the data sheet and design and set up a suitable test bed Investigate the functions of the IC by writing suitable programs Suggestions for investigations include Real time clock Write a clock calendar program to display date and time Investigate the alarm function Install a solid state siren on the test bed and program the clock to sound it at a given time Investigate the alarm interrupt function Use it in a program that say flashes a red LED 203 Programming projects continuously but when interrupted flashes a green LED five times before going back to resume flashing the red LED Set the interrupts to occur every minute Investigate the periodic interrupt function 74HC164 74HC165 74LS244 74LS
82. One reason is that we write assembler in mnemonics instead of in opcodes A mnemonic is a group of up to four letters to represent each opcode and there is a different mnemonic for each opcode The point of mnemonics is that the letters help us remember what the operation does Example The instruction 00110111 on the data bus is the equivalent of 37 The opcode 37 tells the CPU to set the carry bit in the status register If we are programming the system in assembler the corresponding mnemonic is STC This is easier to remember because the three letters remind us of the action SeT Carry So we type in STC and the assembler program converts this to the opcode 37 and adds it to the program Different assemblers may use slightly different mnemonics but the Assembler 113 Instructions Learning assembler The only way to learn how to use an assembler is to practise writing programs There are many suggestions for programming exercises in this book Many different microcontrollers are suitable for this work and each has its own assembler program The example in this section is based on the assembler for one of them the Atmel 1200 microcontroller The assembler you use may differ in detail from the examples given here However the examples used in this chapter are short and they are simple in structure so it is easy for you to find the equivalents on your assembler principle is the same for all Some of the mnemoni
83. Pentium and the AMD Athlon The Z80 is contained in a 40 pin package Its pins fall into seven main groups power lines the Z80 like most microprocessors and other devices in the system runs on a 5 V supply data inputs outputs there are 8 of these allowing a byte of data to be read or written by the IC address outputs there are 16 of these providing 64K see box locations in address space More advanced versions of the Z80 Microprocessors Microelectronics Systems and Devices 26 such as the eZ80 have 24 address outputs system clock input reset input control inputs there are six of these The inputs control the rate of processing or allow processing to be interrupted or slowed control outputs there are seven of these used by the CPU to communicate with the rest of the system The internal structure and activities of the microprocessor are described in Chapter 4 Here we look at the ways in which the microprocessor communicates with the other parts of the microelectronic system Architecture of the system A microcontroller system usually has I O connections radiating to the input and output devices Fig 2 5 By contrast a microprocessor system has a single central connector with branches going to the microprocessor and all other units in the system This arrangement of connectors is known as a bus The bus was first referred to in Fig 1 5 but in the more det
84. QWE bit square wave enable must be set in Register B all other bits being 0 The SQWE bit is B3 Using the write program make location 11 and value 8 When the settings are complete a frequency meter applied to pin 23 shows a frequency of 256 Hz There are several other functions in the 146818 which can all be investigated by using methods similar to those described above It is left to the reader to work out how to implement and demonstrate these functions The hardware A test bed for the IC is constructed on stripboard It is connected to the Stamp prototyping board by two ribbon cables one of 8 lines for the address data bus and one of four lines for the control There are also two power lines a ground 0 V line and the regulated 5 V supply from the Stamp A 100 F electrolytic capacitor is connected across the supply lines where they enter the board to decouple the supply A clock and pull down circuits on pins 18 and 22 are installed as shown in the diagrams in the data sheet see also Fig 12 2 Pin 20 is wired to the positive supply but as the clock output pin pin 21 is not being used pin 20 could equally well be connected to ground 202 Microelectronics Systems and Devices Activity 12 1 Investigating a support IC Investigate the functions of an IC following the same procedure as described above for the real time clock You need IC This could be Real time clock 74HC164 SIPO 74HC165 PIS
85. These are concerned with handling individual bits in a register and setting or resetting some of the flags in the status register ROL is short for ROtate Left Each bit in the register is shifted one place to the left The operation involves the carry bit C of the status register When the bits are shifted the bit in carry is shifted into the register as the LSB The MSB in the register is shifted into the carry bit Suppose that a register holds these bits The register holds 105 in binary A left shift puts 0 into the LSB and shifts 0 into the carry The register now holds 210 Shifting a binary number one place to the left is equivalent to multiplying it by 2 Left shifting is the basis for some multiplying routines After a second left shift we have The LSB holds 0 from the carry but the carry now holds 1 from the previous MSB Including the carry as a ninth bit the value now equals 420 It has doubled again A third shift gives The LSB holds 1 from the carry bit The carry holds 1 from the previous MSB There is no doubling this time because of the rotation The carry bit has gone into the LSB instead of being shifted along to give a tenth bit 142 Microelectronics Systems and devices CLZ or CLear Zero flag changes the zero flag from 1 to 0 or leaves it as 0 if it is already 0 It is an example of several similar instructions for altering the carry flag C the negative flag N and others The programs above
86. They also accept serial data check its parity and produce a parallel output The Z80 SIO serial input output is an IC that includes all the necessary facilities on the same chip Fig 5 10 The SIO is a very versatile IC and has several operating modes We will look at the features that are of general interest The SIO has two channels A and B that are separately programmable As with the PIO the CPU communicates with the IC as if it consists of four registers a data register and control register for Channel A and the same for Channel B The CPU controls the SIO by writing bytes into the control registers With serial communications it is essential that both the transmitting 85 Interfacing Figure 5 10 Comparison with Fig 5 4 shows that many of the connections of the Z80 serial input output device are the same as for the Z80 PIO In the right the figure shows only Channel A The lines of Channel B are identical except that it has no TxC and RxC inputs Both channels share the baud rate generator station and the receiving station are set to receive data at the same rate This is usually specified by stating the baud rate see box p 81 There are a number of standard baud rates ranging from 110 to 38400 The baud rate generator may be a simple crystal controlled TTL oscillator or there is the Z80 CTC generator Also available are special baud rate generators such as the MC14411 The timing chain of the SIO can be set to divide th
87. This may not be enough to affect the operation of a logic gate powered from the track but the voltage spike will be greater and more effective if the current is greater the track is longer the track is narrower Microelectronics Systems and Devices 102 inductance of the track and leads in combination with the capacitor produces a resonant circuit This will cause ringing whenever there it a change of logic level with unpredictable results For the very fastest logic there are IC sockets which include the capacitor directly under the IC For simpler slower circuits there is less problem with decoupling A few 100 nF capacitors should be scattered around the board one capacitor to every 5 or 6 ICs It is also a good idea to decouple the supply with a larger capacitor say a 47 F electrolytic where the supply lines enter the board Another major problem is crosstalk which is the picking up of a signal by an adjacent track This is particularly serious in digital circuits because the signals have rapid rise and fall times Changes in level are equivalent to a very high frequency signal Where two tracks lie side by side for a few centimetres there is capacitance between them At very high frequency the impedance of a capacitor is reduced to a minimum Signals pass freely across from one track to the next To avoid this effect we can increase track spacing keep parallel runs of tracks as shor
88. a general purpose register to a port register A short program demonstrates the action of OUT Demonstrating the action of OUT ldi r19 255 all 1 s out 17 r19 to ddrb ldi r20 165 binary 10100101 out 18 r20 to portb register ldi r20 0 binary 00000000 out 18 r20 to port b register The first step is to configure all lines of Port B as outputs This is done by using out to write 11111111 into data direction register B which has the address 17 in the I O memory space Then the data from r20 is copied to the Port B register which has the address 18 This could be checked by looking at the port register using simulator software or by using a multimeter to measure the output voltage at each pin In the 1200 0 means a logic high output so D1 D3 D4 and D6 should be high and the rest low In the last two lines out is used a second time to make all outputs high You will need to single step the program to give time to check outputs after the sixth line ST tells the CPU to STore data in one of the registers ST is followed by the number of the register in which the data is to be stored However unlike ADI it does not immediately declare the register 138 Microelectronics Systems and devices number Instead it gives the CPU the number of another register r30 also known as register Z which is holding the number This is rather an indirect way of copying data so it is referred to as store indir
89. a maths coprocessor which is specially designed to take over the more complex mathematical operations of the processor This gives an advantage of speed when running programs in which mathematical operations predominate The 80286 known as the 286 was the successor to the 8086 in a 68 pin package with a 24 bit address bus and a 16 bit data bus It can access up to 16K of memory This featured a number of improvements and was succeeded by the 386 the 486 and the Pentium processors The 386 is a true 32 bit processor with all its registers 32 bits wide It can access 4 Gb of memory Some versions of the 486 included the maths coprocessor on the same chip The Pentium includes this as a regular feature At every stage in development from 286 to Pentium there have been improvements in performance There have been increases in the number of transistors now over a million the number of pins the number of instructions in the set and the maximum clocking rate 37 The CPU This chapter describes some of the features of the 1200 the PICs the Z80 the 6502 and the 8086 family Select one or two other popular processors from the same or different families and briefly study the manufacturer s data sheets Write a short account of the main features of each processor and describe their advantages or usefulness An imaginary microprocessor has an 8 bit address bus Design a decoder that responds when t
90. able in a library file Examples of these are printf which prints values and text on the monitor screen The matter to be printed is included inside the brackets This is the way the data is being passed to the function A function of a different kind is isalpha This is passed a value which may or may not be an alphabetic character The output of the funtion is 1 true if the value is a character or 0 false if it is not A function such as this is useful when checking input from a keyboard to make sure that it is of the expected form There is an extensive range of ready made functions in C which make it possible to write programs that are compact and reasonably easy to follow The other kind of C function is written by the programmer An example is a function that is passed the value of a telephone number as input and produces as output the signals to drive the dial tone generator When the functions that a C program requires have either been found to exist already in the standard function library files or have been written by the programmer they are tied together by the function that all C programs must have the main function In length this may be less than a quarter the length of the whole program It consist principally of calls to the other functions These in turn may call the 181 Structured programs functions that they need We can see how this structuring of the program mirrors the breaking down of a complex operation i
91. ailed drawing of Fig 2 8 it can be seen that there are actually three busses running side by side Each bus is connected to all the units of the system The three busses are concerned respectively with data addresses and control match this statement with the list of pin types in the previous section Note that the data bus carries signals from the CPU to other parts of the system and also carries signals to the CPU from other parts There is two way communication The address bus and individual lines of the control bus communicate in only one direction Figure 2 8 A typical microprocessor system has all its parts connected to the data address and control busses 27 The CPU Continual upgrading The reader should be aware that the microelectronics industry is forever active in producing new devices and improving the performance of the old ones It is impossible for a book to be really up to date Specifications can and probably will change between the date this book is written and the date that the reader gets it even though the dates are less than a year apart The best way for the reader to keep up to date is to study the various electronic magazines and to browse the electronic sites on the Internet Arrangements of pins In the Z80 the terminal pins are arranged in two rows along either side of the ICs as in Fig 2 1 except that there are 40 pins more in some versions This is known as the dual in line DIL layout CPUs
92. al to draw a diagram first then write the program 169 Structured programs Diagrams of programs Diagrams help planning and proper planning is essential if programs are to operate correctly operate efficiently be easily readable and understood by other people be easily readable and understood by the program writer a few weeks later be easily adapted and modified at a later date Flowcharts A flowchart is a method of representing the way a program is constructed Each small section of the program perhaps only two or three opcodes or mnemonics long is represented by a box Fig 11 1 Inside the box we write in a concise way what happens at that stage Boxes are connected by arrows which indicate the direction of flow of the program The best way to understand the meaning of a flowchart is to look at one drawn to represent one of the simple programs we have already studied Figure 11 1 Symbols used for drawing flowcharts Microelectronics Systems and Devices 170 Figure 11 2 A flowchart of the delay routine clearly shows the nested loops The line labels are added to the flowchart to make it easier to relate the flowchart to the listed program Fig 11 2 illustrates the delay program of Chapter 10 Begin at the terminal box labelled Start and follow the arrows When you come to a decision box decide whether the answer to the question is yes or no and follow the ap
93. already know how to use the computer Find out how to enter programs into the assembler Try out any working examples for beginners that are explained in the manual You are not expected to memorise or to use all of the instruction mnemonics As you work these activities write on a piece of card the mnemonics and directives that you actually use This will give you a quick reference list of the most useful to you of the mnemonics and directives Look for the mnemonics and directives that most closely match the ones we have used in the example program above Use these to write a version of our program that will make your chosen microprocessor or controller perform the same task The set of registers you have available may be similar to those of our 1200 microcontroller but may be different If they are different remember that the aim of the program is to get the CPU to load a number to increment it and to store it in another register If you do not have enough registers for this store it at an address in RAM Assemble the program you have written and correct any errors that you have made Run your program single stepping it and watching the changes that occur in the register the program counter and the status register Activity 7 2 Adapting the program You need A computer with assembler software installed A user manual for the assembler Your card of frequently used mnemonics 121 Instructio
94. and can appear to have an input of 1 Another point to consider is that if we run the program and then we operate the switch or button the program runs so fast that it is likely to have ended before the switch is closed The input routine must wait for the switch to be closed Here is an assembler input routine One bit input routine equ ddrb 17 ddrb address equ portb 18 port B address equ bit0 1 set a constant ldi r20 254 binary 11111110 out ddrb r20 register 0 as input rest as outputs start in r16 portb read port andi r16 bit0 mask bit 0 brne start if Z 0 back to start ldi r17 255 if Z 1 continue We will discuss this program in a little more detail than there is room for in the comments The first stage in this program is to allocate labels to two addresses in the I O registers The data direction register of port B is at address 17 and this address is now conveniently labelled ddrb The actual I O registers of port B are at 18 which is now labelled portb As well as labelling addresses the equ directive also labels constants The example here called bit0 has bit 0 high and the rest low With the 1200 a register is set as an output by setting its direction register to 1 If the direction register is reset to 0 the register is an input Some processors may work the other way round to this The program begins by making Port B register 0 an
95. and port D larger 40 pin members of this series also have ports A and C Port B has 8 pins PB0 to PB7 while port D has only 7 pins PD0 to PD6 The bits of each port the individual pins can be programmed as inputs or as outputs reset this line is held high when the CPU is running A low level 0 V on this line resets the system Microcontrollers Figure 2 1 The pinout of the 1200 illustrates the way we number the pins of ICs when there is a row of them down each side of the IC As viewed from above pins pointing down to the circuit board pins are numbered from 1 down the left side continuing up the right side A notch indicates the end of the IC where pin 1 is located Some makers mark this end with a dot or a stripe instead Microelectronics Systems and Devices 20 Figure 2 2 Serial transfer of a byte of data uses only one data line but takes about 8 times as long as parallel transfer Numbering digits When a number consists of more than one digit they are numbered from right to left starting with number 0 For example in the number 5239 D0 is 9 D1 is 3 D2 is 2 and D3 is 5 D0 is always the least significant digit LSD In a 4 digit number D3 is the most significant digit MSD In the 1220 IC Fig 2 1 the eight terminals of port B are numbered from PB0 LSD to PB7 MSD using the same rules Parallel and serial data Digital data are transferred from one place
96. approximation or guess at the correct output is 1000 the half way point There is a digital to analogue converter that converts this approxima tion to the equivalent analogue voltage This is then compared with the input voltage If the guess is less than the input voltage we know the approximation is too low The MSB stays at 1 and the control logic goes on to try the next digit If the guess is more than the input voltage it is too high The MSB is reset to zero and the logic goes on to try the next digit If the guess is too low setting the next digit gives 1100 from the SAR This too is converted and tested against the input voltage Again the digit is either retained as 1 or reset to 0 The routine is repeated at each clock cycle working along from the MSB to the LSB After 4 cycles conversion is complete and the END OF CONVERSION output of the IC goes high The data is then present on the parallel outputs of the converter With some ADCs the data is presented at a serial output MSD first The time taken for the conversion is roughly proportional to the number of bits in the output so higher precision is obtained at the cost of longer conversion time One of the problems with successive approximation is that a rapidly changing input may be impossible to evaluate The homing routine does not work properly with a moving target If this is a problem a sample and hold circuit is used to sample the input
97. are intended simply to demonstrate some of the things that a CPU can do Now we will look at ways in which these actions of the CPU can be used to do something useful Before a system can process data it must be able to accept data from the outside world After it has processed the data it needs to be able to pass it back to the outside world The next chapter describe routines for inputting and outputting data A loop The program demonstrating BREQ p 140 includes our first example of a loop The CPU is made to repeat a section of the program This is a conditional loop because the CPU is made to jump out of the loop when a certain condition becomes true In this example the condition is that r16 has been decremented to zero The branch command tests the zero flag Z and makes the CPU jump out of the loop when Z 1 Activity 9 1 Investigating mnemonics The short programs in this chapter are based on the instruction set of the 1200 microcontroller using the Atmel AVR Assembler for Windows It is likely that the reader will be using a different microcontroller or microprocessor and will be programming it with a different assembler or directly in machine code or in a high level language such as C or Basic The programs in this book are intended to illustrate the important features of processors in general and ways of programming They show the reader how to tackle the programming of any processor using any programming system
98. ata bus 3 Three state outputs are used A for putting addresses on the address bus B to send interrupts C for reading data from the data bus D to isolate internal circuits from the data bus 4 A system has an address bus that is 14 bits wide The most significant bit is also known as A A14 B A0 C A13 D A1 5 An example of an integer is A 24 B C 3 76 D 38 6 One of the logic families most suited for high speed operation is A ECL B TTL C 74LS00 D CMOS 7 A logic family most suited for battery powered equipment is A TTL B ECL C CMOS D 74LS00 41 Memory Memory The two main types of memory are random access memory RAM and read only memory ROM Two types of RAM are static RAM SRAM and dynamic RAM DRAM There are several types of ROM including mask programmed ROM PROM EPROM and EEROM Decoders are used to route signals to or from a specified location in memory A real time clock relieves the CPU of timekeeping duties There are two main types of memory Random access memory used for temporary storage Data may be written into it at any time and later read from it The data is lost when the power supply is switched off Used for the storage of data and for programs copied from more permanent data stores such as magnetic disks Read only memory used for permanent storage In most types the data once written into it can not be changed Used for storing prog
99. be seen that except for a CMOS4000 output driving a 74LSXX input fanouts are sufficiently great for most designs A problem arises with certain microprocessors that have low fanout Buffer ICs may be needed to prevent the bus from being overloaded In the case of TTL gates which includes gates of the 74LSXX series the minimum output voltage 2 7 V that counts as logic 1 is less than the minimum logic 1 input voltage 3 5 V to a CMOS gate or 74HCXX gate running on the same 5 V supply This is shown in Figure 2 12 A high output sent from a TTL output to a CMOS or 74HCXX input may fail to be recognised as high Because of this any connection between a TTL output and a CMOS or 74HCXX input needs a pull up resistor of 2 2 k wired to the supply line Alternatively substitute a 74HCTXX device for the CMOS device as the input and output voltages of the HCT series are compatible with TTL Many chips including RAMs and some CPUs are made using NMOS technology NMOS is compatible with CMOS so what has been said 101 Planning the system about fanout of CMOS also applies to NMOS When a gate changes state it produces a change in the amount of current drawn by the gate and by any unit being driven by it The result is a sudden change in voltage on the power lines In other words there is noise The situation is worse with synchronised logic in which many gates change state at the same time Conversely devices such as flip flops and
100. ble for outputs taken directly from a microprocessor begins in the same way as the previous one but then goes on to light a lamp or LED after loading a value into r17 One bit input output routine equ ddrb 17 ddrb address equ portb 18 port B address equ bit0 1 set a constant ldi r20 254 binary 11111110 out ddrb r20 register 0 as input rest as outputs out portb r20 outputs all low start in r16 portb read port andi r16 bit0 mask with bit 0 brne start if Z 0 back to start ldi r17 252 if Z 1 Put binary 11111100 in r17 out portb r17 light the LED 152 Microelectronics Systems and Devices Programming in PBASIC A push button or switch is connected to pin P0 as before An LED is connected to pin P1 as in Fig 10 1 The BASIC version of the one bit input output program is as follows A detailed analysis of the program shows that The program begins the in the same way as before An extra line before the waiting routine makes all outputs low by storing 1 in each register that is an output The waiting routine is as before The input is masked to find the state of bit 0 see box In the final stage when the switch is closed r17 is loaded with a value that has zero at bit 1 A 0 in the output register makes the output go high so lighting the LED This is the other way round from many microcontrollers in which 0 low output and 1 hig
101. by connecting it to the 0 V supply line When the target address is on the bus all three inputs to the 7410 go high Its output goes low This pin is wired to the CHIP ENABLE input of the device that is being addressed This causes that one addressed device to become active There is more about addressing in Chapter 3 Figure 2 10 It takes several logic gates to decode a single address The diagram shows the logic levels present in the decoder when and only when the target binary address 0010 1110 0110 1011 2D6C in hexadecimal is present on the 16 bit address bus The 1 k resistor reduces flow of current to the unused inputs of the 74133 and so reduces the risk of damage from voltage spikes on the positive supply line Microelectronics Systems and Devices 32 The control bus The control bus consists of an assortment of lines with various func tions Some of them carry signals from the CPU to other devices Some carry signals from other devices to the CPU They may run side by side for part of their length but their routing depends mainly on their function The composition of the control bus depends on the needs of the microprocessor The Z80 control bus comprises these six input lines Symbol Symbol FFuFunctionFunction DetaDetails BUSRQ Bus request Forces CPU to let device use the bus CLK System clock Controls timing of cycles INT0 Interrupt request 0 Device interrupts CPU NMI Nonmaskable inter rupt Inte
102. cache first If it is there it uses it If it is not there the CPU looks in the normal RAM In some systems data is stored in Level 1 L1 and Level 2 L2 caches L1 is on the CPU chip so it is almost instantly available The L2 cache is fast acting DRAM on the computer board which is not so quickly read as L1 but is faster than the usual SRAM Some processors have separate caches for data and for instructions The instruction cache holds the data when it is first loaded and from there it goes to the prefetch buffer Floating point unit The floating point format works with large positive and negative numbers that are stored in four bytes but is more complicated to process Processing in the accumulator using software routines is slow A floating point unit is a logic circuit specially for processing numbers in floating point format and is appreciably quicker It may also include circuitry for multiplication and division MMX Multimedia extensions are routines intended for speeding up graphics and sound processing in multimedia applications They include SIMD instructions which stands for single instruction multiple data For example a single SIMD instruction can be used to change the colour of many pixels at the same time Branch prediction When a processor has performed a certain operation it is more than likely that the program will require it to perform that same operation again For example when the program has a loop in it the pro
