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1. fields in a bitfield can be addressed exactly like regular structures For instance the following statements are all valid Siieucie okere Lo rise field flagA 1 Lelek elac 0 field nybbA Ox0A field byteA 2555 The individual fields in a bit field do not take storage types because you are manually defining how many bits each field takes J wish that s how Richie had done it However I m pretty sure that Each bit field requires a storage type such as unsigned See Declaring and Using Bit Fields in Structures Uy Allowable bit field types 1 However the fields in a bitfield may be qualified with the keywords signed or unsigned although signed is implied if neither is specified If a 1 bit field is marked as signed it has values of 1 and 0 Allow me to quote from Wiki BitField A signed I bit bit field that can contain 1 is a bug in the compiler It is important to note that different compilers may order the fields differently in a bitfield so the programmer should never attempt to access the bitfield as an integer object Without trial and error testing on your individual compiler it is impossible to know what order the fields in your bitfield will be in Also bitfields are aligned like any other data object on a given machine to a certain boundary Embedded Systems C Programming 28 const A const in a variable declaration is a promise by the programmer who wrote it that the program will not alte
2. 5 6 e Volatile as a promise article by Dan Saks e Nullstone Volatile 7 Empirical data suggests that incorrect optimization of volatile objects is one of the most common defects in C optimizers Some of many incorrect understandings of combining const and volatile 8 9 e Use of volatile by Ashok K Pathak The Wikipedia Small Device C Compiler supports several popular microcontrollers SDCC is the only open source C compiler for Intel 8051 compatible microcontrollers Free C C Compilers amp Cross Compilers for MicroControllers 1 References 1 http publib boulder ibm com infocenter macxhelp v6v81 index jsp topic com ibm vacpp6m doc language ref clrc03defbitf htm 2 http gcc gnu org onlinedocs gcc 4 2 2 gcc Structures unions enumerations and bit_002dfields implementation html Structures unions enumerations and bit_002dfields implementation 3 Data in Program Space A Note On const http www nongnu org avr libc user manual pgmspace html pgmspace_const 4 BoostC C Compiler for PICmicro Reference Manual http sourceboost com Products BoostC Docs boostc pdf 5 http www arm com support faqdev 1485 html 6 http www embedded com columns programmingpointers 175801310 7 http nullstone com htmls category volatile htm 8 http www openasthra com c tidbits static volatile const int x 1 valid 9 http en allexperts com q C 1587 constant volatile htm 10 htt
3. C compilers assume that they can reorder instructions The compiler may decide to change the order in which variables are assigned to make better use of registers This may fail miserably with IO peripherals where you for example write to one location to acquire a sample then read that sample from a different location Reordering these instructions would mean the old stale fundefined sample is read then the peripheral is told to acquire a new sample which is ignored Depending on your hardware and compiler capabilities other optimizations SIMD loop unrolling parallelizing pipelining may also be affected const volatile Many people don t understand the combination of const and volatile As we discussed earlier in Embedded Systems Memory embedded systems have many kinds of memory Many input peripherals such as free running timers and keypad interfaces must be declared const volatile because they both a change value outside by means outside this C program and also b this C program should not write values to them it makes no sense to write a value to a 10 key keypad compiled and interactive The vast majority of the time when people write code in C they run that code through C compiler on some personal computer to get a native executable People working with embedded systems then download that native executable to the embedded system and run it However a few people working with embedded system
4. Calling C C function from ASM code 6 OS development C to ASM linkage in GCC M References 1 http www nongnu org avr libc user manual FAQ html faq_reg_usage 2 http techref massmind org techref microchip pages htm 3 http www embedded com story OEG20020226S0045 4 http www cypress com psoc2 id 1353 amp rtID 76 amp rID 2610 5 http www nongnu org avr libc user manual inline_asm html 6 http www expertcore org viewtopic php f 12 amp t 478 7 http wiki osdev org C 2B 2B_to_ASM_linkage_in_GCC Embedded Systems IO Programming An embedded system is useless if it cannot communicate with the outside world To this effect embedded systems need to employ I O mechanisms to both receive outside data and transmit commands back to the outside world Few Computer Science courses will even mention I O programming although it is a central feature of embedded systems programming This chapter then will serve as a crash course on I O programming both for those with a background in C and also for those without it Dos h The Dos h header file commonly included in many C distributions especially on DOS and Windows systems This file contains information on a number of different routines but most importantly it contains prototypes for the inp and outp functions that can be used to provide port output directly from a C program Many embedded systems however will not have a Dos h header file in their librar
5. Embedded Systems RTOS Implementation 52 the MP3 player must decode a new sample at 44 1 KHz no faster or it sounds chipmunk like no slower or it sounds like it s underwater Some embedded systems have only one task Other embedded systems have a single microcontroller connected to many different pieces of hardware they need to multi task What is the Scheduler The task scheduler or often scheduler is the part of the software that schedules which task to run next The scheduler is the part of the software that chooses which task to run next The scheduler is arguably the most difficult component of an RTOS to implement Schedulers maintain a table of the current state of each task on the system as well as the current priority of each task The scheduler needs to manage the timer too In general there are 3 states that a task can be in 1 Active There can be only 1 active thread on a given processor at a time 2 Ready This task is ready to execute but is not currently executing 3 Blocked This task is currently waiting on a lock or a critical section to become free Some systems even allow for other states 1 Sleeping The task has voluntarily given up control for a certain period of time 2 Low Priority This task only runs when all other tasks are blocked or sleeping There are 2 ways the scheduler is called the current task voluntarily yield s to the scheduler calling the scheduler directly
6. PIC on a FPGA implementations History A long time ago General Instruments produced a chip called the PIC1650 described as a Programmable Intelligent Computer This chip is the mother of all PIC chips funetionaHy elose to the current H6CS54 It was intended as a peripheral for their CP1600 microprocessor Maybe that is why most people think PIC stands for Peripheral Interface Controller Microchip has never used PIC as an abbreviation just as PIC And recently Microchip has started calling its PICs microcontrollers PICmicro MCU s Which PIC to Use How do you find a PIC that is right for you out of nearly 2000 different models of PIC microcontrollers Rule Number 1 only pick a microprocessor you can actually obtain PICs are all similar and therefore you don t need to be too picky about which model to use If there is only 1 kind of PIC available in your school storeroom use it If you order from a company such as Newark 21 or DigiKey B ignore any part that is out of stock only order parts that are in stock This will save you lots of time in creating your project Recommended first PIC At one time the PIC16F84 was far and away the best PIC for hobbyists But Microchip Parallax and Holtek are now manufacturing many chips that are even better Id like a list of the top 4 or so PIC recommendations and why they were recommended so that when better cheaper chips become available it s easy to conf
7. Whiteknight 3 anonymous edits Embedded Systems Interrupts Source http en wikibooks org w index php oldid 1221443 Contributors DavidCary Maksym yehorov Whiteknight 4 anonymous edits Embedded Systems RTOS Implementation Source http en wikibooks org w index php oldid 1606849 Contributors DavidCary Whiteknight 5 anonymous edits Embedded Systems Locks and Critical Sections Source http en wikibooks org w index php oldid 1715779 Contributors DavidCary Marty Boogaart QuiteUnusual Whiteknight 1 anonymous edits Embedded Systems Common RTOS Source http en wikibooks org w index php oldid 1701649 Contributors Az1568 ChrisRing DavidCary Tomtailor Whiteknight 9 anonymous edits Embedded Systems Common RTOS Palm OS Source http en wikibooks org w index php oldid 1461752 Contributors Whiteknight Embedded Systems Common RTOS Windows CE Source http en wikibooks org w index php oldid 890022 Contributors Hagindaz Whiteknight 2 anonymous edits Embedded Systems Common RTOS DOS Source http en wikibooks org w index php oldid 1627681 Contributors DavidCary Robert Horning Whiteknight Embedded Systems Linux Source http en wikibooks org w index php oldid 1538412 Contributors Adrignola DavidCary Embedded Systems Interfacing Basics Source http en wikibooks org w index php oldid 917624 Contributors DavidCary Hagindaz Whiteknight 2 anonymous edits Embedded Systems External ICs Source http en wikibooks org w index php oldid 8
8. and has been replaced by the ATtiny2313 12 13 e Olimex produces many AVR based development and prototyping boards and has a list of links to example projects as well Programming Interfaces There are many means to get Program code onto the AVR In System Programming Functionally ISP is done through SPI with some twiddling on Reset As long as the SPI pins of the AVR aren t connected to anything disruptive the AVR chip could stay soldered onto a board while reprogramming All that s needed is a 6 pin plug and an affordable PC adapter This is the most common way to develop with an AVR Atmel s AVR ISP mkII connects to a PC s USB port and performs in system programming using Atmel s software avrdude 14 AVR Downloder UploaDEr runs on Linux FreeBSD Windows and Mac OS X and supports a variety of in system programming hardware including Atmel AVR ISP mkII Atmel JTAG ICE older Atmel serial port based programmers and various third party and do it yourself programmers High Voltage Programming HV programming is mostly the backup mode on smaller AVRs An 8 pin package doesn t leave many unique signal combinations to kick the AVR into programming mode A 12 volt signal however is something the AVR should never see in a proper circuit Parallel Programming Parallel is a backup mode on larger AVRs It may be the only way to talk to an AVR that has a crazy oscillator fuse set Parallel programming may also be f
9. and needs only to check a certain input once per second It seems a waste to continue looping the program especially when we don t need to do anything most of the time In this situation the program will loop 1000 times before it needs to read the input and the other 999 loops of the program will just be a countdown to the next read In this case it is sorely inefficient to have the processor chugging away at 100 capacity all the time We will now implement an expanded superloop to build in a delay Function Main_Function Valea lji zenro 2 Do_Forever Check_Status pomestkeuiitorsskonmsti E Output_Response Delay_For_Next_Loop Embedded Systems Super Loop Architecture 42 Notice how we added a delay at the end of the super loop If we build this delay to delay for 999ms we don t need to loop 1000 times we can read the input on every loop Also it is important to note that many microcontrollers have power save modes where they will require less electrical power which can be especially good if the system is running off a battery Power Use Calculations Let s say that we have a microcontroller that uses 20mA of current in normal mode but only needs 5mA of power in Low Power Mode Let s also say that we are using the example superloop above which is in Low Power Mode 99 9 of the time 1ms of calculations every second and is only in normal mode 0 1 of the time 99 9 x 5mA 0 1 x
10. avoided altogether Embedded Systems Embedded System Basics 13 Real Time Issues Embedded systems frequently control hardware and must be able to respond to them in real time Failure to do so could cause inaccuracy in measurements or even damage hardware such as motors This is made even more difficult by the lack of resources available Almost all embedded systems need to be able to prioritize some tasks over others and to be able to put off skip low priority tasks such as UI in favor of high priority tasks like hardware control Fixed Point Arithmetic Some embedded microprocessors may have an external unit for performing floating point arithmetic FPU but most low end embedded systems have no FPU Most C compilers will provide software floating point support but this is significantly slower than a hardware FPU As a result many embedded projects enforce a no floating point rule on their programmers This is in strong contrast to PCs where the FPU has been integrated into all the major microprocessors and programmers take fast floating point number calculations for granted Many DSPs also do not have an FPU and require fixed point arithemtic to obtain acceptable performance A common technique used to avoid the need for floating point numbers is to change the magnitude of data stored in your variables so you can utilize fixed point mathematics For example if you are adding inches and only need to be accurate to the hundret
11. http sensorwiki org index php Building_a_USB_sensor_interface http www parallax com Store Microcontrollers B ASICStampModules tabid 134 txtSearch 604 0005 1 List 1 Default aspx http www parallax com Store Microcontrollers BASICStampModules tabid 134 txtSearch 27937 List 1 Default aspx http micah navi cx 2010 04 its alive bit banging full speed usb host for the propeller Understanding USB On The Go http www edn com article CA181883 html by Kosta Koeman 2001 USB On The Go OTG Basics AN1035 http www cypress com rID 12904 http www fourwalledcubicle com LUFA php http www instructables com id How to choose a MicroController http www esacademy com automation faq primer 6 htm http wsn oversigma com wiki index php Microcontrollers http www ladyada net library picvsavr html http www cnczone com forums showthread php p 2662 17 http www psocdeveloper com forums viewtopic php f 3 amp t 3483 Embedded Systems Intel Microprocessors 71 Embedded Systems Intel Microprocessors When talking about Intel microprocessors the first words that come to mind might be Pentium or Celeron or any of the other high performance expensive PC chips that are on the market today However Intel maintains a very impressive list of legacy parts that can be adapted for embedded systems The beauty of using these microprocessors is that they are frequently very cheap and they will all use the standard x86 assembly la
12. 17 PORT P3 acts as a normal IO port but Port P3 has additional functions such as serial transmit and receive pins 2 external interrupt pins 2 external counter inputs read and write pins for memory access PORT P2 pins 21 to 28 PORT P2 can also be used as a general purpose 8 bit port when no external memory is present but if external memory access is required then PORT P2 will act as an address bus in conjunction with PORT PO to access external memory PORT P2 acts as A8 A15 as can be seen from fig 1 1 PORT PO pins 32 to 39 PORT PO can be used as a general purpose 8 bit port when no external memory is present but if external memory access is required then PORT PO acts as a multiplexed address and data bus that can be used to access external memory in conjunction with PORT P2 PO acts as ADO AD7 as can be seen from fig 1 1 Embedded Systems 8051 Microcontroller 79 Oscillator Circuits The 8051 requires the existence of an external oscillator circuit The oscillator circuit usually runs around 12MHz although the 8051 depending on which specific model is capable of running at a maximum of 40MHz Each machine cycle in the 8051 is 12 clock cycles giving an effective cycle rate at IMHz for a 12MHz clock to 3 33MHz for the maximum 40MHz clock Internal Architecture 1 Bold text Data and Program Memory The 8051 Microprocessor can be programmed in PL M 8051 Assembly C and a number of other high level languages Many c
13. 20mA Power yt 5 015mA Average 0 Notice how we can cut down our power consumption by adding in a substantial delay This is especially important because few embedded applications will require 100 of processor resources Most embedded systems are able to just sit and wait in a low power state until needed Embedded Systems Protected Mode and Real Mode x86 Processor Modes Real mode and protected mode are two operating modes of the Intel x86 processor However there are certain other modes as well V86 Virtual 86 mode This is the mode in which DOS applications run on Windows machine This was done mainly to maintain compatibility with older DOS applications SMM System Management Mode This mode was introduced as the name suggests for managing the system transparently without applications or OS getting the hint of it Its primarily meant to be used by the BIOS code Big Real Mode Now this mode is more like Real mode but in this we can access full 4Gb address space of the 32 bit processor However in this page we will be focussing mainly on Real mode and Protected mode only Embedded Systems Protected Mode and Real Mode 43 Real Mode This is the only mode which was supported by the 8086 the very first processor of the x86 series The 8086 had 20 address lines so it was capable of addressing 2 raised to the power 20 i e 1 MB of memory Protected Mode This is the mode used most commonl
14. 21 20 Programming PICmicros in the C programming language Programming PICmicros in other programming languages Forth JAL BASIC Python etc 22 The 8 bit PIC Microcontroller Solutions brochure describes how big the PIC hardware stack is in each PIC microcontroller family and other major differences between families http holtek com http www newark com http digikey com http techref massmind org techref piclist begin htm http amqrp org elmer160 lessons index html http www voti nl swp n_index html http techref massmind org techref supplies htm http www sparkfun com commerce present php p PIC 20Boot 20Loader http massmind org techref microchip pages htm http www ubasics com adam pic icp84 html http www ic prog com http massmind org techref microchip devprogs htm PIC Microcontroller Programmers http massmind org techref microchip devprogs htm Choosing a PIC programmer http www best microcontroller projects com pic programmer html http massmind org techref microchip http forums parallax com forums default aspx f 7 amp m 7304 1 http microchip com stellent idcplg IdcService SS_GET_PAGE amp nodeld 74 http finitesite com d3jsys 16F628 html http finitesite com d3jsys 16F88 html http www best microcontroller projects com http www microchipc com http massmind org techref microchip languages htm http ww 1 microchip com downloads en DeviceDoc 39630B pdf M
15. 500mA to function properly although there are INTUP3 3 113 28 P2 7 K15 l d like the A 1 2051 which i led d TO P3 4 14 27 P P26 A14 ower powered versions like the Atme which is a scaled down T35 15 96 P2513 version of the 8051 which runs on 3V WRIP3 6 416 25 P24M12 RDIP3 7 417 24 I P23A11 PINS 29 30 amp 31 As described in the features of the 8051 this chip XTAL2 118 23 P2 2 A10 ahs XTAL1 19 22 I gt P21 A9 contains a built in flash memory In order to program this we need to vss 20 21 gt P20A8 supply a voltage of 12V at pin 31 If external memory is connected then PIN 31 also called EA VPP should be connected to ground to indicate the presence of external memory PIN 30 is called ALE Pin diagram of the 8051 DIP address latch enable which is used when multiple memory chips are connected to the controller and only one of them needs to be selected We will deal with this in depth in the later chapters PIN 29 is called PSEN This is program select enable In order to use the external memory it is required to provide the low voltage 0 on both PSEN and EA pins Ports There are 4 8 bit ports PO P1 P2 and P3 PORT P1 Pins 1 to 8 The port P1 is a general purpose input output port which can be used for a variety of interfacing tasks The other ports PO P2 and P3 have dual roles or additional functions associated with them based upon the context of their usage PORT P3 Pins 10 to
16. Digital Signal Processor DSP DSPs are the best of the best when it comes to microcontrollers DSPs frequently run very quickly and have immense processing power for an embedded chip Digital Signal Processors and the field of Digital Signal Processing is so large and involved that it warrants its own book Digital Signal Processing Grades of Microcontrollers Microcontrollers can be divided up into different categories depending on several parameters such as bus width 8 bit 16 bit etc amount of memory speed and the number of I O pins Low end Low end chips are frequently used in simple situations where speed and power are not a factor Low end chips are the cheapest of the bunch and can usually cost less than a dollar depending on the quantity in which they are purchased Low end chips rarely have many I O pins 4 or 8 total and rarely have any special capabilities Almost all low end chips are 8 bits or smaller Mid level chips mid level chips are the basic microcontroller units They don t suffer from the drawbacks of the low end chips but at the same time they are more expensive and larger Mid level chips are 8 bits or 16 bits wide and frequently have a number of available I O pins to play with Mid level chips may come with ADC voltage regulators OpAmps etc Mid level chips can cost anywhere between 1 and 10 for reasonable chips High end chips Embedded Systems Terminology High end ch
17. If the autopilot detects that the plane for some reason is going into a stall then it should take steps to correct this within milliseconds or there would be catastrophic results What are Embedded Systems Used For The uses of embedded systems are virtually limitless because every day new products are introduced to the market that utilize embedded computers in novel ways In recent years hardware such as microprocessors microcontrollers and FPGA chips have become much cheaper So when implementing a new form of control it s wiser to just buy the generic chip and write your own custom software for it Producing a custom made chip to handle a particular task or set of tasks costs far more time and money Many embedded computers even come with extensive libraries so that writing your own software becomes a very trivial task indeed From an implementation viewpoint there is a major difference between a computer and an embedded system Embedded systems are often required to provide Real Time response A Real Time system is defined as a system whose correctness depends on the timeliness of its response Examples of such systems are flight control systems of an aircraft sensor systems in nuclear reactors and power plants For these systems delay in response is a fatal error A more relaxed version of Real Time Systems is the one where timely response with small delays is acceptable Example of such a system would be the Scheduling Display Syste
18. N 6 http antiradio narod ru raznoe cypress tv_generating tvgenerating html 7 http www uelectronics info generating tv signal pics 8 http www acm uiuc edu sigarch projects breakout 9 http interwiki sourceforge net cgi bin picawiki pl StartingPoint Embedded Systems Where To Buy 107 Embedded Systems Where To Buy This page will list some places where the reader can buy some of the hardware discussed in this book Some of the embedded systems can be purchased for relatively cheap some PIC microcontrollers will cost 5 or less and therefore the reader can purchase them for cheap and play around with them Some of the embedded computers are relatively expensive but these ones are very useful for home projects because larger expensive boards will be more versatile and will have more gadgets and gizmos to play with JK Microsystems UI This company has a number of good solid embedded computers many of which are i386 compatable Many of these computers come with DOS preloaded but an RTOS called eRTOS is available for free Many of these computers come with ethernet extensions so they can be used to perform simple web based tasks IRC bot or simple web server for instance e Technologic Inc 2 This company offers many moderate to high end microprocessors with a number of available add ons including PCMCIA cards for things like wireless internet cards Many of these systems are pre loaded with
19. RAM 256 bytes of any 8 pin PSoC CY8CNP102 not yet available as of 2008 09 but expected to have 256 KByte non volatile storage and 2 KByte RAM analogous to Embedded Systems PIC Microcontroller Which PIC to Use other FAQs Q It s not working A Have you gone through the Software Checklist Tips on Using PSoC UI by Zlatko Saravanja 2004 Tips on using the PSoC 2 PSoC R Technical Reference Manual TRM Getting Started with PSoC READ THIS FIRST AN2010 419 Q I m having problems trying to use my PSoC MiniProg to program a CY8C26443 A The Miniprogrammer does not support the 25 26 families You will have to use ICE 4000 or the ICE cube Or switch to a chip that the MiniProg does support such as the 27 29 families FIXME make a list of chips and mark each with Y known to work with MiniProg N doesn t work with MiniProg or unknown http www psocdeveloper com forums viewtopic php t 2057 http www psocdeveloper com forums viewtopic php t 2022 Should we make this a table with the various programmers m8cprogs open hardware 51 MiniProg ICE cube etc vs the various chips interrupt handler Q How do I write an interrupt handler in C A See Pl Software Checklist Tips on Using PSoC UJ Also inside the PSoC IDE choose Help Documentation then C Language Compiler User Guide 6 More discussion http www psocdeveloper com forums viewtopic php t 2089 and Tips on usin
20. Some transistors known as Power Transistors can control a high voltage source using a lower voltage control signal There is also a type of electromechanical device known as a relay that can also be used to control a high voltage source with a relatively small control current Both of these tools can be used to control the flow of a high power electrical flow with an embedded computer Occasionally we need to use multiple stages of amplification To turn on a large motor we need a large relay but to turn on the large relay we need a power transistor but to turn on the large transistor we need at least a small transistor finally we turn on the small transistor with the microcontroller output pin Isolation When working with embedded systems or any expensive piece of equipment we often find that it is a good idea to isolate the expensive components from the high power components We do this through a technique called isolation Isolation in essence is how we keep the high current and or high voltages out of low current low voltage devices There are several types of isolators The isolation barrier is an imaginary line between the high current or high voltage device on one side and low current low voltage devices on the other side Transformers are used to transfer power from one side of the isolation barrier to the other e optoisolators are used to transfer signals across an isolation barrier from one low power dev
21. a linux distro called TS Linux ZWorld Pl This webpage offers a number of embedded systems and development kits The flagship model is called the Rabbit Core and is a very fast and flexible microprocessor These core units can be incorporated into a number of different single board computers for maximum flexibility RabbitCore processors are programming in a proprietary language called Dynamic C which is similar to standard C e Rentron This webpage is basically a large catalogue for a number of different components including microcontrollers microprocessors transmitters receivers encoders decoders etc e http www atmel com This company sells a large number of different types of components including ARM chips an 8051 compatible chip and a proprietary 8 bit microprocessor called the Atmel AVR and a 4 bit microprocessor called the MARC4 Also sells a number of DSP modules and controllers Embedded Planet This company offers numerous fully customizable PowerPC XScale ARM and MIPS based embedded computers with multiple bootloader and operating system choices further reading e Open Circuits wiki list of electronics suppliers 6 References 1 http www jkmicro com 2 http www embeddedx86 com 3 http www zworld com 4 http www rentron com 5 http www embeddedplanet com o 6 http opencircuits com Supplier Embedded Systems Resources 108 Embedded Systems Reso
22. bits 1 and 3 to zeros using 68HC12 ASM Setting Bits BSET PORTH SOA Changes bits 1 and 3 to ones using 68HC12 ASM Embedded Systems C Programming 27 Special Features The C language is standardized and there are a certain number of operators available that everybody knows and loves However many microprocessors have capabilities that are either beyond what C can do or are faster than the way C does it For instance the 8051 and PIC microcontrollers both have assembly instructions for setting and checking individual bits in a byte C can affect bits individually using clunky structures known as bit fields but bit field implementations are rarely as fast as the bit at a time operations on some microprocessors Bit Fields Bit fields are a topic that few C programmers have any experience with although it has been a standardized part of the language for some time now Bit fields allow the programmer to access memory in unaligned sections or even in sections smaller than a byte Let us create an example Suite loiernele 4 ple e 17 flagB lg nybbA 4 byteA 8 The colon separates the name of the field from its size in bits not bytes Suddenly it becomes very important to know what numbers can fit inside fields of what length For instance the flagA and flagB fields are both 1 bit so they can only hold boolean values 1 or 0 the nybbA field can hold 4 bits for a maximum value of 15 one hexadecimal digit