103. calculation particularly in processors that have only a few registers With a simple PSH a value is quickly pushed from the accumulator on to the stack and may be recovered at a later stage with a simple POP instruction The stack is also used for registering the state of the processor when a jump to subroutine instruction is executed or when the microprocessor is responding to an interrupt Before going to the subroutine or interrupt routine the controller stores the current contents of the accumulator the status register and the program counter on the stack As soon as the microprocessor returns from the subroutine or interrupt it recovers all this essential information from the stack It then continues operating at the same point in the program that it had reached before it jumped or was interrupted 187 Structured programs Activity 11 2 Subroutines 1 Rewrite the assembler subroutine program in another assembler or a high level language 2 If you are working in assembler with a simulator program single step the program and watch the program counter as it goes to the subroutine Watch the stack pointer Try to find the location of the stack in your system Watch the program counter as the CPU returns from the subroutine 3 Revise the program so that a value speed is passed to the subroutine to control the length of the delay 4 Revise the flash routine so that the LED flashes faster on each successive loop 5 Find out about th
104. certain what topics it covers Then the reader moves rapidly from one part of the sheet to another picking up facts here and there and gradually summarising the more relevant ones into two or three pages of notes The following paragraphs outline the information which was gleaned from the data sheet and is needed for programming the 146818 as a clock 195 Programming projects Pinout The pinout Fig 12 1 has been marked to show which are inputs and which are outputs This is done because input pins must nearly always be connected to something Output pins can usually be left unconnected at least during the early stages of development There are eight address input pins which are also data output pins These and four of the control pins are to be connected to a Stamp2 which will be used to command the IC The data sheet explains that during read and write cycles the address is placed on the bus by the controller At a later stage in the cycle data for writing is placed on the bus by the controller or data for reading is placed on the bus by the clock The data sheet has diagrams to show what other connections need to be made to the IC The most important is the external clock circuitry consisting of a crystal and a few resistors and capacitors Fig 12 2 There is a choice of frequencies In this project we use a 32 768 kHz crystal Figure 12 1 The pinout of the real time clock IC has been marked to indicate which pins are inpu
105. cessor has to jump back to the beginning of the loop every time it reaches the end of it It may run round the loop hundreds of times At any stage it is safe to predict that it will jump back to the beginning of the loop when it comes to the end It is only on the last time round the loop that this prediction will be wrong Special routines within the CPU are used to store the instructions most recently used on the assumption that they will be used again Coordinated instruction set and compiler As will be explained in Chapter 7 a compiler is a program that lets the programmer key in the program in a form that is more understandable than machine code After it has been typed in the compiler turns the program into machine code that is into instructions that the processor understands Usually the instruction set is designed by the electronics engineeers who design the layout and logic of the chip After that stage is complete the compiler is written by a software designer This may lead to problems when it is found difficult to write an efficient program for some of the 66 Microelectronics Systems and Devices Figure 4 2 Pipelining is a technique for speeding up the reading of data Without pipelining a a byte may take 3 clock cycles to reach the ALU A new byte arrives every 3 cycles With pipelining b a new byte arrives every cycle instructions For many of the more recent processors including the 1200 the instruction
106. ch except to an expert in Z80 machine code 59 Inside the CPU Here is the list of codes with a explanation of what the CPU does as its reads them 06 Load into register B the number that comes next 59 59 78 Copy the value in B to the accumulator A 06 Load into register B the number that comes next 23 23 80 Add register B to register A and leave the result in A 77 Copy the contents of A into the address that is in the HL registers we assume that there is already an address in HL 76 Halt operations This table shows the contents of the registers and memory location at each step of the program Notes 1 numbers are in hex and the addition is hex 2 values are copied from one register to another not moved 3 everything the CPU does is done in a series of many short simple steps The example shows how the CPU is told to add two numbers and store their sum in memory Basically all CPUs perform this operation in the same winding way So all CPUs need a similar structure in order to do it Fig 4 1 illustrates the main features found in most CPUs The CPU comprises Control unit This is a highly complex logic circuit which finds out what the microprocessor must do next and oversees the doing of it There are several stages in its operation and it steps from stage to stage Code A B HL 0173 06 0 0 0173 0 59 0 59 0173 0 78 59 59 0173 0 06 59 59 0173 0 23 59 23 0173 0
107. consists only of the start label and an if then statement in which in0 is the input read from pin P0 There are two debug statements to monitor the action of the program In this program debug is being used to pass messages back to the programmer that certain stages of the program have been reached Each time the CPU reaches a debug statement the message is displayed on the screen in the debug panel When the program is run the CPU goes though the loop repeatedly causing Running to be displayed each time The word Running repeatedly scrolls up the debug panel at high speed When the button is pressed the word Done is displayed just once and the program stops The debug statements are deleted when the program has been found to operate correctly leaving a very short and understandable program Its three lines are equivalent to eight lines of assembler 150 Microelectronics Systems and Devices One bit output One of the simplest output devices is a lamp switched by a transistor Fig 2 7 An even simpler output circuit is a light emitting diode LED with a series resistor to limit the flow of current The circuit of Fig 10 1a requires about 10 mA so this is suitable for a microcontroller output pin which can usually supply up to 20 mA Microprocessor outputs can not usually supply as much current so a transistor switch is needed as in Fig 10 1b Assume in the following program that the LED is connected
108. croelectronic systems to employ special purpose ICs like this where there are simple repetitive tasks to be done The telephone has another special IC to generate the DTMF dialling signal for transmission to the base station Cellphones have circuits similar to cordless phones the main difference being that the cellphone communicates directly with the public system through a base station up to several kilometres distant There is usually an LCD message screen to display numbers dialled and other useful information Controllers in industry Microelectronic systems are widely used in industry This section describes an example of microelectronic control of a chemical process Fig 1 2 by a programmable logic controller or PLC The CPU with system clock its memory keypad and display are part of a single unit Fig 1 3 As in the telephone the heart of the system is a CPU This has access to memory for storage of the program and working data In some systems the whole memory or part of it is included on the CPU chip There is often a keypad by which the operator runs the system and there is a message panel on which the CPU displays information about the current state of the system 6 Microelectronics Systems and Devices Figure 1 2 In the manufacture of chipboard the bonding resin is made by heating a mixture of urea and formaldehyde A slider valve 1 controls the flow of urea from a hopper 2 to the processing kettle 3 The val
109. cs may be identical in different assemblers An assembler is written with a particular CPU in mind It has for example to take into account the widths and numbers of registers that the CPU has and whether or not they can be used for arithmetical and logic operations Some CPUs can do things that others can not do In such cases the assembler may include special mnemonics for managing these particular actions Although an assembler simplifies program writing by using mnemonics it still requires us to give the CPU its instructions systematically Example Here is a short section of program written in the assembler of the 1200 microcontroller ldi r17 37 inc r17 mov r5 r17 In the 1200 assembler numbers prefixed with r refer to one of the CPUs 32 registers Values are taken to be decimal unless prefixed by the symbol Given this information it is easy to see that this sequence means Load register 17 with the value 37 Increment register 17 Move or copy to register 5 the content of register 17 ldi 114 Microelectronics Systems and devices With the assembler program running in a PC or similar computer the program listing given above is typed in It appears on the computer screen just as shown above In this book we use a different style of type to indicate programs and other screen displays This style is the same as used by our assembler and many others to display text on the screen Ano
110. ct to cause a change of input level Then there should be a short delay after which the input is sampled again to make sure the key is still pressed The length of delay is important If it is too short debouncing is not effective If it is too long the response is sluggish A delay of 1 ms is reasonable as a starting point 204 Microelectronics Systems and Devices Lookup tables Sometimes a value that is required may be calculated from another value by using a formula For example to convert a Fahrenheit temperature into a Celsius temperature we can use a formula If it is needed several times in a program it can be defined as a function DEF FN FtoC f 32 5 9 Other pairs of values may not be related in a mathematical way For example the groups of Morse Code are not directly related to the letters and numerals they represent As another example the voltage produced by a circuit using a thermistor is not directly related to the temperature In such cases as these we use a lookup table If you are using assembler this is a table in ROM or it could be downloaded into RAM When using a high level language the data is stored in an array In assembler the table has a starting address incorporated in the program and we can access any particular value by offsetting the starting address by a given amount For example a table of the number of days in a month would be Month RAM Value 1 0100 31 2 0101 28 3 0102
111. d program Up to now we have not used this directive but it is important because in the 1200 the first 4 bytes of program memory are allocated for special purposes If the program is stored from byte 000 onward there is a risk that when the program is run its first four bytes may become overwritten with data The program would be corrupted and would not run correctly The first byte of the four is reserved for a jump instruction telling the CPU where the first byte of the actual program is stored When the assembler gets to the line org 000 it starts storing at 000 There it stores the opcodes for rjump start After that it is told to store the program from 004 onwards so it leaves 001 002 and 003 empty and begins storing the remainder of the program from 004 onwards The effect of this is that if ever the CPU is reset its program counter is automatically cleared to 000 It goes to 000 and there finds the instruction telling it to jump to 004 This jump takes it to the beginning of the program proper From then on it follows the prgram in the usual way subroutine that is to jump to the address where the subroutine begins The delay is called twice in this program By making it a subroutine we need to include it only once This saves both typing and storage space in memory This may be limited in amount in a microcontroller Note that in this program we do not pass any data to the subroutine or receive any
112. d to program them To keep the size of the book within reasonable bounds the book looks at the Zilog Z80 as a typical microprocessor and at the Atmel AT90S1200 as a typical microcontroller Both of these are readily available from the major suppliers such as Farnell RS Components and Maplin as well as from several of the smaller firms Other processors are mentioned where they show interesting differences from these two types With regard to languages the book concentrates on assembler for the Atmel controller BASIC and PBASIC for the Stamp Other languages are described including C The descriptions of these processors and languages are intended to exemplify processors and languages in general They are aimed at giving the student a wide view of the topic but it is not expected that students will centre their studies on these particular processors or languages In keeping with the syllabuses the book leaves the student with an unrestricted choice of devices prototyping systems and programming languages The book is a study guide suitable for class use and also for self instruction The main text is backed up by boxed off discussions and summaries which the student may read or ignore as appropriate There are frequent Test Your Knowledge questions in the margins with answers given at the end of the book Another feature of the book is the placing of short memos in the margins These are intended to remind the student
113. data from it This could have been done had it been necesssary using variables registers or RAM addresses The stack The stack is a region of RAM set aside for the temporary storage of data It works in a last in first out manner like the plate dispenser sometimes seen in canteens The stack pointer in the CPU holds the address of the address next above the top of the stack 185 Structured programs In some microcontrollers such as the 1200 the stack is small and is automatically operated When the CPU goes to a subroutine the address of the program counter is stored in the stack This is how the CPU remembers which address in the main program it jumped from When the CPU comes to the return command ret at the end of the subroutine it goes to the stack recovers the address and replaces it in its program counter The value is then incremented by 1 and the CPU goes to the program line following the one from which it jumped In other microcontrollers and in most microprocessors there are opcodes for using the stack Commonly used mnemonics are psh push to place a given value on the top of the stack and pop to load a value stored at the top of the stack Usually the push and pop operations take place between the top of stack and the accumulator register As an example of the function of the stack this table shows a section of RAM and the hex data stored there Address Stored data 3B2C 0 3B2B 0 3B2A
114. data or program to be put into the PROM It works through the memory locations one at a time either blowing a link or leaving it unblown Figure 3 2 A PROM consists of an address decoder a line for each memory location and a set of data output lines Where they cross there may be a link between the memory line and the data output line Figure 3 3 There is a transistor at every crossing point in Fig 3 2 linking the memory location to the data line through a fusible link This link may be blown when the PROM is programmed 45 Memory Example Fig 3 4 shows the links of memory location at address 011 3 in decimal Two have been blown and two left unblown After the PROM has been programmed it is ready for use as ROM in a microelectronic system The voltage V used for reading the ROM is not as high as it was for programming Turning on the transistor now does not blow the unblown links When a memory line is addressed it goes high and turns on the transistors that have unblown links Currents flow through these to the data bus and are equivalent to a stored 1 No current flows where a link is blown and is equivalent to a stored 0 In Fig 3 4 the blown and unblown links produce a nybble of data equivalent to 1010 Figure 3 4 The PROM is being programmed with data 1010 in loca tions 011 An extra high voltage is applied to the location line while the appropriate data lines D0 and D2 are connected to 0 V This blows
115. de locations in memory for them the first time each variable is used There is no use of punctuation marks at the end of program lines in BASIC Program lines just end with a carriage return ENTER on the keyboard instead of a semicolon There is no use of braces for limiting sections of the program In most versions of BASIC line numbers are used instead of labels Example If when the CPU gets to line 250 we want it to jump back to line 40 to increment counter we can use this statement at line 250 later in the program 250 GOTO 40 Many programmers prefer to avoid using GOTO as it may lead to sloppy programming that is difficult to understand later Instead they would use a properly constructed program loop as we shall describe later The same program written in PBASIC for the Stamp 2 looks different from the program above Part of the reason is that the microcontroller has a limited amount of RAM only 2 K so the program must be as compact as possible One obvious difference is that there are no line numbers This means that there must be labels on certain lines Example 1 written in PBASIC counter var byte trial var byte body CON 37 counter body again counter counter 1 trial counter The program begins with a title and in PBASIC we use an apostrophe to indicate a comment It is so much easier to understand what is happening in PBASIC programs that it is rarely necessary to append remarks to every line
116. e number the bits from D0 the LSB to D7 the MSB However a bus with only 8 bits can transmit only the integer values from 0 to 255 It takes more bits to represent larger numbers or to represent quantities more precisely The main way to allow for larger numbers and greater precision is to increase the number of bits to 16 32 or as in the case of the Intel Pentium and Digital Alpha to 64 The number of lines in the address bus limits the number of different addresses within the system see table p 30 The Z80 and many other microprocessors have an address bus that is 16 bits wide This allows it 64K addresses which is enough for a small system More powerful processors have more address lines such as the 68000 family with 24 bits and the Pentium with 32 bits disconnected from the internal circuit The logical state of the internal circuit can then have no effect on the bus Whether the data pins of a device are in the high impedance state or not is decided by the level applied to its CHIP ENABLE CE input If the chip is enabled CE low the outputs may be 0 or 1 that is the voltage at each output pin is either 0 V or 5 V The device puts this data on to the bus If the chip is disabled CE high the pin is in the high impedance state so it can not place data on the bus A control signal sent by the CPU to the CE input of the device ensures that the device puts data on the bus only when required At other times it is disconnec
117. e code and in what forms does it exist in a microelectronic system 4 Explain the function of the arithmetic logic unit 5 Descibe the status register of the Z80 or other named CPU and the meaning of any three of the flags 6 List the registers of the Z80 or other named CPU and describe their functions 7 List four ways in which the speed of operation of a microelectronic system may be increased 8 Write a general account of the architecture of a named microprocessor or microcontroller illustrated by a block diagram Explain how the architecture is related to the functions of the device 9 Rewrite the table on p 59 to show the computer performing the operation 172 35 these are decimals and storing it in 2A2C 10 Using the information given in the example on p 59 write a program in Z80 machine code to add 45 125 and 23 and store the sum in 143D 1 The binary number 11010011101011 is represented in hex by A BE43 B 34EB C 13547 D D3A3 2 A system is designed to have an address space from zero to 3FFF The number of address lines required is A 16 383 B 16 384 C 14 D 15 3 A processor that is programmed by only a small number of instructions is called a A RISC processor B microcontroller C CISC processor D PLC unit 4 Programs are stored in the SRAM memory of a computer as A bits B a series of 1 s and 0 s C arrays of transistors switched on or off D machine code Multiple c
118. e much more carefully laid out In summary ECL is difficult to use and is restricted to computer circuits where very high speed is the prime factor Problems on the CPU 1 Describe the main features of a named microcontroller details of internal structure not required 2 Explain the difference between parallel and serial transfer of data What are the advantages and disadvantages of each 3 Describe the main features of a named microprocessor details of internal structure are not required 4 Name the three buses of a microprocessor system Outline their features and functions giving examples 5 What are three state outputs and why are they essential on the data bus Why are they not needed on the address bus 6 Explain giving an example what is meant by address decoding 7 Describe the signals present on the control bus when a Z80 or other named microprocessor is writing data into memory 8 What is a system clock and why is it an essential unit in a microelec tronic system 9 Explain what bit nybble byte and word mean Microelectronics Systems and Devices 40 Multiple choice questions 10 What is meant by bus contention and how may it be avoided 1 When it is essential for a Z80 to be interrupted a low signal is put on A the data bus B the NMI line C the control bus D the INT0 line 2 Signals can be sent in either direction on A the address bus B the BUSACK line C the WAIT line D the d
119. e operation of the stack in the processor that you are using Write simple programs to demonstrate its action Interrupts The CPU is always busy handling the program proceeding from one instruction to another perhaps cycling indefinitely round a loop Events sometimes occur which require that the processor should interrupt what it is doing and take some other action Such events include Arithmetic error a typical example is division by zero A divisor in an expression may on occasion evaluate to zero Division by zero is not possible mathematically This kind of error is detected inside the CPU and causes it to interrupt itself Instead of going on to the next instruction in the program it jumps to a special interrupt service routine ISR This might cause an error message to be displayed Division by zero error The program is then halted and the programmer checks to find why such an error occurred and alters the program so as to prevent it Exception error Occasionally a CPU may be given instructions that it is impossible to obey such as storing a batch of data in an area of memory that does not exist An internal interrupt occurs The ISR may instruct the CPU to display an Exception error message and then close down the program Clocked interrupts If the system includes a real time clock this Microelectronics Systems and Devices 188 can be programmed to interrupt the processor at regular
120. e or more low Line 20 uses the command OUT address value The address is the address of the 4 bit I O register The value written is 00000100 in binary The top four bits are not stored see p 93 Bit D2 is high to make the output high Bits 0 1 and 3 are low because they are inverted and the outputs will be high This preliminary setting of the bits is required with this register Line 20 is the actual input line using state INP address This assigns the reading at the port to variable state The following line displays the value of A on the monitor The value that is obtained depends on whether the switch is open or closed If the switch is open the value is amp H04 and the program loops back to line 20 If the switch is pressed state amp H06 and Switch closed is displayed at line 40 149 Input and output Programming in PBASIC This input routine can be tested by connecting a switch to pin P0 as shown in Fig 2 6 The supply voltage must not be more than 5 V DC It is best to use the Stamp s own regulated supply available at pin 21 Here is the One bit input program written in PBASIC for the Stamp2 One bit input routine input 0 Make PO an input start debug Running if in0 0 then start Jump to start if 0 debug Done Print Done if 1 The first line configures pin P0 as an input the push button switch is connected to that pin The waiting routine
121. e program several times In machine code there are instructions to tell the CPU to jump and giving it the address in memory to jump to This is another cause of difficulty Calculating the exact number of bytes to jump over and expressing that number in hex is a frequent source of error If the program is subsequently altered perhaps by inserting extra sections of code many of the jump to addresses will be wrong It is then necessary to work through the program correcting all the addresses There is the further point that an address itself example E28A has little meaning to the programmer or to someone else reading the program Fortunately an assembler allows us to label the lines of the program so that the CPU can be told to jump to this named line Even if we insert new pieces of program the label stays with its line and there is no need to correct it Example The sample program processes the value stored in trial then has to come back to start work on the next incremented value It needs to jump back to the line in which the value in count is incremented We place a label again at the beginning of this line to indicate the line to which the CPU is to jump back Program example 1 introducing assembler def counter r17 def trial r5 equ body 37 ldi counter body loads r17 with body temp again inc counter incrementtemp mov trial counter store in trial again 119 Instructions Later in the p
122. e pulse rate by different amounts so that it is possible to send and receive at a selection of baud rates without having to change the clock The SIO can be programmed by coded bytes written into its control registers to operate according to a number of different protocols see Fig 5 7 It is possible to select for a character of 5 6 7 or 8 bits and to add 1 1 or 2 stop bits It is also possible to choose between even Microelectronics Systems and Devices 86 parity and to disable the addition of a parity bit if preferred If the system is to communicate with another system by way of the public telephone network it is necessary to use a modem This is a device which receives a serial transmission of high and low logical levels as in Fig 5 7 from the SIO and converts it into a form suitable for transmission over the telephone line This is known as modulation The demodulator section of the equipment converts the received modulated signal back into pulses at high and low logic levels One modulation technique is frequency shift keying FSK The logic lows and highs of the serial data are represented in the modulated signal by two different audio frequencies For example in the Kansas City protocol a 0 is represented by four cycles at 1200 Hz and a 1 is represented by eight cycles at 2400 Hz Thus each bit takes the same length of time to transmit Control signals between the SIO and the modem of each channel are sent by wa
123. e system has its own microcontroller as an integral part of it The microcontroller is programmed specifically to manage the action of the sensor or actuator For example consider an electric motor geared to an aileron in the wing of an aeroplane When the aileron is to be moved a command is sent from the central computer in the pilot s cockpit to the controller in the wing The controller is then responsible for moving the aileron to its new angle Normally it will accelerate it as fast as mechanical stresses allow until it reaches the maximum allowable rate of turning This controlled acceleration involves complicated calculations by the microcontroller based on previously determined parameters There must be feedback of the actual position of the aileron to allow for mechanical effects such as wind resistance The controller has been told the angle at which the aileron is to finish so before it reaches that position the controller begins to decelerate it at the maximum allowable rate so that it finally comes to rest at exactly the required angle While the action is in progress and when it is completed the processor reports back to the main computer The main computer may also interrogate it at any stage to find out what angle the aileron has reached This is a reasonably complicated operation in which factors such as wind resistance must be taken into account It is simpler for the task to be undertaken by an independent processor situated
124. e the function of a watchdog timer Questions on planning the system 107 Instructions Instructions The CPU reads a program in a series of fetch execute cycles in which it reads a machine code instruction and then obeys it Writing programs in machine code is difficult but easier if we use an assembler program and write it in mnemonics The assembler turns the mnemonics into machine code A program written in assembler can have a title and comments added to it to make it even easier to understand Part B The Software A CPU can do many different things but it does nothing unless it is told to It needs to be issued with instructions The instructions are issued to it in the form of a program stored in memory The instructions that a given CPU can understand and obey are its instruction set Different types of CPU have a different instruction set that is they have a particular collection of instructions suited to the architecture of the CPU Some kinds of CPU have a large instruction set with several hundred instructions in it CISC Others work on a much smaller set usually fewer than a hundred RISC Usually all the members of a family of CPUs have the same instruction set Some members of the family especially the newer ones may have a few additional instructions in their set 108 Microelectronics Systems and devices The CPU obtains its instructions by going to an address in memory and reading the instructions
125. ears Programs are loaded serially and there are six I O pins Members of this family are RISC processors The 12CE518 has only 33 instructions in its set so learning to program it does not take long All of its instructions except branches take 1 s to execute Branches take 2 s It has a built in timer that can function as a real time clock It also has a watchdog timer that has its own oscillator to ensure reliability Another RISC 35 instructions microcontroller is the 16F872 which is typical of the more advanced members of the PIC family It is in a 28 pin package which allows it to have three I O ports that are 6 8 and 8 bits wide It runs at 20 MHz It has 2K 14 words of FLASH reprogrammable program memory 128 8 bytes of RAM and 64 8 bytes of EEPROM On the same chip there are three timers a watchdog timer with its own oscillator a 10 bit analogue to digital converter a synchronous serial port It can be programmed to capture a 16 bit value at regular intervals which gives it application in data acquisition The captured data can be compared with a value in another register and produce an output signal if the two are equal There is also a pulse width modulator to generate pulses of a set length In total this is a very versatile controller with many applications in control systems and measurement systems There is a wide range of development equipment and software to help the PIC programmer These include as
126. ect The end result is the same as if we used MOV but the indirect feature is useful in certain programs as we shall show later A dry run helps explain the action of ST Indirection This is a technique that is often used in programs Instead of giving the CPU a value directly the CPU is told the number of a register where the value is already stored Example ldi r19 48 is a direct or immediate command It means load register 19 with the value that follows immediately 48 st X r16 is an indirect command It means look in register X actually r30 find the register number that is stored there and then store the contents of r16 in the register that has that number Register X acts as a pointer to another register Indirection makes programming more complicated but it has a useful feature A direct address the 48 in the example above is written into the program It cannot be altered while the program is running With indirection the number in X may be changed as the program runs Then it may point to any of the other registers In this way the value in r16 may be stored in different registers depending on the stage the program has reached Program Z R16 R20 mov ra rb 139 Mnemonics The number of the destination register r20 is placed in the Z register r30 Then we put some data 125 in r16 ST tells the CPU to copy the data present in r16 into the register pointed at by Z Branch inst
127. ed while the system is running A EEROM B SRAM C DRAM D Mask programmed ROM 2 Which type of memory is most suited to store the program in a reprogrammable microcontroller A EEROM B DRAM C PROM D Mask programmed ROM 3 How many address lines are needed to address all the locations in a 4K ROM A 10 B 12 C 14 D 17 4 How many locations can be addressed by a CPU with a 14 bit address bus A 16 384 B 8192 C 8191 D 32 768 5 The number of address lines of a ROM that stores 1 Mbits as bytes is A 16 B 20 C 17 D 10 6 To make a memory chip store data from the bus we must A make the WE line and CE lines low B make the OE line low C make the WE line high D make the CE line low 54 Microelectronics Systems and Devices 7 To make a memory chip place stored data on the bus we must A make the CE line high B make the WE and CE lines low C make the CE and OE lines low D make the OE and WE lines low 8 The typical access time of DRAM is A 70 ns B 100 ns C 25 ms D 250 ns 9 The typical access time of SRAM is A 70 ns B 10 ns C 250 ns D 1 ms 10 A kilobyte is A 1000 bytes B 1024 bits C 1024 bytes D 1000 bits 11 The kind of memory used in a microcomputer for storing the booting up program is A cache B SRAM C DRAM D ROM 55 Inside the CPU Inside the CPU The criteria for the choice of a CPU are briefly set out After an introduction to the natu
128. els on the four control pins programming the rtc writing loc var byte declare variables value var byte loc 2 their values value 12 dirc 15 as outputs to control outc 1 disable chip write routine outc 13 DS and WR high outc 15 AS high outc 14 enable chip dirl 255 as outputs to bus outl loc address on bus outc 12 AS low outc 8 WR low outl value data on bus outc 12 WR high outc 1 disable chip Work through this program to see how the sequence of outc instructions corresponds to the levels shown in Fig 12 2 see box The values of location and value are edited into the program before the it is run It would be an improvement to write a proper routine for inputting this data The program loads the selected location with the given value The data can be read back by using this program which is based on the read routine Fig 12 4 programming the rtc reading loc var byte declare variables value var byte loc 2 their values dirc 15 as outputs to control outc 1 disable chip read routine outc 13 DS and WR high outc 15 AS high outc 14 enable chip 199 Programming projects Stamp I 0 The Stamp has 16 I O pins P0 to P15 divided into a high byte and a low byte The low byte P0 to P7 is accessed by using the label l To set the data direction registers of the low byte
129. ent by the CPU is always immediately latched into the register and can be read by the peripheral at any time Other parallel port ICs There is a wide range of parallel port ICs available similar to the Z80 PIO but usually specialised to work best when connected to one particular CPU The 8255A PPI programmable parallel interface has three 8 bit ports programmable as inputs or outputs The individual pins can not be programmed but the high and low nybbles of Port C can be programmed separately Handshaking is provided The PPI has the same addressing as the Z80 PIO except that addresses F0 to F2 cater for the three ports and all control signals go to F3 As in the Z80 PIO all mode control operations involve sending a byte to the control register The bit set reset mode is of interest It allows the bits of Port C to be set or reset Only one bit can be set or reset at one time This feature is useful for producing strobe signals The Intel 68230 PIT parallel interface timer is designed for 68000 systems Port A and Port B are both definable as 8 bit input output or bidirectional ports Alternatively they may be combined as a single 16 bit input output or bidirectional port The ports may also be programmed for bitwise operation There is a set of lines for handshaking There is a third port Port C which can be used as a I O port but many of its pins are multipexed for other functions including acting as input and output for t
130. eparate lines for enabling reading or writing Some other CPUs such as the 6205 use a single line symbol R W A memory IC interprets this as a read operation when it is high and a write operation when it is low On studying the two tables above it can be seen that the majority of the control lines are used for handshaking between the CPU and other devices In other words the control lines are the means by which the actions of the CPU and of the other devices are co ordinated Microelectronics Systems and Devices 34 Other processors the 6502 Discussion of microprocessors in this chapter is centred mainly on the Zilog Z80 but there are many other processors in use They have individual features which may make them better suited for certain applications The Synertek 6502 is a relatively early processor that has been very successful in the past and is still in use today Its architecture has the basic features internal busses status register stack register program counter address and data buffers ALU that are in the Z80 It differs in having only an accumulator and no general purpose registers This means that all processing centres on the accumulator and ALU a difference that shows in the types of instructions in its instruction set Like the Z80 it has an 8 bit bi directional data bus and a 16 bit address bus It has two interrupt inputs NMI and IRQ which function in a similar way to those in the Z80 The 6502
131. ess allocations of a typical microelectronic system that has a 16 bit address bus are located next above the booting and ISR routines There are spare addresses in this space so that other devices can be added to the system when required The upper part of the address space is occupied wholly or partly by RAM The area is usually divided into two parts the program area and the data area The program area is used for holding the program on which the system is currently working With many complex programs there is not enough address space to hold the whole program at once Different sections of the program are loaded into the program area from the disk drive and replaced with other sections as the microprocessor requires them This is why you often hear the disk drive of a computer switching on and off automatically while you are running a program The top end of memory is generally used as a data storage area Here it stores data on which it is currently working including values obtained in the intermediate stages of calculations Data is being stored read and replaced continually as the system goes about its tasks 51 Memory A system can operate with only a portion of its program and data areas actually occupied with memory The system adjusts itself to work within whatever memory is available The larger the memory the larger the sections of a program that can be held there at one time The larger the sections the less often the micr
132. example it may be set to run in the 12 hour mode or the 24 hour mode Data sheets of computer chips usually include a number of timing diagrams like those in Figs 7 1 and 7 2 Exact timing is important if the clock is to operate at maximum speed The diagrams show the minimum times taken for addresses and data to settle on the bus and the minimum response time of the clock They show how long data and addresses must be left on the bus to ensure that they are loaded by the clock chip In general an IC will work just as well when it is taken through its stages of operation at a slow pace This project used a relatively slow controller programmable in BASIC so there was no need to aim for exact timings The most important point is the order in which control address and data signals are sent to the clock Fig 12 3 shows a write cycle The levels on the DS WR and AS pins have no effect until CE goes low enabling the chip The address must now be on the bus and is latched into the clock when AS goes low Making WR low read with DS high causes the clock to put data on the bus At the end of the cycle the lines return to their original state with CE high and the rest low Figure 12 3 The sequence of voltage levels high or low present on the control bus when the CPU is writing to an address in the RAM of the real time clock 198 Microelectronics Systems and Devices The next step is to write a program to put a sequence of logic lev
133. fected by the values of the top two bits These can take the values 00 01 10 or 11 but they are not decoded and make no difference to the result The effect of this is that there are ghost addresses with different values for the top two bits 49 Memory Example Consider the lowest address in chip no 5 which is 2800 Preceding this address by values of lines A14 and A15 can alter the value of the most significant hex digit The last two bits of this remain the same 10 but the first two may vary We can have 0010 0101 1010 or 1110 In hex these are 2 6 A and E The real address is 2800 and the ghost addresses are 6800 A800 and E800 Chip no 5 is enabled if any one of these ghost addresses is placed on the bus Allocation of addresses With a 16 bit address bus for example a system has 64K different addresses This is the address space of the system This does not mean that there is a device or memory location at every address waiting for the CPU to call it into action The address space is the range of possible addresses The addresses that are actually used depend on what devices are present in the system When you purchase a computer it may often have only a limited amount of memory Later you may want to expand the system by purchasing additional memory boards These plug into sockets on the motherboard which are already provided with decoders to place the new memory in a range of previously unused addresses T
134. fferent codes each corresponding with a different instruction So there can be 256 different instructions to the CPU which is more than enough for most of them Similarly there can be 256 different values 0 1 2 255 which is certainly not enough for the calculations we may want the CPU to perform However there are ways of expressing larger numbers negative numbers and also ways of expressing numbers with greater precision for example numbers such as 23 456789 In Chapter 7 we will explain the ways of coding values For the moment we are thinking only of single byte values The codes are not codes as we write or print them on paper They are the states of sets of eight flip flops if SRAM or eight MOSFET transistors if DRAM or eight fusible links if PROM and so on Programs and processing 57 Inside the CPU CISC and RISC Performing a logical operation by using a circuit of logic gates hardware is about ten times faster than running the equivalent software on a CPU Traditionally manufacturers increased the speed of their processors by including extra logic gates connected to perform a wider range of operations As a result a larger number several hundred of different instructions is needed to operate the CPU This type of organisation is known as a complex instruction set computer shortened to CISC Most of the 8 bit CPUs and more powerful ones such as the Intel Pentium and Motorola 68000 are CISC processors
135. ginal BASIC There have been many versions of BASIC written for different computers This lack of standardisation contrasts with the strict definition of C by ANSI This is one reason why BASIC has been less popular with the professional programmers who like to be able to write a program on one model of computer and quickly adapt it for running on other computers The original BASICs differ from most other languages in that the program lines are numbered Line numbers are keyed in as the program is being written Conventionally the lines are numbered in tens This allows extra lines to be inserted in the program later without having to disturb the earlier numbering Here is the sample program written in BASIC Example 10 REM Example 1 written in BASIC 20 body 37 30 counter body 40 counter counter 1 50 trial counter Line 10 begins with REM which stands for Remark The CPU ignores everything on the line following the REM statement The remainder of this program sample is easily read and understood provided that we remember that the symbol means assign the value of the variable constant or expression on the right to the variable on the left BASIC Microelectronics Systems and Devices 128 Setting out a BASIC program is much simpler than a C program It is not necessary to begin by defining variables and constants though you can do so if you prefer to If you do not define variables BASIC sets asi
136. gister in each to find which has its interrupt flag set Other methods are described later with reference to the Z80 As an alternative to interrupts a system may rely on polling the individual devices Each device has a register in which there is an interrupt flag In between its main activities the CPU interrogates each device to find out which one or more of them has its interrupt flag set It then takes action This system is the slowest of all as a device has to wait to be interrogated and by then it may be too late to avoid a program crash or a loss of data It may happen that the CPU is in the middle of a complicated routine which might fail if interrupted With some CPUs there is a disable interrupts opcode which causes it to ignore all interrupts until interrupts are enabled again by using another opcode We use these instructions to mask shut out all interruptions while a difficult routine is in progress This is a matter for the programmer to decide However there is usually a highest priority interrupt that is non maskable The Z80 has two interrupt input terminals When a logic low level is sent to the NMI terminal it causes a non maskable interrupt The falling edge of the interrupt pulse latches the input so that there can be no further NMIs until the current one has been dealt with As soon as it receives the NMI the Z80 goes to the address 0066 There it finds the address of the non maskable interrupt routine Since a non
137. gram really is It is the physical state of a 58 Microelectronics Systems and Devices set of transistors or other electronic storage devices When the byte is read it becomes a set of high and low voltages on the data bus L L L L L H H L To make it easier to read we represent low voltage by 0 and high voltage by 1 0 0 0 0 0 1 1 0 As the CPU works its way through memory reading the sequence of bytes it finds 00000110 01011001 01111000 00000110 00100011 10000000 01110111 01110110 This is what the CPU reads into its registers one line one byte at a time There the corresponding bits in the register are either set 1 or reset 0 At this stage we are back to flip flops that are set or not set Remember the program is really an array of set or reset fllip flops in memory or their equivalent transistors that are switched on or off Depending on which bits are high or low in each byte the CPU takes action accordingly There are complicated logic circuits in the CPU which on receiving a given combination of 0 s and 1 s cause the CPU to perform a particular action Although they mean a lot to the CPU rows of binary digits mean very little to us The strings of 0 s and 1 s are difficult to read To make them easier to understand we take the rows of bits as binary numbers and then convert them to their equivalents in hexadecimal 06 59 78 06 23 80 77 76 This does not mean mu
138. h output Programming in BASIC The PC has a switch or button connected to bit D0 Pin 1 of the 4 bit I O register and an LED connected to bit D0 of the 8 bit register 10 REM 1 bit input output routine 20 OUT amp H037A amp H04 REM Set up inputs 30 state INP amp H037A 40 IF state amp H04 THEN 20 50 OUT amp H0378 amp H01 Lines 20 to 40 are the same as before Note the reminder to make the outputs high in the 4 bit I O register so that pressing the button can pull D0 down The processor loops around lines 20 to 40 until the button is pressed Then it drops out of the loop and goes to line 50 There an OUT statement sends a byte to the 8 bit output port making D0 output high and turning on the LED 153 Input and output One bit input output routine input 0 output 1 pin P1 is an output out1 0 LED off start debug Running if in0 0 then start out1 1 make P1 high Two additional lines are needed to switch on the LED connected to pin P1 The second debug statement of the previous program is not needed now On running the program the word Running is repeatedly scrolled up the debug panel This stops when the button is pressed at which point the LED comes on The debug line is deleted when the program has been tested Activity 10 2 One bit input output If you have not already done so read the introduction to Activity 9 1 1 Adapt the example of a one bit input outpu
139. has 8 registers A to H each of which holds one bit of data The registers are connected internally so that the data in one register can be shifted into the register on its right It has 8 data inputs which in a computer could be connected to the data bus Bit rate and baud rate Both of these are used to express the rate of transmission of data They are not the same thing Bit rate is the number of binary bits 0 s or 1 s transmit ted in 1 second Faster tranmission rates are often ex pressed as kilobits 1024 bits per second Baud rate is the number of signal events transmitted per second If one signal event for example a pulse of given length or amplitude represents one binary digit the baud rate and the bit rate are equal But in many systems a signal event represents more than one bit Figure 5 8 A shift register is used to convert parallel data into serial data ready for transmission by a serial output interface This is a parallel in serial out PISO shift register Microelectronics Systems and Devices 82 Parity This is a technique for detecting errors in transmission When a group of 7 data bits is transmitted the number of 1 s is counted and an extra bit the parity bit is added at the end so that the total number of 1 s is even Examples If the seven bits are 1101010 the group already contains an even number of bits So the parity bit is 0 and the group transmitted is 11010100 If
140. has a wide range of addressing modes some of which help to make programming simpler One of these is zero page addressing which assumes that the first high byte of an address is 00 Instead of quoting the full address such as 005A the programmer quotes only 5A This shorter form saves program storage space and runs faster The 6502 makes use of its two index registers X and Y for addressing In indexed zero page addressing an address in zero page is given relative to a value stored in the X or Y register For example if the X register holds 24 and the address is given as 57 the actual address is found by adding these together to give 007B This feature has the advantage that the registers can have different values put into them as the program runs If the program runs in a loop X can be changed each time round the loop so that different addresses are accessed each time In particular X and Y can be incremented or decremented so the program can scan a table of data stored in consecutive bytes in zero page Indexed addressing is not restricted to zero page Other modes of addressing apply it to addresses in the whole memory space 35 The CPU The clock is a squarewave generator capable of running at high frequency Some microprocessors such as the 8085 and practically all microcontrollers include the clock circuitry on the chip so all that is necessary is to attach a crystal having the required frequency If a completely separate
141. he rate of addition of urea must be carefully controlled so that it does not overheat The CPU controls another actuator which is a water valve which admits cold water to pipes surrounding the kettle The program continually checks temperature and adjusts the rate of addition of urea and the rate of flow of cooling water accordingly 9 Systems in action continually reading input from the sensors taking decisions and sending the appropriate output to the actuators A typical program has a few hundred or thousand steps and take only a few tens of miliseconds to run so the system responds reasonably quickly to changes in the state of the inputs The example of the valve demonstrates that it is not enough for the CPU to instruct actuators to move the shutter There must also be sensors to check that the shutter is actually open or shut This is to allow for the fact that it may not have had time to move to the required position Or maybe it has jammed A system of the kind shown in Fig 1 3 is common in industrial plant whether it is a simple machine for filling cartridges with toner powder a vast printing press or a chemical plant producing insecticides The main difference from one system to another is in the types and numbers of sensors and actuators attached to the system The other main difference is the program that directs it The program for the PLC is written by the operator using special software running on a microcomputer The PL
142. he PIO and then to the periph eral A similar technique is used in Mode 1 the port as a data input If the PIO is ready to receive a data byte from the peripheral its RDY output is high The peripheral puts data on the port bus then sends a low pulse on the STR line This latches the data in the port register The RDY line is made low so that the peripheral does not send another byte until this byte has been read by the CPU The STR pulse causes the PIO to interrupt the CPU by putting a low level on the INT line The CPU is programmed to read the data from the register when it is interrupted The control signals at the end of the read operation cause the RDY line to go high indicating to the peripheral that it can now send the next byte The above description shows how the data is transferred with hand shaking so that if inputting the CPU will not miss any data Con versely when outputting the CPU will not send any data until the previous byte has been acknowledged These procedures can be sim plified in some circumstances At an input port the STR line can be held permanently low perhaps connected directly to the 0 V line Then any data that the peripheral sends appears immediately in the register and can be read by the CPU at any time For example the 77 Interfacing peripheral could be a DAC continuously providing data which is sampled by the CPU at regular intervals The output routine may similarly be simplified Data s
143. he appearance of a ladder with four rungs which is why PLC programs are described as ladder logic PLCs uses the X prefix for inputs and the Y prefix for outputs Annotation on the diagram helps the user to understand how the system functions Reading across from left to right in Fig 8 1 we see that if the receptacle is in place a proximity sensor detects this and location X1 goes logic high we can trace a continuous path across its symbol Then if the temperature is correct another continuous path if it is we can continue across to the symbol representing the pump motor This complete the path from left to right Current flows along the path and pumping begins The diagram shows an alternative path through a manually operated switch This means that an operator can start the pump provided the receptacle is in place but it is not necessary for the temperature to be correct Fig 8 2 sets out an elementary program for controlling the entrance to a car park In the first line of the excerpt shown here if an optical sensor detects that a car is waiting to enter and the barrier is down as sensed by a limit switch and the program is in Phase A barrier down ready for car to arrive a memory location M41 in the controller is set to a 1 The next line is line 70 Note that the lines need not be numbered Microelectronics Systems and Devices 132 consecutively This makes it easy to interpose additional
144. he binary address on the bus is 1100 0101 Build the decoder on a breadboard using CMOS or TTL ICs and test its action Logic families There are three main logic families Transistor transistor logic TTL Complementary MOSFET logic CMOS Emitter coupled logic ECL Each of these families has its own versions of the standard logic gates such as NAND and NOR Each family includes a range of ICs with more complex functions such as flip flops adders and counters The families differ in the way the basic gates are built and this gives rise to family differences in operating conditions and performance Fig 2 12 shows the logic levels used by TTL and CMOS families The main features of each family are as follows TTL Based on bipolar transistors Operates on 5 V DC which must be regulated to within 0 25 V The original 74XX series all type numbers begin with 74 is almost completely replaced by newer series with improved performance One of the most popular is the 74LSXX series which has a lower power requirement of about 2 mW per gate It has faster operation than standard TTL the typical propagation time per gate being 9 ns It operates with clock speeds up Microelectronics Systems and Devices 38 Bit slicing techniques While most systems are based on processors with a data bus width of 8 bits or more another approach uses 4 bit controllers Such a system may have 2 4 or more controllers
145. he data is transferred to the internal bus of the CPU If the data is an instruction it is stored in the instruction register While the last two states are occurring the CE line goes high and the data is no longer present on the data bus Assuming that the data is an instruction the logic of the control unit causes one of many different actions to occur Example The data register of an 8085 CPU holds the following byte 01001111 Fetch execute cycle 109 Instructions Figure 7 1 Reading a byte from memory involves a sequence of logic levels on two lines of the control bus The WRITE ENABLE line stays high as this is a read operation Data from memory is put on the bus when the CHIP ENABLE line goes low The latches of the register are either set 1 or reset 0 When the control unit receives this data it copies to register C a general purpose register the data that is in register A the accumulator This is an internal movement of data so a single byte is enough to tell the control unit what to do How the control logic works that is to say how the input of 01001111 makes the control logic take the action we have described is outside the scope of this book Example At another time the data register holds 00110111 This instructs the control unit to set the carry bit of the status register to 1 As above a single byte is all that is necessary for this instruction It is obvious that if