23. by the OS from untrusted programs and Most embedded systems do not have a MMU We discuss the two versions of Linux that can run on a system that does not have a MMU in Embedded Systems Linux x86 Memory Layout Reserved Memory Reserved memory is memory which is reserved for some purpose like additional software installation and startup Segmented Memory Old x86 processors were only 16 bit processors and if a flat memory scheme was used those processors would only be able to support 65 Kilobytes of memory The system engineers behind the old 8086 and 80286 processors came up with the idea to segment memory and use a combination of segment pointers and offset pointers to access an effective 20 bit address range for a maximum of 1 megabyte of addressable memory Address Segment register 16 pointer register New 32 bit processors allow for 4 Gigabytes of addressable memory space and therefore the segmented memory model was all but abandoned in current 32 bit machines although the segment registers are still used to implement paging schemes Memory Mapped I O Memory Mapped I O is a mechanism by which the processor performs I O access by using memory access techniques This is often put into effect because the memory bus is frequently much faster then the I O bus Another reason that memory mapped I O might be used is that the architecture in use does not have a separate I O bus In memory mapped IO certain range of
24. call e R13 SP stack pointer for nested subroutine calls e R12 R4 long term variables used by a subroutine only if it restores the original values before it returns e R3 RO scratch pad variables and subroutine call parameters and subroutine return results T O hardware is typically memory mapped Word Length Modern desktop PCs are almost all 32 bit machines and the next generation of processors is going to be fully 64 bit This is all well and good for the average programmer but what do you do when you are in an embedded situation with a microcontroller the size of your finger nail that is capable of only 4 bit arithmetic 32 bits may be the norm in the desktop market but there is no gold standard in embedded chips more bits take up more space and costs more money In essence it is the job of a good embedded systems engineer to find the smallest cheapest microcontroller that does the job that needs to get done Consider the following table Embedded Systems Assembly Language 33 bits biggest unsigned number biggest signed number smallest signed number 4 15 7 8 8 255 127 128 16 65 535 32 767 37 768 2 s compliment format Even the 16 bit processor is a far cry from the 4 billion integer range of a standard 32 bit processor Let s say that we have a 4 bit microcontroller with 4 available internal registers 4 bit each and 256 bytes of onboard programmable memory This
25. compiler for the AVR recognizes the compiler specific keyword __eeprom on a variable declaration Thereafter a person writes code to read and write that variable with the same standard C syntax as normal variables in RAM but the compiler generates code to access the EEPROM instead of regular data memory Atmel s datasheets indicate that the EEPROM can be re written a minimum of 100 000 times An application must implement a wear leveling scheme if it writes to the EEPROM so frequently that it will reach the write limit before it reaches the expected lifetime of the device AVRs ship from the factory with the EEPROM erased i e the value in each byte of EEPROM is FF 16 Many of the AVRs have errata about writing to EEPROM address 0 under certain power conditions usually during brownout and so Atmel recommends that programs not use that address in the EEPROM Fuse Settings A Fuse is an EEPROM bit that controls low level features and pin assignments Fuses are not accessible by the program they can only be changed by a chip programmer Fuses control features which must be set before the chip can come out of reset and begin executing code The most frequently modified fuses include 1 Oscillator crystal characteristics including drive strength and start up time 2 JTAG pins used for JTAG or GPIO 3 RESET pin used as a reset input debugWire or GPIO 4 Brown Out Detect BOD enable and BOD voltage trigger points There is a also a fus
26. does not fit in the microcontroller e If you run out of RAM sometimes you only need 2 bytes or 1 byte or 4 bits or 1 bit to store a particular variable If yourun out of time sometimes you can add lower precision math routines that quickly calculate the results needed for that inner loop even though other parts of the code may need higher precision math routines e If you run out of ROM sometimes you can trade time for ROM space Rather than a collection of sets of math routines each one customized to a slightly different width you can use a single set of math routines that can handle the maximum possible width If you have some variables less than that width to save RAM then you typically sign extend variables into a full size register or global buffer do full width calculations there and then truncate and store the result to the small size Embedded Systems Floating Point Unit 20 FFT Many people do FFT using fixed point arithmetic more tips and hints here Develop FFT apps on low power MCUs 7 by Paul Holden 2005 Comparing Floating Point and Fixed Point Implementations on ADI Blackfin Processors with LabVIEW 8 Fixed Point Fast Fourier Transform FFT 9 program listed for a fixed point FFT 10 EE 18 Choosing and Using FFTs for ADSP 21xx a fixed point DSP Further reading Floating Point Fixed Point Numbers Floating Point Fixed Point Numbers AN660 floating point routines for the Microc
27. down the system and prevents other time sensitive tasks from running Next we will show a method by which critical sections can be implemented to allow for preemption Critical Section Objects Critical Sections like any other term in computing can have a different definition than simply an operation that prevents preemption For instance many systems define a CS as being an object that prevents multiple tasks from entering a given section of code Let us say that we are implementing a version of malloc on our system We want to ensure that once a memory allocation attempt has started that no other memory allocation attempts can begin Only 1 memory allocation attempt can be happening at one time However we want to allow for the malloc function to be preempted just like every other function To implement this we need a new data object called a CRITICAL_SECTION or CRIT_X or something of that nature Our malloc function will now look like this CRITI SECI mallocts a global CS variable mor use in All casks int RTOS_main void we register our CS in the beginning of the RTOS main routine AllocCS mallocCS register our critical section with the OS to prevent duplicates void malloc size_t size VOLG SECA EnterCS mallocCS we enter the CS and no other instance of malloc can enter it ptr FindFreeMemory size ExitCS mallocCS other malloc attempts can now proceed REP Iput If two tasks call mall
28. hpinfotech ro html cvavr htm Embedded Systems Atmel AVR 98 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 http www arduino cc http www harbaum org till nanovm index shtml http mcujavasource sourceforge net http e lab de http www mikroelektronika co yu english product compilers mikropascal_avr index htm http users iafrica com r ra rainier http forthfreak de index cgi ByteForth http www forth hccnet nl pr bytef html http www forth hccnet nl downloads avrb207 pdf http www forth hccnet nl downloads eng avrf zip http amforth sourceforge net http www cinetix de avise index htm http claymore engineer gvsu edu steriana S oftware index html http www softsynth com pforth http krue net avrforth http pymite python hosting com wiki PyMite http 1010 co uk atmegascheme html http www atmel com products AVR http www atmel no beta_ware http support atmel no bin customer http wiki avrfreaks net http massmind org techref http massmind org techref atmel avr index htm http avrwiki com http www avrbeginners net http avr 15 forumer com http extremeelectronics co in avr tutorials http www avrfreaks net http groups yahoo com group AVR Chat https savannah nongnu org projects avr http groups yahoo com group
29. http en wikibooks org w index php title File Wikibooks logo en noslogan svg License logo Contributors User Bastique User Ramac File LED circuit svg Source http en wikibooks org w index php title File LED_circuit svg License Creative Commons Attribution Sharealike 2 5 Contributors Dmccreary 1 anonymous edits Image Pinouts8051 jpg Source http en wikibooks org w index php title File Pinouts8051 jpg License Public Domain Contributors Az1568 Mike lifeguard Nkrypt 1 anonymous edits File Heckert GNU white svg Source http en wikibooks org w index php title File Heckert_GNU_white svg License Free Art License Contributors Aurelio A Heckert lt aurium gmail com gt License 114 License Creative Commons Attribution Share Alike 3 0 Unported http creativecommons org licenses by sa 3 0
30. important first to cover exactly what embedded systems are and how they are used This wikibook will attempt to cover a large number of topics some of which apply only to embedded systems but some of which will apply to nearly all computers embedded or otherwise As such there is a chance that some of the material from this book will overlap with material from other wikibooks that are focused on topics such as low level computing assembly language computer architecture etc But we will first start with the basics and attempt to answer some questions before the book actually begins What is an Embedded Computer The first question that needs to be asked is What exactly is an embedded computer To be fair however it is much easier to answer the question of what an embedded computer is not than to try and describe all the many things that an embedded computer can be An embedded computer is frequently a computer that is implemented for a particular purpose In contrast an average PC computer usually serves a number of purposes checking email surfing the internet listening to music word processing etc However embedded systems usually only have a single task or a very small number of related tasks that they are programmed to perform Embedded Systems Embedded Systems Introduction Every home has several examples of embedded computers Any appliance that has a digital clock for instance has a small embedded microcontroll
31. including DragonBall ColdFire PowerQUICC and others Fujitsu FRV Hitachi H8 MIPS and Xilinx MicroBlaze processors Wikipedia Embeddable Linux Kernel Subset ELKS is a small subset of Linux that like uClinux can run even on machines that lack a MMU It apparently only supports x86 machines including the 8088 based original IBM PC the 80286 based original IBM PC AT the NEC V30H based Psion Series 3 etc Real Time and Embedded Guide rtHOWTO 7 by Herman Bruyninckx 2002 claims that standard Linux in 2002 is not a true real time OS nor an embedded OS The coreboot project formerly known as the LinuxBIOS project is developing firmware that replaces a standard BIOS boots out of motherboard Flash just like standard BIOS and boots into almost any modern 32 bit operating system much faster than a standard BIOS by cutting out most of the device detection and hardware initialization a standard BIOS does since the OS needs to do that all over again anyway e Wikipedia coreboot LinuxBIOS wiki e Stallman calls for action on Free BIOS P Reducing OS Boot Times for In Car Computer Applications 10 by Damien Stolarz 2004 Comparing real time Linux alternatives 11 by Kevin Dankwardt 12 sells a DO 178B certifiable RTOS and also BlueCat embedded Linux 13 Lynux Works hard real time Linux technology modifications to the Linux kernel in order to provide a real time operating system 14 Em
32. jour be it RS 232 PC Oy or wireless USB Further Reading e Wikipedia RedBoot e x86 Assembly Bootloaders AVR bootloader e PIC bootloaders 3 e PIC16f877 Monitor on Linux USB PIC18 microcontroller bootloader 5 download new firmware over USB e USB bootloader for Cypress PSoC microcontrollers 6 Embedded Systems Bootloaders and Bootsectors 44 References 1 2 3 4 5 6 http en wikipedia org wiki I2C http www cmeter org CVmegaload index html http massmind org techref microchip devprogs htm bootloaders http www wsu edu jackdoll jmon index htm http www diolan com pic bootloader html http www psocdeveloper com forums viewtopic php t 1021 Embedded Systems Terminate and Stay Resident In the original DOS operating system there was no capability for multi threading or multi process mechanisms However it was found to be very beneficial to leave certain components in memory even after the process that created it ended These program fragments were known as Terminate and Stay Resident modules and were the precursors to the dynamic library infrastructure of current Windows operating systems TSR routines are often used to implement device drivers or common library functions 45 Real Time Operating Systems Embedded Systems Real Time Operating Systems A Real Time Operating System RTOS is a computing environment that reacts to input within a specific tim
33. means that every embedded system needs to be tested and analyzed extensively An embedded system will have very few resources when compared to full blown computing systems like a desktop computer the memory capacity and processing power in an embedded system is limited It is more challenging to develop an embedded system when compared to developing an application for a desktop system as we are developing a program for a very constricted environment Some embedded systems run a scaled down version of operating system called an RTOS real time operating system Why Study Embedded Systems Embedded systems are playing important roles in our lives every day even though they might not necessarily be visible Some of the embedded systems we use every day control the menu system on television the timer in a microwave oven a cellphone an MP3 player or any other device with some amount of intelligence built in In fact recent poll data shows that embedded computer systems currently outnumber humans in the USA Embedded systems is a rapidly growing industry where growth opportunities are numerous Who is This Book For This book is designed to accompany a course of study in computer engineering However this book will also be useful to any reader who is interested in computers because this book can form the starting point for a bottom up learning initiative on computers It is fundamentally easier to study small limited simple computers th
34. microcontroller wiki e Whirlwind Tour of ARM Assembly e GCC ARM Improvement Project BI at the University of Szeged The ARM Linux Project 4 Linux for all ARM based machines ARM ARM developers discussion forums 6 References 1 http www open research org uk ARMuC 2 3 http www inf u szeged hu gcc arm 4 5 http arm com 6 2 http www coranac com tonc text asm htm http www arm linux org uk docs whatis php fa http sevensandnines com Embedded Systems AT91SAM7S64 100 Embedded Systems AT91SAM7S64 The AT91SAM7S64 is a noteworthy instance of the ARM processor architecture because of the availability of affordable prototyping hardware 1 i 2 2 3 BH and of on line tutorial information 1 4 2 Bh There is a bootloader for it as well Image of Olimex board at http www olimex com dev images ARM ATMEL SAM7 Hxxx 1 jpg There are a number of interesting projects documented for this controller 1 U 2 My hope for this page is to work up a getting started guide for the cheapest SAM7 board I can find which is thusfar the Olimex board H References 1 http www olimex com dev sam7 h64 html 2 http www sparkfun com commerce product_info php products_id 614 3 http www atmel com dyn products tools_card asp tool_id 3784 4 http www doctort org adam nerd notes getting started with the olimex sam7 p256 html 5 http www atmel com dyn resources prod_documents d
35. never wear out no matter how many times they are written External Data Memory Some of the higher pin count AVR microcontrollers allow for external expansion of the data space addressable up to 64KB When enabled external SRAM is overlaid by internal SRAM an access to address 0000 16 in the data space will always resolve to on chip memory Depending on the amount of on chip SRAM present in the particular AVR anywhere from 512 bytes to several KB of external RAM will not be accessible This usually does not cause a problem The support circuitry required is described in the datasheet for any device that supports external data memory such as the Mega 162 i in the External Memory Interface section The support circuitry is minimal consisting of a 573 or similar latch and potentially some chip select logic The SRAM chip select may be tied to a logic level that permanently enables the chip or it may be driven by a pin from the AVR For an SRAM of 32KB or less one option is to use a higher order address line to drive the chip select line to the SRAM EEPROM Storage Almost all AVR microcontrollers have internal EEPROM memory for non volatile data storage Only the Tiny11 and Tiny28 have no EEPROM EEPROM memory is not directly mapped in either the program or data space but is instead accessed indirectly as a peripheral using I O registers Many compilers available for the AVR hide some or all of the details of accessing EEPROM IAR s C
36. obstacle to mixed language programming is the issue of function calling conventions C functions are all implemented according to a particular convention that is selected by the programmer if you have never selected a particular calling convention it s because your compiler has a default setting This page will go through some of the common calling conventions that the programmer might run into and will describe how to implement these in assembly language Code compiled with one compiler won t work right when linked to code compiled with a different calling convention If the code is in C or another high level language or assembly language embedded in line to a C function it s a minor hassle the programmer needs to pick which compiler optimization switches she wants to use today and recompile every part of the program that way Converting assembly language code to use a different calling convention takes more manual effort and is more bug prone Unfortunately calling conventions are often different from one compiler to the next even on the same CPU Occasionally the calling convention changes from one version of a compiler to the next or even from the same compiler when given different optimization switches Unfortunately many times the calling convention used by a particular version of a particular compiler is inadequately documented So assembly language programmers are forced to use reverse engineering techniques to figure ou
37. operations These topics will all be discussed in the following chapters Embedded Systems Threading and Synchronization 48 Mutexes The term Mutex is short for Mutual Exclusion and is a type of mechanism used in a preemptive environment that can prevent unauthorized access to resources that are currently in use Mutexes follow several rules 1 Mutexes are system wide objects that are maintained by the kernel 2 Mutexes can only be owned by one process at a time 3 Mutexes can be aquired by asking the kernel to allocate that mutex to the current task 4 If a Mutex is already allocated the request function will block until the mutex is available In general it is considered good programming practice to release mutexes as quickly as possible Some problems with mutexes will be discussed in the chapter on deadlocks Spin Locks Spin locks is a quick form of synchronization methods It is named after its behavior spin in the loop while the condition is false To implement spin lock system should support test and set Ul idiom or give exclusive access to a locking thread by any means masking interrupts locking bus An advantage of spin locks is that they are very simple A disadvantage is that they waste CPU cycles in loop waiting Most common use of spin locks is to syncronize quick access to objects It is not advisable to do a long computations while spin locked a section Critical Sections A critical se
38. other signal conditioning must be interposed so that the signal from the button does not violate the setup and hold times required on the interrupt pins General Purpose I O Ports General Purpose I O or GPIO pins are the digital I O for the AVR family These pins are true push pull outputs The AVR can drive a high or low level or configure the pin as an input with or without a pull up GPIOs are grouped into ports of up to 8 pins though some AVRs do not have enough pins to provide all 8 pins in a particular port e g the Mega48 88 168 does not have a PortC7 pin Control registers are provided for setting the data direction output value or pull up enabled and for reading the value on the pin itself An individual pin can be accessed using bitwise manipulation instructions Each port has 3 control registers associated with it DDRx PORTx and PINx Each bit in those registers controls one GPIO pin i e bit 0 in DDRA controls the data direction for PortAO often abbreviated PAO and bit O in PORTA will control the data or pullup for PAO Embedded Systems Atmel AVR 85 The DDR Data Direction Register controls whether the pin is an input or an output When the pin is configured as an output the corresponding bit in the PORT register will control the drive level to the pin high or low When the pin is configured as an input the bit in the PORT register controls whether a pull up is enabled or disabled on that pin The PIN P
39. performance with low power consumption such as PDA s Other Chips Intel does sell embedded varieties of all its chips from the 486 up to the Pentium 4 Keep in mind however that these chips have all the power of their PC cousins but in a smaller package Therefore it can be expected that they will all be considerably more expensive then the desktop chips Also with some of the higher performance chips pentium and up since the size has been aggressively reduced and because they have been highly integrated for embedded environments heat can become an issue meaning you will need to invest in fans and heat sinks as well Further reading ARM e x86 Assembly Embedded Systems PIC Microcontroller 72 Embedded Systems PIC Microcontroller Manufactured by Microchip the PIC Programmable Intelligent Computer or Peripheral Interface Controller microcontroller is popular among engineers and hobbyists alike PIC microcontrollers come in a variety of flavors each with different components and capabilities Many types of electronic projects can be constructed easily with the PIC family of microprocessors among them clocks very simple video games robots servo controllers and many more The PIC is a very general purpose microcontroller that can come with many different options for very reasonable prices 1 Other microprocessors in this family include the Parallax SX the Holtek HT48FxxE Series I and some
40. speed 12 Mbps USB 2 0 with USB Host slave or On The Go OTG capability other details on USB For more details on USB see the Serial Programming USB Technical Manual Further reading e Instructables How to choose a MicroController 115 by westfw e Once you ve picked out a processor you ll want to know Embedded Systems Where To Buy it e Robotics Single Board Computers discusses processor modules that include the CPU and a few support chips in a convenient package e Getting started with microcontrollers 16 part of the Microcontroller Primer FAQ by Russ Hersch 17 e microcontrollers for wireless sensor network devices USI Embedded Systems Particular Microprocessors 70 PIC vs AVR HSI OK I know what you people want You want ultimate fighting embedded E E style You want to know WHICH IS BETTER PIC OR AVR CNCzone Microchip vs Atmel 19 PSoC Developer PSoC VS PIC AVR ATMEL 8051 120 has a brief comparison review of a few Freescale Microchip and Cypress CPUs References 16 17 18 19 20 http www circuitcellar com archives viewable Bachiochi204 index html http calewatch com http www psocdeveloper com forums viewtopic php t 2953 http atmel com dyn corporate view_detail asp FileName AT32AP7001_6_4 html http opencircuits com RS232_RS485_USB_Converter_Board http www dontronics shop com product php productid 16141 http www parallax com detail asp product_id 28024
41. wikibooks org w index php oldid 1758203 Contributors Adrignola DavidCary Mikiemike Rajdivecha Recent Runes Whiteknight 17 anonymous edits Embedded Systems IO Programming Source http en wikibooks org w index php oldid 1555639 Contributors DavidCary QuiteUnusual Whiteknight 2 anonymous edits Embedded Systems Serial and Parallel IO Source http en wikibooks org w index php oldid 1407949 Contributors Darklama DavidCary Kwhitefoot Mikiemike Robert Horning Whiteknight 9 anonymous edits Embedded Systems Super Loop Architecture Source http en wikibooks org w index php oldid 894156 Contributors Darklama Fpeelo Whiteknight 5 anonymous edits Embedded Systems Protected Mode and Real Mode Source http en wikibooks org w index php oldid 890012 Contributors RohitUpadhyay Whiteknight Embedded Systems Bootloaders and Bootsectors Source http en wikibooks org w index php oldid 1292577 Contributors DavidCary Jamin1001 Whiteknight 2 anonymous edits Embedded Systems Terminate and Stay Resident Source http en wikibooks org w index php oldid 890014 Contributors Tannersf Whiteknight Embedded Systems Real Time Operating Systems Source http en wikibooks org w index php oldid 1731460 Contributors DavidCary Fxyoung IRelayer Jomegat Whiteknight Zephram Stark 24 anonymous edits Embedded Systems Threading and Synchronization Source http en wikibooks org w index php oldid 1087168 Contributors DavidCary Derbeth Maksym yehorov
42. with all the goodies and a subset of variant members that lack one thing or another For prototyping we generally use the full version to make sure we can get the prototype working at all During prototyping we want to tweak code reprogram and test over and over until it works Se we ase one of the above Flash families not the OTP families Each member of each family generally comes in several different packages Hobbyists generally use the plastic dual inline package often called DIP or PDIP because it s the easiest to stick in a solderless breadboard and tinker with The wide DIP works just as well They avoid using ceramic dual inline package CDIP not because ceramic is bad it s just as easy to plug into a solderless breadboard but because the plastic parts work just as well and are much cheaper Later for mass production we may figure out which is the cheapest cut down version that just barely has enough goodies to work and comes in the cheapest package that has just barely enough pins for this particular application perhaps even a OTP chip And then each different package for each member of each family comes in both a commercial temperature range and a industrial temperature range Embedded Systems PIC Microcontroller 74 PIC 16x The PIC 16 family is considered to be a good general purpose family of PICs PIC 16s generally have 3 output ports to work with Here are some models in this family
43. 2693 10 http www psocdeveloper com forums viewtopic php t 186 amp postdays 0 amp postorder asc amp start 60 11 http casainho net tiki index php page Homemade midi turntable 106 Appendices Embedded Systems Common Protocols This is a list of common protocols used in embedded systems Eventually this list will become hyperlinks to sources of information on each rc CAN BlueTooth e InfraRed e ZigBee e SPI e RS 232 USB MINES Microcontroller Interpreter for Networked Embedded Systems was designed for very small embedded systems see Gallery of MINES Devices 12 the Tiny Embedded Network B IEEE Standard for Sensor Transducer Interface e the three byte Mini SSC protocol 5 JTAG NTSC PAL television video output w TV Typewriter Generating TV signal by PSoC 6 Generating TV signal 4 with the PICs VI PIC Breakout 8 Parallax Propeller has a video generator If you are designing a new protocol because none of these meet your needs which are what exactly you may want to consider the w Network protocol design principles and post rough drafts to the PICA standards wiki PI for expert review References http www artisllc com solutions embedded mines http www artisllc com solutions embedded mines mines_gallery html http members tripod com mdileo http standards ieee org announcements electindustrange html http seetron com ssc htm m r n A U
44. 5 536 locations however systems using these processors often have much more physical RAM and ROM than that They use paging hardware that swaps in and out banks of memory into the directly accessible space Early Microchip PIC processors had 2 completely separate set of banking registers one for swapping in different banks of program ROM the other for swapping in different banks of RAM Embedded Systems Memory 23 memory management All too often programs written for embedded systems grow and grow until they exceed the available program space There are a variety of techniques for dealing with the out of memory problem e re compile with the Os optimize for size option find and comment out dead code e refactor repeated sections into a common subroutine e trade RAM space for program space e put a small interpreter in internal program memory that loads and interprets instructions use instructions perhaps p code or threaded code that are more compact than directly coding it in assembly language Or place these instructions can be placed in EEPROM or external serial Flash that couldn t otherwise be used as program memory Or e Both This technique is often used in stamp style CPU modules e add more memory perhaps using a paging or banking scheme Most CPUs used in desktop machines have a memory management unit MMU The MMU handles virtual memory protects regions of memory used
45. 5V Our LED is green which implies a forward voltage drop of about 2 V However we also need to consider that our LED requires at least 10mA to light and our LED can not exceed 20 mA If the current through the LED gets too high the LED could pop it s an actual pop cover your eyes Using ohm s law on the pin we can find the minimum resistance for the circuit 5V 2V 20MA rmin gt r 1509 Now if we use Ohm s law on the diode we can figure out the maximum resistance the resistance that makes the LED not light up 5V 2V 10MA rmar gt r 3000 So we know that our resistance r needs to be between 150 and 300 Ohms Any less than that and we can permanently destroy the LED or the microcontroller or probably both Any more than that and no damage is done but the LED will be too dim to see further reading Analog and Digital Conversion Wikipedia ballast resistor Embedded Systems High Voltage Circuits 66 Embedded Systems High Voltage Circuits Often we use embedded systems to control high power devices For example maybe we want to program a microcontroller to turn on and off standard light bulbs As we discussed earlier typical microcontroller output pins switch between 0 V and 5 V and can drive a maximum of 0 025 A But a typical light bulb requires 120 VAC at 0 5 A to turn on We can t connect the microcontroller up to the 120 VAC directly What do we do Transistors and Relays