146. he on chip timer The timer is a 24 bit counter that has a value loaded into it and then counts down Depending on the programming it triggers various events when the count reaches zero The timer can be programmed to generate periodic interrupts a square wave of selected frequency or a single interrupt afer a preset period of time In addition to these functions it can be used to measure elapsed time As is usual with such devices the timer is programmed by the CPU writing codes into its registers The registers can also be read to discover for example the length of time elapsed since it was reset Microelectronics Systems and Devices 78 Signal Signal name 9 pin 25 pin TD Transmitted data 3 2 RD Received data 2 3 RTS Request to send 7 4 CTS Clear to send 8 5 DSR Data set ready 6 6 SG Signal ground 5 7 CD Carrier detect 1 8 DTR Data terminal ready 4 20 RI Ring indicator 9 22 The peripheral reads the data whenever it needs it ignoring the RDY line and not signalling back on the STR line The INT routine is disabled in the CPU so that it sends data whenever it has data to send In Mode 2 Port A is bidirectional and four handshaking lines are available The operating method is a combination of the input and output sequences described above When Port A is in mode 2 Port B must be assigned to Mode 3 the bitwise I O There is no handshaking on Port B in this mode Signals placed on the output
147. he program will remain in the processing stage indefinitely as in the cooler program The output stage is often a simple sequence because all the work of processing has been done and it remains only to light an LED or switch on a motor After that the program may come to an end or there may be a loop back to input more data process it and produce new output However output is not always simple The complex actions of a robot are an example of this Whatever the level of complexity the main point is to break everything input processing output down to the level of individual mnemonics or high level commands Draw diagrams Do a dry run Then program it It will still need to be tested which is one of the subjects of the next chapter Modular programming The discussion above assumed that the program is a single input processing output chain Most programs are more complicated than this For example the program for a cellphone would have several distinct parts waiting to receive a call receiving a call handling a call in progress terminating a call initiating a call recovering a number from data store dialling displaying data checking battery level Microelectronics Systems and Devices 180 Each of these activities can have a self contained program Such separate programs within the main program are referred to as modules Each module is programmed individually
148. he same applies with new input output devices that you may wish to add to the system Each must be allocated its individual address or range of addresses Often the operating system takes care of this automatically but there can be occasions on which two different devices are allocated the same address This always leads to problems of conflict The allocation of addresses may be visualised by drawing an address map This is often called a memory map because most of it is occupied by memory of various kinds The map in Fig 3 6 is typical of a small computer or microprocessor system with a 16 bit address bus capable of addressing up to 64 kilobytes The first kilobyte is given over to one or more memory chips that store the booting routines When we start or boot a system the microprocessor needs to be told how to get the system ready for operation It automatically goes to address 0000 and there reads the first byte of the first instruction Reading on through the bottom kilobyte it finds further instructions for getting the system booted up This first kilobyte also contains interrupt service routines The purpose of these is explained later Note that the routines in the first kilobyte must be available when the system is first switched on For this reason the memory in this range of addresses is ROM Usually the input output devices such as the printer and the disk drives 50 Microelectronics Systems and Devices Figure 3 6 The addr
149. her with precautions that may be taken to ensure correct operation Breadboarding techniques are outlined A digital system in general has fewer problems than an analogue system One reason is that a digital system operates on binary data Voltages are either high if they exceed a certain minimum high value or they are low if they are less than a certain maximum low value There is usually no difficulty in outputting voltages that are comfortably within the specified high or low ranges and there is no difficulty in inputting these voltages and having them correctly interpreted The second advantageous feature of digital circuits is that most of them are driven by a system clock We make sure that voltages have had time to settle and then the changing clock edge triggers off the next state of the system Both voltage levels and time are accurately managed so eliminating many of the uncertainties prevalent in analogue circuits Digital systems 97 Planning the system For the reasons stated above minor variations in power supply ambient temperature and component values have little effect if any on the operations of a digital circuit For the same reasons noise in the form of voltage spikes is largely ignored by digital circuits This is why digital circuits have become so widely used even for applications such as audio recording and playback in which the original input and final output are necessarily analogue In sp
150. hich bits are inputs and which are outputs The port registers and port control registers are allocated a range of Microelectronics Systems and Devices 76 four addresses These are fixed addresses which are automatically put on the bus by the Z80 CPU when accessing the port They can not be chosen by the user The addresses are F0 Port A data register F1 Port A control register F2 Port B data register F3 Port B control register All four addresses have the same top six bits 111100 The decoder circuit receives address lines AB2 to AB7 and makes the CE input low when these carry 111100 The register selected depends upon the bottom two bits AB0 and AB1 which go directly to another decoder inside the PIO In Mode 0 the port as a data output the CPU writes data into the data register This causes the RDY output to go high which tells the peripheral that there is data in the register waiting to be read The peripheral reads this data when it is ready to do so and then puts a low pulse on the STR line This tells the PIO that the data has been read and this message must now be passed on to the CPU The PIO first makes the RDY output low as there is no new data to read Then the INT output of the PIO is made low This interrupts the CPU which is programmed to respond in some way usually to send the next byte of data By using this simple handshaking procedure successive bytes of data are transferred from the CPU to t
151. his is that as the input voltage increases from 0 V the outputs become 0000000 0000001 0000011 0000111 and so on to 1111111 There are 8 possible outputs which are sent to a Data converters 87 Interfacing Figure 5 11 A flash analogue to digital converter is based on a chain of resistors connected to an array of comparators priority encoder This is a logic circuit which determines which is the highest 1 bit counting from the LSB on the left that is from the bottom of the chain The output of the encoder ranges through all the binary values from 000 001 010 011 and so on to 111 This output is proportional to the input voltage and the conversion of analogue to digital is complete The conversion time of a flash converter is the time taken for the comparators to settle plus the propagation delay in the gates of the encoder Typical conversion times lie in the range 10 ns to 2 s which is fast enough for the conversion of audio signals in real time The ADC of Fig 5 11 has seven comparators and there are only 8 possible output readings This has to cover the entire input range from 0 V up to the reference voltage As a simple example if the reference Microelectronics Systems and Devices 88 voltage is 8 V the eight possible values from the encoder correspond to 0 V 1 V 2 V 8V We can read the voltage only to the nearest 1 volt If we want higher precision we must have more comparators The rule
152. his takes 765 s This delay is too short for operations such as flashing LEDs but it could be useful when waiting for the output voltage levels of an ADC to settle before sampling its output For a shorter delay load r16 with a smaller number For a longer delay we need to load a larger number Like many other microcontrollers 1200 has only 8 bit registers If your microcontroller has 16 bit registers it can produce delays of up to 65536 3 21845 s or 22ms With only 8 bit registers to work with we adopt another strategy We use two registers and program two loops one inside the other These are known as nested loops ldi r16 255 the first number startouter start of outer loop startinner start of inner loop ldi r17 255 the second number dec r17 dec second number brne startinner end of inner loop dec r16 dec first number brne startouter end of outer loop nop do nothing more The loop in which r16 is counted down remains as before except that its label has been renamed startouter The CPU will run this loop 255 times as before The difference is that within this outer loop there is an inner loop It starts at startinner and is based on decrementing r17 Each time the CPU runs the outer loop it comes to the beginning of the inner loop Then it has to decrement r17 from 255 to 0 before it can continue with the outer loop It takes 756 cycles to decrement r17 to zero and it has to do
153. hoice questions 69 Inside the CPU 5 The program counter of a processor holds 14C2 The processor reads an instruction that tells it to jump to a new address ten bytes further on The address in the program counter will change to A A B 14C3 C 14CC D 0000 6 A register that holds the flags may be called a A status register B program counter C index register D instruction register 7 If the zero flag is set 1 it indicates that A the result of the previous operation was not zero B the result of the previous operation was zero C the carry out bit is 1 D the result of the previous operation as 0000 8 With respect to CISC processors RISC processors A are slower B may be faster in many applications C are more difficult to program D are faster 9 A place where data is stored for fast access is A cache memory B DRAM C the prefetch buffer D a CD ROM drive 10 In bitwise logic 59 AND 33 is A 92 B 8C C 7B D 11 Microelectronics Systems and Devices 70 Interfacing Microcontrollers generally have built in I O terminals that can be directly connected to low voltage low current peripherals It is usually necessary to provide I O ports to interface a microprocessor system to its peripherals The ports may operate as parallel or serial interfaces Ports may be built from TTL or CMOS logic or special ICs are available such as the Z80 PIO and the Z80 SIO Other I O ports are
154. ibed in Chaper 10 For more extensive interfacing there are I O cards that can be plugged into slots inside the computer and which have their own special addresses and decoding circuits 95 Interfacing 3 The RS 232 standard covers serial transmission for distances up to A 15 m B 1 km C 200 m D 8 m 4 Which of these does the RS 232 standard not specify A Serial signalling protocol B Types of connector C Maximum frequency D Allocation of connector pins 5 Ashift register A converts parallel data to serial data B moves bits from one register to the one next to it C has three state outputs D converts serial data to parallel data 6 In an even parity system which one of these bytes is in error A 11001100 B 11110011 C 01101011 D 00110101 7 An R 2R ladder is used in a A DAC B flash converter C successive approximation converter D operational amplifier 8 One of the disadvantages of a successive approximation converter is that A it is very expensive B the input must not change too fast C its output is limited to 8 bits D it is inaccurate Microelectronics Systems and Devices 96 Planning the system The advantageous feature of digital circuits are described Discussion of noise including EMI leads on to choosing a suitable logic family for building a project Connections within families and between families are considered The design of the circuit board is discussed toget
155. ication of each 2 List three output devices and describe examples of systems in which they could be used 3 Which programming language do you prefer to use Give reasons 4 What is masking Give an example of a program in which masking is used 5 How can we make the CPU wait for a given input or wait until a particular event has occurred 6 Why is is sometimes necessary to have a delay in a program Describe how a delay of 10 s could be produced 7 Choose one of the following tasks and outline the design of a microelectronic system to perform it using only 1 bit inputs and outputs a radio controlled mechanism for opening and closing a garage door a system to switch on lights at the front of a house whenever someone approaches it but only at night an automatic coffee vending machine a sliding valve with proximity detectors as in Fig 1 3 a task of your own choosing List the input and output devices that you would use Outline in words how you would program the system to operate you are not expected to write the actual program 8 List the things that should be done at the beginning of a program 9 Write a delay routine for use in BASIC programs that uses time but does not upset the Windows date and time routine 10 Basing your answer on the cooler system design and program a system to control a heater to keep a room at constant temperature 11 Extend the programming of the security
156. ich will improve the performance of a microelectronic system Unused inputs It may often happen that one or more inputs to a gate or logic device are left unused Sometimes a chip contains more gates than are required so there are whole gates unused There are rules about what to do with unused gate inputs and other unused inputs such as resets presets and chip enable inputs The rule is simple with CMOS All inputs must be connected to the positive supply line the negative supply line or the output of another gate You must always do this even when you are breadboarding an experimental circuit and are using only one gate out of the four on the chip If the inputs of the other three are left unconnected the gate you are using may not behave correctly and may take a large amount of current A rule related to this is that all connections to the power line must be made before the power is switched on Conversely the power supply must be switched off before any alterations are made to the circuit With TTL inputs of gates or other circuits that are left unconnected act as if they have a logic high input However if a gate that is being used has spare inputs it is better not to leave these unconnected Unconnected inputs reduce noise immunity Instead connect unused inputs to the positive supply through a 1 k resistor the 0 V line no resistor required one of the used other inputs of the same gate Fig 2 10 shows
157. ics of three TTL devices that can be used as input output ports 4 Explain how you would use a Z80 parallel input output IC or any named device that is similar to interface a microprocessor to a a printer and b a 4 key keypad 5 What hardware would you use to send signals from a CPU over the telephone network 6 What is an analogue to digital converter How would you interface it to a named processor Give an example of a practical use for such a circuit 7 Describe how you would interface a digital to analogue converter to a named processor 8 Explain the difference between a latch and a flip flop 9 Describe the action of a shift register For what purposes are shift registers used in microelectronic systems 10 Explain the difference between bit rate and baud rate When are they equal 11 What is meant by parity How and why is it checked in serial data transmission 12 Describe a method for interfacing devices to a personal computer How could you use this interface to receive data from an ADC 1 When it has a logic low output a TTL gate can sink up to A 24 A B 8 mA C 1 mA D 16 A 2 Buffers are used is certain systems because they A prevent voltage spikes from passing B change state very rapidly C change a logic high to a logic low D sink and source larger currents Multiple choice questions Apart from the complications mentioned above the ports are suitable for the 1 bit interfacing descr
158. in Chapter 2 the noise immunity of ECL is less than that of TTL or CMOS making system design with this family a difficult task One of the first decisions to be made after deciding on the processor Chapter 4 is what IC family is to be the main one used The choice is between 74LSXX series low power Schottky TTL A wide range of ICs available but must have a regulated 5 V supply The original 74XX series is now available only in a restricted range of types There are other TTL sub families for special applications CMOS 4000 series A wide range of ICs is available including many complex ones It operates on 3 V to 15 V supply It has a large fanout and good noise immunity CMOS is slower than TTL but fast enough for most applications CMOS versions of TTL series the 74HCXX and 74HCTXX sub families They have greater noise immumity than regular TTL ECL is fast but has low noise immunity and circuits are troublesome to design Another choice to be made is the speed of the system clock A safe general rule is to make the clock as slow as possible In addition use the slowest family that will do the job Usually this means settling on the CMOS 4000 series The reason for keeping the system speed as low as possible is that this minimises EMI emission which may affect the operation of nearby circuits and perhaps other sections of the same circuit Conversely low speed often makes the system less susceptible
159. in checking the short example program above is the Register window This shows the content of all 32 registers It makes it simple to follow the changes in the values held in registers as the program is run If you simply run the program by clicking on the run icon in the toolbar the program runs so fast that it is impossible to keep track of all the changes Instead click on the single step icon Then the program runs one line each time you click and you can look at the results of each step You can see the changes in the values stored in the registers and at the same time watch for changes in the program counter and in the flags in the status register This is another big advantage of using an assembler program 115 Instructions Errors There are three main kinds of programming error Telling the CPU to do something it can not do Telling the CPU to do something it can not understand Telling the CPU to do something it can understand and do but which produces a result you did not intend The assembler and most other software for producing programs will report the first two kinds of error An example of the first kind is mentioned in the text An example of the second kind is if you make a typing error typing ICN instead of INC As another example you may follow the opcode with too few or too many operands These errors are often reported as syntax errors Errors of the third kind are the prog
160. is best avoided For example a program is downloaded from a computer into a microcontroller using a serial port Another example is a digital camera where the image is fed to a computer through its serial port Parallel ports built from TTL A port could be built from individual TTL gates but more often we use members of the TTL family specially designed for interfacing to microelectronic systems One such IC is the 74244 octal buffer Fig 5 2 To make the diagram simpler we have drawn only one of the eight identical buffers The function of the buffers is to make more power available for driving external devices A typical TTL output of Parallel ports Figure 5 2 The 74244 IC contains eight identical buffer gates with 3 state outputs The outputs are enabled in two groups of fourgates when the two ENABLE inputs are made low 73 Interfacing the 74LSXX series can sink up to 8 mA when the output is low It can source up to 0 4 mA when the output is high By comparison for a buffer in the same series the currents are 24 mA and 2 6 mA Note that the buffer shown in Fig 5 2 is logic TRUE or non inverting The output level is always the same as the input level Inverting buffers are also available and may be useful in certain circumstances Buffers are used as data output buffers The inputs are connected to the data bus and the outputs to a data output socket or to the input of a peripheral device The three state outp
161. is that given n comparators the number of possible steps in the output is 2 n 1 For example a flash ADC with 8 bit output requires 256 converters Such an ADC often has a 2 56 V reference so that each step in the output is 0 01 V Flash ADCs are made with 4 bit outputs and 8 bit outputs for low precision applications For higher precision there are 12 bit converters but these work by a technique known as half flash This is a compro mise that requires fewer comparators It works in two stages and therefore takes longer Successive approximation converters These provide greater precision than flash converters at relatively low cost Converters with 16 bit precision are available Conversion times of 20 s are achieved by some types though some take as long as 100 s Fig 5 12 illustrates the principle on which the successive approxima tion works This example has only four bits to make the explanation easier Conversion takes place in a number of steps one per clock cycle As the name suggests the converter works by gradually homing on the correct output To begin the START CONVERSION input of Figure 5 12 At each stage of conversion the analogue equivalent of the output value in the SAR is compared with the analogue input voltage 89 Interfacing the IC is made low At the next clock step the control logic sets the first bit the MSB of successive approximation register SAR to 1 In other words the first
162. ite of the essential robustness of digital circuits there can be problems with these too The problems are more severe when we try to push the circuits to the limits of their capabilities particularly as we increase processing speeds to their maximum In this chapter we look at some of the problems that may arise with digital circuits in microelectronic systems and how we may try to overcome them Many of the problems arise because of two related reasons The signals in microelectronic systems are mostly of radio frequency The signals being of high frequency their rise times and fall times are extremely short Putting this the other way round there are high rates of change of voltage and current A third source of difficulty is electromagnetic interference EMI Noise Any unwanted electrical signals present in a circuit are referred to as noise If the amplitude of the noise is great enough it degrades the wanted signals In logic circuits it may cause a logic low level to be read as a logic high level or the other way about so that data is corrupted It may have a similar effect on control signals causing a device to reset for example or to latch its input at the wrong time Similar effects can result from noise on the supply lines It is usually possible to minimise noise to an acceptable level but impossible to remove it altogether Noise may be generated in the circuit itself especially in resistances and semic
163. its D3 to D0 in a 4 bit binary number Similarly the sum of the currents is proportional to the sum of the bits In the figure S3 and S1 are switched to the op amp so the equivalent binary number is 1010 and the sum of the currents is 250 62 5 312 5 A The op amp is connected as an inverting summer Its output swings negative by an amount proportional to the sum of the currents It takes only a second inverting amplifier with a fixed gain of 1 to make the output positive The output appears as a voltage proportional to the reference voltage Figure 5 13 The R 2R ladder produces currents that are weighted on the binary scale They are summed by the operational amplifier Precision The precision of the flash and R 2R converters depends on the precision of the resistors Their exact values do not matter but it is important for the ratios between them to be exact This is relatively easy to achieve because all the resistors are fabricated at the same time on the same chip 91 Interfacing Activity 5 1 ADCs Study the data sheet of an analogue to digital converter and note its main operating conditions Set up the ADC on a breadboard and provide it with A suitable power supply A variable input voltage for conversion A set of 8 LEDs switched by transistors to display the digital output If it is a successive approximation converter it will need a clock pulse generator a low pulse generator
164. known as ANSI C compilers There is also a version of C with additional features and this is known as C There is also a version of C called Visual C This is a Windows program primarily intended for writing applications based on a graphical user interface It is more difficult to learn than a simple C compiler program There are rules for typing in a C program and these rules must be strictly followed if the program is to be compiled without error messages appearing As with an assembler all variables used in the program must be declared preferably at the beginning of the program Example Here is the sample program of Chapter 7 re written in C Example 1 re written in C int counter int trial define BODY 37 main counter BODY trial counter C compilers Microelectronics Systems and Devices 126 After the initial declarations and a definition the main program is very short and consists almost entirely of readable and memorable words It is enclosed in braces but as this is only a section of the sample program the final brace is not shown in the listing above The program declares two variables counter and trial as integer variables Note that we do not have to specify where these variables are to be stored or even whether they are to be stored in a register of the CPU or in a location in RAM The compiler takes care of all the details and always knows where to find the values of counter
165. lectronics Systems and Devices 122 A high level language program produces machine code from a program written in a form that is close to ordinary English The many small steps of machine code and assembler are replaced by keywords that call up ready made routines Some languages use an interpreter and others use a compiler Two widely used high level languages suitable for programming microelectronic devices are BASIC and C Ladder logic is often used with PLCs Just as assembler is one stage removed from machine code so the high level languages are one stage removed from assembler There are many high level languages a few of them very widely used and several that have more specialist applications CPUs can not understand high level languages but there are several programs that allow us to write a program in a high level language and then have it turned into machine code In this chapter we look at three such programs using C BASIC and ladder logic The main difference between assembler and high level languages is that assembler is related to machine code on a one to one basis Each line of an assembler program corresponds to one fetch execute cycle of the CPU Because a CPU operates in short simple steps programming in assembler can be very tedious and repetitive With a high level language a single command may be the equivalent of several tens of fetch execute cycles 123 High level languages Example A program written in
166. led Protective Ground connected to the metal chassis of the terminal at either end This uses pin 1 TD is sometimes known as TXD and RD as RXD SG is sometimes called GND In practice many systems use the 9 pin connectors with only three lines TD RD and SG for two way communication One way connec tion needs only two lines TD at the transmitting end is wired to RD at the receiving end Interfaces can be built from TTL gates as in Figs 5 5 and 5 6 The TTL input to the interface Fig 5 5 comes from the transmitting system possibly through its output port The voltage levels of the system are 0 V logic 0 and 5V logic 1 The 74LS06 gate inverts these levels Note that the gate has an open collector output requiring a pull up resistor At this stage the voltage for logic 0 is higher than that for logic 1 As the signal passes through the circuit it is inverted by each of the transistors so is still inverted at the output Here the levels are 12 V with 12V being equivalent to logic 0 and 12 V being equivalent to logic 0 These levels are valid RS232 levels Figure 5 5 This interface converts a TTL signal 0 V 0 5 V 1 to an RS232 signal 12 V 1 12 V 0 Figure 5 6 The INVERT gate used in this RS 232 TTL interface has an open collector output Microelectronics Systems and Devices 80 At the receiver the signal first passes through a network comprising a resistor and two diodes to limit the voltage s