46. 90025 Contributors Whiteknight Embedded Systems Low Voltage Circuits Source http en wikibooks org w index php oldid 1601709 Contributors DavidCary Whiteknight 1 anonymous edits Embedded Systems High Voltage Circuits Source http en wikibooks org w index php oldid 1 169531 Contributors DavidCary Whiteknight 3 anonymous edits Embedded Systems Particular Microprocessors Source http en wikibooks org w index php oldid 1792735 Contributors Adrignola Daleh DavidCary Whiteknight 11 anonymous edits Embedded Systems Intel Microprocessors Source http en wikibooks org w index php oldid 1693437 Contributors DavidCary Hagindaz Watom Whiteknight 5 anonymous edits Embedded Systems PIC Microcontroller Source http en wikibooks org w index php oldid 1791205 Contributors A B Az1568 DavidCary Imp Wit Jomegat Stingraze Unihedron Whiteknight 44 anonymous edits Embedded Systems 8051 Microcontroller Source http en wikibooks org w index php oldid 1784282 Contributors AdRiley Adrignola Derbeth Eibwen Enator24 Herbythyme Jomegat Kayau Mattb1 12885 Nkrypt Robert Horning Webaware Whiteknight 56 anonymous edits Embedded Systems Freescale Microcontrollers Source http en wikibooks org w index php oldid 1755143 Contributors Daleh DavidCary Pharuo Whiteknight 8 anonymous edits Article Sources and Contributors 112 Embedded Systems Atmel AVR Source http en wikibooks org w index php oldid 1765318 Contributors And
47. AVRButterFly http www smileymicros com http www cbaird net http mech143 engr scu edu http www stanford edu class ee281 http instruct1 cit cornell edu courses ee4 76 http www engr sjsu edu bjfurman courses ME 106 index htm http www cse unsw edu au cs2121 http www siwawi arubi uni kl de avr_projects http avenhaus de EEG MMC MMC shtml Embedded Systems ARM Microprocessors 99 Embedded Systems ARM Microprocessors The ARM architecture is a widely used 32 bit RISC processor architecture In fact the ARM family accounts for about 75 of all 32 bit CPUs about 90 of all embedded 32 bit CPUs Wikipedia ARM architecture ARM Limited licenses several popular microprocessor cores to many vendors ARM does not sell physical microprocessors Originally ARM stood for Advanced RISC Machines Some cores offered by ARM e ARM7TDMI e ARM9 e ARMII Some examples of ARM based processors Intel X Scale PXA 255 and PXA 270 used in Palm PDAs e Philips LPC2000 family ARM7TDMI S core LPC3000 family ARM9 core e Atmel AT91SAM7 ARM7TDMI core ST Microelectronics STR710 ARM7TDMI core e Freescale MCIMX27 series ARMO core The lowest cost ARM processors in the the LPC2000 series have dropped below US 5 in ones which is less than the cost of many 16 bit and 8 bit microprocessors For further reading e Embedded Systems Assembly Language e Embedded_Systems Mixed_C_and_Assembly_Programming ARM the ARM
48. CPU s address space is kept aside for the external peripherals These locations can be accessed using the same instructions as used for other memory accesses But instead the read writes to these addresses are interpreted as access to device rather than a location on the main memory A CPU may expect a particular device at a fixed location or can dynamically assign a space for it The way this works is that memory interfaces are often designed as a bus a shared communications resource where many devices are attached These devices are usually arranged as master and slave devices where a master device Embedded Systems Memory 24 can send and receive data from any of the slave devices A typical system would have A CPU as the master One or more RAM and or ROM devices for program code and data storage Peripheral devices for interfacing with the outside world Examples of these might be a UART serial communications Display device or Input device Further reading Some popular interpreters for small systems some of which we briefly mentioned before include Forth one Forth wiki ists many ports of Forth to many embedded systems a subset of Python a subset of BASIC Programming typically a tokenized BASIC such as PBASIC or PICAXE BASIC Wikipedia CHIP 8 Wikipedia XPLO a subset of Lua Functional Programming 3 a subset of Objective Caml 4 a subset of Embedded Systems C Programming a C interpreter such a
49. Contents Articles Wikibooks Collections Embedded Systems Wikibooks Collections Preface Embedded Systems Embedded Systems Introduction Embedded Systems Terminology Microprocessor Basics Embedded Systems Microprocessor Introduction Embedded Systems Embedded System Basics Embedded Systems Microprocessor Architectures Embedded Systems Programmable Controllers Embedded Systems Floating Point Unit Embedded Systems Parity Embedded Systems Memory Embedded Systems Memory Units Programming Embedded Systems Embedded Systems C Programming Embedded Systems Assembly Language Embedded Systems Mixed C and Assembly Programming Embedded Systems IO Programming Embedded Systems Serial and Parallel IO Embedded Systems Super Loop Architecture Embedded Systems Protected Mode and Real Mode Embedded Systems Bootloaders and Bootsectors Embedded Systems Terminate and Stay Resident Real Time Operating Systems Embedded Systems Real Time Operating Systems Embedded Systems Threading and Synchronization Embedded Systems Interrupts Embedded Systems RTOS Implementation Embedded Systems Locks and Critical Sections Embedded Systems Common RTOS Embedded Systems Common RTOS Palm OS ao A o 11 11 12 14 17 19 21 22 25 26 26 32 33 38 39 41 42 43 44 45 45 47 50 51 53 56 58 Embedded Systems Common RTOS Windows CE Embedded Systems Common RTOS DOS Embedded Systems Linux Interfacing Embedded Systems Interfacing Basic
50. Embedded Systems PIC Microcontroller 75 insufficient the compiler automatically switches over to using a software stack Assembly language programmers are forced to do such analysis by hand What else do you need Compilers Assemblers Versions of BASIC C Forth and a few other programming languages are available for PICmicros See Embedded Systems PIC Programming downloaders You need a device called a downloader to transfer compiled programs from your PC and burn them into the microcontroller Unfortunately programming has 2 meanings see Embedded_Systems Terminology programming There are 2 styles of downloaders If you have your PIC in your system and you want to change the software e with a IC programmer style device you must pull out the PIC plug it into the IC programmer reprogram then put the PIC back in your system e with a in circuit programmer style device ICSP you don t touch the PIC itself you plug a cable from the programmer directly into a header that you have hopefully placed next to the PIC reprogram then unplug the cable An incomplete list of programmers includes e In Circuit Programmer for PIC16F84 PIC16F84 Programmer 10 IC Programmer ICProg 11 Programs 12Cxx 16Cxxx 16Fxx 16F87x 18Fxxx 16F7x 24Cxx 93Cxx 90Sxxx 59Cxx 89Cx051 89553 250x0 PIC AVR 80C51 etc Many other programmers are listed at MassMind 121 Many people prefer to use a boot
51. EnterCS and ExitCS using serializing tokens in such a way that when a process attempts to get a lock on another data structure the OS briefly releases all locks that process holds before giving that process a lock on all requested locks Further Reading Barr Michael Multitasking Alternatives and the Perils of Preemption U Embedded Systems Design January 2006 References 1 http netrino com Articles RTOS Alternatives Embedded Systems Common RTOS 56 Embedded Systems Common RTOS This chapter will discuss some particular RTOS implementations We may use some technical terms described in the Real Time Operating Systems chapter Requested RTOS Use this page to request or suggest a new chapter about a new RTOS Add new RTOS s here before adding them to the main page Do not list an RTOS on the main page if you do not intend on writing a chapter for it yourself There are far too many different RTOS s in this world to list every instance on the main table of contents and expect other users to fill in the blanks Many RTOS are designed for a particular purpose and few are common enough that other contributors can be expected to have some experience with them e INTEGRITY e velOSity e u velOSity e QNX Operating System Design Case Studies QNX e VxWorks e eCos e ST OS20 e FreeOSEK 1 e DSPnano 2 e Unison 3 e Atomthreads 4 Many embedded systems have no operating system other
52. Free open source versions and commercially supported versions for MCUs DSCs and DSPs Wikipedia Unison RTOS Ultra Tiny Embedded Linux and POSIX compatible RTOS for 32 Bit MCUs with Dual Licensing Free open source versions and commercially supported versions for MCUs DSCs and DSPs References 1 2 3 4 5 6 http marte unican es o http opensek sf net e http rowebots com products dspnano http rowebots com products unison http atomthreads com iz E http picoos sourceforge net Embedded Systems Common RTOS Palm OS 58 Embedded Systems Common RTOS Palm OS For many years the Palm OS was the defacto RTOS used in hand held devices primarily the Palm handheld PDAs However Palm has lost a large amount of market share in recent years and has lost dominance in the PDA market However many Palm devices still exist in this world and an intrepid engineer can still find and use an old palm device as the primary microcontroller for other projects This page will discuss Palm OS and to a lesser extent Palm PDAs Further Reading e PalmOS Guide e http pluggedin palm com e A Waba Powered Palm Pilot Robot H by James Caple 2001 discusses how to make your Java application run on a Palm Pilot and control a robot RoboPilot allows you to use the serial port on your Palm Pilot Palm Pro or later model to control a robot that uses the Lynxmotion Inc Serial Servo Controller SSC Th
53. NING Those who know a thing or two about copyright issues should check if I didn t just do something wrong The next segment was copy pasted from http www nongnu org avr libc user manual FAQ html faq_reg_usage 1 What registers are used by the C compiler Data types char is 8 bits int is 16 bits long is 32 bits long long is 64 bits float and double are 32 bits this is the only supported floating point format pointers are 16 bits function pointers are word addresses to allow addressing the whole 128K program memory space on the ATmega devices with gt 64 KB of flash ROM There is a mint8 option see Options for the C compiler avr gcc to make int 8 bits but that is not supported by avr libc and violates C standards int must be at least 16 bits It may be removed in a future release Call used registers r18 r27 r30 r31 May be allocated by gcc for local data You may use them freely in assembler subroutines Calling C subroutines can clobber any of them the caller is responsible for saving and restoring Call saved registers r2 r17 r28 r29 May be allocated by gcc for local data Calling C subroutines leaves them unchanged Assembler subroutines are responsible for saving and restoring these registers if changed r29 r28 Y pointer is used as a frame pointer points to local data on stack if necessary The requirement for the callee to save preserve the contents of these registers even applies in situations
54. R has thirty two general purpose eight bit registers RO to R31 six of which can be used in pairs as sixteen bit pointers X Y and Z All AVR microcontrollers have some amount of RAM from 32 bytes up to several KB This memory is byte addressable The register file both general and special purpose is mapped into the first addresses and thus accessible also as RAM Some of the tiniest AVR microcontrollers have only the register file as their RAM The data address space consists of the register file I O registers and SRAM The working registers are mapped in as the first thirty two memory spaces 0000 6 01F P followed by the reserved space for up to 64 I O registers 0020 6 005F 6 The actual usable SRAM starts after both these sections address 0060 6 Note that the I O register space may be larger on some more extensive devices in which case the beginning address of SRAM will be Embedded Systems Atmel AVR 83 higher Even though there are separate addressing schemes and optimized opcodes for register file and I O register access they can still be addressed and manipulated as if they were SRAM The I O registers and the program counter are reset to their default starting values when a reset occurs The registers and the rest of SRAM have initial random values so typically one of the first things a program does is clear them to all zeros or load them with some other initial value The registers I O registers and SRAM
55. ROGMEM or rom l on variable definitions and function parameters to indicate value resides in program Flash not RAM Unfortunately different compilers have different incompatible ways of specifying that data may be placed in ROM Typically such people use function libraries that 2 copies of functions that deal with strings etc one copy is used for strings in RAM the other copy is used for strings in ROM This technique uses the minimum amount of RAM but it usually requires more ROM than other techniques 2 Some function libraries assume the data is in RAM When a programmer wants to call such functions with data that is actually in ROM the data is first temporarily copied to a buffer in RAM This technique uses the minimum amount of ROM to hold the library but it uses more ROM and RAM than the other techniques at every function call that involves data in ROM 3 Some function libraries use functions that can handle being called from one place with a string in RAM and from other places with a string in ROM This typically requires fat pointers that have extra bits that indicate whether the pointer is pointing to something in RAM or ROM Every time such a library uses a pointer the executing code checks those bits to see whether to execute the read from RAM or read from ROM instructions volatile A volatile in a variable declaration tells us and the compiler that the value of that variable may change at any time by som
56. Resources 109 Technical Report on C Performance 19 by Dave Abrahams et al has a lot of tips for using C in embedded systems Books Barr Michael et al Embedded Systems Dictionary ISBN 1578201209 Predko Myke Programming and Customizing PICmicro Microcontrollers McGraw Hill 2002 ISBN 0071361723 Pont Michael J Embedded C Addison Wesley 2002 ISBN 020179523X Berger Arnold S Embedded Systems Design An Introduction to Processes Tools and Techniques CMP Books 2001 ISBN 1578200733 References 16 17 18 19 http www st com stonline books pdf docs 4966 pdf http www vlsichipdesign com http embedded system net reference what is embedded system http www nabble com MicroControllers f2055 html http www embeddedstar com http www endtas com robot http embedded com http embedded com design multicore 201200638 http www eg3 com http www norcom electronics com electronics_tutorials php http omimo be http www rtemagazine com http www cotsjournalonline com http www portabledesign com http www pkgmagazine com http www embeddedcommunity com http www ganssle com tools htm http embeddedsystemnews com http www research att com bs performanceTR pdf Embedded Systems Licensing 110 Embedded Systems Licensing GN Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentation License V
57. Systems Embedded Systems C Programming Embedded Systems Assembly Language Embedded Systems Mixed C and Assembly Programming Embedded Systems IO Programming Embedded Systems Serial and Parallel IO Embedded Systems Super Loop Architecture Embedded Systems Protected Mode and Real Mode Embedded Systems Bootloaders and Bootsectors Embedded Systems Terminate and Stay Resident Real Time Operating Systems Embedded Systems Real Time Operating Systems Embedded Systems Threading and Synchronization Embedded Systems Interrupts Embedded Systems RTOS Implementation Embedded Systems Locks and Critical Sections Embedded Systems Common RTOS Embedded Systems Common RTOS Palm OS Embedded Systems Common RTOS Windows CE Embedded Systems Common RTOS DOS Embedded Systems Linux Interfacing Wikibooks Collections Embedded Systems Embedded Systems Interfacing Basics Embedded Systems External ICs Embedded Systems Low Voltage Circuits Embedded Systems High Voltage Circuits Particular Microprocessor Families Embedded Systems Particular Microprocessors Embedded Systems Intel Microprocessors Embedded Systems PIC Microcontroller Embedded Systems 8051 Microcontroller Embedded Systems Freescale Microcontrollers Embedded Systems Atmel AVR Embedded Systems ARM Microprocessors Embedded Systems AT91SAM7S64 Embedded Systems Cypress PSoC Microcontroller Appendices Embedded Systems Common Protocols Embedded Systems Where To Buy Resources and Licensing Embedded S
58. Technology produces a carrier board for the Butterfly OI Which provides a power supply convenient connections to I O ports a DB 9 serial port with level translator and a large prototyping area Embedded Systems Atmel AVR 91 STK500 starter kit The STK500 starter kit and development system features ISP and high voltage programming for all AVR devices either directly or through extension boards The board is fitted with DIP sockets for all AVRs available in DIP packages Several expansion modules are available for the STK500 board These include e STKS501 Adds support for microcontrollers in 64 pin TQFP packages e STK502 Adds support for LCD AVRs in 64 pin TQFP packages e STKS503 Adds support for microcontrollers in 100 pin TQFP packages e STK504 Adds support for LCD AVRs in 100 pin TQFP packages e STKS505 Adds support for 14 and 20 pin AVRs e STK520 Adds support for 14 and 20 pin microcontrollers from the AT9OPWM family Third Party Boards There are many AVR based development and or application boards available from third parties far too many to list all of them here e Arduino l is built around an ATmega8 or ATmegal68 and is designed to be used with an open source development environment AVR Based Support and Application Boards DPI by Mr Pascal Stang from Stanford University e GPMPU40 H supports many different Atmel AVR chips e Futurlec 2313 Board 1 Note that the AT90S2313 is obsolete
59. all the possibilities We promise that we won t bite Who are the authors The volunteers at Wikibooks come from around the world and have a wide range of educational and professional backgrounds They come to Wikibooks for different reasons and perform different tasks Some Wikibookians are prolific authors some are perceptive editors some fancy illustrators others diligent organizers Some Wikibookians find and remove spam vandalism and other nonsense as it appears Most wikibookians perform a combination of these jobs Wikibooks Collections Preface It s difficult to say who are the authors for any particular book because so many hands have touched it and so many changes have been made over time It s not unheard of for a book to have been edited thousands of times by hundreds of authors and editors You could be one of them too if you re interested in helping out At the time this book was prepared for print there have been over edits made by over 0 registered users These numbers are growing every day Wikibooks in Class Books at Wikibooks are free and with the proper editing and preparation they can be used as cost effective textbooks in the classroom or for independent learners In addition to using a Wikibook as a traditional read only learning aide it can also become an interactive class project Several classes have come to Wikibooks to write new books and improve old books as part of their normal course work I
60. an it is to start studying the big PC behemoths that we use on a daily basis Many topics covered in this book will be software topics as well so this book will be the most helpful to people with at least some background in programming especially C and Assembly languages Having a prior knowledge of semiconductors and electric circuits will be beneficial but will not be required What Will This Book Cover This book will focus primarily on embedded systems but the reader needs to understand 2 simple facts 1 This book cannot proceed far without a general discussion of microprocessor architecture 2 Many of the concepts discussed in this book apply equally well if not better to Desktop computers than to embedded computers In the general interests of completeness this book will cover a number of topics that have general relevance to all computers in general Many of the lessons in this book will even be better applied by a desktop computer programmer than by an embedded systems engineer It might be more fair to say that this book is more about Low Level Computing than Embedded Systems Embedded Systems Embedded Systems Introduction This book will of course cover many embedded systems topics that are irrelevant when programming desktop computers such as cross compilers Real Time Operating Systems EEPROM storage code compression bit banging serial ports umbilical development etc Where to Go From Here Afte
61. and allow it to be reprogrammed EEPROM A step up from EPROM is EEPROM Electrically Erasable Programmable Read Only Memory EEPROM can be erased by exposing it to an electrical charge This means that EEPROM can be erased in circuit as opposed to EPROM which needs to be removed from the circuit and exposed to UV An appropriate electrical charge will erase the entire chip so you can t erase just certain data items at a time Many modern micrcontroller have an EEPROM section on booard which can be used to permanently store system parameters or calibration values These are often referred to as non volatile memory NVM They can be accessed read and write as single bytes or blocks of bytes Like Flash memory EEPROM allows only a limited number of write cycles usually several ten thousand Write access to on board NVM tends to be considerably slower than RAM Embedded software must take this into account and queue write requests to be executed in background RAM Random Access Memory RAM is a temporary volatile memory that requires a persistant electric current to maintain information As such a RAM chip will not store data when you turn the power OFF RAM is more expensive than ROM and it is often at a premium Embedded systems can have many Kbytes of ROM sometimes Megabytes or more but often they have less then 100 bytes of RAM available for use in program flow FLASH Memory Flash memory is a combination of the b
62. ard Function Libraries Now we will talk about booting and bootloaders later and we will also devote several chapters to task scheduling So we should mention at least one thing about standard function libraries In an embedded system there is rarely enough memory if any to maintain a large function library If functions are going to be included they must be small and important In an embedded system frequently the kernel will boot the system initialize the ports and the global data items Then it will start the scheduler and instantiate any hardware timers that need to be started After all that the Kernel basically gets dumped out of memory except for the library functions if any and the scheduler will start running the child tasks Basic Kernel Services In the discussion below we will focus on the kernel the part of an operating system that provides the most basic services to application software running on a processor The kernel of a real time operating system RTOS provides an abstraction layer that hides from application software the hardware details of the processor or set of processors upon which the application software will run For further reading Operating System Design e Operating systems on the rise UI by Jim Turley Embedded Systems Design 2006 06 21 Survey results show that about 3 4 of all embedded system projects use some kind of an operating system About 1 4 of all embedded system projec
63. aster good if you have a modest production line going Embedded Systems Atmel AVR 92 Bootloader Programming Most AVR models can reserve a bootloader region 256B 2KB where re programming code can reside At power on the Bootloader runs first and does some user programmed determination whether to re program or jump to the main application The code can re program through any interface available it could read an encrypted binary through an ethernet adapter if it felt like it Atmel has application notes and code pertaining to any interface from RS 232 onwards Bootloaders are covered in detail in chapter Bootloaders and Bootsectors No Programming at All The AT9OSC series of AVRs are available with a mask ROM rather than flash for program memory 15 Because of the large up front cost and minimum order quantity mask ROM is only cost effective for a large production run Debugging Interfaces The AVR offers several options for debugging mostly involving on chip debugging while the chip is in the target system JTAG JTAG provides access to on chip debugging functionality while the chip is running in the target system JTAG allows accessing internal memory and registers setting breakpoints on code and single stepping execution to observe system behaviour Atmel provides a series of JTAG adapters for the AVR 1 The JTAGICE adapter 16 interfaces to the PC via a standard serial port It is somewhat expensi
64. at doesn t natively support them Python compilers are available for some popular microcontrollers Pyastra 2 compiles for all Microchip PIC12 PIC14 and PIC16 microcontrollers PyMite 3 compiles for any device in the AVR family that has at least 64 KiB program memory and 4 KiB RAM PyMite also targets some ARM microcontrollers Notice that these Embedded Systems Embedded Systems Introduction embedded Python compilers typically can only compile a subset of the Python language for these devices Further reading e Robotics Design Basics Design software Programming Languages e Embedded Systems PIC Programming Compilers 2C_Assemblers References 1 http imaginetools com support downloads 2 http pyastra sourceforge net 3 http pymite python hosting com wiki Embedded Systems Terminology This page will try to discuss some of the different important terminology and it may even contain a listing of some of the acronyms used in this book Types of Chips There are a number of different types of chips that we will discuss here Microprocessors These chips contain a processing core and occasionally a few integrated peripherals In a different sense Microprocessors are simply CPUs found in desktops Microcontrollers These chips are all in one computer chips They contain a processing core memory and integrated peripherals In a broader sense a microcontroller is a CPU that is used in an embedded system
65. be moved from one process to another process at any given time The process that was preempted would not even know that anything had happened except maybe there would be a larger then average delay between 2 instructions Preemptive multithreading allows for programs that do not voluntarily give up control and it also allows a computer to continue functioning when a single process hangs There are a number of problems associated with preemptive multithreading that all stem from the fact that control is taken away from one process when it is not necessarily prepared to give up control For instance if one process were writing to a memory location and was preempted the next process would see half written data or even corrupted data in that memory location Or if a task was reading in data from an input port and it was preempted the timing would be wrong and data would be missed from the line Clearly this is unacceptable The solution to this new problem then is the idea of synchronization Synchronization is a series of tools provided by the preemptive multithreaded operating sytem to ensure that these problems are avoided Synchronization tools can include timers critical sections and locks Timers can ensure that a given process may be preempted but only for a certain time Critical sections are commands in the code that prevent the system from switching control for a certain time Locks are commands that prevent interference in atomic
66. bedded Systems Linux 62 U Boot the Universal Bootloader and Embedded Linux 15 RED Linux Real time and Embedded Linux 119 KURT Linux Kansas University Real Time Linux 18 n 17 the Realtime Linux Security Module selectively grants realtime permissions to specific user groups or applications Real Time Linux H by Alex Ivchenko 2001 for Linux to be a true alternative to traditional real time operating systems its lack of determinism must be dealt with Real time extensions have recently made this easy Linux Realtime Approaches 12012005 embeddedTUX org 21 the companion site to Karim Yaghmour s book Building Embedded Linux Systems The Linux Kernel References 16 17 18 19 20 21 http www linuxdevices com news NS3989618385 html http www linuxdevices com news NS9566944929 html http en wikipedia org wiki ECos http rt wiki kernel org http realtimelinuxfoundation org http linuxdevices com articles AT8073314981 html http people mech kuleuven be bruyninc rthowto http linuxbios org http www fsf org news freebios html http www linuxjournal com article 7857 http www linuxdevices com articles AT4503827066 html http lynuxworks com http www windriver com announces rtlinux http www realtimelinuxfoundation org variants variants html http denx de wiki http linux ece uci edu RED Linux http ittc ku edu kurt http sourceforge net projects realtime Ism htt
67. bits Early computers used a variety of floating point number formats Each one required slightly different subroutines to add subtract and do other operations on them Because some computer applications use floating point numbers a lot Intel standardized on one particular format and designed floating point hardware that calculated much more quickly than the software subroutines The 80186 shipped with a floating point co processor dubbed the 80187 The 80187 was a floating point math unit that handled the floating point arithmetic functions In newer processors the floating point unit FPU has been integrated directly into the microprocessor Many small embedded systems however do not have an FPU internal or external Therefore they manipulate floating point numbers when necessary the old way They use software subroutines often called a floating point emulation library However floating point numbers are not necessary in many embedded systems Many embedded system programmers try to eliminate floating point numbers from their programs instead using fixed point arithmetic Such programs use less space fixed point subroutine libraries are far smaller than floating point libaries especially when just one or two routines are put into the system On microprocessors without a floating point unit the fixed point version of a program usually runs faster than floating point version However these embedded system programmers must f
68. ble time to defragment and really is a major problem for tasks that are time sensitive This page then will talk about how to implement a memory management scheme in an RTOS and will talk through to a basic implementation of malloc and free There are a variety of ways to deal with memory Some systems never do a malloc or free all memory is allocated at compile time Some systems use malloc and free with manual garbage collection e Some early automatic garbage collection schemes did a stop the world for several seconds during garbage collection and or memory defragmentation Such a system could miss real time deadlines which would be bad e Some later automatic garbage collection schemes do incremental garbage collection and memory defragmentation What is a Task Embedded systems have a microprocessor connected to some piece of hardware LEDs buttons limit switches motors serial port s battery chargers etc Each piece of hardware is generally associated with a little bit of software called a task For example Check the keyboard and figure out which if any key has been pressed since the last check Or Check the current position of the spindle and update the PID Often a task has a real time limits such as the motors must be shut off within 1 10 second after hitting the limit switch to avoid permanent damage the PID loop must be updated at least every 1 100 second to avoid oscillation
69. bust while other implementations are very simple and suited for only one particular purpose Embedded Systems Real Time Operating Systems 46 An RTOS may be either event driven or time sharing An event driven RTOS is a system that changes state only in response to an incoming event A time sharing RTOS is a system that changes state as a function of time The Fundamentals To most people embedded systems are not recognizable as computers Instead they are hidden inside everyday objects that surround us and help us in our lives Embedded systems typically do not interface with the outside world through familiar personal computer interface devices such as a mouse keyboard and graphic user interface Instead they interface with the outside world through unusual interfaces such as sensors actuators and specialized communication links Real time and embedded systems operate in constrained environments in which computer memory and processing power are limited They often need to provide their services within strict time deadlines to their users and to the surrounding world It is these memory speed and timing constraints that dictate the use of real time operating systems in embedded software Real Time Kernel The heart of a real time OS and the heart of every OS for that matter is the kernel A kernel is the central core of an operating system and it takes care of all the OS jobs 1 Booting 2 Task Scheduling 3 Stand
70. can add USB functionality with a fixed function chip such as the FTDI232 USB to RS232 converter chip or a general purpose USB interface such as the PDIUSB11 Adding additional electronics is in fact necessary for some supported communication protocols e g standard compliant RS232 communication requires adding voltage level converters like the MA X232 The number of serial communication possibilities supported by a particular AVR can be confusing at times in particular if the pins are shared with other chip functions An intensive study of the particular AVR s datasheet is highly recommended The serial communication features most commonly to be found on AVRs are discussed in the following sections Universal Synchronous Asynchronous Receiver Transmitter USART Recent AVRs typically come with a Universal Synchronous Asynchronous Receiver Transmitter USART built in A USART is a programmable piece of hardware which is capable of generating and decoding various serial communication protocols USART is an acronym from the following words Universal Can be used in a lot of different serial communication scenarios Synchronous Can be used for synchronous serial communication sender and receiver are synchronised by a particular clock signal Asynchronous Can be used for asynchronous serial communication sender and receiver are not explicitly synchronised via a clock signal but synchronise on the data signal Receiver The hardware in th
71. cess memory and 4K bytes of Flash ROM Read only memory The Hc08 cores offer a maximum bus speed of 8MHz a 20MHz crystal may be used as the external clock source as the oscillator is internaly divided by 4 to give 8MHz bus speed Typical peripheral components of the microcontroller include Two 16 bit free running timers SCI serial communications interface RS232 e 12 channel 8 bit Analogue to digital converters A D The HC08 microcontrollers are usually supplied in 28 pin or 32 pin DIL packages but can also be obtained in serface mount SOIC footprints 16 bit MCUs 32 bit Embedded Processors 68k ColdFire The 68k family and the nearly binary compatible ColdFire family are 32 bit processors capable of running Linux There is a Debian Linux port to 68k processors with a MMU A Debian Linux port to ColdFire processors with a MMU is in progress There are several ColdFire chips that as of 2008 are available for under 5 in qty 1 Those low cost chips do not include a MMU and so cannot run a full version of Linux w uClinux runs on chips without a MMU and has been ported to some ColdFire chips 1 on platforms with at least 1 MB of RAM Yes but does uClinux actually run on a chip that costs less than 5 Embedded Systems Freescale Microcontrollers 81 Most all currently manufactured ColdFire and 68k chips are available only in surface mount packages not in any DIP package PowerPC First Generat
72. ch means added expense and more time It is very important that you try to limit the amount of current flowing into and out of your pin Ohms Law Some people would like to never see Ohm s law again and some people have never seen it before Ohm s law relates the voltage and the current of a given circuit together as such v 1r Where v is the voltage i is the current and r is the resistance of the circuit Let s do an example We have a microcontroller with output pins that can source 20mA mA milliamps and goes from OV for a logical 0 to 5V for a logical 1 Using Ohm s law 5V 20mA r gt r 2500 keep in mind that the resistance is the minimum value necessary to meet the requirements we could easily pick a resistor with 300Ohms or even IKOhm if that was all we had It is very important to note that diodes transistors and relay circuits all of which are common in embedded systems can be considered to have an effective resistance of 0 Therefore it is very important to place resistors in a circuit to control the current flow Embedded Systems Interfacing Basics 64 further reading Analog and Digital Conversion e Electronics Digital to Analog amp Analog to Digital Converters Embedded Systems External ICs Integrated Circuits IC frequently have very similar operating characteristics to microcontrollers It is often possible to connect different pins directly to each other without resistors to
73. com approx htm Embedded Systems Parity 21 Embedded Systems Parity In many instances especially in transmission it is important to include some amount of error checking information so that incorrect information can be determined and discarded One of the most simple method of error checking is called parity Parity can be broken up into Even Parity and Odd Parity schemes A parity check consists of a single bit that is set depending on a certain condition Even Parity In an even parity scheme the parity bit is set if an odd number of bits in the data are set to 1 to make the total number of 1 bits even For instance 01001100 would generate an even parity bit while 11001100 would not generate one Odd Parity The opposite of Even parity odd parity generates a parity bit if there are an even number of high bits in the data to create an odd number of 1 s Limitations of Parity Simple 1 bit parity is only able to detect a single bit error or an error in an odd number of bits If an even number of bits 2 4 6 8 are transmitted in error the parity check will not catch the mistake However chances of getting 2 errors in 1 transmission is much much smaller than getting only 1 error in 1 transmission so parity checks serve as a cheap and easy way to check for errors More advanced error detection ECC codes are often used for the same reasons as parity bits These codes use more bits however they allow mu
74. control the current flow because the ICs will not draw much current Further Reading Digital Circuits e Semiconductors Embedded Systems Low Voltage Circuits Low voltage circuits in this field of consideration can essentially be considered circuits that never exceed the pin voltage or exceed it by small amounts If the voltage goes higher or if we keep our current under control we risk damage to our embedded systems Example sensor As a simple example we would like to measure the temperature The simplest and one of the cheapest ways to measure the temperature is to use a thermistor connected to GND a resistor connected to VCC and connect the other ends of each to the analog input pin of a microcontroller Because the thermistor and resistor are connected to the same power supply as the microcontroller we can guarantee that the signal voltage is no higher than the VCC of the microcontroller and no lower than GND Because the analog input pin of a microcontroller inherently has high input resistance we can guarantee that very little current flows So in this case we don t need any other components to protect the microcontroller from damage Embedded Systems Low Voltage Circuits 65 Example Lighting LEDs Anode gt Cathode g As a more complex example we would like to light a LED light emitting diode from an output pin on an embedded computer Consider that our output pin can source 20mA at
75. ction is a sequence of computer instructions that may malfunction if interrupted An atomic operation is a sequence of computer instructions that cannot be interrupted and function correctly In practice these two subtly different definitions are often combined Operating systems provide synchronization objects to meet these o requirements and some actually call these objects as critical sections atomic operations or monitors An example of a critical section is code that removes data from a queue that is filled by an interrupt If the critical section is not protected the interrupt can occur while the dequeuing function is changing pointers and corrupt the queue pointers An example of an atomic operation is an I O read where the process must read all the data at a particular rate and cannot be preempted while reading A generally good programming practice is to have programs exit their critical sections as quickly as possible because holding a critical section for too long will cause other processes on the system not to get any time and will cause a major performance decrease Critical sections should be used with care Priority Scheduling Many RTOS have a mechanism to distinguish the relative priorities of different tasks High priority tasks are executed more frequently than the low priority tasks Each implementation of priority scheduling will be a little different however Deadlock A deadlock occurs when a series
76. d that high priority inputs are never ignored Hardware and Software There are two types of interrupts Hardware and Software Software interrupts are called from software using a specified command Hardware interrupts are triggered by peripheral devices outside the microcontroller For instance your embedded system may contain a timer that sends a pulse to the controller every second Your microcontroller would wait until this pulse is received and when the pulse comes an interrupt would be triggered that would handle the signal Interrupt Service Routines Interrupt Service Routines ISR are the portions of the program code that handle the interrupt requests When an Interrupt is triggered either a hardware or software interrupt the processor breaks away from the current task moves the instruction pointer to the ISR and then continues operation When the ISR has completed the processor returns execution to the previous location Many embedded systems are called interrupt driven systems because most of the processing occurs in ISRs and the embedded system spends most of it s time in a low power mode Sometimes ISR may be split into two parts top half fast interrupt handler First Level Interrupt Handler FLIH and bottom half slow interrupt handler Second Level Interrupt Handlers SLIH Top half is a faster part of ISR which should quickly store minimal information about interrupt and schedule slower bottom half at a later t
77. d to change the current bank number in the flag status register A and B Registers The A register is located in the SFR memory location 0xEO0 The A register works in a similar fashion to the AX register of x86 processors The A register is called the accumulator and by default it receives the result of all arithmetic operations The B register is used in a similar manner except that it can receive the extended answers from the multiply and divide operations When not being used for Embedded Systems 8051 Microcontroller 80 multiplication and Division the B register is available as an extra general purpose register References 1 http www embedds com wp content uploads 2007i 0710 8051 jpg Embedded Systems Freescale Microcontrollers Freescale Semiconductor formally Motorola Semiconductor Products Sector spun off from Motorola in July 2004 Freescale makes many microcontrollers MCU s and also a whole host of other devices such as sensors DSP s and memory to name a few The Freescale Microcontrollers come in 5 families 6800 descendents 8 bit or 16 bit 68000 descendents 32 bit e MCORE 32 bit PowerPC family 32 bit e ARM family 32 bit We discuss ARM core Freescale microcontrollers in another chapter Embedded Systems ARM Microprocessors 8 bit MCUs Freescale HC08 There are many variations on the HC08 CPU core The 68HC908JL8 is one example the HC908J13 offer 256 bytes of RAM random ac
78. ddresses instead of over a special bus This means you ll have simpler software but it also means main memory will get more access requests Programming the IO Bus When programming IO bus controls there are 5 major variations on how to handle it the main thread poll the multithread poll the interrupt method the interrupt thread method and using a DMA controller Main thread poll In this method whenever you have output ready to be sent you check if the bus is free and send it Depending on how the bus works sending it can take a large amount of time during which you may not be able to do anything else Input works similarly every so often you check the bus to see if input exists Pros e Simple to understand Cons e Very inefficient especially if you need to push the data manually over the bus instead of via DMA Ifyou need to push data manually you are not doing anything else which may lead to problem with real time hardware e Depending on polling frequency and input frequency you could lose data by not handling it fast enough In general this system should only be used if IO only occurs at infrequent intervals or if you can put it off when there are more important things to do If your system supports multithreading or interrupts you should use other techniques instead Multithread polling In this method we spawn off a special thread to poll If there is no IO when it polls it puts itself back to sleep fo
79. de to tell the microcontroller what to do Firmware for AVRs can be written in many different languages Atmel published The Novice s Guide to AVR Development 28 29 part of Atmel Applications Journal 2001 Summer which provides a brief tutorial in assembly language programming using AVR Studio AVR Assembly Language e AVR Studio 4 Assembler Simulator amp WinAVR Compatible Project Editor 30 free download AVR Instruction Set User Guide BH AVR Assembler Site H e AVR Assembler Forum AVR Assembler Tutorial 132 32 Some features of the AVR microprocessor can only be accessed with assembly language Assembly language will almost always produce the smallest code as compared to other compiled languages and for this reason it is a popular choice for applications that must fit into a very small code space AVR Studio is free of charge but the program runs only on Windows and its source code is not available Two particular free open source assemblers for AVR are AVRA 134 and Toms AVR Assembler 151 Embedded Systems Atmel AVR 94 Ada AVR Ada 1551 at Sourceforge Ada compiler from GCC and libraries for AVR BASIC The BASCOM AVR development environment 37 is a BASIC Compiler for the AVR family The IDE includes an editor compiler simulator and a lot of library functions The demo version is limited to 4K code The bascomp exe command line works in Wine 38 BASIC to C 39 has a free starte
80. e period A real time deadline can be so small that system reaction appears instantaneous The term real time computing has also been used however to describe slow real time output that has a longer but fixed time limit Learning the difference between real time and standard operating systems is as easy as imagining yourself in a computer game Each of the actions you take in the game is like a program running in that environment A game that has a real time operating system for its environment can feel like an extension of your body because you can count on a specific lag time the time between your request for action and the computer s noticeable execution of your request A standard operating system however may feel disjointed because the lag time is unreliable To achieve time reliability real time programs and their operating system environment must prioritize deadline actualization before anything else In the gaming example this might result in dropped frames or lower visual quality when reaction time and visual effects conflict Methods An operating system is considered real time if it invariably enables its programs to perform tasks within specific time constraints usually those expected by the user To meet this definition some or all of the following methods are employed The RTOS performs few tasks thus ensuring that the tasks will always be executed before the deadline The RTOS drops or reduces certain functions wh
81. e AVR can receive serial data Transmitter The hardware can send serial data Embedded Systems Atmel AVR 87 Earlier AVRs had a UART that did not support synchronous serial communication hence the absence of the S in the acronym USARTs or UARTs work with logic voltage levels while e g the RS232 protocol requires much different voltage levels than the 5V or 3 3V supplies found on AVR circuits The conversion from and to such voltage levels is performed by an additional chip which is commonly called a line driver or line interface With the right line interface an AVR s USART can for example be used to communicate with RS 232 RS 485 MIDI LIN bus or CANbus devices to name some of the popular protocols See Robotics Computer Control The Interface Networks for more details RS 232 Signalling The RS 232 specification calls for a negative voltage to represent a 1 bit and a positive voltage to represent a 0 bit The spec allows for levels from 3 to 15V and 3 to 15V but 12V is commonly seen The AVR does not have the ability to drive a negative output voltage on any GPIO pin and so a level converter such as the MAX232 is used to talk to PCs and strict RS 232 devices See Serial Programming RS 232 Connections for more detail on RS 232 wiring RS 232 has a relatively short maximum cable length For longer cabling distances consider using RS 485 signaling on your USART Two Wire Interface TWI is a variant of Phi
82. e Palm Pilot Robot Kit PPRK Build your own Palm powered robot 3 by Greg Reshko Matt Mason and Illah Nourbakhsh e PPRK Overview ll Carnegie Mellon University Palm Pilot Robot Kit 5 Robot ASCII Serial Command Interpreter RASCI References 1 2 http www taygeta com robots robopilot html 3 4 http www acroname com robotics info PPRK overview html 5 6 http www mrrobot com rasci html mar http java sys con com node 36769 http www palmpower com issues issue200012 robot001 html http www cs cmu edu pprk Embedded Systems Common RTOS Windows CE 59 Embedded Systems Common RTOS Windows CE Windows CE has been gaining a large market share in the high end PDA market and can even be found occasionaly on cell phones as well Windows CE is very similar to other Windows implementations and actualy has a nearly complete implementation of the Win32 programming API for developers to tap into This page will briefly discuss Windows CE because further discussions of it are heavily related to discussions of the desktop breeds of Windows and this book does not have enough scope to discuss windows architecture the Windows API or windows programming in general Currently Windows CE implementations ship with the NET Compact framework This means that Windows CE applications can be programmed in traditional languages C C etc but also NET languages such as C and VB NET Fur
83. e doing embedded programming quickly become masters at using serial IO channels and error message style debugging Resources To save costs embedded systems frequently have the cheapest processors that can do the job This means your programs need to be written as efficiently as possible When dealing with large data sets issues like memory cache misses that never matter in PC programming can hurt you Luckily this won t happen too often use reasonably efficient algorithms to start and optimize only when necessary Of course normal profilers won t work well due to the same reason debuggers don t work well So more intuition and an understanding of your software and hardware architecture is necessary to optimize effectively Memory is also an issue For the same cost savings reasons embedded systems usually have the least memory they can get away with That means their algorithms must be memory efficient unlike in PC programs you will frequently sacrifice processor time for memory rather than the reverse It also means you can t afford to leak memory Ul Embedded applications generally use deterministic memory techniques and avoid the default new and malloc functions so that leaks can be found and eliminated more easily Other resources programmers expect may not even exist For example most embedded processors do not have hardware FPUs l Floating Point Processing Unit These resources either need to be emulated in software or
84. e means outside this C program The volatile keyword tells the compiler not to make certain optimizations that only work with normal variables stored in RAM or ROM that are completely under the control of this C program The entire point of embedded programming is its communications with the outside world and both input and output devices require the volatile keyword There are at least 3 types of optimizations that volatile turns off read optimizations without volatile C compilers assume that once the program reads a variable into a register it doesn t need to re read that variable every time the source code mentions it but can use the cached value in the register This works great with normal values in ROM and RAM but fails miserably with input Embedded Systems C Programming 30 peripherals The outside world and internal timers and counters frequently change making the cached value stale and irrelevant write optimizations without volatile C compilers assume that it doesn t matter what order writes occur to different variables and that only the last write to a particular variable really matters This works great with normal values in RAM but fails miserably with typical output peripherals Sending turn left 90 go forward 10 turn left 90 go forward 10 out the serial port is completely different than optimizing it to send 0 out the serial port instruction reordering without volatile
85. e most important tools of an embedded systems engineer is the Watch Dog Timer WDT A WDT is a timer with a very long fuse several seconds usually The WDT counts down toward zero like the big red numbers counting down on the bombs in the movies Left to itself eventually the counter will reach zero When the counter reaches zero the WDT resets the microcontroller as if the power were turned off then turned back on When the system is running normally you don t want it to randomly reset itself so you need to make sure that your program always feeds the watch dog long before time runs out Good practice is to reset the WDT less than halfway through it s countdown For instance if the WDT has a timer of 20 seconds then you will want to feed the WDT at least once every 10 seconds Unlike when our hero deals with bombs in the movies feeding the watch dog doesn t stop the countdown When the code uses a reset or clear command to feed the watchdog it merely sets the WDT back to some large number and then the watchdog timer immediately starts counting down from there If the programmer fails to feed the watchdog in time or if the program hangs for any reason then sooner or later WDT will time out and the program will reset hopefully getting your system unstuck Some watchdogs count up With this kind of watchdog feeding the watchdog resets it to zero If it ever reaches some high limit it resets the sys
86. e non volatile memory of the embedded system After a person tweaks the source code and compiles a new executable image on the PC that person connects a downloader between the PC and the embedded system and clicks the go button Then the PC streams the image into the downloader and the downloader burns the image into the embedded system n A downloader is variously called a downloader burner flasher flash downloader programming interface or confusingly a programmer e Embedded_Systems PIC_Microcontroller downloaders e Embedded_Systems Atmel_AVR Programming_Interfaces programming Sometimes programming means the overall process of a person writing software on a PC going through many edit compile download burn test cycles Other times programming means the specific step of the programmer device burning the compiled code into the chip Please help us make this book less confusing In this book we use the term programming to describe what a human being does to create and test software source code Be aware that other texts may use the term programming such as when talking about high voltage programming gang programming etc to describe what we would call installing They may call the piece of hardware that does it a programmer Embedded Systems Terminology 10 n o Texts that talk about C programming assembly programming pair programming etc use progra
87. e parameters to the stack before calling it The parameters are pushed from right to left and MSB first Example C function to be called void foo WORD Argl WORD Arg2 assembly code mov A Arg2 MSB push A mov A Arg2 LSB push A mov A Argl MSB push A mov A Argl LSB Embedded Systems Cypress PSoC Microcontroller 104 push A CELL O0 add SP 4 interrupts Q How do I call a C function from an assembly ISR A1 The simple method Use pragma interrupt_handler to mark the C function From the ISR side LJUMP to the C function don t bother pushing anything on the stack Refer to the answer of question How do I write an interrupt handler in C The C function marked with pragma interrupt_handler can call normal C functions and normal assembly functions but normal C functions cannot call any C function marked with pragma interrupt_handler A2 If you insist that your assembly ISR must call a normal C function it gets tricky You need to take care of saving and restoring virtual registers used by the C function Open the lst file and check what are the virtual registers used by the C function For example if the C function foo uses virtual registes __ 10 and __r1 assumes void foo void See How do I call a C function from assembly if foo has parameters imena Ay LE push A mow A Teil push A KCELL EOG pop A mow FL pop A none 1210 _ 7 Apart from savi
88. e the microcontroller at the device end and a PC on the host end These adapters are full speed 12Mbps USB devices but don t expect them to be fast most of them emulate a serial port with baud rates up to about 1 Mbps e Many microcontrollers such as the Atmel ATmega16 can be programmed to be a Low speed USB device with a few external passive components 8 microcontroller as host connecting to some USB device There are a variety of ways to connect a microcontroller to a USB device e practically all microcontrollers have a UART and some USB adapters 9 10 can be set up with a microcontroller as the host and some USB device a mouse keyboard or flash drive on the device end e afew microcontrollers such as the Parallax Propeller can be programmed to talk to a few USB peripherals with a few external passive components 11 microcontroller as both a device connected to a standard PC and a host connected to one or more USB devices How USB on the go OTG defines a single socket that automatically switches between host and device for example a camera with a single USB socket that acts as a device when plugged into a PC for uploading photos but acts like a host when plugged into a printer for printing photos directly without a PC 121 13 e The LUFA library allows the USB enabled AVR microcontrollers to act as a USB Host slave or OTG device 14 e Most Atmel 32 bit AVR UC3 microcontrollers support full
89. e to enable serial in system programming which is set by default If it is set incorrectly the only way to program the chip is by using a high voltage programmer such as the STK 500 AVR Dragon or third party Embedded Systems Atmel AVR 84 programmer A developer is therefore cautioned to be careful when manipulating fuses Reset The AVR s RESET pin is an active low input that forces a reset of the processor and its integrated peripherals The line can be driven by an external power on reset generator a voltage supervisor which asserts RESET when the power supply voltage drops below a predefined threshold or another component in a larger system For example if the AVR is managing a few sensors and servos as part of a large integrated system another controller might observe some condition that justifies resetting the AVR it could do so by asserting the AVR s RESET line AVRs also include a watchdog timer which can reset the processor when it times out The watchdog timer must be reset periodically to prevent it from timing out Failure to reset the watchdog timer is usually an indication that the program code has failed locked up entered an infinite loop or otherwise gone astray and the processor should be reset On some AVRs the watchdog can be programmed to issue an interrupt instead of resetting the processor This functionality can be used to wake up the AVR from a sleep mode The RESET pin is used for in system ser
90. eDOS project e http freedos 32 sourceforge net home of the FreeDOS 32 project e http reactos org ReactOS Development Wiki Embedded Systems Linux A few of the many versions of Linux are designed for embedded systems Unlike the majority of desktop or server distributions of Linux these versions of Linux either a support real time tasks or e b run in a small embedded system typically booting out of Flash no hard drive no full size video display and take far less than 2 minutes to boot up or e c both Linux and MMU Linux was originally designed on a processor with a memory management unit MMU Most embedded systems do not have a MMU as we discussed earlier Embedded Systems Memory Benefits of using a processor with a MMU can isolate running untrusted machine code from running critical code so the untrusted code is guaranteed in the absence of hardware failures not to interfere with the critical code makes it easier for the OS to present the illusion of virtual memory e can run normal Linux could also run uClinux but what s the point Benefits of using a processor without a MMU e typically lower cost and lower power e can still run the uClinux version of Linux specifically designed to run on processors without a MMU Embedded Systems Linux 61 Linux and real time People use a variety of methods to combine real time tasks with Linux Run the real time tasks on a d