167. lications Instruction set The choice between a CISC processor see opposite and a RISC processor depends partly on the application In general RISC processors are considered to be the faster choice I O facilities Microcontrollers differ widely in the number of I O pins or ports On chip facilities Microcontrollers vary in the amount of RAM none or 25 bytes to 1 K PROM none to 8 K EPROM EEPROM none to 8 K they have and whether or not they have an ADC a DAC and one or more timers Although there are many different kinds of CPU as the previous section made clear there are many similarities between them This is because although some CPUs are faster than others some address more memory than others and some have a bigger instruction set than others they all basically do the same thing Before we go on to look inside the CPU we will outline what it does A computer program consists of a sequence of codes For long term storage these may be held on a disk floppy disk hard disk or compact disc or tape They are transferred to a block of RAM when the CPU is to read them Alternatively the codes are stored in ROM and read from there Each code group is either an instruction to the CPU or a value for it to use For simplicity assume that each code group consists of a single byte This is actually the case in many CPUs but some read longer codes If a code is 8 bits long there can be 256 di
168. lse Another disadvantage is that the action of a processor can be interrupted when it has to attend to something urgent Interrupting the clock action would make the clock lose time It is better to give the time keeping task to an independent IC There is a detailed study of an RTC in Chapter 10 Activity 3 1 Investigating memoryActivity Connect up a 6116 or another DRAM chip on a bread board It needs connections to the positive and ground 0 V rails of a 5 V supply Input to the address data and control terminals of the IC can be provided through switches Fig 2 6 or by simply plugging in wires to 52 Microelectronics Systems and Devices 1 Describe RAM and the way that it is used in microelectronic systems 2 Compare the functions of RAM and ROM 3 List the different kinds of ROM and give examples to illustrate the ways in which each kind is used 4 Draw a diagram similar to Fig 3 4 but with 4 address lines and 6 data lines Show the state of the links when the value decimal 35 has been blown into address 5 and the value decimal 21 has been blown into address E 5 On a copy of Fig 3 5 write in the voltage levels 0 or 1 that would be present when the value decimal 99 is being written into address 0372 connect them directly to the supply line or 0 V line Output from the data terminals may be read by connecting LEDs as in Fig 2 7 Either use a low voltage filament lamp as shown or LEDs in series with a
169. m as a diagram Give short examples 3 Draw a flowchart or DSD of a program that you have written Add notes to point out structural features such as sequences iterations and selections 4 What are nested loops Give an example to illustrate this structure 5 What is initialisation List the actions that the CPU performs in a typical initialisation sequence Why is it advisable to repeat some of the initialisation steps later in a long program 6 Explain what is meant by the input processing and output stages of a program or routine 7 What is the top down approach to programming Give an example device may be left unconnected if there is only one device in the system However if there is a second device the EIO is connected to the EIE of the second device which has lower priority Both devices have their INT output connected to the INT input pin of the CPU The EIO output is normally high but goes low when the first device is interrupting The low input to the IEI of the second device disables it It can not interrupt until the first device has finished The low EIE input also makes the EIO go low The chain can be extended to three or more devices so that if the second device for example is interrupting the third and subsequent devices can not interrupt However in this case the first device can still interrupt and take priority over the others This system of connecting the EIO to the EIE of the device of next lower priorit
170. m from 600 to 680 then at line 690 jumps back to the line after the one it came from 183 Structured programs Demonstrating ORG and subroutines Initialisation equ ddrb 17 equ portb 18 equ bit0 1 def ledoff r18 def ledon r19 org 000 program starts 000 reset rjmp start reset routine org 004 program starts 004 start ldi ledoff 2 ldi ledon 0 ldi r20 FE out ddrb r20 wait in r16 portb input button andi r16 bit0 button pressed breq wait flash out portb ledon LED on rcall delay to subroutine out portb ledoff LED off rcall delay to subroutine rjmp flash repeat for ever Delay subroutine same as delay program delay ldi r16 FF startouter ldi r17 FF startinnner dec r17 brne startinner dec r16 brne startouter ret return to main program The reason that the flash sequence is so easily understood is that the reader is not confused by having to read twice through the delay routine The five steps of the routine are clearly set out with a line for each The delay routine has been placed at the end of the listing as a subroutine Whenever a delay occurs the CPU is told simply to call the Microelectronics Systems and Devices 184 org directive A directive tells the assembler how to assemble the program It is not part of the program The org directive tells the assembler where in memory to start storing the assemble
171. mmer to disable interrupts when the CPU is 33 The CPU engaged in a complicated routine that might crash if interrupted The I flag can be set after the routine is complete to enable interrupts again The seven output pins of the Z80 control bus are all 3 state outputs Symbol Symbol FunctionFunction DetailsDetails BUSACK Bus acknowledge Responds to BUSREQ by making this low and making its data and address out puts high impedance HALT Halt The Z80 has stopped be cause of a HALT instruction INSTRD or M1 Instruction read The Z80 is reading an in struction from memory MREQ and RD also low IORQ I O request The Z80 is reading or writ ing data to I O MRQ Memory request The Z80 is reading or writ ing data to memory RD Reading data Causes device to put data on bus RFSH Refresh Signal to refresh dynamic memory WR Writing data The data bus has data ready for a device to copy All of these outputs are active low Their symbols should have a line drawn above them to indicate this but they were omitted from the table for clarity When the Z80 is about to read or write to memory it makes the MRQ line low This is the equivalent of a CHIP ENABLE signal to the memory chip At the same time it makes the RD or WR lines low depending on whether it wants to read or write data Similarly when it wants to read or write to a port it makes the IORQ line low and makes the RD or WR low at the same time The Z80 has s
172. ms Fig 11 7 each path leads to an appropriate action Note that in Fig 11 7 the selection is within an iteration Figure 11 7 The cooler program drawn as a DSD can be seen to run forever never reaching the end Microelectronics Systems and Devices 176 For reference the three basic structures are shown in Fig 11 8 as they appear in flowcharts and in Fig 11 9 as they appear in DSDs Figure 11 8 Flowcharts of program structures a sequence of processes A B and C b iteration of A and B indefinitely c iteration of A until condition is true d selection between performiing process A or not e selection between performing process A or process B 177 Structured programs Figure 11 9 DSDs of program structures a sequence of processes A B and C b iteration of A and B indefinitely or iteration of A and B until condition is true c selection between performing process A or not e selection between performing process A or process B Microelectronics Systems and Devices 178 It is generally stated that the best way to design a program is from the top down The process starts with an outline definition of what the program is to do With a little experience it becomes clear that most programs can be divided into a number of stages Usually the first stage which could be called initialisation is concerned with defining variables their types and possibly their initial values Then come definitions of constant
173. n a cordless telephone this is a microcontroller complete with memory It may be wondered why a cordless telephone needs a CPU yet an old fashioned corded telephone can operate without One of the reasons is that the corded telephone is wired directly to the public network but the cordless telephone has to make radio contact with its base before a call can be received or made The handset of a cordless telephone Fig 1 1 consists essentially of a radio transceiver that is under the control of a CPU The radio has limited range and communicates with the base unit which may be up to 200 m away normally in the same building The base unit Figure 1 1 A cordless telephone handset is under the control of its central processing unit CPU This may be a microprocessor or more often a specialised microcontroller 4 Microelectronics Systems and Devices Cordless telephones Fig 1 1 shows that the radio circuits are under the direct control of the CPU The double headed arrow between the CPU and radio circuits indicates that signals may pass in both directions though not both ways at exactly the same time The radio circuits of the base station and the handset are permanently switched on waiting for a call When the base station receives an incoming call from the public network it sends out a digital signal by radio This signal includes a code that identifies the base station The handset receives this signal However it is als
174. n then be called from any point in the program as many times as it is needed The box explains what happens On p 183 is an example of using a subroutine in an assembler Subroutines Microelectronics Systems and Devices 182 program The reader will recognise lines and routines from the LED switching programs and the delay program of Chapter 10 The point of interest in this listing is the main program These are the 5 lines following the flash label These few lines clearly show the sequence of the program The LED is repeatedly turned on then off with a delay in each state so that the flashes are visible Note the essential mnemonic ret return from subroutine at the end of the subroutine This is rts in some assemblers Subroutines This illustrates the path of the microprocessor as it runs through a BASIC program that calls a subroutine three times 10 REM Demonstrating subroutines First part of program 150 GOSUB 600 REM goes to subroutine 160 Comes back to second part of program 210 GOSUB 600 REM goes to subroutine again 220 Comes back to third part of program 350 GOSUB 600 REM goes to subroutine again 360 Comes back to fourth part of program 590 END REM Stops CPU running on 600 Start of subroutine 680 End of subroutine 690 RET REM returns to line after Each time the CPU finds GOSUB 600 it jumps to that line executes the progra
175. n this program we set it the task of decrementing a number until the number becomes zero delay routine ldi r16 255 the number to work on start dec r16 decrementing brne start and again nop until Z 1 The branch instruction brne is the opposite of breq It means branch as long as the number is not equal to zero In other words branch as long as Z 0 To give the CPU something to jump to when it has finished running the loop we use the mnemonic nop which means no operation The CPU does nothing at this instruction Inserting one or more nop commands in a program can be used to 6 Install two switches on two input pins and program the system so that closing either of these turns the LED on 7 Install two switches on two input pins and program the system so that the LED lights only when both switches are closed 8 Install two switches and two LEDs Write a program to turn one LED on with one of the switches and to turn the other LED on with the other switch 155 Input and output create very short delays but only of a few microseconds The program above loads r16 with 255 and then decrements it by 1 each time round the loop until it reaches zero It takes one machine cycle to decrement r16 and two cycles to fetch Z and check whether or not it is 1 This makes 3 cycles for the whole loop and the CPU runs round the loop 255 times This takes 3 255 765 cycles If the system is running at 1 MHz t
176. n which the CPU can operate reliably 7 Systems in action Figure 1 3 Like almost every other control system a PLC is centred on a CPU The dotted line indicates that the CPU memory keypad and display are normally installed as a single general purpose unit Interfaces to sensors and actuators may be separately installed and there may be several hundred in the system Smaller systems may use PLCs with a dozen or so built in interfaces A Seimens 95U PLC can be seen in Fig 1 4 The PLC is wired to a number of input and output interfaces which are mounted on the rack in the cabinet Cables run from these to the sensors and actuators on the plant A few others run to control switches and indicator lamps on the door of the cabinet The door is normally closed when the plant is operating so acting as a control panel The program of a PLC runs continuously in a loop for as long as it is switched on The first stages of the program read the state of each sensor and store the results in a special area of memory Then the program examines the input data and decides what action is to be taken As an example take the valve mechanism of Fig 1 2 If the proximity sensor 7 shows that a shutter has reached the far end of its travel the valve 5 admitting compressed air to the nearer side of the cylinder must be closed A message indicating close valve is stored in the output area of memory of the PLC When all the logical decisions have been
177. nd BASIC programs for microcontroller systems microprocessor systems and PCs using single bit input and output are presented and discussed Delay routines are introduced Two routines suitable for microelectronic control systems are described with programs in assembler and BASIC A 1 bit input or a set of 1 bit inputs is all that is needed in many control applications In Fig 1 3 one of the two proximity sensors 7 and 8 has a high output logic 1 when the sliding valve is at one end of its track At other times their outputs are logic 0 Similarly an output may need only one bit to perform a vital task It takes only a 1 bit output to sound a fire alarm Although we are mainly concerned with control applications in this chapter it must be realised that there are many instances of 1 bit input and output in measuring communications and commercial microelectronic systems A 1 bit input is either 0 or 1 The simplest way of inputting this is to use a switch or push button as in Fig 2 6 This is connected to one of the lines of an I 0 port for example to the LSB line D0 Before an input can be accepted this line must be configured as an input If only One bit input 146 Microelectronics Systems and Devices this line is being used the remaining lines could be inputs or outputs However it is preferable to configure all unused lines as outputs If they are inputs and unconnected it can happen that they pick up stray charge
178. noise spike on an output signal can be as much as 1 45 V without affecting its apparent logical value This means that CMOS has good noise immunity 39 The CPU microelectronic applications Several of the 74XX series are available in CMOS versions known as the 74HCXX and 74HCTXX series Logic 0 and logic 1 voltage levels are widely separated see Fig 2 12 making the series highly immune to noise Because of the small size of CMOS gates it is possible to fabricate complicated logic circuits on the chip The series includes many complex devices such as the 4020 14 stage counter which are not available in the 74XX series ECL This is specially designed for high speed operation The transistors are never driven into saturation so they quickly change state when logic inputs change In addition the circuits have low input impedances to avoid the speed reducing effects of capacitance This increases power requirements to around 30 mW per gate Propagation time is about 1 ns per gate and the maximum clock rate is in the region of 500 MHz Because the transistors are operated in the non saturated state logic levels have to be carefully controlled at 0 8 V and 1 6 V respectively The small difference between these levels means that ECL is much more affected than the other families by noise spikes on the lines Specially filtered power supplies are essential Because of the high frequency of the signals tracks on the circuit board have to b
179. nother Fig 2 2 It can be seen that the pins of the 1200 provide all the essential access to the system a power supply a timing crystal too big to go on the chip a way of quickly resetting the system and connections for sensors and actuators for inputting or outputting data Data can be fed to the 1200 in parallel Fig 2 3 That is we simultaneously send a 1 or 0 along its 8 Port B lines and so load it with 8 bits a byte of data in a single operation We can do the same thing with Port D except that this has only 7 bits Similarly we can read in parallel a byte from Port B or a 7 bit group from Port D As might be expected from their name microcontrollers are mainly used in control applications A port may receive a byte of data from a temperature sensor the value of the byte representing the temperature In the other direction the port may output a byte of data to control the speed of a motor However the input from a sensor may not be a byte but a single bit Microelectronics Systems and Devices 22 Other controllers the PIC family The Microchip PIC family of microcontrollers has many and varied members Some of the smallest are in a standard 8 pin IC package Fig 2 4 The 12CE518 runs on 2 5 V to 5 V with a frequency up to 4 MHz It has a program memory PROM or EPROM of 512 bytes It also has 25 bytes of RAM and 16 bytes of EEPROM which can retain stored data for as long as 40 y
180. ns If you have not already done so improve the program you wrote in Activity 7 1 by adding a title and comments Re write the program to make the CPU do something slightly different For example make it load a different number Make it store the incremented number in a different register Make it increment the number twice before storing it Make it decrement the number instead of incrementing it Using the mnemonics from your version of our program and perhaps one or two others from your instruction set there are many different ways of programming the CPU to do something slighty different Add comments to each short program you write so that others will be able to understand what it is supposed to do 1 What is a fetch execute cycle Give examples based on a CPU that you are studying 2 Describe a read cycle illustrating it by a timing diagram not necessarily to scale for the CPU you are studying 3 Describe a write cycle illustrating it by a timing diagram not necessarily to scale for the CPU you are studying 4 What is machine code Give examples of a few opcodes and explain the operands that they require 5 What is an assembler program and what are its advantages over writing a program in machine code 6 List a short assembler program that you have written Explain concisely what the CPU does at each stage 7 Do a dry run of the program you described in your answer to Question 6 Problems on instructions Microe
181. nstruction or vector on the data bus The Z80 reads this single byte instruction which tells it the address of the appropriate ISR There are eight different restart instructions sending the CPU to a particular one of eight addresses 0000 0008 and so on to 0038 These 8 byte blocks of memory can be used to service the routine or send the CPU on to another address where a longer routine is stored Having these eight address means that up to eight different ISRs can be programmed and the interrupting device can indicate which one of these should be used Direct interrupts If the Z80 is in Mode 1 it is sent automatically to address 0038 without the need for a restart instruction Indirect interrupts In Mode 2 the Z80 receives a vector as in Mode 0 and adds to this a 16 bit value that is already stored in its I register By setting different values in the I register this allows the Z80 to be directed to different blocks of memory each holding eight restart instructions This gives the Z80 the flexibility to process a large number of different ISR routines Several peripheral devices can be connected to the INT pin leaving the CPU the problem of finding out which one is interupting If two devices interrupt at the same time the CPU has to give one of them priority It is not necessary for them to interrupt at exactly the same time The CPU does not check for interrupts until the end of its current execute cycle so if two interrupts occ
182. nto its smallest parts Functions in most versions of BASIC are more limited than those in C BASIC provides a few useful functions such as SIN COS and TAN but the user is able to define other functions as required Example A measurement system is reading data from an ADC To convert the data adc into a reading of temperature it has to have 0 5 subtracted from it is then divided by 5 2 and finally have the room temperature room added to it temp adc 0 5 5 2 room Data is read many times during the program but instead of having to type in this equation after every reading it can be defined as a function using DEF FN temp adc 0 5 5 2 room This is included early in the program preferably in the initialisation stage After that every time the equation has to be used all that needs be typed is FN temp adc room It could be used in a statement such as 300 IF FN temp adc room gt 29 THEN 460 Unfortunately most versions of BASIC limit function to one line though some versions allow multi line definitions There are programs in which a particular sequence of operations has to be repeated at several different points in the program It may for example be a routine to set a data direction register of a port to all inputs read the input data AND with a mask then store the result in a given register Much time and memory space can be saved by entering this routine just once as a subroutine It ca
183. nvironment The change may only be a single bit that changes from 0 to 1 or from 1 to 0 but such a change may have a serious effect on the program For example if a port register is set as an input and the data in the direction register becomes corrupted the line may change from becoming an input to becoming an output This can have unforeseen effects on the running of the program It may be be worth while to re initialise control registers and also essential data such as constants by sending the CPU back to initialising routines at regular intervals 179 Structured programs usually involves iteration while waiting for input If more than one form of input can be made for example when the input device is a keypad or when it is an analogue quantity that is converted by an ADC the program may use selection to check that the input is valid It will also use selection to follow different paths depending on the input values The processing stage may be anything from a simple sequence perhaps only of one or two program lines to a complex combination of sequences iterations and selections Here it is important to use a graphical technique such as DSDs to structure the program Then the pathways through the program can be checked to confirm that there are no short cuts and no dead ends For further confirmation especially where there are selections a dry run will help show how the program behaves for different input values In some cases t
184. o able to receive signals from any other base stations within range The signal is sent to the CPU which then checks to find out if this is the code of its own base station If it is not it ignores it and nothing further happens If it is recognised the CPU makes the radio circuit send an acknowledging signal The signal includes a code to identify the handset The base station has been waiting to receive this signal which is checked by its own CPU to make sure that it comes from a handset with which it is allowed to communicate Then the CPUs both open up the radio channels for two way conversation between the handset and the base and by land line to the remote caller The procedure is similar for an outgoing call The CPU makes the radio circuit transmit a series of code groups including its own identity code and the number to be dialled On confirming that the identity of the handset is acceptable the CPU of the base station dials the number In practice dialling the number means generating a sequence of pairs of DTMF tones that code the required telephone number When the number answers it replies to the handset and radio channels are opened for two way conversation The lower part of the diagram shows the input and output that links the CPU to the user The keypad is used to send input to the CPU to tell it what number to call It is also used for operations such as storing frequently needed numbers in memory The CPU has output to
185. of facts recently encountered but probably not yet learnt They also provide definitions of terms particularly of some of the useful jargon associated with microelectronics and computing Each chapter ends with a batch of examination type questions and in most instances with a selection of multiple choice questions Answers to the multiple choice questions appear at the end of the book Owen Bishop This Page Intentionally Left Blank 1 Systems in action Part A The Hardware Microelectronic systems are digital systems built from one or more integrated circuits They contain a central processing unit CPU which is programmed to make the system perform its tasks The CPU is either a microprocessor or a microcontroller or in complex systems there may be more than one Microelectronic systems are widely used in equipment and installations such as washing machines automatic teller machines personal computers production line packaging machines printing presses and radio telescopes The CPU has such complicated tasks to perform that is is made up of hundreds of thousands or even millions of transistors as well as resistors and other components all assembled on the same silicon chip There are two main types of CPU Microprocessors Microcontrollers Systems in action The essential features of a microelectronic system are described These are illustrated by descriptions of typical systems a cordless telephone prog
186. of the CPU to the address bus register The address is placed on the address bus Fig 7 2 The address is decoded by the logic partly on the memory chip to select the location for writing to The data from the register often the accumulator is placed on the internal bus The data is latched into the data bus buffer The data appears on the data bus Fig 7 2 Writing is enabled In Fig 7 2 the WRITE ENABLE line is made low The CHIP ENABLE line goes low to store the data from the bus in the addressed location The CE line goes high followed by the WE line The address and data are no longer present on their busses The instructions for a program are stored in memory RAM or ROM as a sequence of consecutive bytes which are the opcodes and operands of a program This is known as machine code It is the only form of program on which the machine the CPU can work If the Writing to memory 111 Instructions Bitwise logic CPU logic is always done bitwise that is corresponding bits in two values are subjected to the logical operation Example Suppose that as the result of a previous operation register A holds the value AC To AND this bitwise with 4A we must first write out the values in binary 10101100 01001010 The rules for the AND operation are 0 0 0 0 1 0 1 0 0 1 1 1 Note that in these equations represents the o
187. oller that would be suitable for a switching traffic lights with presettable delays at each stage b controlling an automatic weighing machine Weights are displayed on a liquid crystal display c a hand held stock logger such as is used in supermarkets for shelf stock taking d controlling a printer that is attached to a computer e monitoring the water level in a tank and warning when the rate of rise is too fast Use manufacturers data sheets and other reference materials to help you make your choice Make a list of the reasons for your choice Activity 4 2 Inside the CPU Investigate the internal structure of a microprocessor or microcontroller using the manufacturers data sheets and other reference materials Draw a diagram to show the main units and the way data flows between them Show the ports and indicate the direction s of flow of signals Write a brief account of the internal structure outlining what each unit does List the flags of the status register or equivalent register and explain what each flag means Note any features of the device which suit it for special applications Note any features that give the device high operating speed 68 Microelectronics Systems and Devices Problems on the CPU 1 What are the differences between CISC and RISC processors 2 List the kinds of additional units that are included on the chip of several named microcontrollers 3 What is machin