91. edicated microcontroller communicate with a non real time PC that handles non real time tasks This is pretty much your only choice if you need real time response times below 1 microsecond Run the real time tasks in a underlying dedicated real time operating system run Linux as a nested operating system inside one low priority task on top of the real time operating system Some of these systems claim real time response times below 500 microseconds use a Linux kernel designed to emphasize real time tasks and run the real time tasks with a high priority perhaps even as a kernel thread As Linux develops it seems to be getting better response times Preemptible kernel patch makes it into Linux kernel v2 5 4 pre6 ll Linux kernel gains new real time support 12 Typically embedded Linux needs a minimum of about 2 MB of RAM not including application and service needs 3 further reading using Linux with hard real time tasks the Real Time Linux wiki e Wikipedia Xenomai Wikipedia RTLinux e Wikipedia RTAI e Wikipedia MontaVista Linux Real Time Linux Foundation P Real time Linux Software Quick Reference Guide l6 discribes many projects that try to bring real time systems and Linux together non real time Linux distributions designed for embedded systems Wikipedia uClinux MicroController Linux is a version of the Linux kernel that supports Altera NIOS ADI Blackfin ARM ETRAX Freescale M68K
92. emented using bit banging of the I O lines An efficient implementation of a slave can be done by connecting SCLK to an external interrupt source The datasheet for a particular AVR provides a block diagram of the SPI or USI controller on that chip Protocol Issues SPI RS 232 UC and other serial interfaces only define the method by which bits and bytes are transmitted they correspond to layer 1 in the OSI model the physical layer The bytes could be anything temperature readings in Centigrade or Fahrenheit depending on your sensor readings from a pressure sensor control signals to turn off a pump or the bytes of a JPEG image Some of this meaning may be assigned by the use of a serial communications protocol A serial protocol must handle a wide variety of usage conditions as well as provide for recovering from failures For example if two sensors are connected to a single microcontroller such as inside and outside temperature the protocol provides a way for the receiver on the other end of the serial line to discern which reading belongs to which sensor If a cable is unplugged during transmission or a byte is lost due to line noise the protocol can provide a way to re synchronize the transmitter and the receiver The Serial Programming wikibook contains more discussion of serial protocols Analog Interfaces Analog to Digital Analog to digital conversion uses digital number to represent the proportion of the analog si
93. ems have DMA controllers This is especially true of the more basic 8 bit microcontrollers e Parallel nature may complicate a system Embedded Systems Programmable Controllers The original 8086 processor shipped with a number of peripheral chips that each performed different tasks Among these chips were programmable Interrupt controllers programmable timers and programmable I O chips that could handle many of the tasks of the original computer to take some of the computational strain off the 8086 In new versions of Intel chips 486 Pentium etc many of the peripheral chips have been integrated into the processor in an attempt to speed up the entire computer However much of the functionality remains even in today s high end computer systems Timers Timers are incredibly useful for performing a number of different operations For instance many multi threaded operating systems operate by setting a timer and then switching to a different thread every time the timer is triggered Programmable timer chips can often be programmed to provide a variety of different timing functions to take the burden off the microprocessor Another common application of a timer is to keep track of the time in human units of hours and minutes and often years months and days Often this real time clock RTC has a battery to keep it running even in systems that are usually plugged into line power Such a timer can save power in two ways When we wa
94. en they cannot be executed within the time constraints load shedding The RTOS monitors input consistently and in a timely manner The RTOS monitors resources and can interrupt background processes as needed to ensure real time execution The RTOS anticipates potential requests and frees enough of the system to allow timely reaction to the user s request The RTOS keeps track of how much of each resource CPU time per timeslice RAM communications bandwidth etc might possibly be used in the worst case by the currently running tasks and refuses to accept a new task unless it fits in the remaining un allocated resources Chapters in this section will discuss how an RTOS works some general methods for working with an RTOS and a few popular RTOSes Finally in some later chapters we will discuss how to write your own RTOS Objectives An RTOS must respond in a timely manner to changes but that does not necessarily mean that an RTOS can handle a large throughput of data In fact in an RTOS small response times are valued much higher than power or data speed Sometimes an RTOS will even need to drop data to ensure that it meets its strict deadlines In essence that provides us with a perfect definition an RTOS is an operating system designed to meet strict deadlines Beyond that definition there are few requirements as to what an RTOS must be or what features it must have Some RTOS implementations are very powerful and very ro
95. er be bitten by the read modify write problem Warning you cannot read a port that is configured for interrupt on change from last read from main code and have the ISR feature work reliably See http www psocdeveloper com forums viewtopic php t 2094 How to set a single port pin http www cypress com portal server pt gateway PTARGS_0_652034_739_205_211_43 http 3B sjapp20 cf_apps design_supports forums messageview cfm catid 3 amp threadid 18055 PSoC I O Pin Port Configuration AN2094 http www cypress com portal server pt space CommunityPage amp control SetCommunity amp CommunityID 285 amp PageID 552 amp shortlink DA_240474 Mr Zee s intro to GPIO Basic fundamentals of GPIO by mrzee http www psocdeveloper com forums viewtopic php t 2058 http www psocdeveloper com forums viewtopic php t 1667 Q What should I do with pins I m not using A QThe XRES should be connected to what A Although it has an internal pull down it is good practice to connect it to ground via a 470 1K ohm resistor Embedded Systems Cypress PSoC Microcontroller 103 I O pins Q How do I read the state of a single digital input pin A Usually you connect the input pins to some digital module such as a timer block If none of the modules do what you want you can set them in software see the GPIO references in the previous question interrupts Q How do I set up a digital input pin to t
96. er that performs no other task than to display the clock Modern cars have embedded computers onboard that control such things as ignition timing and anti lock brakes using input from a number of different sensors Embedded computers rarely have a generic interface however Even if embedded systems have a keypad and an LCD display they are rarely capable of using many different types of input or output An example of an embedded system with I O capability is a security alarm with an LCD status display and a keypad for entering a password In general an Embedded System e Isa system built to perform its duty completely or partially independent of human intervention Is specially designed to perform a few tasks in the most efficient way e Interacts with physical elements in our environment viz controlling and driving a motor sensing temperature etc An embedded system can be defined as a control system or computer system designed to perform a specific task Common examples of embedded systems include MP3 players navigation systems on aircraft and intruder alarm systems An embedded system can also be defined as a single purpose computer Most embedded systems are time critical applications meaning that the embedded system is working in an environment where timing is very important the results of an operation are only relevant if they take place in a specific time frame An autopilot in an aircraft is a time critical embedded system
97. ersion 1 2 or any later version published by the Free Software Foundation with no Invariant Sections no Front Cover Texts and no Back Cover Texts A copy of the license is included in the section entitled GNU Free Documentation License Article Sources and Contributors 111 Article Sources and Contributors Wikibooks Collections Embedded Systems Source http en wikibooks org w index php oldid 1339120 Contributors Whiteknight Wikibooks Collections Preface Source http en wikibooks org w index php oldid 1795903 Contributors Adrignola Jomegat Magesha Mike lifeguard RobinH Whiteknight Embedded Systems Embedded Systems Introduction Source http en wikibooks org w index php oldid 1826627 Contributors AdRiley Az1568 DavidCary Hagindaz Herbythyme Jedikaiti Jomegat Mcleodm QuiteUnusual Recent Runes SPat Webaware Whiteknight YMS 33 anonymous edits Embedded Systems Terminology Source http en wikibooks org w index php oldid 1604590 Contributors DavidCary Intgr Manikandan ts Whiteknight 7 anonymous edits Embedded Systems Microprocessor Introduction Source http en wikibooks org w index php oldid 1826643 Contributors Adrignola DavidCary Jedikaiti Whiteknight 2 anonymous edits Embedded Systems Embedded System Basics Source http en wikibooks org w index php oldid 1816057 Contributors DavidCary Hagindaz Icewedge Javariel Mbluett Recent Runes Whiteknight 8 anonymous edits Embedded Systems Microprocess
98. est parts of RAM and ROM Like ROM Flash memory can hold data when the power is turned off Like RAM Flash can be reprogrammed electrically in whole or in part at any time during program execution Flash memory modules are only good for a limited number of Read Write cycles which means that they can burn out if you use them too much too often As such Flash memory is better used to store persistant data and RAM should be used to store volatile data items 26 Programming Embedded Systems Embedded Systems C Programming The C programming language is perhaps the most popular programming language for programming embedded systems Earlier Embedded Systems Embedded Systems Introduction Which Programming Languages Will This Book Use we mentioned other popular programming languages Most C programmers are spoiled because they program in environments where not only is there a standard library implementation but there are frequently a number of other libraries available for use The cold fact is that in embedded systems there rarely are many of the libraries that programmers have grown used to but occasionally an embedded system might not have a complete standard library if there is a standard library at all Few embedded systems have capability for dynamic linking so if standard library functions are to be available at all they often need to be directly linked into the executable Often times because of space concerns i
99. formation about embedded systems hardware and software Massmind is almost a wiki AVR wiki offline as of 2010 02 15 Programming amp Educational Websites AVRBeginners 7 AVR Assembly Language Site UI AVR Machine Language 78 Free AVR Tutorials and Projects 79 Mailing List amp Forums The AVR Forum l AVRFreaks 1801 AVRbeginners 77 AVR Chat Egroup com AVR Chat amp AVR GCC mailing list B AVR Butterfly Group 83 81 Books Dhananjay Gadre Programming and Customizing the AVR Microcontroller McGraw Hill 2000 Richard H Barnett Sarah A Cox Larry D O Cull Embedded C Programming and the Atmel AVR Thomson Delmar Learning 2002 John Morton AVR An Introductory Course Newnes 2002 Claus Kuhnel AVR RISC Microcontroller Handbook Newnes 1998 Joe Pardue C Programming for Microcontrollers featuring ATMEL s AVR Butterfly and the free WinAVR Compiler Smiley Micros 2005 Smiley Micros l8 Chuck Baird Programming Microcontrollers using Assembly Language Lulu com 2006 cbaird net 85 Richard H Barnett Embedded C Programming And The Atmel AVR Delmar Cengage Learning 2 edition June 5 2006 University Courses The following courses are known to use the Atmel AVR as part of the curriculum Introduction to Mechatronics Santa Clara University 86 Embedded System Design Laboratory Stanford University 87 Designing with Microcontrollers Cornell University 88 San J
100. g Normally parameters are passed between functions either written in C or in Assembly via the stack For eg if a function foo1 calls a function foo2 with 2 parameters say characters x and y then before the control jumps to the starting of foo2 two bytes normal size of a character in most of the systems are filled with the values that need to be passed Once control jumps to the new function foo2 and you use the values passed as parameters in the function they are retrieved from the stack and used There are two parameter passing techniques in use 1 Pass by Value 2 Pass by Reference Parameter passing techniques can also use right to left C style left to right Pascal style On processors with lots of registers such as the ARM and the Sparc the standard calling convention puts all the parameters and even the return address in registers On processors with inadequate numbers of registers such as the 80x86 and the M8C all calling conventions are forced to put at least some parameters on the stack or elsewhere in RAM Some calling conventions allow re entrant code Pass by Value With pass by value a copy of the actual value the literal content is passed For example if you have a function that accepts two characters like void foo char x char y x x 1 y y 2 putchar x putchar y and you invoke this function as follows char a b a A b B foo a b then the program pushes a copy of the ASCII
101. g the PSoC http www psocdeveloper com old index php page 8 amp mode article amp k 11 http www psocdeveloper com forums viewtopic php t 1902 http www psocdeveloper com forums viewtopic php t 2102 http www psocdeveloper com forums viewtopic php t 2099 http www psocdeveloper com forums viewtopic php t 2089 warning area myproject_RAM not defined Q I m getting the message warning area myproject_RAM not defined in startup file obj boot o and does not have an link time address How do I get rid of that warning A from http www psocdeveloper com forums viewtopic php t 1761 Open the boot tpl In the end you will see many areas defined in RAM Add the following line below the already existing RAM area declarations just above the AREA bss declaration AREA myproject_RAM RAM REL CON Then save the boot tpl file and Config gt gt generate application Embedded Systems Cypress PSoC Microcontroller 102 lookup table Q How do I create a lookup table in assembly language A See http www psocdeveloper com old index php page 8 amp mode article amp k 10 Um don t you also have to disable code compression during that table gotchas Q Any gotchas I should watch out for A Tips on using the PSoC http www psocdeveloper com old index php page 8 amp mode article amp k 11 simulator and other programming languages Q What alternatives are there to the free PSoC Designe
102. gital camera to global networks or even interplanetary communication for space probes however some recent CPU bus architechtures are even using serial methodologies as well Serial Transmission RS 232 See Also Serial Programming Book RS 485 See also Serial Programming RS 485 I2C Inter Integrated Circuit See Also I2C Inter Integrated Circuit Bus Technical Overview and Frequently Asked Questions 1 Embedded Systems Serial and Parallel IO 40 Ethernet As on chip memory increases it is becoming more common to see Ethernet support in small system on chip embedded systems New Ethernet ASIC products are also on the market This allows an embedded system to have it s own IP address on a network or on the internet It can act as a server for its own webpage to implement a GUI or general purpose I O and to display any relevant information such as sensor data or even as a portal to remotely upgrade firmware For example many network routers have these features USB See Also Serial Programming USB Currently Q1 2006 the module is a stub Serial ATA See Also Serial Programming Serial ATA Currently Q1 2006 the module is a stub Parallel Transmission Centronics Centronics is synonomous with the 1980 s PC standard parallel printer interface For further reading e Robotics Computer Control The Interface Networks References 1 http www esacademy com faq i2c Embedded Systems Super Loo
103. gnal sampled For example by applying a 3V to the input of an ADC with a full scale range of 5 V will result as a digital output of 60 of the full range of the digital output The digital number can be represented in 8 or 10 bits by the ADC An 8 bit converter will provide output from 0 to 23 _ 1 or 255 10 bits will provide output from 0 to 210 _ 1 1023 7 10 bit sample o 0 6 x 1023 614in ADCH ADCL or V 8 bit sample ue 0 6 x 255 153in ADCL 5V Many AVRs include an ADC specifically a successive approximation ADC The ADC reference voltage 5V in the example above can be an external voltage an internal fixed 1 1 V reference AVRs with an ADC have several analog inputs which are connected to the ADC via an analog multiplexer Only one analog input can be converted at any given time The ADC controller provides a method for sequentially converting the inputs so that an AVR can easily cycle through multiple sources thousands of times a second AVRs can run Embedded Systems Atmel AVR 89 ADC conversions continuously in the background or use a special ADC sleep mode to halt the processor while a conversion is taking place to minimize voltage disturbances from the rest of the MCU Analog Comparator Peripheral Nearly all AVR microcontrollers feature an Analog Comparator which can be used to implement an ADC on those AVRs which do not have an ADC or if all of the ADC inputs are already in use Atmel provides
104. gram flash memory EEPROM Fuse bits All these memories are on the same chip as the CPU core Each kind of memory is separated from each other in different locations on the chip Address 0 in data memory is distinct from address 0 in program flash and address 0 in EEPROM Program Memory All AVR microcontrollers have some amount of 16 bit wide non volatile flash memory for program storage from 1KB up to 256KB or 512 128K typical program words The program memory holds the executable program opcodes and static data tables Program memory is linearly addressed and so mechanisms like page banking or segment registers are not required to call any function regardless of its location in program memory AVRs cannot use external program memory the flash memory on the chip is the only program memory available to the AVR core The flash program memory can be reprogrammed using a programming tool the most popular being those that program the chip in situ and are called in system programmers ISP Atmel AVRs can also be reprogrammed with a high voltage parallel or serial programmer and via JTAG again in situ on certain chips The flash memory in an AVR can be re programmed at least 10 000 times Many of the newer AVRs MegaAVR series have the capability to self program the flash memory This functionality is used mainly by bootloaders Data Memory Data Memory includes the registers the I O registers and internal SRAM The AV
105. h hand shaking References 1 Technical Report on C Performance http www research att com bs performanceTR pdf by Dave Abrahams et al 2003 Embedded Systems Serial and Parallel IO This page of the Embedded Systems book is a stub You can help by expanding this section Data Transmission Data can be sent either serially one bit after another through a single wire or in parallel multiple bits at a time through several parallel wires Most famously these different paradigms are visible in the form of the common PC ports serial port and parallel port Early parallel transmission schemes often were much faster than serial schemes more wires more data faster but the added cost and complexity of hardware more wires more complicated transmitters and receivers Serial data transmission is much more common in new communication protocols due to a reduction in the I O pin count hence a reduction in cost Common serial protocols include SPI and IC Surprisingly serial transmission methods can transmit at much higher clock rates per bit transmitted thus tending to outweigh the primary advantage of parallel transmission Parallel transmission protocols are now mainly reserved for applications like a CPU bus or between IC devices that are physically very close to each other usually measured in just a few centimeters Serial protocols are used for longer distance communication systems ranging from shared external devices like a di
106. h of an inch you could store the data as hundreths rather than inches This allows you to use normal fixed point arithmetic This technique works so long as you know the magnitude of data you are adding ahead of time and know the accuracy to which you need to store your data We will go into more detail on fixed point and floating point numbers in a later chapter further reading e Floating Point Fixed Point Numbers References 1 http en wikipedia org wiki Memory_leak 2 http en wikipedia org wiki Floating_point_unit Embedded Systems Microprocessor Architectures 14 Embedded Systems Microprocessor Architectures The chapters in this section will discuss some of the basics in microprocessor architecture They will discuss how many features of a microprocessor are implemented and will attempt to point out some of the pitfalls speed decreases and bottlenecks specifically that each feature represents to the system Memory Bus In a computer a processor is connected to the RAM by a data bus The data bus is a series of wires running in parallel to each other that can send data to the memory and read data back from the memory In addition the processor must send the address of the memory to be accessed to the RAM module so that the correct information can be manipulated Multiplexed Address Data Bus In old microprocessors and in some low end versions today the memory bus is a single bus that will carry both
107. he electronics needed in an embedded system in a single integrated circuit chip 0 CPU e T O ports e RAM contains temporary data ROM contains program and constant data the firmware Starting in 1993 many microcontrollers use Flash memory instead of true ROM to hold the firmware but many engineers still refer to the Flash memory that holds the firmware as ROM from force of habit e timers we discuss these later at Embedded Systems Programmable Controllers Timers e serial interface often a USART we discuss these later at Embedded Systems Serial and Parallel IO e EEPROM contains permanent data e analog to digital converter This list is roughly in order of integration The earliest microcontrollers contained only the CPU and I O ports modern microprocessors typically contain the CPU some I O ports and a lot of cache RAM the cost of a microcontroller dropped dramatically once the CPU I O RAM and ROM could all be squeezed onto the same chip because such a microcontroller no longer needs address pins etc The most highly integrated microcontrollers include all these parts on one chip Should we say something about Harvard architecture here Further reading Learn Electronics Microprocessors 1 Microcontrollers made easy http www st com stonline books pdf docs 4966 pdf ST AN887 Embedded Systems Embedded System Basics 12 Embedded Systems Embedded System Basics Embedded
108. he PDIUSB12 or for a low speed and minimal functionality device a firmware only approach Two firmware only USB drivers are obdev BI which is available under an Open Source compliant license with some restrictions and USBtiny 4 which is licensed under the GPL Although these software implementation provide a very cheap way to add USB connectivity they are limited to low speed transfers and tie up quite some AVR resources Other hardware ICs which translate USB signals to RS 232 serial for the AVRs are available from vendors such as FDTI These ICs have the advantage of offloading the strenuous task of managing the USB connection with the disadvantage of being limited to the speed of the AVR s serial port Embedded Systems Atmel AVR 90 Temperature Sensor Some newer models have a built in temperature sensor hooked up to the ADC AVR Selection The AVR microcontrollers are divided into three groups e tinyAVR e AVR Classic AVR e megaAVR The difference between these devices mostly lies in the available features The tinyAVR microcontrollers are usually devices with lower pin count or reduced feature set compared to the megaAVRs All AVR devices have the same basic instruction set and memory organization so migrating from one device to another AVR is usually trivial The classic AVR is mostly EOL d and so new designs should use the Mega or Tiny series Some of the classic AVRs have replacement parts in the mega
109. he better choice than building an RTOS from scratch The Perfect RTOS by Colin Walls 2004 link not working Really simple memory management Fat Pointers D describes a simple garbage collection and memory defragmentation scheme that is compatible with small real time systems it never does a stop the world Embedded Systems RTOS Implementation 53 e FLIRTing with 8 bit MCU OSes 6 by Dave Armour 2009 describes implementing just about the minimum functionality required for a pre emptive OS TaskCreate TaskDestroy and a very simple timer driven task switcher FLIRT uses 144 bytes of flash References 1 2 3 4 5 6 http www awprofessional com articles article asp p 30188 amp seqNum 3 amp rl 1 http microchip com stellent idcplg IdcService SS_GET_PAGE amp nodeld 1490 amp filterID 388 http www embedded com 1999 9903 9903sr htm http www techonline com learning techpaper embedded 37804 http www logarithmic net pfh blog 01184061815 http www embedded com design opensource 2 18600135 printable true Embedded Systems Locks and Critical Sections This page of the Embedded Systems book is a stub You can help by expanding this section An important part of an RTOS is the lock mechanisms and the Critical Section CS implementation This section will talk about some of the problems involved in creating these mechanisms Basic Critical Sections Most embedded systems have at
110. hip PICmicro 12 AN617 fixed point routines for the Microchip PICmicro 13 Algorithm ArcTan as Fast as You Can AN2341 UA fixed point routine for the Cypress PSoC Floating Point Approximations 15 collected by the Ganssle Group giving code and test cases Assumes you already have floating point add subtract multiply and divide and gives formulas for trig roots logarithms and exponents various formulas with different tradeoffs between accuracy speed and range References http archive chipcenter com eexpert rashby rashby059 html http archive chipcenter com eexpert rashby rashby002 html http massmind org techref method math muldiv htm http massmind org techref method bits htm http c2 com cgi wiki MakeltWorkMakeltRightMakeltFast http c2 com cgi wiki MakeltWorkMakeltSmallMakeltFast http www embedded com columns technicalinsights 172302493 _requestid 742859 http zone ni com devzone cda tut p id 3 115 http www mathworks com products demos shipping fixedpoint fi_radix2fft_demo html product PO 10 http www ceet niu edu faculty kuo exp exp7 html 11 http www analog com UploadedFiles Application_Notes 275080109ee_18 pdf 12 http www microchip com stellent idcplg IdcService SS_GET_PAGE amp nodeId 1824 amp appnote en010982 13 http www microchip com stellent idcplg IdcService SS_GET_PAGE amp nodeId 1824 amp appnote en010962 14 http www cypress com design AN2341 15 http www ganssle
111. ial programming as a GPIO or for debugWIRE low pin count debugging depending on the chip and the programming of the fuse bits If the reset functionality of that pin is disabled it cannot be recovered by in system serial programming and another method such as high voltage programming must be used Interrupts AVRs support multiple interrupt sources both internal and external An interrupt could be from an internal peripheral reaching a certain state i e character received on UART or from an external event like a certain level on a pin Each interrupt source causes a jump to a specific location in memory That location is expected to contain either a RETI Return from Interrupt instruction to essentially ignore the interrupt or a jump to the actual interrupt handler Most AVRs have at least one dedicated external interrupt pin INTO Older AVRs can trigger an interrupt on a high or low level or on a falling edge Newer AVRs add more options such as triggering on the rising edge or either edge Additionally many of the newer AVRs implement pin change interrupts for all pins in groups of eight eliminating the need for polling the pins The pin change interrupt handler must examine the state of the pins that are associated with that interrupt vector and determine what action to take Due to button bounce issues it is considered poor design to connect a push button or other user input directly to an interrupt pin some debouncing or
112. ice to another low power device e relays allow a microcontroller on one side of the isolation barrier to switch on and off high power devices on the other side Transformers Transformers use magnetic fields to move a voltage from one coil to another over simplification There is no direct wire connection between the input and the output terminals and therefore transformers can help to prevent spikes on one side from damaging expensive equipment on the other Opto Isolators Opto Isolators are the ultimate in isolation needed One half of the Opto Isolator OD is an LED The circuit connected to that side turns the light on and off The other half of the optoisolator is a phototransistor When the light is on the phototransistor absorbs the light and acts like a closed switch When the light is off the phototransistor acts like an open switch Because light is used instead of electricity and because the light can only go in one direction from LED to phototransistor they provide a very high level of reliable isolation The hardware MIDI interface is an example of a good opto isolator interface Embedded Systems High Voltage Circuits 67 Relays Relays can also be used to isolate because they act very similarly to transformers The current flow in one wire is controlled by a magnetic field generated by a second wire A relay controls whether electrons flow or not by allowing a small current to the input coil produc
113. icro amp Robot 877 http www scmstore com english robotic programmable microrobot877 asp robot kit with self programmable PIC Microcontroller You don t need a PIC programmer Programming the PIC16f628a with SDCC http burningsmell org pic 16f628 An occasionally updated list of examples demonstrating how to use the PIC s peripherals and interface with other devices with the free SDCC Embedded Systems PIC Microcontroller 77 pic compiler Embedded Systems 8051 Microcontroller The Intel 8051 microcontroller is one of the most popular general purpose microcontrollers in use today The success of the Intel 8051 spawned a number of clones which are collectively refered to as the MCS 51 family of microcontrollers which includes chips from vendors such as Atmel Philips Infineon and Texas Instruments About the 8051 The Intel 8051 is an 8 bit microcontroller which means that most available operations are limited to 8 bits There are 3 basic sizes of the 8051 Short Standard and Extended The Short and Standard chips are often available in DIP form but the Extended 8051 models often have a different form factor and are not drop in compatable All these things are called 8051 because they can all be programmed using 8051 assembly language and they all share certain features although the different models all have their own special features Some of the features that have made the 8051 popular are e 8 bit data bu
114. igure out exactly how much accuracy a particular application needs and make sure their fixed point routines maintain at least that much accuracy math routines Is there a better place in this wikibook for this discussion It doesn t even mention floating point Low end embedded microcontrollers typically don t even have integer multiply in their instruction set 1 So on low end CPUs you must use routines that synthesize basic math operators multiply divide square root etc from even simpler steps Practically all microprocessors have such routines posted on the internet by their manufacturer or other users Multiplication and Division Made Easy 2 by Robert Ashby Novel Methods of Integer Multiplication and Division BI i A Following the advice known as Make It Work Make It Right Make It Fast DI and Make It Work Make It Small Make It Fast 61 many people pick one or two number resolutions that are adequate for the largest kind of data efficient bit twiddling methods etc handled in a program and used that resolution for everything For desktop machines often 32 bit integers and 64 bit double precision floating point numbers are more than adequate For embedded systems often 24 bit integers and 24 bit fixed point numbers are more than adequate If the software fits in the microcontroller and is plenty fast enough it is a waste of valuable human time to try to optimize it further Alas sometimes the software
115. ime Embedded Systems Interrupts 51 Interrupt Vector Table The Interrupt Vector Table is a list of every interrupt service routine It is located at a fixed location in program memory Some processors expect the interrupt vector table to be a series of call instructions each one followed by the address of the ISR Other processors expect the interrupt vector table to hold just the ISR addresses alone You must make sure that every entry in the interrupt vector table is filled with the address of some actual ISR even if it means making most of them point to the do nothing and return from interrupt ISR further reading e Wikipedia interrupt vector e Embedded_Control_Systems_Design Operating_systems Interrupts Operating System Design Processes Interrupt Embedded Systems RTOS Implementation The chapters in this section will discuss some of the general concepts involved in writing your own Real Time Operating System Readers may be able to read and understand the material in these pages without prior knowledge in operating system design and implementation but a background knowledge in those subjects would certainly be helpful Memory Management An important point to remember is that some embedded systems are locked away and expected to run for years on end without being rebooted If we use conventional memory management schemes to control memory allocation we can end up with fragmented memory which can take valua
116. include tiny 8 pin SOIC microprocessors If you want a 32 bit processor some Philips ARM processors and Freescale Coldfire processors are now under 5 for one only comes in LQFP64 The only 32 bit processor currently being manufactured in a DIP package is the Parallax Propeller w Parallax Propeller 13 for one Many people and several commercial products run Linux on a XScale microprocessor or a Atmel AT91RM9200 ARM core without a heatsink or fan Linux has also been ported to the Atmel AVR32 AP7 family 4 only comes in a 208 pin VQFP Linux has also been ported to Freescale 68k ColdFire processors I don t think Linux has been ported to any of the other processors mentioned above Embedded Systems Particular Microprocessors 69 USB interface Main page Serial Programming USB Technical Manual FIXME very incomplete standard PC as host microcontroller as device There are a variety of ways to connect a microcontroller to a USB host e Some microcontrollers such as some 18x series PICmicro 24x94 series x 7 8 9 PSoC and some Philips ARM microcontrollers and some Atmel ARM microcontrollers and the Freescale MC9S08JS16 that have a built in Full Speed USB device interface e practically all microcontrollers have a UART You can add a USB adapter DI that interfaces between that UART and USB such as some based on the CP2102 chip 6 and some based on the FTDI chips 7 Most of these adapters are designed to hav
117. ing a magnetic field in which operates the switch For further reading e SCRs and triacs include several power transistors in a convenient package and often cost less than buying equivalent transistors seperately e Wikipedia relay e So called solid state relays Wikipedia SSR are a convenient combination of an opto isolator and some power transistors Some SSRs include a Wikipedia Zero cross circuit References 1 1 Occasionally someone does accidentally connect an integrated circuit to 120 V The integrated circuit immediately self destructs If you re lucky it cracks in half and lets off a small puff of smoke If you re unlucky it will still look like a good chip leading to hours of frustration trying to figure out why the system isn t working 68 Particular Microprocessor Families Embedded Systems Particular Microprocessors This module of Embedded Systems is a very brief review of the most popular microprocessor families used in embedded systems We will go into more detail in the next few modules Each one of these microprocessor families has an entire module dedicated to that family of processors The microprocessor families we will discuss are 8051 Microcontroller 8 bit Atmel AVR 8 bit Atmel AVR32 Microchip PIC Microcontroller this family includes the code compatible Parallax SX chips 8 bit Microchip dsPIC microcontroller 16 bit review Circuit Cellar Are You Up for 16 Bits A look a
118. ion G1 601 Second Generation G2 603 603e 604 Third Generation G3 750 750CX 750CX3 750FX 750GX Fourth Generation G4 7400 7450 further reading w Freescale 68HC12 w Freescale ColdFire w PowerPC The 68HC12 discussion forum at EmbeddedRelated l is still pretty active apparently because 68HC12 dev boards such as those from EVBplus Bh are typically lower cost than dev boards with most other microcontrollers EE Compendium resources for using Freescale s HC12 family 4 w Motorola 68000 family References 1 http www uclinux org ports coldfire 2 http www embeddedrelated com groups 68hc12 1 php 3 http evbplus com 4 http ee cleversoul com hc12 html Embedded Systems Atmel AVR 82 Embedded Systems Atmel AVR The Atmel AVR is a family of 8 bit RISC microcontrollers produced by Atmel The AVR architecture was conceived by two students at the Norwegian Institute of Technology NTH and further refined and developed at Atmel Norway the Atmel daughter company founded by the two chip architects Memory The memory of the Atmel AVR processors is a Modified Harvard architecture in which the program and data memory are on separate buses to allow faster access and increased capacity The AVR uses internal memory for data and program storage and so does not require any external memory The four types of memories in a Atmel AVR are Data memory registers I O registers and SRAM e Pro
119. ips are used in situations where power and speed are a must but a conventional microprocessor board think a computer motherboard is too large or expensive High end chips will have a number of fancy features more available memory and a larger addressable memory range High end chips can come in 8 bit 16 bit 32 bit or even 64 bit and can cost anywhere between 10 to 100 each Acronyms This will be a functional list of most of the acronyms used in this book ADC ADC stands for Analog to Digital Converter ADCs are also written as A D or A2D in other literature DAC The exact opposite of an ADC a DAC stands for Digital to Analog Converter May also be called D A or D2A RAM Random Access Memory RAM is the memory that a microcontroller uses to store information when the power is on When the power goes off RAM is erased ROM Read Only Memory ROM is memory that can be read but it cant be written or erased ROM is cheaper than RAM and it doesnt lose its information when the power is turned off OTP OTP means One Time Programmable OTP chips can be programmed once and only once usually by a physical process or burning extra wires inside the chip If an OTP chip is programmed incorrectly it can t be fixed so be careful with them downloading no In this book the terms burning flashing installing or downloading all mean the same thing the automated process of putting the executable image into th
120. irm and add them to the list ummarizin 1st eginners checklist for icrocontrollers A mer H endix ther S izing PICList Begi hecklist for PIC Mi llers l PIC Elmer 160 Appendix A Oth PICs 2003 BI and Wouter van Ooijen 6 PIC Select a chip and buy one 7 Many people recommend the following PICs as a good choice for the first PIC for a hobbyist e PIC18F4620 has 13 analog inputs Wouter van Ooijen recommends that hobbyists use the largest and most capable chip available 6 and this is it as of 2006 01 9 e PIC16F877A the largest chip of the 16F87x family has 8 analog inputs recommended by Wouter 2 AmQRP PICList 8 e PIC16F88 has 7 analog inputs recommended by AmQRP SparkFun 8 5 e PIC16F628 Cheaper than the PIC16F84A with a built in 4MHz clock and a UART but lacks any analog inputs recommended by Wouter 3 AmQRP 4 Embedded Systems PIC Microcontroller Of the many new parts Microchip has introduced since 2003 are any of them significantly better for hobbyists in some way than these chips There are several different families More selection tips The F Suffix implies that the chip has reprogrammable Flash memory PIC1OF in super tiny 6 pin packages RILICLIZT in tiny g ia packages PIC14F PIC16F PIC ALE dsPIC30F i ROM memory A few of these chips can be erased with an 1 Each family has one full member
121. is only one master and one slave the slave s SS line could be tied low and the master would not need to drive it If there are two or more slaves then the master must use a separate slave select signal to each slave The downside of this approach is that the master can only address as many slaves as it has extra outputs without the use of a separate decoder Embedded Systems Atmel AVR 88 Hardware Implementation The larger AVR microcontrollers have built in SPI transceivers from the ATMEGA8 upwards The serial clock is derived from the processor clock with several divisors available The data length is always 8 bits The clock polarity and phase may be configured leading to four possible combinations of when the data is clocked in and out of the chip This interface is very popular and is widely available on a variety of other processors and peripherals The pins used for the SPI bus are also used as a way of programming the chip via ISP In System Programming Except on the megal 28 Universal Serial Interface Some AVRs particularly in the Tiny family provide a Universal Serial Interface USI instead of an SPI The USI is capable of operating as an SPI but also as an I2C controller and with a little extra effort a USART The bit length of the transfer is configurable as is the clock driver The clock can be driven by software by the timer 0 overflow or by an external source Software Implementation SPI can be impl
122. is useful to point out some features of popular programming languages that are especially important for embedded systems ANSI C programming language Many microprocessors and microcontrollers can be programmed in C and a number of C cross compilers exist for that purpose C is perhaps the most frequently used language for new embedded system development The const and the volatile keywords rarely used in desktop app programming become very important in Embedded Systems C Programming Assembly language There are many different microcontroller families each with their own assembly language with its own unique quirks This book will cover some basics of assembly language common to most microcontrollers Unlike desktop app programming embedded system programs generally must set up an interrupt vector table This book will discuss at least briefly some techniques for multi language programming specifically C and assembly There are some instances where microcontrollers are better programmed in a different language BASIC and Forth come to mind Some controllers are even programmed in their own proprietary languages PIC Basic and Dynamic C U for instance Some extremely well known languages such as C and Java are rarely used in embedded systems because C and Java compilers are simply unavailable for popular microcontrollers However this book may occasionally describe how to implement C and Java features in an environment th
123. least one data structure that is written by one task and read by another task With a preemptive scheduler it is all too easy to write software that seems to work fine most of the time but occasionally the writer will be interrupted right in the middle of updating the data structure the RTOS switches to the reader task and then the reader chokes on the inconsistent data We need some way of arranging things so that a writer s modification appears atomic a reader always sees only the consistent old version or the consistent new version never some partially modified inconsistent state There are a variety of ways to avoid this problem including Design the data structure so that the writer can update it in such a way that it is always in a consistent state This requires hardware that supports atomic primitives powerful enough to update the data structure from one consistent state to the next consistent state in one atomic operation Wikipedia lock free and wait free algorithms Have the writer turn off the task scheduler while it is updating the data structure Then the only time the reader could possibly see the data structure the data structure is in a consistent state Have the writer turn off all interrupts including the timer interrupt that kicks off the task scheduler while it is updating the data structure Then the only time the reader could possibly see the data structure the data structure is in a consistent state B
124. led then resumed by re selecting the previous clock input PWM Many of the AVRs include a compare register for at least one of the timers The compare register can be used to trigger an interrupt and or toggle an output pin i e OCIA for Timer 1 when the timer value matches the value in the compare register This may be done separately from the overflow interrupt enabling the use of pulse width modulation PWM Some AVRs also include options for phase correct PWM or phase and frequency correct PWM The Clear Timer on Compare CTC mode allows for the timer to be cleared when it matches a value in the compare register before the timer overflows Clearing the timer prior to overflow manipulates the timer resolution allowing for greater control of the output frequency of a compare match It can also simplify the counting of an external event The ATtiny26 is unique in its inclusion of a 64MHz high speed PWM mode The 64MHz clock is generated from a PLL and is independent of and asynchronous to the processor clock Some AVRs also include complementary outputs suitable for controlling some motors A dead time generator DTG inserts a delay between one signal falling and the other signal rising so that both signals are never high at the same time The high end AT90PWM series allows the dead time to be programmed as a number of system clock cycles while other AVRs with this feature simply use clock cycle for the dead time E
125. lement this with Flash While your program is running it is impossible to change any of the data in program memory But at least when the power comes back on it s all still there RAM which holds the variables and stack Initial values for variables are copied from ROM Forgets everything when power is lost e EEPROM Used kind of like the hard drive in a personal computer to store settings that might change occasionally and that need to be remembered next time it starts up e I O This is really the entire point of a microcontroller Many popular microcontrollers including the 8051 the Atmel AVR the Microchip PIC the Cypress PSoC have a Harvard architecture meaning that programs can only execute out of ROM You can copy bytes from ROM or elsewhere into RAM but it s physically impossible to jump or call such code in RAM This is exactly the opposite of the situation on desktop computers where the code you write cannot be executed until after it is copied into RAM A few popular microcontrollers such as the 68HC11 and 68HC12 and have a unified address space a von Neumann architecture You can jump or call code anywhere although jumping to an address in I O space is almost certainly not what you really wanted to do paging and banking Often software applications grow and grow Ancient processors such as the 8085 used on the Mars rover Sojourner with 16 bit address registers can directly access a maximum of 6
126. llers such as the Microchip PIC32 and Freescale 68k ColdFire processors Embedded Systems C Programming 31 The people who write compilers have also with more difficulty ported GCC to target the Texas Instruments MSP430 16 bit MCUs the Microchip PIC24 and dsPIC 16 bit Microcontrollers the 8 bit Atmel AVR microcontrollers the 8 bit Freescale 68HC11 microcontrollers Other microcontrollers are very different from a 32 bit Princeton architecture CPU Many compiler writers have decided it would be better to develop an independent C compiler rather than try to force the round peg of GCC into the square hole of 8 bit Harvard architecture microcontroller targets SDCC Small Device C Compiler for the Intel 8051 Maxim 80DS390 Zilog Z80 Motorola 68HC08 Microchip PIC16 Microchip PIC18 http sdcc sourceforge net There are some highly respected companies that sell commercial C compilers You can find such a commercial C compiler for practically every microcontroller including the above listed microcontrollers Popular microcontrollers not already listed i e microcontrollers for which the only known C compiler is a commercial C compiler include the Cypress M8C MCUs Microchip PIC10 and Microchip PIC12 MCUs etc Further reading e Wikipedia bit field e Wiki BitField e C Programming Variables and C Programming Variables also discuss const and volatile variables e ARM technical support FAQ Use of const and volatile
127. llips IZC bus interface I C consists of two wires known as SDA serial data and SCL serial clock which use open drain drivers and therefore require pull ups to a logic 1 state I2C uses a common ground so all devices on the bus should be at the same ground potential to avoid ground loops TWI uses 7 bit addressing which allows for multiple devices to connect to the bus Many TWI devices have at least the top four bits of the address hard coded and the remaining bits configurable by some means such as connecting dedicated address pins to power or ground this often allows for only 2 8 model X devices on the bus The AVR s TWI hardware can act as Master or Slave and can meet the 400Kbit s spec Serial Peripheral Interface SPI SPI the Serial Peripheral Interface Bus is a master slave synchronous serial protocol This means that there is a clock line which determines where the pulses are to be sampled and that one of the parties is always in charge of initiating communication It uses at least three lines which are called MISO Master In Slave Out MOSI Master Out Slave In SCK Serial Clock Conceptually SPI is a bidirectional shift register as bits are shifted out on either MISO or MOSI bits are shifted in on the other line The master always controls the clock An SPI slave has a Slave Select SS signal which signals to the slave that it should respond to messages from the master SS is almost always active low If there
128. loader for programming whenever possible Bootloaders are covered in detail in chapter Bootloaders and Bootsectors Power Supply The most important part of any electronic circuit is the power supply The PIC programmer requires a 5 volt and a 13 volt regulated power supply The need for two power supplies is due to the different programming algorithms e High Power Programming Mode In this mode we enter the programming mode of the PIC by driving the RB7 Data and RB6 CLOCK pins of the PIC low while driving the MCLR pin from 0 to VCC 13v e Low Power Programming Mode This alogrithm requires only 5v for the programming operation In this algorithm we drive RB3 PGM from VDD to GND to enter the progamming mode and then set MCLR to VDD 5v This is already taken care of inside the PIC burner hardware If you are curious as to how this is done you might want to look at the various PIC burner hardware schematics online 14 Oscillator Circuits The PIC microcontrollers all have built in RC oscillator circuits available although they are slow and have high granularity External oscillator circuits may be applied as well up to a maximum frequency of 20MHz PIC instructions require 4 clock cycles for each machine instruction cycle and therefore can run at a maximum effective rate of SMHz However certain PICs have a PLL phase locked loop multiplier built in The user can enable the Times 4 multiplier thus yielding a virt
129. lti bit error detection and also correction of single bit errors CRC checks are usually used at the end of data blocks These are carefully designed to give a very high probability of detecting corruption of the data block no matter how many bit errors are in the block Embedded Systems Memory 22 Embedded Systems Memory On an Embedded System memory is at a premium Some chips particularly embedded VLSI chips and low end microprocessors may only have a small amount of RAM on board built directly into the chip and therefore their memory is not expandable Other embedded systems have a certain amount of memory and have no means to expand In addition to RAM some embedded systems have some non volatile memory in the form of miniature magnetic disks FLASH memory expansions or even various 3rd party memory card expansions Keep in mind however that a memory upgrade on an embedded system may cost more then the entire system itself An embedded systems programmer therefore needs to be very much aware of the memory available and the memory needed to complete a task Memory is frequently broken up into a number of different regions that are set aside for particular purposes addressable areas There are typically 4 distinct addressable areas each one implemented with a different technology e program memory which holds the programs you write often called ROM although most developers prefer to use chips that actually imp
130. m on the railway platforms In technical terminology Real Time Systems can be classified as Hard Real Time Systems systems with severe constraints on the timeliness of the response Soft Real Time Systems systems which tolerate small variations in response times Hybrid Real Time Systems systems which exhibit both hard and soft constraints on its performance Embedded Systems Embedded Systems Introduction What are Some Downfalls of Embedded Computers Embedded computers may be economical but they are often prone to some very specific problems A PC computer may ship with a glitch in the software and once discovered a software patch can often be shipped out to fix the problem An embedded system however is frequently programmed once and the software cannot be patched Even if it is possible to patch faulty software on an embedded system the process is frequently far too complicated for the user Another problem with embedded computers is that they are often installed in systems for which unreliability is not an option For instance the computer controlling the brakes in your car cannot be allowed to fail under any condition The targeting computer in a missile is not allowed to fail and accidentally target friendly units As such many of the programming techniques used when throwing together production software cannot be used in embedded systems Reliability must be guaranteed before the chip leaves the factory This
131. mbedded Systems Atmel AVR 86 Output Compare Modulator An Output Compare Modulator OCM which allows generating a signal that is modulated with a carrier frequency OCM requires two timers one for the carrier frequency and the second for the signal to be modulated OCM is available on some of the Mega series Serial Communication AVR microcontrollers are in general capable of supporting a plethora of serial communication protocols and serial bus standards The exact types of serial communication support varies between the different members of the AVR microcontroller family On top of support in hardware there is also often the option to implement a particular serial communication mechanism entirely in software Typically this is used in case a particular AVR controller does not support some serial communication mechanism in hardware the particular hardware is already in use e g when two RS232 interfaces are needed but only one is supported in hardware or the chip s hardware can t be used because it shares pins with other chip functions and such a function is already in used for the particular hardware The latter often happens with the low pincount AVRs in DIP packages Finally there is also the possibility to use additional logic to implement a serial communication function For example most AVRs don t support the USB bus some later ones do so however When using an AVR which doesn t support USB directly a circuit designer
132. mmercial C compiler for the AVR C GCC also has support for C on the AVR Certain features of C are unsuitable for use on a smaller micro like the AVR due to the amount of memory required to implement them these include exceptions and templates However when using a suitable subset of C the resultant code is of comparable size to its C language equivalent One notable use of C on the AVR is the Arduino 53 Embedded Systems Atmel AVR 95 Java NanoVM el Java virtual machine written in C for Atmel AVR microcontrollers with at least 8k flash MCU Java source l Java source to C source translator which allows to write MCU programs in Java Pascal 56 AVRco development environment IDE also includes simulator and HLL debugger with JTAG ICE Includes numerous library functions MikroPascal P Includes AVR specific libraries plus help and examples Free version is limited to 4KB Embedded Pascal 15 AVR assembly in pascal code IDE running under Windows 95 98 and NT Language extensions provide for mixing Forth ByteForth wiki 59 purchase 60 _ Includes dis assembler simulator ISP programmer and supports almost any AVR to date Many library functions and example programs Comes with complete Dutch language manual 61 There is however an English language version with crash course included in the free but complete 2 kByte demo version 1971 ByteForth runs under DOS or any sys
133. mming the same way we do C programmer Python programmer pair programmer etc refer to human beings that do the programming for further reading We mentioned op amps A long time ago analog computers were once built entirely out of such operational amplifiers resistors and capacitors Today most embedded systems have few if any op amps they are still useful for some sensor signal amplifiers anti aliasing filters before the ADC anti aliasing filters after the DAC and a few op amps embedded in power supply circuits 11 Microprocessor Basics Embedded Systems Microprocessor Introduction Effectively programming an embedded system and implementing it reliably requires the engineer to know many of the details of the system architecture Section 1 of the Embedded Systems book will cover some of the basics of microprocessor architecture This information might not apply to all embedded computers and much of it may apply to computers in general This book can only cover some basic concepts because the actual embedded computers available on the market are changing every day and it is the engineer s responsibility to find out what capabilities and limitations their particular systems have As people continue to pack more and more transistors onto a single chip more and more of the stuff that was once peripheral logic has been integrated on the same chip as the CPU A microcontroller includes most or all t
134. n some cases the books written by students one year are used to teach students in the same class next year Books written can also be used in classes around the world by students who might not be able to afford traditional textbooks Happy Reading We at Wikibooks have put a lot of effort into these books and we hope that you enjoy reading and learning from them We want you to keep in mind that what you are holding is not a finished product but instead a work in progress These books are never finished in the traditional sense but they are ever changing and evolving to meet the needs of readers and learners everywhere Despite this constant change we feel our books can be reliable and high quality learning tools at a great price and we hope you agree Never hesitate to stop in at Wikibooks and make some edits of your own We hope to see you there one day Happy reading Embedded Systems Embedded Systems Introduction Embedded Technology is now in its prime and the wealth of knowledge available is mindblowing However most embedded systems engineers have a common complaint There are no comprehensive resources available over the internet which deal with the various design and implementation issues of this technology Intellectual property regulations of many corporations are partly to blame for this and also the tendency to keep technical know how within a restricted group of researchers Before embarking on the rest of this book it is