188. ollowed by a night watchman who has to report to a supervisor or automatic security system at frequent intervals while on his rounds If he fails to check in at the expected time it indicates that something is wrong and action is taken Similarly the CPU is programmed to trigger the timer at frequent intervals perhaps every millisecond The period of the timer is a little longer than the interval at which it is triggered When the timer has been triggered its output goes high and it should be triggered again before the period is over The output of the timer is connected to the reset input of the processor and as long as the timer holds this high the processor runs normally If a fault develops and the CPU is no longer following the program correctly it no longer triggers the timer After a millisecond or so the timer output falls to logic low This automatically resets the CPU which jumps back to the beginning of the program and starts again Most if not all of the microelectronic systems you build program and test in your practical classes will be assembled on a breadboard From what has been said about the problems of digital circuits it might be wondered if you will ever get your breadboarded circuits to work There is not much need to worry The systems you will build have a clock of relatively low frequency so that there is little EMI The slow action also gives the circuits time to respond and settle Connecting wires are short alwa
189. onductors It may be picked up from other parts of the same circuit especially where tracks on the circuit board run side by side crosstalk It may also be picked up from outside as electromagnetic interference EMI Microelectronics Systems and Devices 98 Electromagnetic interference Any flow of electric current generates electromagnetic radiation radio waves the larger the current the stronger the radiation When this radiation passes through a piece of electronic equipment it generates voltages in the signal lines and in the power lines These may cause the circuit to operate incorrectly EMI may pass directly as radiation or may be transmitted along the mains power lines There are many sources of EMI one of the worst being switching currents on or off The switching of the clock in a microelectronic system generates high frequency radio waves which may interfere with the operation of the system itself or of a neighbouring system The switching of signals in the data and address busses has the same effect EMI can also arise from outside sources such as domestic equipment switching the motors of refrigerators washing machines in industrial plant and in the ignition systems of motor vehicles In these examples the loads are inductive so a high voltage is built up as the switch contacts open This causes arcing at the contacts which is a powerful source of EMI Another source of EMI is the electric mains where the 5
190. oprocessor has to jump to a different part of memory and continue its reading from a new address On these occasions the control unit puts the new address in the program counter to direct the microprocessor to the different part of the program Stack pointer Another register is used to hold the address of the top of the stack The stack is a small block of memory where very important data is stored temporarily The action of the stack is described in Chapter 11 Status register In some CPUs this is called a flag register It holds essential information about the result of an operation that has just occurred The flag register usually has a capacity of one or two bytes but the information is stored as the separate bits within the bytes These flag bits are individually set to 1 or reset to 0 to indicate certain events For example every time a calculation gives a zero result the zero flag Z is set to 1 If we want to know if the most recent calculation gave a zero result or not the CPU can look at this particular bit in the status register to see if it is 1 or 0 Another flag the sign flag S may be set when a calculation gives a positive result zero counts as positive A further example of a flag is the carry flag C which is set when there is a carryout from a calculation The use of this flag is illustrated in Chapter 7 The HC or half carry flag is set when there is a carryover 62 Microelectronics Systems and Devices Bit
191. oprocessor has to pause in its operations while the next section and its data are loaded Real time clock Every system must have a system clock to drive the processor A real time clock is optional It functions like a digital watch keeping account not only of the time but also the date the day of the week the day of the month the month and the year It includes logic to make it give February twenty nine days in a Leap Year It also has an alarm function to produce a signal at any preset time In addition it can be programmed to generate a pulse to interrupt the CPU at regular intervals To the CPU the real time clock is a block of addresses in which time and date are updated automatically once they have been set The CPU writes into or reads from these addresses just as it does with memory In some types of RTC the addresses used for times and dates are part of a larger block of RAM that is available for other purposes The RTC always has to have its own back up battery so that it does not stop working when the system is switched off This makes the clock s spare RAM useful for storing data that must not be lost All of the functions of the real time clock except interrupt generation could be performed by the processor itself if it was programmed to do so This would have two serious disadvantages One is that the processor would be kept so busy with time keeping functions that it might have too little time to spare for doing anything e
192. p to eight different sensors or eight different actuators or a mixture of both connected to Port B and seven more connected to Port D When it is important to keep the size of the IC and therefore the number of pins to a minimum it is possible to transfer data serially into or out of the microcontroller Then we need only a single input output pin An example of this is seen in the 1200 microcontroller In programming mode the pin PB5 see Fig 2 1 is used for serial input of programs Pin PB6 is used for serial output when the program is read back into the programming PC to verify it PB7 is used as a clock input for the serial data transfers The main disadvantages of serial data transfer are that it takes longer than parallel transfer does and that the programming is more complicated Microelectronics Systems and Devices 24 The layout of a typical microcontroller system is represented in Fig 2 5 The input devices may be directly connected to the I O pins Switches for example can be wired directly to the IC as in Fig 2 6 Figure 2 5 A microcontroller has the CPU and most other units of the system on a single chip It communicates with the external world through the I O pins of its one or several ports The input and output lines radiate from the microcontroller Figure 2 6 A simple one bit input to a microcontroller The resistor holds the pin high logic 1 except when the switch is closed when the input is low
193. perator AND The result on the right is true 1 only if both bits on the left are 1 In the example only the fourth pair of bits from the right is 1 1 The result of ANDing is 00001000 In hex this is 08 which is the value that would be left in register A at the end of the operation Opcodes and operands The first byte of an instruction tells the CPU what type of operation to perform It is known as the operational code or opcode Sometimes the opcode is followed by one or two more bytes which give the CPU something on which to operate It may be a value to work on or it may be an address where a value will be found stored This byte or pair of bytes is known as the operand 112 Microelectronics Systems and devices Figure 7 2 The timing of signals on the busses and control lines must allow logic levels to settle and for logical responses to be completed The sequences illustrated here and in Fig 7 1 normally take at least 3 or 4 cycles of the system clock possibly up to 12 If the system clock is running at 100 MHz four cycles take 40 ns program is first written in machine code this is also known as the source code Writing in machine code is not easy Fortunately there is software that makes it possible to write the source code in other forms and have it automatically turned into machine code The simplest type of software is assembler It is much easier to write in assembler than in machine code
194. pins by the CPU go straight out to the peripheral Signal placed on the input pins by the peripheral are put on to the data bus when the PIO is enabled In Mode 3 it is possible to configure all eight lines in the same direction in which case the port becomes an input or output port with no handshak ing Alternatively the pins may be configured separately and up to eight 1 bit inputs or outputs may be connected to the port For transmission of data over short distances of a metre or two an ordinary TTL or CMOS gate or buffer can be used Additional gates are needed if any handshaking signals are to be transmitted as well For longer distances it is preferable to use a serial port based on the RS 232 standard This allows for transmission over distances up to 15 m The driver output signal is between 5 V and 15 V The maximum data rate is 20 Kbit per second Other RS standards exist for higher rates of transmission The RS standards specify the types of connector to be used and the functions of each signal line The standard specifies the use of nine lines for carrying the serial signal and the handshaking signals Fig 5 5 shows the connections between a transmitting computer and receiving device such as a modem A 25 pin D type connector is used at both ends of the cable More recently a 9 pin D type connector has been used The pin connections are Serial ports 79 Interfacing When using a 25 pin socket there is often a line cal
195. propriate arrow Eventually after going round the inner loop 65025 times and round the round the outer loop 255 times you will finish at the terminal box labelled End You will find that there is no other route from Start to End There is no way in 171 Structured programs which the CPU can take a short cut This proves that the program if it follows the structure shown in Fig 11 2 will operate as required Fig 11 3 illustrates the cooler program of Chapter 10 The structure of this is different from the delay program It has a single loop but the decision box leads the CPU one way or the other depending on the temperature of the sensor Note that there is no End terminal in this program It runs indefinitely for ever Figure 11 3 The alternative states of the cooling system are evident in the flowchart Microelectronics Systems and Devices 172 Activity 11 1 Flowcharts 1 Draw a flowchart of the security program in Chapter 10 2 Draw a flowchart of any other program that you have written 3 Add a third loop to Fig 11 2 Run it for longer delays Design structure diagrams DSDs are another way of representing programs Some of the more useful symbols for DSDs are given in Figs 11 4 and 11 5 The program is represented by an unbranched vertical line or stem running from the Start box at the top to the End box at the bottom Program flow is always from top to bottom so there is no
196. put output pins so that the microprocessor can either write or read a byte at a time Whether there is a read or a write operation depends on the state of the two control lines shown in Fig 3 5 If the OUTPUT ENABLE is made low the addressed data is put on the bus If the WRITE ENABLE is made low data from the bus is stored on the chip at the addressed location 47 Memory Figure 3 5 Decoding the 16K addresses within the eight 6116 memory ICs is made simpler because each 6116 has its own inter nal decoder and the 74137 contains all the logic needed to decode the upper three address lines The 2K storage locations of a 6116 require an address bus 11 bits wide to address every location The 6116 has a built in decoder with 11 address inputs A0 to A10 To increase the amount of memory in a system we can use several 6116s and access these one at a time Each 6116 is connected to the data bus to the control bus and to bits A0 to A10 of the address bus The eleven address bits address the corresponding location in every one of the eight chips The next problem is to select just one of the eight chips To select one out of eight requires three lines which we will name A11 A12 and A13 The figure shows how we use them They go to a decoder IC known as a 3 line to 8 line decoder or multiplexer The three address lines are connected to the input lines A to C Normally the outputs of 48 Microelectronics Systems and Devices the decoder
197. rals 0 to 9 then there are some punctuation marks Capital letters of the alphabet begin at code 65 and lower case letters at 97 The codes in between these blocks are used for punctuation command and turns it into machine code It sends this to the CPU The CPU has to do four things at once Read a line of the source program Use the interpreter program to find out what the line means Turn it into machine code Obey the machine code The need to perform four tasks simultaneously makes running an interpreted BASIC program relatively slow In addition it is wasteful for the CPU to have to interpret a line every time it is used Imagine a program in which the same sequence of calculations is repeated 100 times with the CPU jumping back to the start every time it finishes The lines of that section of the program are interpreted 100 times even though they always produce the same result It would be far quicker if the lines were interpreted just once This is the function of a compiler A compiler is a program that takes a program written in a high level language and converts it directly into machine code It does this in just one session and the machine code is stored in memory or on a disk When the compiled program is run the CPU has only to read the machine code and obey it This is much faster than having to interpret every line as it comes to it C is a compiled language There are also compiled versions of BASIC
198. rammable logic controllers in industry a personal computer measuring instruments and data loggers the control room of a power station and distributed processing in flight control of aeroplanes 2 Microelectronics Systems and Devices These have much in common although there are important differences between them A microprocessor is just what its name says It is a data processor It is designed to be able to process large quantities of complex data at high speed It needs the support of other units such as memory and input output devices to make up a complete system A microcontroller usually operates more slowly and has less processing capability but it has the advantage of having the other units on the same chip It is a computer on a chip and as such is used on its own to take full control of a piece of equipment or installation Because CPUs are programmable the behaviour of the system can be changed in many ways simply by revising the program This gives microelectronic systems a big advantage over hardwired logical systems such as are used in the less expensive home security systems the older types of washing machines and in remote control systems Such systems may perform their intended task well but it is not easy to alter the system once it has been wired Any major change of action usually involves rewiring possibly changing some of the ICs and frequently making so many alterations that it is simpler to scrap the
199. rammer s responsibility and are not reported Provided that the CPU can do what you tell it it will do it This is why you need to debug your programs thoroughly The assembler program may include a section for debugging or you may use a separate debugging program Example The easiest way to follow the changes is to run through the program one line at a time At the start of the program all registers hold zero After the first line r17 holds 25 although we are working in decimal the assembler is working in hex After the second line r17 changes to 26 After the third line r17 still holds 26 and the same value is copied to r5 At the same time the program counter in the Processor window begins at 0 then changes to 1 then to 2 and then to 3 There are no changes in the flags As a reminder that the essential thing about an assembler is that it assembles machine code for us it is possible to display the actual machine code of the short program The memory of the 1200 is 16 bits wide so it can store both the opcode and the operand in a single 16 bit word The numbered memory locations are listed on the left and in the next column we see the machine code The original assembler listing is repeated on the right 116 Microelectronics Systems and devices Example The display of memory content of the example program looks like this 00000000 E215 LDI R17 0x25 00000001 9513 INC R17
200. rams and data tables This occupies most of the available address space in a typical micro computer A PC when purchased may be equipped with 32 Mb of RAM with room for expansion to 64 Mb or more The RAM is used for the temporary storage of programs and data There is usually less need for RAM in a microcontroller system as their programs are permanent and are stored in ROM instead see below The RAM is Random access memory 42 Microelectronics Systems and Devices usually included on the microcontroller chip and may consist of as few as 64 bytes Whenever data is read from RAM it is copied in a register of the CPU Conversely it is copied from the CPU when it is written into RAM In either case the original data remains unaltered after copying This is known as non destructive readout Once data is stored in RAM it remains there until it is either Overwritten and replaced by new data or The power supply is switched off There are two types of RAM Static RAM SRAM Each SRAM IC contains an array of flip flops each one of which can be set or reset to one of its two states and so represent a logic 0 or a logic 1 The flip flops may be individually addressable or they may be addressed in groups of 4 a nybble or 8 a byte or 16 a double byte also known as a word An example is the 6116 SRAM which has its 16K flip flops arranged in 2K groups of eight Fig 3 1 The address decoder is included on the
201. rations data transfer branching and bit handling Whatever language is used for programming the program used by the CPU consists of a series of opcodes and operands In an assembler program the opcodes are represented by mnemonics These may be divided into a number of types according to what they do Below we will look more closely at a few of each type of mnemonic They are demonstrated by using them in short assembler programs These assembler programs are written for the 1200 microcontroller Similar operations are available on all microprocessors and microcontrollers though they may differ in the mnemonics used to identify them Arithmetical mnemonics These cover such operations as addition subtraction incrementing decrementing and clearing or setting registers Addition and subtraction can only be done with two numbers Adding two numbers in binary may produce a 1 to carry over to the next stage This is 134 Microelectronics Systems and devices adc r17 r16 Architecture and programming CPUs vary in the ways they handle data Some do almost all the processing in a special register known as the accumulator Some have a few additional registers which assist the accumulator Others for example the 1200 microcontroller have no accumulator Instead it has 32 general purpose registers of which 16 can be used for arithmetical and logical operations The instruction set of a given CPU and the corre
202. re of a program the functions of the units within the CPU are discussed in detail Measures to increase operating speeds are discussed When we are choosing a CPU for a microelectronic system there are a number of points to consider Microprocessor or microcontroller If we are building a system that is required to process a large amount of data very quickly then we choose a microprocessor We can back up its powerful features with plenty of memory and a wide range of peripherals and input output devices If high performance and complex data processing is not essential and particularly if the system is to take up only a small amount of space and is to be inexpensive or is an embedded system then we chose a microcontroller Clock speed High speed may be essential for large scale processing especially real time control and complex graphics However high speed makes the CPU expensive and may cause problems in the design of the circuit board For many applications a clock speed of 1 MHz is more than adequate Bus width The wider the address bus the larger the address 56 Microelectronics Systems and Devices space The wider the data bus the easier and faster it is to process large numerical values more precisely However wide busses make it more difficult to lay out the circuit board Wide busses are used in powerful computer systems but an 8 bit data bus and a 16 bit address bus is sufficient for very many app
203. rmation are Books in the public library and your departmental library Back issues of technical periodicals AActivity 1 1 Microelectronic systems 16 Microelectronics Systems and Devices Manufacturers advertising matter and brochures Manufacturers and other sites on the Internet Arranged visits to local factories and businesses 1 List the stages in making a call from a cordless phone referring to the parts of the system that are pictured in Fig 1 1 Cover the action from the time the handset is switched on until the first words are spoken 2 Outline the structure and action of a programmable logic controller 3 Explain why special interfaces are needed between a PLC and the attached sensors and actuators 4 Describe the sequence of actions as a PLC runs its program 5 In what ways does a digital multimeter based on a microcontroller differ from an analogue multimeter with a moving coil microammeter What are the advantages of the digital multimeter 6 List the devices peripherals that may be attached to a PC and explain briefly what they do 7 Describe the features and action of a data logger 8 What is meant by distributed processing 9 Write an essay under the title Microelectronic systems in everyday life 1 Lamps motors and solenoids are examples of A sensors B interfaces C actuators D outputs 2 A CPU is A a microelectronic system B the heart of a microelectronic system C unit
204. rocontroller is as follows Security routine equ ddrb 17 equ portb 18 equ bit0 1 Sensor input equ bit2 4 Button input ldi r20 250 out ddrb r20 out portb r20 Siren off waiting in r16 portb Waiting for intruder andi r16 bit0 brne waiting ldi r17 248 out portb r17 Siren on alarm in r16 portb Siren sounding until andi r16 bit2 button is pressed brne alarm ldi r17 250 out portb r17 Siren off rjmp waiting Note that after initialising labels and constants which takes only a few milliseconds the first action is to turn off the siren This is to guard against the output going high when power is applied and switching the siren on This precaution may be unnecessary but it takes care of a possible glitch Fig 10 4 The light sensitive circuit has a high output when light is shining on the LDR It goes low when the LDR is shaded 164 Microelectronics Systems and Devices Programming in PBASIC Connect the light sensor Fig 10 4 to pin P0 of the Stamp and a siren switched by a transistor to P1 The first version of the security program is similar to the I O routine on p 153 Security system V 1 input 0 output 1 out1 0 ensure LED off onalert debug On alert if in0 0 then onalert debug Intruder out1 1 switch on siren If a delay is added it makes the system insensitive to short breakages of the beam and so avoids false alarms
205. rogram when the CPU has finished working on the current value of trial it is told to jump back to again Then trial is incremented and the CPU does the processing again using the new value This completes our account of the principles of assembler programs Summing up its main advantages are Easily remembered mnemonics Can use decimal numbers Comments Automatic assembly Error reporting Labels for addresses registers and variables There is more to be learned about assembler but this is best done by working the practical examples in the Activity sections below A dry run In the early stages of writing a program or a section of a program it often helps to take paper and pencil and work out how the values in the registers and in memory will change line by line This is known as a dry run The same kind of tryout can be run using the assembler program or some of the high level languages but things may seem clearer to you on paper than on the screen In addition it is possible to cross out and scribble comments on paper but less easy to do this on the screen Here is a dry run of the assembler program The columns show the contents of the registers after each line has been run 120 Microelectronics Systems and devices Activity 7 1 Starting assembler You need A computer with assembler software installed The manual for the assembler assuming that you
206. rogrammer and to others who may want to study it and perhaps modify or extend it This is done by adding a title and some comments or remarks These are preceded by a semi colon When assembling anything between the semi colon and the end of the line is ignored by the assembler However it may be of great interest to programmers and others Improving the program 117 Instructions Example This is one way in which the sample program can be improved by a title and comments Program example 1 introducing assembler ldi r17 37 loads register 17 with blood temperature inc r17 increments temperature mov r5 r17 stores incremented temp in base register Note the positions of the semi colons The title explains what the program is about The comments on the right explain what happens at each step Long comments can be carried on to the next line after a semi colon Comments are important because an uncommented program of more than a few lines is extremely difficult to understand even by the person who wrote it and especially a few weeks after it is written Writing useful comments is an acquired skill They must not be so brief as to be unintelligible They must not be so long that the actual program becomes lost in a mass of multi line comments Another advantage of assembler compared with machine code is that assembler allows us to use labels A label is a word used to identify a program line
207. rrupt of higher priority than INT0 RESET Master reset Initialises CPU and other devices WAIT Wait Devices extend bus cycle to more than 1 clock period All of these inputs except CLK are active low Their symbols should have lines drawn above them to indicate this but they were omitted from the table for clarity In this table the term devices includes memory I O and various peripheral devices An example of the use of the BUSAK input is in direct memory access DMA Some systems include an IC known as a DMA controller It is used when a large block of data such as a word processing file or a program has to be transferred to memory from storage on a disk Transfer in the normal way in which the CPU requests each byte reads it and then writes it to RAM would take far too long Instead the DMA controller requests the CPU to give up control of the address and data busses while it transfers the data directly from the disk to memory In this CPU there are only two levels of interrupt but other micro processors may have more A low input to NMI can not be ignored by the CPU unless a BUSRQ signal has been received On receiving an NMI signal the CPU completes the instruction it is working on and then jumps to the address of the interrupt request routine IRQ The INT0 signal is the one normally used for requesting interrupts This can be ignored if the interrupt flag in the status register has not been set This allows the progra
208. ructions A branch instruction tells the CPU to stop working its way systematically along the program and to jump to some other part of the program instead The jump may be forward or backward RJMP or Relative JuMP is the simplest of the branch instructions Normally the program counter is incremented after each fetch execute cycle so that the CPU automatically fetches the next byte in the program for execution RJMP puts a completely new address in the program counter so the next fetch execute cycle starts at a different place in the program The program below explores the action of RJMP in the forward direction Demonstrating the relative jump RJMP ldi r16 10 rjmp land ldi r16 20 ldi r17 20 ldi r18 20 ldi r19 20 land ldi r20 20 ldi r21 20 ldi r22 20 The CPU jumps to land on the third line up so registers r20 r21 and r22 are loaded with 20 Registers r16 r17 r18 and r19 are left unchanged Sometimes we want the CPU to continue working its way along the program provided that a given condition is true However if the condition is not true we want it to branch to some other part of the program instead This is known as a conditional branch or jump BREQ or BRanch if EQual makes the CPU jump to another point in the program if the result of a calculation is zero Before making the jump the CPU goes to the status register to find out if Z is set 1 or reset 0 The zero flag is set to 1 if the result of a
209. s including masking constants where used Often certain addresses such as the addresses of ports and data direction registers are labelled at this stage and then the ports are configured as inputs and or outputs In some programs it may be advisable to repeat at least part of the initialisation at later stages see box In some high level languages there is a fixed order in which the definitions must be made This first part of the program consists of a sequence as defined above and programming it presents no difficulty except that of making sure that the definitions are correct and complete In its simplest form the remainder of the program falls under three headings input processing output This is certainly true for the control programs used with programmable logic devices as explained in Chapter 6 It is also true for many other control programs and for measurement programs too The idea of breaking down into shorter simpler sections is not restricted to the whole program For example the input section may in turn be broken down into smaller sections especially if a certain sequence of key presses has to be made In this stage the program Designing programs Countering corruption It is a mistake to assume that it is sufficient to initialise control registers for example the data direction registers once and for all at the beginning of a program Stored data may change especially in an electrically noisy industrial e