135. ng and restoring virtual registers A and X also have to be saved and restored In case you are using a program with LMM enabled then the paging mode has to be restored to native paging before calling the C function and also the paging registers have to be saved and restored This is all stuff that the compiler would have handled automatically for you if you had marked that C function with pragma interrupt_handler and had your assembly language LJUMP to that C function object code size Q How can I optimize object code size generated by the PSoC C Compiler Al On MCUs with more than 256 bytes of SRAM do not use LMM if at all possible The overhead from page management is significant A2 Do not use 32 bit variables unless absolutely necessary A3 If possible stick to 8 bit variables of type BYTE and only use 16 bit WORDs if necessary A4 If readability does not overly suffer try to only use global variables AS Stay away from pointers to structures and arrays of structures This particularly means avoiding the passing of structure pointers to functions A6 Consider limiting the number of function parameters and making common data global that can be accessed directly by all functions Embedded Systems Cypress PSoC Microcontroller 105 JTAG Q Does the PSoC do JTAG A No But much of the JTAG functionality can be done other ways PSoC boundary scan using m8cbscan PI Q Is there a way that the program runni
136. ng in the PSoC can modify the flash in that same PSoC instead of the normal process of burning the program into the PSoC then never modifying the program Al use the EEPROM module A2 some tips in the Flash Write Routine 10 thread A3 Forth PSoC Forth gives an example of a program running in the PSoC that modifies the flash in that same PSoC further reading e Forth PSoC Forth gives an example of PSoC assembly language e Homemade MIDI turntable HH by casainho very briefly shows a schematic and a photo of a PCB with a CY7C63723 microcontroller on it inside an optical mouse although the final project ends up using an Atmel AVR instead References 1 http www psocdeveloper com docs articles tips_on_using_the_psoc software_checklist 2 http www psocdeveloper com articles tips on using the psoc introduction html 3 http www cypress com portal server pt space Community Page amp control SetCommunity amp CommunityID 285 amp PageID 552 amp shortlink DA_550707 4 http www cypress com design AN2010 5 http www psocdeveloper com forums viewtopic php t 2075 6 http www cypress com portal server pt space Community Page amp control SetCommunity amp CommunityID 285 amp PageID 552 amp shortlink DA_242412 7 http www psocdeveloper com forums viewtopic php t 1744 8 http www psocdeveloper com forums viewtopic php t 1685 9 http www psocdeveloper com forums viewtopic php t
137. nguage so that developers can program assemble and test from the comfort of a PC 8086 and 80186 8086 and 80186 processors are available in heavily integrated packages They are usually available in DIP form and are relatively cheap 10 or 15 range These processors might not be as good as an 8051 in an embedded environment but the ease of programming and the familiarity that many programmers will feel for these chips can more then make up for the cost 1386 Embedded Processors The i386 microprocessor is a modified form of the Intel 80386 microprocessor with a few notable differences an integrated FPU originally wasnt standard until th 80486 and a variety of different small form factors One of the major benefits of an i386 microprocessor is that it can be programmed easily using most standard C compilers and x86 assembly language In fact many times no additional settings need to be changed in the compiler except maybe to not link to the standard libraries on the host system 1386 processors are 32 bit processors and are frequently very economical choices when a 32bit processor is required Also i386 processors frequently have very low power consumption and generate very little heat Remember Intel has been working on this architecture and the general design of this chip continuously for many years now X Scale Embedded Processors The X Scale processor is an ARM based device designed for embedded systems requiring high
138. nt to know what time it is for example when a digital camera time stamps a picture it just took the system can read it from the real time clock Other ways of figuring out the time require more energy When a system needs to do something periodically for example measure the outside temperature every 10 seconds and transmit it wirelessly to an indoor display the system can turn off power to everything except the real time clock and then wait for the clock to wake it up You can see some of the 8086 compatible timer chips like 8253 54 also they are the same have three independent timers internally but 8254 can work with higher frequencies and is used to generate interrupt like Memory refresh interrupt Time of day TOD interrupt and the last one is used to generate the speaker frequencies Practically all microcontrollers sold today include integrated timer peripherals on the same chip Most embedded systems either a have no external timer chip at all using only the internal timers b an external real time clock or c attempt to be PC compatible with a southbridge chip that emulates both the 8253 compatible timers and the real time clock Embedded Systems Programmable Controllers 18 Interrupt Controllers The original 8086 processor had only a single pin used for signaling an interrupt so a programmable interrupt controller would handle most of the messy details of calling the interrupt Also a programmable i
139. nterrupt controller could be used to monitor an input port for instance and triggering an interrupt routine when input is received Direct Memory Access Since memory read write operations take longer than other operations for the microprocessor one should avoid moving large blocks of memory Luckily the original 8086 came with a programmable direct memory access controller DMA for use in automatically copying and moving segments of memory DMAs could also be used for implementing memory mapped I O by being programmed to automatically move memory data to and from an output port DMA memory copies can also greatly enhance system performance by allowing the CPU to execute code in parallel with a DMA controller automatically performing the memory copy Peripheral Interface Controllers Peripheral interface controllers take a number of different forms Each different type of port has a different controller that the microprocessor will interface with to send the output on that port For instance there are controllers for parallel ports and more modern USB ports These controllers are used for controlling settings on output such as timing and setting different modes on the output input port Further reading e x86 Assembly Programmable Interrupt Timer Embedded Systems Floating Point Unit 19 Embedded Systems Floating Point Unit Floating point numbers are Like all information floating point numbers are represented by
140. oc at nearly the same time the first one will enter the critical section while the second one will wait or be blocked at the EnterCS routine When the first malloc is finished the second malloc s EnterCS function will return and the function will continue To allow other processes looking at other data structures to continue even though this data structure has been locked EnterCS is typically redefined something like Embedded Systems Locks and Critical Sections non blocking attempt to enter critical section ime WeylnmeeccS Carr suecr imas int success 0 DisableScheduler if this gt lock 0 this gt lock 1 mark structure as locked success 1 EnableScheduler return success blocking attempt to enter critical section Enbereo lt CRU SEGE waite ji ime Sieceass Op do success TryEnterCS this gt lock if success Yield tell scheduler to run some other task for a while while success return release lock JE G9 KERES E Guliaer feline sam ASSERT 1 this gt lock this gt lock 0p return The value to creating a Critical Section object and using that to prevent preemption of sensitive areas is that this scheme does not slow down the system the way the first scheme does by disabling the scheduler and preventing other tasks from executing Some OSes such as Dragonfly BSD implement
141. oc6293 pdf 6 http claymore engineer gvsu edu 7Esteriana Software Sam_I_Am index html 7 http www siwawi arubi uni kl de avr_projects arm_projects index_at91 html Embedded Systems Cypress PSoC Microcontroller The Cypress PSoC microcontroller like all microprocessors has its own quirks When you can t figure something out and this FAQ doesn t help the forums at e http www psocdeveloper com under Forums e http www psoczone com under Forums e http www cypress com under Technical Support then Discussion Boards are a good place to start User Documentation e Embedded Systems Cypress PSoC Microcontroller Application Notes e Embedded Systems Cypress PSoC Microcontroller User Modules Cypress PSoC FAQ If the answers I ve sketched in are incorrect please correct them Which PSoC to use Some people recommend that hobbyists use the largest and most capable chip available in a DIP package as of 2008 09 that is the CY8C29466 28 DIP CY8C29466 28 DIP I suspect this is the same chip as in the CY8C29466 28 SSOP or 28 SOIC CY8C29566 44 TQFP CY8C29666 48 SSOP or 48 QFN CY8C29866 100TQFP These parts have more Flash 32 KByte Flash and more RAM 2 KByte RAM and more digital blocks and analog blocks than any other PSoC available Embedded Systems Cypress PSoC Microcontroller 101 CY8C27143 8 DIP more Flash 16 KByte Flash and more digital blocks and analog blocks and just as much
142. of synchronization objects are held in a preemptive MT system in such a way that no process can move forward Let us take a look at an example Let s say that we have 2 threads T1 and T2 We also have 2 mutexes M1 and M2 1 T1 asks for and acquires mutex M1 2 T2 acquires M2 3 T1 asks for M2 and the system transfers control to T2 until T2 releases M2 Embedded Systems Threading and Synchronization 49 4 T2 asks for MI and the system is in deadlock neither thread can continue until the other releases it s mutex This is a very difficult problem to diagnose and an even more difficult problem to fix This chapter will provide some general guide lines for preventing deadlocks Watchdog Timer In an embedded environment far away from the lab and far away from the programmers engineers and technicians all sorts of things can go wrong and the embedded system needs to be able to fix itself Remember once you close the box and shrink wrap your product it s hard to get back in there and fix your mistakes In a typical computer systems cosmic rays flip a bit of RAM about once a monthl ation needed Ig that happens to the wrong bit the program can hang stuck in a short infinite loop Turning the power off then on again gets it unstuck But how do you jiggle the power switch when you are on Earth and your embedded system is near Neptune Or you are in Paris and your embedded system is in Antarctica One of th
143. ompilers even have support for compiling C for an 8051 Program memory in the 8051 is read only while the data memory is considered to be read write accessible When stored on EEPROM or Flash the program memory can be rewritten when the microcontroller is in the special programmer circuit Program Start Address The 8051 starts executing program instructions from address 0x00 in the program Direct Memory The 8051 has 256 bytes of internal addressable RAM although only the first 128 bytes are available for general use by the programmer The first 128 bytes of RAM from 0x00 to 0x7F are called the Direct Memory and can be used to store data Special Function Register The Special Function Register SFR is the upper area of addressable memory from address 0x80 to OxFF This area of memory cannot be used for data or program storage but is instead a series of memory mapped ports and registers All port input and output can therefore be performed by memory mov operations on specified addresses in the SFR Also different status registers are mapped into the SFR for use in checking the status of the 8051 and changing some operational parameters of the 8051 General Purpose Registers The 8051 has 4 selectable banks of 8 addressable 8 bit registers RO to R7 This means that there are essentially 32 available general purpose registers although only 8 one bank can be directly accessed at a time To access the other banks we nee
144. or the current task has run long enough the timer hardware interrupts it and the timer interrupt routine calls the scheduler The scheduler must save the current status of the current task save the contents of all registers to a specified location it must look through the list of tasks to find the highest priority task in the Ready state and then must switch control back to that task by restoring it s register values from memory The scheduler should first check to ensure that it is enabled If the scheduler is disabled it shouldn t preempt the current thread This can be accomplished by checking a current global flag value Some functions will want to disable the scheduler so this flag should be accessable by some accessor method An alternate method to maintaining a global flag is simply to say that any function that wants to disable the scheduler can simply disable the timer This way the scheduler never gets called and never has to check any flags The scheduler is generally disabled inside a critical section where we do not want to OS to preempt the current thread Otherwise the scheduler should remain active Further reading Design Patterns for Real Time Systems Resource Patterns H by Bruce Powel Douglass 2002 e Microchip RTOS app notes 2 include Microchip AN585 A Real Time Operating System for PIC16 17 which describes writing your own RTOS e Greg Hawley 15 notes that buying your RTOS in most cases is t
145. or Architectures Source http en wikibooks org w index php oldid 1394714 Contributors DavidCary Jamin1001 Rhartness Whiteknight 1 anonymous edits Embedded Systems Programmable Controllers Source http en wikibooks org w index php oldid 1603920 Contributors DavidCary Herbythyme Jamin1001 Panic2k4 Uhussein Whiteknight 3 anonymous edits Embedded Systems Floating Point Unit Source http en wikibooks org w index php oldid 1826685 Contributors DavidCary Jedikaiti Whiteknight 2 anonymous edits Embedded Systems Parity Source http en wikibooks org w index php oldid 1826714 Contributors DavidCary Dragontamer Jedikaiti Whiteknight 3 anonymous edits Embedded Systems Memory Source http en wikibooks org w index php oldid 1826719 Contributors DavidCary Jedikaiti Webaware Whiteknight 5 anonymous edits Embedded Systems Memory Units Source http en wikibooks org w index php oldid 1286869 Contributors Whiteknight 3 anonymous edits Embedded Systems C Programming Source http en wikibooks org w index php oldid 1800294 Contributors Adrignola DavidCary Eigendude Herbythyme Jomegat Mikiemike Panic2k4 Special Ed Thenub314 Tigerraj32 Whiteknight Zohair ahmad 9 anonymous edits Embedded Systems Assembly Language Source http en wikibooks org w index php oldid 1792729 Contributors Adrignola Darklama DavidCary NipplesMeCool Whiteknight 4 anonymous edits Embedded Systems Mixed C and Assembly Programming Source http en
146. ort Input register was read only on earlier AVRs and was used to read the value on the port pin regardless of the data direction Newer AVRs allow a write to the PIN register to toggle the corresponding PORT bit which saves a few processor cycles when bit banging an interface Timer Counters All AVRs have at least one 8 bit timer counter For brevity a timer counter is usually referred to as simply a timer Some of the Tiny series have only one 8 bit timer At the high end of the Mega series there are chips with as many as six timers two 8 bit and four 16 bit A timer can be clocked directly by the system clock by a divided down system clock or by an external input rising or falling edge Some AVRs also include an option to use an external crystal asynchronous to the system clock which can be used for maintaining a real time clock with a 32 768KHz crystal The basic operation of a timer is to count up to FF or FFFF i 6 roll over to zero and set an overflow bit which may cause an interrupt if enabled The interrupt routine reloads the timer with the desired value in addition to any other processing required The value of a timer can be read back at any time even while it is running There is a specific sequence documented in the datasheets to read back a 16 bit timer so that a consistent result is returned since the AVR can only move 8 bits at a time A timer can be halted temporarily by changing its clock input to disab
147. ose State University 89 Microprocessors and Interfacing UNSW 90 Embedded Systems Atmel AVR 97 AVR Projects Siwawi AVR projects 91 e MMC SD memory cards for Atmel AVR 92 References 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 45 46 47 48 49 50 51 52 http www atmel com dyn resources prod_documents doc25 13 pdf http www atmel com dyn products devices asp family_id 607 1761 http www obdev at products avrusb index html http www xs4all nl dicks avr usbtiny index htm http www atmel com dyn products param_table asp family_id 607 amp OrderBy part_no amp Direction ASC http www atmel com dyn resources prod_documents doc2521 pdf http ecrostech com AtmelAvr Butterfly index htm http www arduino cc http hubbard engr scu edu embedded avr boards index html http awce com avrhome htm http www futurlec com ATDevBoard shtml http www olimex com http olimex com dev avrprojects html http savannah nongnu org projects avrdude http www atmel com products secureavr overview asp http www atmel com dyn products tools_card asp tool_id 2737 http www atmel com dyn products tools_card asp tool_id 3353 http www atmel com dyn products tools_card asp tool_id 389 1 http www siwawi arubi uni kl de avr_projects evertool http aquaticus info jtag http atmel com dyn corporate view_de
148. p www embedded com story OEG20010418S0044 http kerneltrap org node 5291 http embeddedtux org 63 Interfacing Embedded Systems Interfacing Basics Having our embedded system with a fancy operating system is all well and good However embedded computers are worthless if they can t interface with the outside world The chapters in this section will talk about some of the considerations involved with interfacing embedded systems Pins and Ports Many embedded systems will provide a series of output pins for transmitting data to the outside world These pins are arranged into groups called ports Ports will frequently but not always consist of a power of 2 number of pins For instance common ports might be 4 pins 8 pins 16 pins etc It is rare to see ports with fewer then 4 pins because in that case they probably aren t called ports anymore Current and Power When working with a particular microcontroller it is very important to read the datasheet to find out how much current different pins can handle When talking about current datasheets will usually mention 2 words Sink and Source Sink The sink value is the amount of current that can flow into the pin and therefore into the microcontroller safely Source The source current is the amount of current that can be pulled out of the pin safely If you exceed the sink current or the source current on a given pin you could fry the entire processor whi
149. p www programmersheaven com articles pathak article 1 htm 11 http www thefreecountry com compilers cpp microcontrollers pda shtml Embedded Systems Assembly Language 32 Embedded Systems Assembly Language This book will demonstrate techniques for programming embedded systems in assembly language x86 Assembly Review Main page X86 Assembly The x86 microprocessor has at least 4 general purpose registers AX BX CX and DX AX is the fast accumulator 4 segment registers CS code section DS data section ES extra section SS stack section 5 pointer registers SI source index DI destination index IP instruction pointer SP stack pointer BP base pointer 1 flag register Flags 7 flags like Zero flag Carry flag Unlike memory mapped processors the x86 has special I O instructions inp outp intended to talk to I O hardware ARM assembly review Main page Embedded Systems ARM Microprocessors At any one time 17 registers can be accessed RO to R14 which have identical hardware R15 and the status register CPSR To reduce latency on interrupt handling these registers and the SPSR saved program status register are shadowed during interrupts Including those shadows the typical ARM processor has a total of 37 registers The standard C calling convention for ARM lgiranion needed i e R15 PC program counter e R14 LR link register holds return address for most recent subroutine
150. p Architecture 41 Embedded Systems Super Loop Architecture When programming an embedded system it is important to meet the time deadlines of the system and to perform all the tasks of the system in a reasonable amount of time but also in a good order This page will talk about a common program architecture called the Super Loop Architecture that is very useful in meeting these requirements Definition A super loop is a program structure comprised of an infinite loop with all the tasks of the system contained in that loop Here is a general pseudocode for a superloop implementation Function Main_Function Tairere izere ron Do_Forever Check_Status DORCEL OE Output_Response We perform the initialization routines before we enter the super loop because we only want to initialize the system once Once the infinite loop begins we don t want to reset the values because we need to maintain persistent state in the embedded system The loop is in fact a variant of the classic batch processing control flow Read input calculate some values write out values Do it until you run out of input data cards So embedded systems software is not the only type of software which uses this kind of architecture For example computer games often use a similar loop There the loop is called tight main game loop Power Save Super Loop Let s say we have an embedded system which has an average loop time of Ims
151. ple requires only 1 function Cons If dealing with IO takes a long time you can starve other things This is especially dangerous if your handler masks interrupts which can cause you to miss hardware interrupts from real time hardware If your handler takes so long more input is ready before you handle existing input data can be lost This technique is great so long as dealing with the IO is a short process such as when you just need to set up DMA If its a long process use multithreaded polling or interrupts with threads Interrupts and threads We discuss this technique in more detail in Embedded Systems Interrupts In this technique you use an interrupt to detect when IO is ready Instead of dealing with the IO directly the interrupt signals a thread that IO is ready and lets that thread deal with it Signalling the thread is usually done via semaphore the semaphore is initialized to the taken state The IO thread tries to take the semaphore which fails and the OS puts it to sleep When IO is ready the interrupt is fired and releases the semaphore The thread then wakes up and handles the IO before trying to take the semaphore and being put back to sleep The routine the interrupt vector points at is the first level interrupt handler The thread that the OS later wakes up to handle the rest of the work is the second level interrupt handler Pros e minimum latency instead of all other interrupts being disabled until
152. processor cannot handle anything but the most simple tasks What if we need to manipulate an 8 bit number on this little microprocessor for instance what if we want to make a digital clock with it the 4 bit microprocessor is going to need to handle numbers up to and including 59 the number of minutes displayed before the next hour This is going to require more then the 4 bits alotted in fact it is going to require at least 6 bits of space What we need to do then is come up with a way to treat 2 separate small registers as if they are a single large register This chapter will talk about that subject a little bit For further reading e Assembly Language AVR Assembler I References 1 http avr asm tripod com Embedded Systems Mixed C and Assembly Programming C and Assembly Many programmers are more comfortable writing in C and for good reason C is a mid level language in comparison to Assembly which is a low level language and spares the programmers some of the details of the actual implementation However there are some low level tasks that either can be better implemented in assembly or can only be implemented in assembly language Also it is frequently useful for the programmer to look at the assembly output of the C compiler and hand edit or hand optimize the assembly code in ways that the compiler cannot Assembly is also useful for time critical or real time processes because unlike with high level language
153. r ATMEL AVR BASIC called RVK BASIC which runs on Windows The downloaded version is limited to 100 lines of code EEBasic 1 is an implementation of BASIC on a AVR Mega644 which only requires a terminal or terminal emulator to program no PC based compiler or other IDE is used Language extensions provide for use of the on chip peripherals GCC the Gnu Compiler Collection has thorough AVR support for C language Windows WinAVR development tools 41 the Windows port of GCC Now it can even plug into the latest AVR Studio e WinAVR and Butterfly Quickstart Guide e Linux Introduction to using AVR GCC under Linux 42 43 e Debian and Ubuntu users simply apt get install binutils avr gcc avr avr libc e Gentoo emerge avr libc avrdude crossdev then crossdev target avr softfloat linux gnu e Development upon AVR GCC itself happens at the avr gcc maillist bas Libraries for GCC e The avr libc project 45 GCC e Procyon AVRIib 147 An extensive AVR C Code library with example application code included A Doubly Linked Memory Manager for WinAVR 48 e Itis possible to use C on the Tiny series which have no RAM aside from the 32 registers as demonstrated by 49 or avr libe manual 15 describes the library you probably found bundled with AVR Bruce Lightner ImageCraft C 1501 is an inexpensive commercial compiler IAR P is an expensive commercial compiler CodeVisionAVR is a relatively inexpensive co
154. r a predefined amount of time If there is IO it deals with it on the IO thread allowing the main thread to do whatever is needed Pros Does not put off the main thread e Allows you to define the importance of IO by changing the priority of the thread Cons Still somewhat inefficient IfIO occurs frequently your polling interval may be too small for you to sleep sufficiently starving other threads e If your thread is too low in priority or there are too many threads for the OS to wake the thread in a timely fashion data can be lost e Requires an OS capable of threading This technique is good if your system supports threading but does not support interrupts or has run out of interrupts It does not work well when frequent IO is expected the OS may not properly sleep the thread if the interval is too small and you will be adding the overhead of 2 context switches per poll Embedded Systems Microprocessor Architectures 16 Interrupt architecture The interrupt architecture uses interrupts which we discuss in more detail in chapter Embedded Systems Interrupts In this method the bus fires off an interrupt to the processor whenever IO is ready The processor then jumps to a special function dropping whatever else it was doing The special function called an interrupt handler or interrupt service routine takes care of all IO then goes back to whatever it was doing Pros Very efficient Very sim
155. r and the ImageCraft PSoC C compiler Because they haven t yet been ported to Linux Al m8cutils http m8cutils sourceforge net Werner is developing an assembler and a simulator and a programmer for Linux 7 8 http www psocdeveloper com forums viewtopic php t 1616 A2 Forth PSoC Forth Christopher Burns wrote a PSoC Forth The source code is available at http www psocdeveloper com tools misc_dev_tools As of 2007 David Cary is maintaining it at Forth PSoC Forth was http forthfreak net index cgi M8cForth This Forth compiler will work with Linux Mac Windows Solaris Palm and even VT 100 dumb terminals T O pins Q How do I make output pins Hi and Lo A Usually you connect the output pins to some digital module such as a PWM block If none of the modules do what you want you can set them in software see The GPIO reference http www psocdeveloper com articles fundamentals_of_psoc_gpio the GPIO Read Write example project http www psocdeveloper com docs example project html GPIO Help http www psocdeveloper com forums viewtopic php t 83 http www psocdeveloper com forums viewtopic php t 1950 Warning Be aware that the PRTxDR register is write only you can t read back from that register If you read from the PRTxDR address you are directly reading the value at the pins which is often not what you just wrote If you incorrectly assume they will be the same you will sooner or lat
156. r reading this book there are a number of potential fields of study to continue learning For people interested in operating systems and hardware software interfacing read the Operating System Design wikibook For people interested in C programming or Assembly Programming see the Programming C and X86 Assembly wikibooks respectively For people interested in digital control systems there will eventually be a book on that topic here Programmable Logic For people interested in digital signal processing there will eventually be a book on that subject For people interested in a further study of more advanced computer systems there will eventually be books on computer hardware and microprocessors here Microprocessor Design For people interested in an even lower level understanding of electronics see Digital Circuits Wikiversity School of Very Small Information Systems is a course that includes using Java running on a FPGA For people interested in designing motion control systems that is computer controlled machines such as robots machine tools cars buses airplanes ships satellites telescopes etc see Embedded Control Systems Design Which Programming Languages Will This Book Use We try to make this wikibook language neutral It is not fair to focus on one language when all embedded computers can t be programmed in that language However it is nice to have functional example code in some real language Also it