210. s debouncing circuit mean that it responds instantly to a change in the position of the switch charge and discharge Switching on and off can not be repeated as rapidly as required In this case the logic of Fig 6 2 is be used The set reset flip flop is made to change state by bringing one or the other of its inputs low The change over occurs the first time the switch makes contact and the flip flop does not change state again during the subsequent bounces In Chapter 12 wew discuss how to debounce a switch by using software instead of hardware Watchdog timer In spite of all the precautions it may still happen that a bit in memory or in one of the registers of the CPU may become 105 Planning the system altered by electromagnetic means Substituting a 0 for a 1 or 1 for a 0 can provide the CPU with false data or worse still may make it jump from its present place in the program to some unpredictable place in another part of RAM where there may be no program The result may be errors in output or the program may crash completely The watchdog timer is a technique for avoiding such dangers It keeps a watch on the CPU making sure that it is operating correctly The timer can be an ordinary electronic timer circuit such as one based on the popular 555 or 7555 timer IC Or it can be a timer built in to the processor Some I O ICs also have one or more timers included The principle of the technique is very similar to the routine f
211. s its own special features and it is unlikely to work unless it is set up in exactly the right way There is not enough space in this book to describe how to set up and use the many types of I O ports and other microprocessor support devices The details are available in the manufacturers data sheets which may be obtained from suppliers of components or often downloaded from the World Wide Web This case study describes the initial stages in the design and programming of a system based on a real time clock IC There are many different real time clock ICs available The type chosen is the Hitachi 146818 This is a 24 pin IC intended to be part of a system based one of the 6800 family of microprocessors It is also suitable for Programming a support device 194 Microelectronics Systems and Devices use with many other microprocessors and microcontrollers Having chosen this IC it is investigated by following the stages described below Data sheet Data sheets usually begin with a list of the special features of the device The data sheet of the 146818 explains that this is a time of day clock and calendar It counts seconds minutes and hours It registers days of the week the date the month and the year It can deal with months of variable length and with leap years It can also be used to generate interrupts at regular timed intervals and can be programmed to produce a square wave of a number of different frequencies The 14
212. same two values used in the operations above After the values are loaded the registers hold Flags I T H S V N Z C 0 0 0 0 0 0 0 0 R16 1 0 0 1 0 1 0 1 R17 0 0 1 0 1 1 1 1 Flags I T H S V N Z C 0 0 0 0 0 0 0 0 R16 0 0 0 0 0 1 0 1 R17 0 0 1 0 1 1 1 1 Flags I T H S V N Z C 0 0 0 0 0 0 0 0 R16 1 0 0 1 0 1 0 1 R17 0 0 1 0 1 1 1 1 After the final line of the program the registers hold this Only in the first and third columns from the right is there a 1 in r16 AND r17 EOR called XOR in some instruction sets is the logical operation of exclusive OR The result is 1 if either of the two bits but not both of the bits are 1 Before operating on the same two values as before we have 137 Mnemonics Flags I T H S V N Z C 0 0 0 1 1 0 0 0 R16 1 0 1 1 1 0 1 0 R17 0 0 1 0 1 1 1 1 The result of EOR is Register r16 now holds the exclusive OR result and r17 still holds the value with which it was loaded Data transfer There are instructions for transferring or copying to be more correct data from one register of the CPU to another There are also instructions for sending output to a port and for reading in data received by a port With many CPUs data can also be copied from a register to an address in RAM or from an address in RAM to a register OUT copies data from
213. se there is a carry over from bits 3 to bit 4 3 r17 would hold 48 5 ldi r19 15 out 17 r19 6 ldi r20 128 out 18 r20 7 ldi r19 8B mov r17 r19 8 r17 to r22 would all hold 20 r16 would remain unchanged 9 right shift its digits by one place Test your knowledge 1 The gate of the MOSFET is insulated so only a very small current is drawn from theoutput pin 2 Z is the zero flag it goes low when the resultof an operation is not zero 3 4 4 ms 4 The first number Chapter 8 Chapter 9 Chapter 10 211 Index Index Accumulator 62 185 Actuator 6 ADC analogue to digital converter 86 Address bus 26 Address bus register 61 Address space 49 Addresses 46 196 205 ALU arithmetic logic unit 67 Architecture 26 134 Arithmetical instructions 133 ASCII code 124 Assembler 112 Assigning 126 Atmel AT90S1200 microcontroller 19 BASIC see also PBASIC 123 127 148 152 156 161 Baud rate 81 Bit 20 Bit rate 81 Bit slicing 38 Bit test instructions 141 Bitwise logic 111 Branch instructions 139 Branch prediction 65 Breadboard 105 Buffer 73 Bus 10 26 28 60 Bus width 29 55 64 Byte 20 C 124 180 Cache memory 65 Chip enable 29 CISC processor 57 64 CMOS logic 38 99 Comment 116 Compiler 65 124 Contact bounce 103 Control bus 32 Control unit 59 CPU central processing unt 1 3 7 Crosstalk 102 DAC digital to analogue converter 89 Daisy chaining 190 Data bus
214. sembler programs and software for programming in the C or BASIC languages The Stamp 2 is a complete microcontroller system based on the PIC16C57 It includes a compiler for PBASIC which is a version of BASIC devised for making best use of the features of the 16C57 Some programs for the Stamp 2 are provided later in this book 23 The CPU Figure 2 4 The smaller PIC microcontrollers have only 8 pins Some pins share several functions Example In an industrial plant either a valve is closed or it is not closed The sensor is a microswitch on the valve which closes when the valve is closed The signal is sent from the microswitch to one of the port pins of the CPU It is a high voltage logic 1 when the valve is closed and is a low voltage logic 0 when the valve is not closed The signal is a single bit Example Either the temperature of a vessel has reached the required level or it has not We can represent these conditions by input from a thermal sensor at just one pin of Port B The input is high when the temperature has reached the required level and is low when it has not Again the signal is a single bit The same applies to many actuators If a motor is either running or not running only a single bit is required to switch the motor on 1 or off 0 For this reason the individual bits of each port may be configured as inputs or outputs and the output and input values set individually On the 1200 we could have u
215. set and the compiler are both designed at the same time The hardware and software engineers work as a team The result is a processor compiler combination that operates with the best possible efficiency to give faster running programs Pipelining In a conventional computer a byte of data on its way from RAM to the ALU of the CPU is copied from one location to another Fig 4 2a There is one transfer per clock cycle so the whole process in the example takes 3 cycles Bytes arrive at the ALU every 3 cycles With pipelining Fig 4 2b the second byte and third bytes begin their journey before the first byte has reached the ALU A byte arrives at the ALU every cycle Pipelining is advantageous but does not work well with certain kinds of instruction Moreover if the CPU is using branch prediction see above and the prediction is wrong all the data in the pipeline has to be discarded a b 67 Inside the CPU Out of order execution If a calculation has several steps in it the values to be used at a given step depend on values obtained in previous steps The steps must be executed in the correct order However in a system with two or more processors it is possible for an idle processor to check ahead to find an instruction that does not depend on previous calculations and execute that instruction ahead of time The result will then be ready when required Activity 4 1 Selecting a processor Select a microprocessor or microcontr
216. sors include a light dependent resistor a vibration detector a pressure mat and a proximity switch First test the sensor circuit without connecting it to the processor and adjust resistance values if necessary Then decide on a suitable application for the sensor and devise an appropriate actuator Finally connect a sensor and an actuator to the system and write a program for the application 6 Design and build a system with up to three 1 bit inputs and up to three 1 bit outputs For inputs use circuits sensitive to light temperature sound position or magnetic fields There are many ways in which switches keys and buttons of different kinds may be used to provide input This includes the use of microswitches as limit switches or to indicate whether doors are open or closed For outputs use lamps LEDs sirens buzzers solenoids electric motors small ones working on 6V or less DC or relays Aim to control your system with a program not much longer than the programs in this chapter If it seems that the program needs to be longer wait until you have studied the next chapter before you write it It is intended that you should build the systems on a breadboard to begin with though some of them will be suitable for transferring to a more permanent form such as stripboard or pcb 167 Input and output Problems on input and output 1 List three different ways of providing input to a microelectronic system and describe one appl
217. sponding mnemonics of the assembler depend very much on whether processing is centred on the accumulator or spread over several registers Whatever the architecture the general principles of processing in short simple steps is the same for all CPUs The short programs in this chapter are easily adaptable to a range of CPUs stored as the carry digit C in the status flag register ADd with Carry ADC is used at the second and subsequent stages of a multi stage addition A short program to test this is Demonstrating ADC adding 149 47 1 carry sec set carry flag ldi r16 149 load r16 with 149 ldi r17 47 load r17 with 47 adc r16 r17 add all three and put the sum in r16 Addition in stages CPUs operate on pairs of numbers They can add a and b to obtain their sum If there are three numbers a b and c to be added it has to be done in two stages a b sum1 sum1 c sum There is an exception when two numbers and a carry bit are being added All three are added in a single operation as in the ADC operation described on this page 135 Mnemonics inc r17 Flags I T H S V N Z C 0 0 0 0 0 0 0 1 R16 1 0 0 1 0 1 0 1 R17 0 0 1 0 1 1 1 1 Flags I T H S V N Z C 0 0 1 1 0 1 0 0 R16 1 1 0 0 0 1 0 1 R17 0 0 1 0 1 1 1 1 Flags I T H S V N Z C 0 0 0 0 0 0 0 0 1 R16 1 0 0 1 0 1 0 1 R17 0 0 1 0 1 1 1 1
218. ss a given number of bytes further on or further back This is only applicable to machine code programming A person using assembler would not be concerned with the actual addresses of the opcodes Investigate the addressing modes available on the CPU you are using and write short routines to try them out In assembler given the number of the month the stored value of the number of days can be obtained by indexed addressing In a high level language it can be accessed by the commands used for manipulating arrays It is a fairly easy project to program the month days conversion Slightly more difficult is to convert letters of the alphabet into the equivalent Morse Code 206 Microelectronics Systems and Devices positive number using 4 bits Then write a 0 on the left to represent a positive sign Next write the one s complement by writing 1 for every 0 and 0 for every 1 Form the two s complement by adding 1 to this number Ignore any carry digits This is the binary equivalent of the negative of the original number Examples 1 To find the 4 bit equivalent of 3 Write 3 in binary 0011 Write the sign digit for 00011 Find the one s complement 11100 Add 1 1 Result is two s complement 11101 This is the equivalent of 3 and the 1 on the left indicates that it is negative 2 Adding two negative numbers for example adding 2 to 3 Two s complement of 3 11101 Two s complement of 2 11110 Add
219. stored there There are very fast CPUs that read more than one at a time but most CPUs read one instruction at a time and act on it before fetching the next one This is the type of CPU we describe in this chapter We shall also confine our descriptions to systems in which the data is operated on as single bytes When it is operating a CPU repeatedly goes through a cycle known as the fetch execute cycle As its name implies this cycle has two stages The CPU fetches an instruction from memory The CPU executes it that is it obeys the instruction The cycles are repeated continuously as the CPU works its way from the beginning of the program to the end The instruction is a byte of data stored in memory Reading a byte from memory takes place in ten stages The address of the byte to be read is transferred along the internal bus of the CPU to the address bus register The address is placed on the address bus Fig 7 1 The address is decoded by the logic partly on the memory chip to select the location for reading Reading is enabled In Fig 7 1 the WRITE ENABLE line is already high so reading is already enabled The CHIP ENABLE line goes low to switch the memory chip outputs from the high impedance state to a low impedance state outputs high or low Data from the memory location appears on the data bus The CPU stores the data from the bus in its data bus register T
220. system to switch the siren off after it has been sounding for 15 minutes Microelectronics Systems and Devices 168 Flowcharts and design structure diagrams are two techniques for displaying the structure of a program They help the programmer to write efficient programs that will work as intended Long programs are best broken down into a number of sub programs known as modules Subroutines and functions are modules of a special kind The operation of the stack and the action of interrupts are two other features of importance to programmers The programs described in Chapter 10 are so short that is it easy to read through them and understand what they do Mostly they list a sequence of steps that the processor must take starting at the beginning continuing to the end and then stopping Even in some of these simple programs it is necessary for the processor to jump to different parts of the program or to spend some time running round loops As soon as we introduce jumps and loops into a program it becomes more difficult for the person reading the program to understand what is going on One way of making the action of a program clearer is to draw a diagram to show the paths that the CPU takes while working its way through the program A diagram is useful after a program has been written It is also useful to draw the diagram when the program is being designed and written Indeed with a program of more than 10 or so lines it is essenti
221. t as possible avoid running signal tracks parallel to power line tracks large voltage fluctuations on power lines could swamp the signals keep tracks carrying input signals away from tracks carrying output signals This applies to the board as a whole keep input circuits away from output circuits and to individual ICs keep tracks leading to input pins away from tracks leading from output pins When very high frequency signals are used in a microelectronic system the wavelength of the signals may be comparable to the lengths of some of the tracks Then the normal rules of conduction no longer apply Instead the tracks must be treated as transmission lines A transmisson line has a characteristic impedance depending on its dimensions and other factors If the output impedance of a signal source and the input impedance of the receiver both match the characteristic impedance of the line the signal is transmitted along the line with virtually no loss If they do not match the signal may not reach the far end or may be received with much reduced power A portion of it may be reflected back from the receiving end causing standing waves in the line and also resulting in ringing and other effects These make the operation of the computer very erratic The design of systems involving transmission lines is outside the scope of this book Crosstalk Transmission lines 103 Planning the system Here are a number of design points wh
222. t is helpful to beginners to explore the capabilities of the CPU and the programming language in this way Here is the third though not necessarily the final version of the security program Security system V3 input 0 output 1 input 2 add input for button out1 0 ensure LED off onalert if in0 0 then onalert pause 500 if in0 0 then onalert out1 1 switch on siren alarm if in2 1 then alarm keep siren sounding out1 0 siren off goto start On the alert again 166 Microelectronics Systems and Devices security system you should use it to switch on an LED while you are developing it This is less irritating to other people working in the same room Later when the design is perfected and the program has been debugged you can substitute a siren for the LED 2 Write new versions of the cooler program to make it do something slightly different For example revise the program so that it responds to a sensor connected to bit 3 of Port B instead of to bit 0 Change the program to control a motor through an output at bit 7 3 Adapt the cooler program to switch on a siren when the temperature rises above the set point This could be a fire alarm system 4 Adapt the cooler program to flash an LED repeatedly when the temperature falls below the set point This could be a frost warning system 5 Substitute other sensors for the thermistor in the circuit of Fig 10 2 Suitable sen
223. t program to the system you are using 2 Write new versions of the input output program to make it do something slightly different For example revise the program so that it responds to a switch connected to bit 3 of Port B instead of to bit 0 Change the output pin to bit 5 3 Start with the LED on at the beginning of the run and let closing the switch turn it off 4 Make the user close the switch or press a button twice to light the LED 5 Program a toggle action As the switch is repeatedly closed the LED goes on then off indefinitely 154 Microelectronics Systems and Devices Delays Processors work so fast that a short program such as the input output program seems to run instantaneously The LED seems to come on as soon as we press the button The speed of action is advantageous but sometimes we want to hold the CPU back a little If the command that switches the LED on is followed immediately by a command to switch it off the action takes place too quickly for us to see the LED flash To be able to see the flash we need to introduce a delay between switching it on and switching it off again In this way a delay is useful to hold the CPU back for a short time to let the human operator keep up with it A delay may sometimes be used to make an action repeat at regular intervals One way to produce a pause in a program without actually stopping it is to keep the CPU busy doing something that has no apparent effect I
224. taken and the future output state is stored in memory the program reaches its third and final stage It sends the stored output data to the actuators The actuators are switched on or off in response to the latest state of the system The program repeats immediately so it is 8 Microelectronics Systems and Devices Figure 1 4 A PLC system is housed in a cabinet shown here with the door open It controls the resin production plant illustrated in Fig 1 2 The PLC controller is the small box mounted at the top left of the cabinet with its control keys situated below its LED display The low voltage power supply is mounted to the right of the PLC The input and output interface units are mounted on the rack below the PLC Each may be connected to up to eight sensor or actuator devices On the right is a laptop PC being used for writing programs and downloading them into the PLC By courtesy of Kronospan Ltd Chirk Controlling a chemical reaction The use of PLCs in industry is illustrated by a stage in the manufacture of urea formaldehyde resin Fig 1 2 There are several factors that determine whether the shutter should be opened or closed For example the shutter must be opened when the process begins and must be closed when the kettle is full Weighing sensors tell the CPU how much urea has been added to the kettle Mixing urea with formaldehyde causes heat to be generated so a thermal sensor provides essential input to the CPU T
225. ted Only one device must be enabled at any one time When one is enabled the others must be disabled otherwise two or more devices will try to put signals on the bus at the same time This is known as bus contention The address bus and address decoding Only the CPU puts addresses on the address bus The other devices in the system simply wait until they are addressed before doing anything A device recognises when it is being addressed that is when its address is on the address bus by means of a decoder circuit This is a logic circuit that produces a low usually output when and only when its own Microelectronics Systems and Devices 30 Memory size Every location in the memory of a computer must be identified by its own unique address The number of different addresses that are possible in a given system depends on the number of bits it its addresses For example many microprocessors use 16 bit addresses The addresses run from 0000000000000000 zero decimal to 1111111111111111 65535 in decimal giving a total of 65536 possible addresses With such large numbers we usually quote them in K Really K stands for kilo which means thousand but in quoting addresses and sizes of memories it means times 1024 In the example 65536 is quoted as 64K Here are some sizes of whole memories or of individual memory chips The bottom line of the table explains why the numbers of addresses shoot up
226. tely after the Start pulse the transmitter sends eight data bits of length equal to the start pulse These may be low level or high level and are followed by a low Stop pulse After this the transmitter may send another Start pulse followed by the next byte of data or it may remain in the marking state for a while if there is nothing to send Figure 5 7 This serial interface protocol is called asynchronous be cause it does not need the transmitter and receiver to have their sys tem clocks synchronised In some systems the Stop bit must be at least two bit periods long 81 Interfacing The eight data bits usually consist of seven data bits D0 to D6 followed by a parity bit see box For data to be read correctly the transmitter and receiver must both be operating at the same baud rate However there is no need for precise timing because each group begins with a Start pulse which resets the clock of the receiver It then has to keep time for only 8 pulses This method of signalling is described as asynchronous because it does not require the clocks at the transmitter and receiver to be permanently synchronised Data is processed in parallel form in microelectronic systems Before it can be sent to a serial interface such as the one shown in Fig 5 5 it must be converted to serial form There are several ICs that can do this including the 74LS165 parallel serial converter This IC is described as a shift register Fig 5 8 It
227. that are more powerful have many more pins This is mainly because of extra pins in the data I O and address output groups The extra data pins bringing the total to 16 or 32 connect to additional circuitry on the chip They allow the CPU to deal with much larger numbers and longer codes The extra address output pins allow the CPU to address a much larger memory see box p 30 The DIL layout is not suitable for more than about 40 pins ICs with larger numbers of pins are usually square with the pins arranged along all four sides in a single or double row The most powerful CPUs such as the Pentium and Athlon have hundreds of pins and special layouts are used The data bus and three state outputs If we look even more closely at a bus we see that each bus consists of many tracks running side by side Fig 2 9 shows a data bus that is eight bits 1 byte wide This is typical of most microprocessors Voltages representing logic low 0 or high 1 are placed on this bus by the CPU or other devices in the system They represent a number or an operational code This is how the CPU reads data from units such as the memory or sends data to units such as an output port Microelectronics Systems and Devices 28 Figure 2 9 A bus consists of a number of conductors running side by side on the circuit board Here we illustrate a data bus that is 8 bits wide The figure shows the voltage levels on the data bus when the binary value 1011 1001
228. the display It is automatically updated several times per second The display usually consists of 4 digits with a movable decimal point and a polarity indicator for negative values The quantities that can be measured include voltage current resistance capacitance and frequency With an ordinary multimeter the user has to select the range but range selection is automatic with a meter based on a microcontroller Figure 1 7 A multimeter is no longer a switched network of resistors and capacitors connected to a sensitive moving coil microammeter It still has the resistor and capacitor network but now most of the switching is done by CMOS gates managed by a microcontroller 13 Systems in action Given a constant current source a timing circuit and a voltage measuring circuit it is possible to measure all the other quantities Resistance can be measured by finding the voltage drop across the resistor when a given current flows through it Capacitance can be measured by finding how long it takes a constant current to charge the capacitor to a given voltage Frequency can be measured by timing the changes in voltage These tests are easily automated for different ranges One of the few disadvantages of the numeric display of a digital meter is that with a varying quantity the rapidly changing figures do not help the user to visualise the way in which the value is changing The needle of the conventional electromagnetic meter is much
229. the equivalent of an output port and used to transfer addresses from the internal bus to the external address bus Arithmetic logic unit ALU The second complex logic circuit in the CPU is concerned with processing data Its operations are controlled by the control unit Given a single value it can increment it add 1 or decrement it subtract 1 Given two values it can add them or subtract them As explained in Chapter 7 multiplication and division is often done by repeated addition or subtraction However in some processors the ALU can perform multiplication and division directly which is faster The ALU is also able to perform logical operations such as bitwise AND and OR on a pair of values The remaining units of the CPU are registers How many there are and the way they are used varies widely from one CPU to another but there are four registers that are nearly always present Program counter The CPU must always keep track of where it has got to in a program The program counter is a register holding the most recently accessed address As soon as the CPU has read the byte stored in that address the address stored there is incremented by 1 We say that the program counter now points to the next address This address is put on the internal bus then into the address bus register and finally on to the address bus itself In this way the CPU works its way through a block of memory reading each byte as it goes Occasionally the micr
230. the seven bits are 0110111 the group contains an odd number of bits The parity bit is 1 to make the number of bits even The group transmitted is 01101111 The receiving system counts the number of bits in each group and rejects any group that contains an odd number Then the parity bit is removed from the end of the group and the remaining seven bits are sent on for processing The receiver may be able to request the transmitter to send any rejected groups again This technique is known as even parity Some systems use odd parity in which the number of 1 s is made up to an odd number Parity checking will detect a single error in a group but does not detect two compensating errors Adding a parity bit to each group means that the rate of data transfer is slightly reduced When its LOAD input is made low the data present on each data line is loaded into the corresponding register The IC has a data output and an ENABLE input If the ENABLE input is held low the data is shifted one step to the right every time the clock input rises from low to high On the first rising clock edge the data in H D0 equal to 0 or 1 appears at the serial output terminal and can be sent to a serial output interface The data in G is transferred to H and the other transfers are F to G E to F D to E C to D B to C and A to B Each bit has been shifted one place to the right On the next rising clock edge D1 originally in G but now in H appears at
231. ther advantage of assembler is that we can use decimal numbers instead of having to convert every value into hex The assembler does all the converting when it turns our source code program into machine code When the program is typed in it appears in a window on the screen If it appears to be correct clicking on the Assemble button at the top of the screen causes the program to assemble it that is to turn it into machine code This is much quicker than looking up a table of opcodes The machine code is stored in a file with the extension obj At this stage instead of assembling the assembler may report that there are errors in the program It states the number of line or lines on which the errors occur and gives a brief description of the type of error Example The first version of this program used register 2 where it now uses register 17 On assembling an error was reported in line 1 This was because only registers 16 to 31 can be used for arithmetical operations including loading numerical values After the program had been edited to change r2 to r17 assembly proceeded without error When a program is being typed in it appears in a special window on the screen It is possible to have other windows open on the screen at the same time One useful window is the Processor window This displays the state of the program counter the stack pointer the flags and other useful data Another window of special use
232. ther built in routines save the programmer much time Visual BASIC is one of the relatively new languages that exploit the facilities of the Microsoft Windows environment It has most of the keywords of conventional BASIC and a great many more of its own Most of these are aimed at providing the user with the means of setting up the complicated and visually effective windows menus radio buttons sliders and other graphic devices that are associated with Windows programs For those wanting to write their own Windows utilities and games programs Visual BASIC is an ideal platform Unfortunately its keywords do not include the INP and OUT functions of conventional BASIC To access the ports of the computer requires Microelectronics Systems and Devices 130 Ladder logic Ladder logic is derived from relay logic which was widely used in industrial control systems The plant was controlled by a system of hard wired relays for switching on lamps motors and other items of equipment The diagrammatic form of the program is based on the symbols formerly used in USA for relays and associated devices Actuators are represented by circles Fig 8 1 shows part of a PLC program With the control unit in programming mode the diagram is entered on the screen The keyboard of the control unit has special keys for each of the relay logic symbols If a PC is being used for programming certain keys are allotted to display the symbols The program
233. to another either in parallel or in series Serial transfer Fig 2 2 requires only one line along which the voltage levels are sent one after the other In parallel transfer Fig 2 3 a binary number is represented by high and low voltages placed on a set of conductor lines at the same time Transfer requires a separate line for each bit Parallel transfer is quicker but requires more tracks on the circuit board more terminals on the ICs and may require more buffers at each stage to relay the signals It is used inside computers and other data processing equipment such as modems to provide speed In contrast serial transfer needs only one track one terminal and one buffer at each stage but it is much slower It is often used for communicating between microelectronic systems particularly by the telephone system or by radio links where only one channel is available 21 The CPU Figure 2 3 Parallel transfer of 1 byte 8 bits of data requires 8 data lines D0 to D7 In order to keep the size or more exactly the number of pins of the IC reasonably small manufacturers often allocate two or more functions to the same pin In the instance of the 1200 the I O pins PB5 PB6 and PB7 together with the RESET pin can also be used as a serial port for downloading a program from a computer such as a PC to the memory inside the 1200 In serial transfer of data high or low pulses are fed into or out of the pin one after a
234. to noise from adjacent circuits and other sources of EMI Sometimes the choice of family is determined by what functions are available For example if long counter chains are needed the 4000 series is the best For octal buffers and latches the 74LSXX series has more types to offer Chapter 5 On occasions it happens that the two functions required are not both available in the same family In such Logic and clock rate selection Microelectronics Systems and Devices 100 Driving output Driven input 74LSXX All CMOS 74LSXX 20 74LS buffers 60 CMOS4000 1 50 74HCXX and 74HCTXX 10 74HC HCT buffers 15 cases it is necessary to use an interface between them The supply voltage must be compatible with both families which usually means operating at 5 V When connnecting gates and other logic inputs and outputs remember that there is a limit to the amount of current that an output can provide in the logic high state There is also a limit usually larger in the amount of current a gate output can sink is the low logic state There are also limits on the amounts of current to be sourced or sunk to make an input register as low or high The result is that there are definite numbers of gates that can be reliably driven by the output from a given type of gate This number is known as the fanout The table below shows the fanout of various combinations of families a virtually unlimited number of gates It can
235. tronic system interfaced to the computer Control inputs set the device to operate in one direction or the other The 74LSXX ICs mentioned in this section are also available in CMOS versions Parallel port ICs There are several different types of parallel port IC with very similar features and a few minor differences Fig 5 4 shows an IC intended for use with microprocessors of the Z80 family the Z8420 more usually known as the Z80 PIO parallel I O 75 Interfacing Fig 5 4 illustrates the more important features of the Z80 PIO It has a bi directional connnection with the system data bus which is 8 bits wide It has two 8 bit ports referred to as Port A and Port B Each port has two handshaking lines for communicating with devices attached to the ports The lines are RDY ready and STB strobe Fig 5 4 The Z80 PIO provides two ports which may be configured as inputs or outputs The decoder enables the IC when the upper six bits of an address in the range F0 to F3 is presented to it Inputs marked with small circles are active low The PIO operates in one of four modes for each of the ports Mode 0 As an 8 bit output port Mode 1 As an 8 bit input port Mode 2 As an 8 bit bidirectional port Port A only Mode 3 Port B as a bitwise I O port when Port A is in mode 2 Each port has a port control register to which a byte is sent to select the mode If the port is programmed as mode 3 a second byte must be sent to define w
236. ts dark grey and which are both input and output light grey The pins are also labelled with the names of the pins they are going to be connected to on the Stamp2 196 Microelectronics Systems and Devices Figure 12 2 With an INVERT gate connected internally between pins 2 and 3 of the 146818 this circuit oscillates at 32 768 kHz Also connected internally is a 15 stage binary divider This divides the crystal frequency by 2 15 which is 32768 The result is a squarewave with a frequency of exactly 1 Hz giving a period of 1 s This provides the basic timing period for the clock Periods of 1 min 1 h and longer are derived from this by further division Address map There are 64 bytes of RAM on the chip The first 14 bytes are reserved as registers and the remaining 50 bytes are available for any purpose that the user requires The functions of the first 14 bytes are 0 Seconds 1 Seconds alarm 2 Minutes 3 Minutes alarm 4 Hours 5 Hours alarm 6 Day of the week 7 Day of the month 8 Month 9 Year 10 13 Registers A to D 197 Programming projects The first 10 registers are loaded with the current times day and dates When the clock runs a series of divider counters update the registers as time passes At any future instant the registers are read to provide data on time and date Registers A to D hold flags to indicate various states of the clock Some of these flags may be set to control the operation of the clock For
237. ture control system 158 213 Index Three state output 27 Timing 156 200 Transmission line 102 TTL transistor transistor logic 37 72 99 Two s complement 205 Unused inputs 103 Vectored interrupt 190 Visual BASIC 129 Watchdog timer 104 Write operation 33 110 112 197 Z80 microprocessor 25 32 189 Z80 PIO 75 Z80 SIO 84
238. two examples of this technique Debouncing inputs When a switch or key closes the unevenness of the contact surfaces on a microscopic scale cause it to close and open again several times before it eventually closes permanently We say that there is contact bounce The rapid succession of on off states is not noticeable when for example we switch on a lamp or a motor But a logic gate reponds so rapidy to changes in input that it detects every closure and opening If for example a key is intended to send one pulse to a counter circuit every time it is pressed the counter may register five or more presses each time it is pressed once There are times when contact bounce does not matter For instance when we reset a flip flop it is reset on the first contact and repeating the action half a dozen times has no further effect When contact bounce does matter we use special circuits to debounce the switch In Fig 6 1 the capacitor slows down the rate of change of logic level so that the noise from the switch is absorbed This technique is effective but may make the action of the switch too slow If a large value capacitor is used to thoroughly debounce the switch it takes longer to Design points Microelectronics Systems and Devices 104 Figure 6 1 This debouncing circuit gives a low output when the push button is pressed This is a suitable output for triggering an active low input Figure 6 2 The absence of capacitors in thi
239. ur during a cycle they will both be pending when the cycle ends One solution to this problem is by polling Either the CPU can interrogate each device in order of priority or there is a register that is attached to the data bus and wired as an input port When a device interrupts it sets a bit in this register The priority of each device can be established by the order of bits in the register On being interrupted the CPU reads this register finds out which devices are interrupting and attends to the interrupt of higher priority A second approach to the problem of multiple interrupts is daisy chaining An example of this is seen with the Z80 PIO and SIO Figs 5 4 and 5 10 These have an interrupt enable input IEI and an interrupt enable output IEO The device can interrupt only if its IEI input is at logic high In the figures we show IEI connected to the 5 V line so the chip is enabled for interrupts The EIO terminal of the 191 Structured programs Activity 11 3 Interrupts Find out the interrupt facilties of your CPU Devise a simple program to demonstrate its action For example run a program to flash an LED continually When interrupted the CPU is to sound a siren once then return to flashing If there is more than one level of interrupts devise a test program to demonstrate this too Problems on structured programs 1 What are the advantages of structured programming 2 Describe two ways of representing a progra
240. utput Q The latches are all controlled by the ENABLE LATCH and ENABLE OUTPUT inputs Microelectronics Systems and Devices 74 Latches and flip flops Both of these are data storage devices but they act in slightly different ways Latches There may be several often 8 latches on the same chip and these are all controlled by the STORE or LATCH ENABLE input While the STORE input is high data at the output of each latch follows the data at the in put When the STORE input is made low data present at the output at that instant remains unchanged It remains unchanged until STORE goes high again when it then fol lows input again Some types of latch have three state outputs Some latch when the STORE input goes high instead of low Flip flops The type used in ports is the D type and there may be several often 8 on the same chip They are con trolled by the CLOCK input Data at the output of the flip flop remains unchanged until the CLOCK input goes from low to high The data present at the input at that instant is then transferred to the output There is no stage at which output follows input as in the latch Summing up Latches follow or latch D flip flops change only at the clock edge We may also build a port using an octal bus transceiver As the name implies they provide two way buffered communication between two busses One may be the internal data bus of a computer the other may be the bus of a microelec
241. uts are normally not re quired in this application so the ENABLE pins are connected to the positive supply through a resistor The peripheral device can read from the data bus at any time as data input buffers The inputs are connected to a data input socket and the outputs to the data bus The outputs are enabled by a line from an address decoder only when the buffers are to place data on the bus as address bus buffers If there are many address decoders on the address bus the CPU address outputs may not be able to drive them all Instead the CPU address outputs go to a buffer and from there the bus continues to the decoders Another type of port IC is an octal latch An example is the 74LS373 illustrated in Fig 5 3 This allows data to be stored latched at any time by making the ENABLE LATCH input low In an input port data from the peripheral can be latched at the instant the peripheral is ready to deliver it The data is placed on the bus whenever the CPU is ready to receive it by making the ENABLE OUTPUT input low Similarly in an output port data from the CPU can be transferred to a peripheral when the peripheral is ready A third type of port uses an octal D type flip flop such as the 74LS374 The pinout of the IC is the same as that of the 74LS373 Fig 5 3 except that the EL input is replaced by a clock input Figure 5 3 There are eight identical latches in the 74373 each with a data input D and a 3 state o
242. ve is opened or closed by a shutter that is moved by a piston enclosed in a cylinder 4 The piston is moved by admitting compressed air into the cylinder on one side of the piston or the other side The flow of air is controlled by two solenoid operated air valves 5 and 6 which are switched on or off by the microcontroller Proximity sensors detect when the valve is fully open 7 or fully closed 8 and supply this information to the microcontroller By courtesy of Kronospan Ltd Chirk In the cordless telephone previously described currents are small and can usually be fed directly to the inputs of the CPU Similarly the outputs of the CPU can provide sufficient current at the correct voltage to drive logic circuits including those driving display circuits and tone generators This is rarely the case in industrial plant Motors often operate on a 24 V DC supply or even run on alternating current at mains voltage Similarly signals from sensors may be at voltages higher than those acceptable by the CPU and may sometimes be AC signals Industrial sites are well known for generating strong electromagnetic interference so the input signals from sensors may carry high voltage spikes EMI may also be picked up by the output circuits and could get back to the processor For this reason interface circuits see Fig 5 1 are needed both on the input and output sides to provide a low voltage low current electrically quiet environment i
243. we always had to write out data as 8 bit binary numbers as above it would be all too easy to make mistakes Instead we express the numbers in hexadecimal So the two instructions described above become 4F for copying data from A to C and 37 for setting the carry bit 110 Microelectronics Systems and devices Some operations require two bytes The first byte states what is to be done On loading this byte the CPU loads the next byte in memory to find out what value to operate on Example The code E6 actually 11100110 tells the control unit to AND register A with the binary value that is in the next byte The control unit has to send the CPU back to memory to find out the value of this byte which is stored in the next location to the instruction E6 Therefore the full instruction might be E6 4A This tells the control unit to AND the content of register A with the value 4A A sequence of operations may often end in the storing of a result in memory There are opcodes for storing the contents of various registers The operand is the address at which the data is to be stored In a 16 bit system it takes two bytes to specify the address This means that the opcode is followed by two bytes making the instruction three bytes long The sequence of storing data is almost the reverse of the sequence for reading data It has nine stages The address to which the byte is to be written is transferred along the internal bus
244. we use dirl To output a value we use outl and to input a value we use inl The lower nybble of the upper byte P8 to P11 is labelled c To access and use these pins as a block we use dirc outc and inc but not in this program The pins of the control nybble are P11 MSB P10 P9 P8 LSB DS WR AS CE 1 1 0 1 The bottom row shows the value to allocate to the nybble when AS is low and the other pins are high When programming in decimal this is expressed as outc 13 Figure 12 4 During a read cycle the WRITE line remains high when the RTC puts data on the bus 200 Microelectronics Systems and Devices UIP DV2 DV1 DV0 RS3 RS2 RS1 RS0 0 0 1 0 0 0 0 0 dirl 255 as outputs to bus outl loc address on bus outc 12 AS low outc 4 DS low dirl 0 as inputs to read data value inl reading data from bus outc 12 WR high outc 1 disable chip debug dec value displays value in register This is similar to the write program but uses inl to load the data from the bus The value read from the register is displayed on the computer screen Configuring the clock Before the IC can be used as a clock the values in registers A and B must be set appropriately In Register A UIP is a flag update in progress which is read only DV2 to DV1 set the length of the divider chain A table in the data sheet shows that these bits should be set to 010 for a 32
245. which stores a program D a computer on a chip 3 When a PLC is running its program it is directly connected to A sensors B actuators C a PC D interfaces Problems on systems in action Multiple choice questions 17 Systems in action 4 A microcontroller A has its CPU and memory on the same chip B has only a CPU on its chip C controls a PC D is designed to process data at high speed Microelectronics Systems and Devices 18 The CPU The controlling centre of a microelectronic system is its central processing unit or CPU A typical microcontroller the Atmel AT90S1200 and a typical microprocessor the Zilog Z80 are examined in detail and compared with other popular units of the same type The architecture of microcontroller systems is compared with that of microprocessor systems The bus of microprocessor systems is seen to comprise a data bus an address bus and a control bus The functions of each are described In connection with the data bus the purpose of three state outputs is described Discussion of the address bus centres on address decoding The functions of the signals on the control bus of the Z80 are explained The system clock co ordinates the activities of the CPU and the other units in the system As we have seen in Chapter 1 a microelectronic system has a number or fairly standard parts the CPU the system clock conductors for carrying signals between the CPU and the other
246. wing to between 0 V and 5 V This is fed to the 74LS06 gate which inverts it so restoring the original logic levels Now 5V again corresponds to logic 1 and 0 V to logic 0 Once again the gate needs a pull up resistor Its TTL signal may now be fed to any TTL input including the input port of the receiving system A number of ICs are available for transmitting and receiving RS 232 signals An example is the MAX232 IC which provides for two RS 232 lines in each direction In includes a voltage doubling circuit to generate 10 V from the 5 V supply This means that only the usual 5V supply is needed to produce the RS 232 levels in the chip It also includes an inverter circuit to generate 10 V The IC has two TTL to RS232 inverter gates and two RS232 to TTL inverter gates When data is being sent without using handshaking lines it is impor tant to have a protocol for data transmission RS 232 does not include any such protocol but there are a number of data transmission proto cols agreed by international bodies These are used not only for RS 232 transmissions but for other interfacing techniques as well In one of the more common procedures a transmitter that is waiting to transmit is in the marking state Its output signal is at a continuous low level Fig 5 7 As soon as it has data to transmit it places a high pulse on the line This is called the Start pulse and warns the receiver that a message is about to arrive Immedia
247. working in parallel To process a 16 bit value for example the data bits may be divided into four slices each four bits wide The slices are processed simultaneously and the results combined into a 16 bit word The advantage of this technique is that the 4 bit processors are faster than larger processors and working together at the same time produce the result much more quickly to 40 MHz Some of the newer series operate on 3 3 V making them suitable for battery powered equipment TTL is widely used in microelectronic systems CMOS Based on MOSFETs and therefore have very low power requirements typically 0 6 mW per gate Operating voltage is in the range 3 V to 15 V absolute maximum 18 V which makes it ideal for battery powered equipment With FET inputs the gates require so little input current that the output from a gate can be fanned out to 50 or more gates The most popular series is the 4000B series Its main limitation is speed its propagation time is in the region of 125 ns and the maximum clock rate is 5 MHz This may not matter in many Figure 2 12 Voltage levels for TTL and CMOS are specified as shown in these diagrams For example with a 5 V supply a CMOS gate interprets an input below 1 5 V as a logic low input An input above 3 5 V is taken to be a logic high input A low output is always between 0V and 0 05 V and a high output is between 4 95 V and 5 V In either case there is a 1 45 V margin for error A
248. y creates a daisy chain in which only the device of highest priority can interrupt when more than one is attempting to interrupt Microelectronics Systems and Devices 192 8 What is modular programming Give an example of a program that would best be written in modular form 9 What is a function Give examples in a high level language with which you are familiar 10 Describe the sequence of events when a program calls a subroutine and executes it What are the advantages of using subroutines in the structure of a program 11 Describe the operation of the stack In what ways is the stack used 12 Under what circumstance may an interrupt be generated 13 Explain the sequence of events when an interrupt is generated basing your description on a named CPU 14 Explain how different interrupting devices may be allocated different levels of priority 193 Programming projects A detailed study shows how a typical support device may be programmed to demonstrate its facilities The remainder of the chapter comprises a collection of programming problems based on the techniques described in earlier chapters but also introducing some new programming methods When designing and building a microelectronic system it may sometimes be necessary to incorporate one or more complex ICs into the system The system may need a parallel input port or a serial output port or it may need an analogue to digital converter Each of these devices ha
249. y large part of it has been completely typed in using the Editor you click on the Compile button The compiler will attempt to compile your program but will report any errors you have made that prevent it from compiling If there are errors you return to the Editor to correct them When all 127 High level languages errors have been eliminated the program compiles the complete listing into machine code and produces a new file with the C extension This is an executable file that can then be run perhaps on the computer or after downloading into the ROM of a microcontroller It can also be saved on to disk There is a lot more to writing programs in C than this example illustrates but it is enough to help you compare C with the other languages described here There is more on C in Chapter 11 The acronym BASIC stands for Beginners All purpose Symbolic Instruction Code It is probably the easiest language to learn so it is ideal for beginners but this has led many people to reject its use for serious programming However the fact remains that one can do almost anything with BASIC Moreover several of the recent prototyping systems use BASIC as their programming language A notable one is the BASIC Stamp Its two versions Stamp 1 and Stamp II program the PIC16C56 and the PIC16C57 microcontrollers respectively using a special Stamp version of BASIC We shall return to Stamp BASIC later but first we will look at the ori
250. y of four handshaking lines There are also two lines for sending and receiving the serial data Some systems need to be able to accept analogue input The input usually a voltage varies smoothly over a given range The output must be a digital quantity For this purpose the input interface includes an IC known as an analogue to digital converter or ADC Examples of systems needing an ADC for input are audio systems such as a digital tape recorder and many instrumentation systems such as a digital multimeter There are several types of ADC but the two most popular types are flash converters and successive approximation converters Flash converters Flash converters are faster than the other types but have the disadvan tage of being more expensive than other converters with comparable precision Fig 5 11 demonstrates why A flash converter consists of a number of comparators connected by their inverting inputs to a chain of resistors The chain of resistors is connected at one end to 0 V and at the other end to a reference voltage This produces a fixed range of voltages along the chain The input voltage is fed to the non inverting inputs of all the comparators Each comparator compares this voltage with the voltage it is receiving from the chain The outputs of the comparators then swing either low 0 or high 1 depending on whether the input voltage is less than or greater than the voltage from the chain The result of t
251. y software writers to produce programs intended for their operations Fig 1 6 Figure 1 6 Using a mouse to control a power station The state of the art control room at Ironbridge Power Station Shropshire uses computer monitors to display data readings taken at dozens of points in the steam producing plant in the turbines and in the electricity generators The operator controls the power station by calling up a virtual control panel on the monitor On this the usual control switches variable resistors indicator lamps and meters are displayed in diagrammatic form There is no keyboard on the computer Instead the operator uses a mouse to operate the controls clicking on buttons or dragging sliders in just the same way as when playing a computer game The station also has a basic conventional control panel with real switches for back up in case of computer failure By courtesy of Eastern Generation Ltd Measuring instruments Except in the cheapest models the circuit of a digital multimeter Fig 1 7 centres on a microcontroller This makes it possible for the multimeter to perform a range of functions quite beyond the scope of the conventional analogue multimeter based on a moving coil 12 Microelectronics Systems and Devices microammeter The user of the digital meter simply selects what quantity is to be measured applies the two probes to two test points in the circuit and the reading automatically appears on
252. ys use wires as short as possible on the breadboard Also your systems are small rarely consisting of more than three or so ICs One of the more serious problems with using a breadboard for microelectronics systems is the large number of connecting wires that is sometimes required The situation is worse if the circuit includes busses Inserting the wires systematically is easy but problems arise if the circuit fails to work correctly It then becomes difficult to follow the connections among a cluster of wires Worse if a few of the wires accidentally come out of their sockets it is sometimes difficult to know where to replace them without carefully checking through the whole Breadboards Microelectronics Systems and Devices 106 system If this seems likely to happen in your project it is worth considering building at least part of the circuit on a rectangle of stripboard Mount all ICs in sockets so that they may easily be removed for testing the connections 1 What is meant by noise in electronic systems How can it be reduced 2 What is noise immunity Explain why CMOS has good noise immunity 3 Discuss the points that have to be considered when deciding on what logic IC family to use in a microelectronics project 4 Explain with examples the meaning of the term fanout 5 What is crosstalk and how may it be avoided 6 What is meant by contact bounce Describe two circuits which reduce or eliminate it 7 Describ

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