157. r the variable s value There are 2 slightly different reasons const is used in embedded systems One reason is the same as in desktop applications Often a structure array or string is passed to a function using a pointer When that argument is described as const such as when a header file says more prime Strine Chear COmsie CaS _Sicwesliag ip it is a promise by the programmer who wrote that function that the function will not modify any items in the structure array or string If that header file is properly included in the file that implements that function then the compiler will check that promise when that implementation is compiled and give an error if that promise is violated On a desktop application such a program would compile to exactly the same executable if all the const declarations were deleted from the source code but then the compiler would not check the promises When some other programmer has an important piece of data he wants to pass to that function he can be sure simply by reading the header file that that function will not modify those items Without that const he would either have to go through the source code of the function implementation to make sure his data isn t modified and worry about the possibility that the next update to that implementation might modify that data or else make a temporary copy of the data to pass to that function keeping the original version unmodified s
158. rect procedural method instead Equipment vendors supporting DOS tend to follow an approach of either providing raw spec sheets for their equipment or writing a pre compiled binary object library that has to be linked into your software using a specific compiler Embedded Systems Common RTOS DOS 60 Software Base There is a huge software base for developing software in DOS which is another major strength Pre written even free libraries for doing things like event scheduling and multi threading do exist for DOS as well as GUI interface models and support libraries for most standard equipment peripherals You can even find good compilers for DOS environments that compile to 32 bit protected mode as well so you are not restricted to just the 8086 instruction set either Conclusion DOS is a good base OS to build a custom RTOS that has specific features that you need without the extra cruft that you don t It does require a little bit more time to put these extra features that you may need on a specific project so it is more a trade off of time vs money If you have the time to make a well crafted piece of software fit into a very small memory footprint DOS as a RTOS is the way to go It also allows a generally long shelf time for a project that once completed doesn t have to be changed as often to fit obsoleting chip technologies Further Reading A Neutral Look at Operating Systems DOS http www freedos org The home of the Fre
159. rewHarvey4 DavidCary DepartedUser3 Derbeth G schmidt hp Geocachernemesis Herbythyme Iamunknown Jguk Jomegat Killertoffy Krischik Mike lifeguard Mrbill Opodeldoe Reece Stevenyu Sunlight2 Suruena Villagplh Webaware Whiteknight Yan 308 anonymous edits Embedded Systems ARM Microprocessors Source http en wikibooks org w index php oldid 1857066 Contributors Adrignola DavidCary Eigendude Jguk QuiteUnusual 5 anonymous edits Embedded Systems AT91SAM7S64 Source http en wikibooks org w index php oldid 1538408 Contributors Adrignola WillWare Embedded Systems Cypress PSoC Microcontroller Source http en wikibooks org w index php oldid 1752297 Contributors Aki009 DavidCary Graaja KaiMartin Recent Runes 9 anonymous edits Embedded Systems Common Protocols Source http en wikibooks org w index php oldid 989 112 Contributors DavidCary Robert Horning Whiteknight 2 anonymous edits Embedded Systems Where To Buy Source http en wikibooks org w index php oldid 1287282 Contributors Aaronsells DavidCary Whiteknight Embedded Systems Resources Source http en wikibooks org w index php oldid 1563941 Contributors DavidCary Embeddedsystemnews Whiteknight 1 anonymous edits Embedded Systems Licensing Source http en wikibooks org w index php oldid 890037 Contributors Whiteknight Image Sources Licenses and Contributors 113 Image Sources Licenses and Contributors Image Wikibooks logo en noslogan svg Source
160. rigger an interrupt Where do I put the code to handle that interrupt A 227 http www psocdeveloper com forums viewtopic php t 1586 How to determine source of GPIO interrupt http www psocdeveloper com forums viewtopic php t 1524 How many bytes of stack do I really need Q I m running out of RAM How many bytes of stack do I really need A the stalkwalk utility does static stack analysis http www psocdeveloper com forums viewtopic php t 9 gives a conservative count of how many bytes are needed It still needs some work RS485 Q How do I connect my PSoC to a RS485 bus A1 http www psocdeveloper com forums viewtopic php t 1640 mentions Interrupt on 9th bit Application Note AN2269 Implement 9 Bit Protocol on the PSoCTM UART A2 Half duplex issues are discussed at http www psocdeveloper com forums viewtopic php t 1731 http www psocdeveloper com forums viewtopic php t 2397 UART Q The UART isn t working I m pulling out my hair A Please restate in the form of a question mixing C and assembly language Q How do I call a C function from assembly A You can call C functions in assembly by adding an underscore before the function name For example if you want to call a C function called foo from assembly you would use allll Soo This applies for C functions that do not take any parameters If you want to call functions that take parameters then you need to pass thes
161. rm other tasks while they are waiting for the memory access to complete faster Old microprocessors and low end microprocessors will always take the first option so again limit the number of memory access operations while newer and high end microprocessors will often take the second option T O Bus Any computer be it a large PC or a small embedded computer is useless if it has no means to interact with the outside world I O communications for an embedded computer frequently happen over a bus called the I O Bus Like the memory bus the I O bus frequently multiplexes the input and output signals over the same bus Also the T O bus is moving at a slower speed than the processor is so large numbers of I O operations can cause a severe performance bottleneck Embedded Systems Microprocessor Architectures 15 It is not uncommon for different IO methods to have separate buses Unfortunately it is also not uncommon for the electrical engineers designing the hardware to cheat and use a bus for more than purpose Doing so can save the need for extra transistors in the layout and save cost For example a project may use the USB bus to talk to some LEDs that are physically close by These different devices may have very different speeds of communication When programming IO bus control make sure to take this into account In some systems memory mapped IO is used In this scheme the hardware reads its IO from predefined memory a
162. rts which cannot provide the four spare pins needed for JTAG The JTAGICE mkII and the AVR Dragon support debugWIRE debugWIRE was developed after the original JTAGICE release and none of the JTAG clones support it Simulation Simulation is not a debugging interface per se but simulation in software can be an effective debugging aid prior to committing a design to physical hardware AVR Studio 77 simulates the AVR core at the assembly language level and allows viewing and manipulation of all internal registers HAPsim 23 is a set of virtual devices that plug into AVR Studio It provides LCDs LEDs buttons and dumb terminals Other software packages exist which provide software simulation of the AVR core and peripherals are available VMLab P single step C code as well as edit and rebuild winAVR programs As of version 3 12 VMLab is freeware provides full circuit simulation as well as a virtual oscilloscope The debugger offers the ability to AVRora gt is an AVR simulation and analysis framework Proteus 26 provides schematic capture PCB editing and microcontroller simulation including the AVR The simulator downloads code into simulated AVR core There is also support for a variety of virtual peripherals within the simulator Simulavr P is a free GPLv2 simulator working with GDB and commonly used with avr gcc Firmware Programming A microcontroller won t do much without firmware program co
163. s e 16 bit address bus 32 general purpose registers each of 8 bits e 16 bit timers usually 2 but may have more or less e 3 internal and 2 external interrupts e Bit as well as byte addressable RAM area of 16 bytes e Four 8 bit ports short models have two 8 bit ports e 16 bit program counter and data pointer 8051 models may also have a number of special model specific features such as UARTs ADC OpAmps etc Embedded Systems 8051 Microcontroller 78 Typical applications 8051 chips are used in a wide variety of control systems telecom applications robotics as well as in the automotive industry By some estimations 8051 family chips make up over 50 of the embedded chip market Basic Pins 8051 P1 0 1 40 vec PIN 9 PIN 9 is the reset pin which is used reset the microcontroller s P1 1 2 39 PO OADO P12 _ P0 1 AD1 3 38 internal registers and ports upon starting up 2 machine cycle should be pi3 4 37 PO NAD2 high at this pin P14 J5 36 P0 3 AD3 cae Ma P15 46 35 I POA ADA PINS 18 amp 19 The 8051 has a built in oscillator amplifier hence we P16 l7 34 I POSADS need to only connect a crystal at these pins to provide clock pulses to P1 7 1g 937 RUSADE aa RST lg 32 POJIADT the circuit RxDIP3 0 149 31 EA E TxD P3 1 411 30 AE PIN 40 and 20 Pins 40 and 20 are VCC and ground respectively The NTOP3 2 12 29 PSEN 8051 chip needs 5V
164. s Embedded Systems External ICs Embedded Systems Low Voltage Circuits Embedded Systems High Voltage Circuits Particular Microprocessor Families Embedded Systems Particular Microprocessors Embedded Systems Intel Microprocessors Embedded Systems PIC Microcontroller Embedded Systems 8051 Microcontroller Embedded Systems Freescale Microcontrollers Embedded Systems Atmel AVR Embedded Systems ARM Microprocessors Embedded Systems AT91SAM7S64 Embedded Systems Cypress PSoC Microcontroller Appendices Embedded Systems Common Protocols Embedded Systems Where To Buy Embedded Systems Resources Embedded Systems Licensing References Article Sources and Contributors Image Sources Licenses and Contributors Article Licenses License 59 59 60 63 63 64 64 66 68 68 71 72 77 80 82 99 100 100 106 106 107 108 110 111 113 114 Wikibooks Collections Embedded Systems Wikibooks Collections Embedded Systems Embedded Systems Building and Programming Embedded Devices Wikibooks Collections Preface Embedded Systems Embedded Systems Introduction Embedded Systems Terminology Microprocessor Basics Embedded Systems Microprocessor Introduction Embedded Systems Embedded System Basics Embedded Systems Microprocessor Architectures Embedded Systems Programmable Controllers Embedded Systems Floating Point Unit Embedded Systems Parity Embedded Systems Memory Embedded Systems Memory Units Programming Embedded
165. s there is no ambiguity about how the code will be compiled The timing can be strictly controlled which is useful for writing simple device drivers This section will look at multiple techniques for mixing C and Assembly program development Inline Assembly One of the most common methods for using assembly code fragments in a C programming project is to use a technique called inline assembly Inline assembly is invoked in different compilers in different ways Also the assembly language syntax used in the inline assembly depends entirely on the assembly engine used by the C compiler Microsoft C for instance only accepts inline assembly commands in MASM syntax while GNU GCC only accepts inline assembly in GAS syntax also known as AT amp T syntax This page will discuss some of the Embedded Systems Mixed C and Assembly Programming 34 basics of mixed language programming in some common compilers Linked assembly When an assembly source file is assembled by an assembler and a C source file is compiled by a C compiler those two object files can be linked together by a linker to form the final executable The beauty of this approach is that the assembly files can written using any syntax and assembler that the programmer is comfortable with Also if a change needs to be made in the assembly code all of that code exists in a separate file that the programmer can easily access The only disadvanges of mixing assembly and C in
166. s PicoC BI or Wikipedia Interactive C What are the available interactive languages that run in tiny memory at Stack Overflow 61 References 1 2 3 4 5 6 ea i Si Data Compression executable software compression http wiki forthfreak net http sourceforge net projects hplua http www calcwatch com forum viewtopic php pid 789 p789 http code google com p picoc http stackoverflow com questions 108275 1 what are the available interactive languages that run in tiny memory Embedded Systems Memory Units 25 Embedded Systems Memory Units ROM One type of memory that is as cheap as it is useless is Read Only Memory ROM I say that it is useless because you can program it once and then you can never change the data that is on it This makes it useless because you can t upgrade the information on the ROM chip be it program code or data you can t fix it if there is an error etc Because of this they are usually called Programmable Read Only Memory PROM because you can program it once but then you can t change it at all EPROM In contrast to PROM is EPROM Erasable Programmable Read Only Memory EPROM chips will have a little window made of either glass or quartz that can be used to erase the memory on the chip To erase an EPROM the window needs to be uncovered they usually have some sort of guard or cover and the EPROM needs to be exposed to UV radiation to erase the memory
167. s do things a little differently Some use a C interpreter such as PicoC DI or Wikipedia Interactive C They download the C source code to the embedded system then they run the interpreter in the embedded system itself Some people have the luxury of working with large embedded systems that can run a standard C compiler it runs the standard GCC on Linux or BSD or it runs the Wikipedia DJGPP port of GCC on FreeDos or it runs the Wikipedia MinGC port of GCC on Windows or it runs the Wikipedia Tiny C Compiler on Linux or Windows or some other C compiler They download the C source code to the embedded system then they run the compiler in the embedded system itself C compilers for embedded systems Perhaps the biggest difference between C compilers for embedded systems and C compilers for desktop computers is the distinction between the platform and the target The platform is where the C compiler runs perhaps a laptop running Linux or a desktop running Windows The target is where the executable code generated by the C compiler will run the CPU in the embedded system often without any underlying operating system The GCC compiler igleitation needed the most popular C compiler for embedded systems GCC was originally developed for 32 bit Princeton architecture CPUs So it was relatively easily ported to target ARM core microcontrollers such as XScale and Atmel AT91RM9200 Atmel AVR32 AP7 family MIPS core microcontro
168. sample code and documentation for using the comparator as a low speed ADC The signal to be measured is connected to the inverted input and a reference signal is connected to the non inverting input The AVR generates an interrupt when the signal falls below or rises above the reference value A common use for the analog comparator is sensing battery voltage to alert the user to a low battery Other Integrated Hardware Aside from what might be considered typical peripherals for a microcontroller UART SPI ADC some AVRs include more specialized peripherals for specific applications LCD Driver In larger models like the ATMega169 as seen in the AVR Butterfly an LCD driver is integrated The LCD driver commandeers several ports of the AVR to drive the column row connections of a display One particular trait of Liquid Crystal that must be taken care of is that no DC bias is put through it DC bias or having more electrons passing one way than the other when pumping AC chemically breaks apart the liquid crystal The AVR s LCD module uses precise timing to drive pixels forwards and backwards equally USB Interface The AT90USB series includes an on chip USB controller Some models are function only while others have On The Go functionality to act as either a USB host for interfacing with other slave devices or as a USB slave for interfacing with a USB master AVRs without built in USB can use an external chip such as t
169. selling old versions when a new one comes out What is this book This book was generated by the volunteers at Wikibooks a team of people from around the world with varying backgrounds The people who wrote this book may not be experts in the field Some may not even have a passing familiarity with it The result of this is that some information in this book may be incorrect out of place or misleading For this reason you should never rely on a community edited Wikibook when dealing in matters of medical legal financial or other importance Please see our disclaimer for more details on this Despite the warning of the last paragraph however books at Wikibooks are continuously edited and improved If errors are found they can be corrected immediately If you find a problem in one of our books we ask that you be bold in fixing it You don t need anybody s permission to help or to make our books better Wikibooks runs off the assumption that many eyes can find many errors and many able hands can fix them Over time with enough community involvement the books at Wikibooks will become very high quality indeed You are invited to participate at Wikibooks to help make our books better As you find problems in your book don t just complain about them Log on and fix them This is a kind of proactive and interactive reading experience that you probably aren t familiar with yet so log on to http en wikibooks org and take a look around at
170. series e g the AT90S8515 is replaced by the mega8515 Atmel provides a Parametric Product Table 51 Which compares the memory peripherals and features available on the entire line of AVRs Hardware Design Considerations 6 Atmel provides the AVR Hardware Design Considerations to assist the hardware designer This document also shows the standard in circuit serial programming connector AVR development application boards Butterfly Demo Board The AVR Butterfly is a self contained battery powered demonstration board running the ATMEL AVR ATmegal69V Microcontroller The board includes an LCD screen joystick speaker serial port RTC flash chip temperature light amp voltage sensors The board has a shirt pin on its back and can be worn as a name badge The AVR Butterfly comes preloaded with software to demonstrate the capabilities of the microcontroller Factory firmware can scroll your name display the sensor readings and show the time Also the AVR Butterfly has a piezo buzzer that can reproduce sound The AVR Butterfly demonstrates LCD driving by running a 14 segment 6 alpha numeric character display However the LCD interface consumes many of the I O pins The Butterfly s ATmega169 CPU is capable of speeds up to 8Mhz however it is factory set by software to 2Mhz to preserve the button battery life A pre installed bootloader program allows the board to be re programmed with a standard RS 232 serial plug Ecros
171. stack overflows and data is lost People who program 12 bit PICs spend a lot of effort working around this limitation These people are forced to rely heavily on techniques that avoid using the hardware stack For example macros state machines and software stacks e 14 bit A PIC microcontroller with a 14 bit core most PIC16 has 8 registers in the hardware stack This makes function calls much easier to use even though people who program them should be aware of some remaining gotchas 9 e 16 bit A PIC microcontroller with a 16 bit core all PIC18 has a 31 level deep hardware stack depth This is more than deep enough for most programs people write Many algorithms involving pushing data to then later pulling data from some sort of stack People who program such algorithms on the PIC must use a separate software stack for data reminiscent of Forth People who use other microprocessors often share a single stack for both subroutine return addresses and this stack data Call tree analysis can be used to find the deepest possible subroutine nesting used by a program Unless the program uses w recursion As long as the deepest possible nesting of the main program plus the deepest possible nesting of the interrupt routines give a total sum less than the size of the stack of the microcontroller it runs on then everything works fine Some compilers automatically do such call tree analysis and if the hardware stack is
172. systems programming is not like normal PC programming In many ways programming for an embedded system is like programming a PC 15 years ago The hardware for the system is usually chosen to make the device as cheap as possible Spending an extra dollar a unit in order to make things easier to program can cost millions Hiring a programmer for an extra month is cheap in comparison This means the programmer must make do with slow processors and low memory while at the same time battling a need for efficiency not seen in most PC applications Below is a list of issues specific to the embedded field Tools Embedded development makes up a small fraction of total programming There s also a large number of embedded architectures unlike the PC world where 1 instruction set rules and the Unix world where there s only 3 or 4 major ones This means that the tools are more expensive It also means that they re lower featured and less developed On a major embedded project at some point you will almost always find a compiler bug of some sort Debugging tools are another issue Since you can t always run general programs on your embedded processor you can t always run a debugger on it This makes fixing your program difficult Special hardware such as JTAG ports can overcome this issue in part However if you stop on a breakpoint when your system is controlling real world hardware such as a motor permanent equipment damage can occur As a result peopl
173. t Microchip s Family of 16 Bit Microcontrollers UJ by Jeff Bachiochi 2007 example application uWatch D I Y open source scientific calculator watch 2 Freescale Microcontrollers The Zilog Z8 Series Z8 Z8encore Z8XP Cypress PSoC Microcontroller Texas Instruments MSP430 microcontrollers 16 bit ARM Microprocessors this family includes the Philips LPC210x ARM microcontrollers the discontinuedHnte w StrongARM Atmel AT91RM9200 and the Intel XScale microprocessors x86 microprocessors brief selection guide For many embedded systems any of these microcontrollers would be more than adequate TI MSP430 has the lowest power consumption In sleep mode 0 3 uW 3 V 0 1 uA Some chips in 2xx and 4xx series include 12 bit DACs The Cypress PSoC has more than one true analog output Using sleep mode power consumption as low as 21 uW 5 V 4 2 uA 3 You can get analog output from the other chips by using an external ADC or by faking it with a PWM output and some low pass filtering Most Cypress PSoC microcontrollers come in both DIP and SMT versions Many of these series include microcontrollers with integrated 10 bit ADCs but Atmel AVR 8 bit series as of early 2006 had the lowest price chip that included such an ADC as well as another chip with the lowest cost ADC Most Atmel AVR 8 bit microcontrollers come in both DIP and SMT versions If you need a very tiny chip the Atmel AVR PIC and Freescale microcontroller lines all
174. t is not possible to link in an entire library file and programmers are often forced to brew their own standard c library implementations if they want to use them at all While some libraries are bulky and not well suited for use on microcontrollers many development systems still include the standard libraries which are the most common for C programmers C remains a very popular language for micro controller developers due to the code efficiency and reduced overhead and development time C offers low level control and is considered more readable than assembly Many free C compilers are available for a wide variety of development platforms The compilers are part of an IDEs with ICD support breakpoints single stepping and an assembly window The performance of C compilers has improved considerably in recent years and they are claimed to be more or less as good as assembly depending on who you ask Most tools now offer options for customizing the compiler optimization Additionally using C increases portability since C code can be compiled for different types of processors Example An example of using C to change a bit is below Clearing Bits PORTH amp OxF5 Changes bits 1 and 3 to zeros using C PORTH amp 0x0A Same as above but using inverting the bit mask easier to see which bits are cleared Setting Bits PORTH NOO ALE A Sei bits lands st omone UsmMom cnc OR In assembly this would be Clearing Bits BCLR PORTH SOA Changes
175. t the exact details they need to know in order to call functions written in C and in order to accept calls from functions written in C The typical process is Embedded Systems Mixed C and Assembly Programming 35 e write a c file with stubs details exactly the same number and type of inputs and outputs that you want the assembly language function to have e Compile that file with the appropriate switches to give a mixed assembly language with c in comments file typically a cod file If your compiler can t produce an assembly language file there is the tedious option of disassembling the binary obj machine code file Copy that cod file to a asm file Sometimes you need to strip out the compiled hex numbers and comment out other lines to turn it into something the assembler can handle Test the calling convention compile the asm file to an obj file and link it instead of the stub c file to the rest of the program Test to see that calls work properly Fill in your asm file the asm file should now include the appropriate header and footer on each function to properly implement the calling convention Comment out the stub code in the middle of the function and fill out the function with your assembly language implementation Test Typically a programmer single steps through each instruction in the new code making sure it does what they wanted it to do Parameter Passin
176. tail asp ref amp FileName JTEGICE html amp SEC_NAME product http atmel com dyn products tools_card asp tool_id 2725 http www helmix at hapsim http www amctools com vmlab htm http compilers cs ucla edu avrora index html http www labcenter co uk http www nongnu org simulavr http www atmel grenoble com dyn resources prod_documents novice pdf http www atmel com journal documents issue 1 journal pdf http www atmel com dyn products tools_card asp tool_id 2725 http www atmel com dyn resources prod_documents DOC0856 PDF http avr 15 forumer com index php showforum 1 http www avr asm tutorial net index html http avra sourceforge net http www tavrasm org http avr ada sourceforge net http www mcselec com http en wikipedia org wiki Wine_ 28software 29 http www bastoc com http www dandysolutions com http winavr sourceforge net http www smileymicros com QuickStartGuide pdf http www linuxfocus org English November2004 article352 shtml http lists gnu org archive html avr gcc list http savannah nongnu org projects avr libc http www nongnu org avr libc user manual index html http hubbard engr scu edu embedded avr avrlib index html http www jennaron com au avr DoublyLinkedMemory html http lightner net avr ATtiny projects README txt http www imagecraft com software http www iar com index php show 5921_ENG amp amp page_anchor http www iar com p5921 p5921_eng php http www
177. tem further reading e Massmind watch dog timers 2 e Wikipedia watchdog timer e Embedded Control Systems Design DesignPatterns W atchdog timer e Linux Kernel Drivers Annotated Character Drivers Softdog Driver the Grenade Timer Fortifying the Watchdog Timer Against Malicious Mobile Code 3 by Frank Stajano and Ross Anderson 2000 gives most of the benefits of protected mode hardware to very low cost microcontrollers that don t have protected mode hardware using very frugal hardware resources Embedded Systems Threading and Synchronization 50 References 1 http en wikipedia org wiki Test and set 2 http massmind org techref wdts htm 3 http www cl cam ac uk rja14 Papers grenade pdf Embedded Systems Interrupts Interrupts Sometimes things will happen in a system when the processor is simply not ready In fact sometimes things change that require immediate attention Can you imagine sitting at your PC that you were to hit buttons on the keyboard and nothing happens on your computer Maybe the processor was busy and it just didnt check to see if you were hitting any buttons at that time The solution to this problem is something called an Interrupt Interrupts are events that cause the microprocessor to stop what it is doing and handle a high priority task first After the interrupt is handled the microprocessor goes back to whatever it was doing before In this way we can be assure
178. tem that supports a working DOS box as Linux Windows 95 Windows 98SE etc amforth atmega forth 63 is a compact Forth for AVR atmega micro controllers It is released under the GPL 2 and is modeled after ANS 94 Avise Atmel VIrtual Stack Engine 164 is a modified version of the Forth programming language Avise is only available as HEX files to program into one of the supported AVRs source code is not available The author s web site also includes some PCB layouts for use with Avise PFAVR 191 is a port of pForth 155 to the AVR h 167 avrfort is a 16 bit subroutine threaded forth kernel for atmel s avr series of microcontrollers Note that some Forth environments run interactively on the AVR For example Avise presents a console on the AVR s UARTO which can accept new word definitions and execute operations No software other than a terminal emulator is required on the PC Python PyMite 68 is a subset of Python that runs on any device in the AVR family that has at least 64 KiB program memory and 4 KiB RAM Scheme Scheme on the ATmega ly References Official Atmel Websites Atmel AVR Device Site 17 Atmel AVR Beta Site 7 Atmel AVR Support Site 72 Embedded Systems Atmel AVR 96 Wiki AVRfreaks wiki 173 Serial Programming MAX232 Driver Receiver Wikiversity Embedded System Engineering The massmind technical reference 74 has an AVR section tel plus a lot of general in
179. than a Forth or BASIC interpreter Common embedded operating systems In this book we discuss these operating systems commonly used in embedded systems Palm OS Windows CE e MS DOS or DOS Clones e Linux including RTLinux and MontaVista Linux and Unison OS For further reading A variety of embedded systems and RTOS are based on Linux see Embedded Systems Linux for details Embedded Control Systems Design Operating systems e Wikipedia INTEGRITY A small message passing hard real time micro kernel with memory protection designed for safety critical and high security devices e Wikipedia Contiki a small open source operating system developed for use on a number of smallish systems ranging from 8 bit computers to embedded microcontrollers Wikipedia eCos embedded Configurable operating system an open source royalty free real time operating system intended for embedded systems and applications eCos was designed for devices with memory footprints in the tens to hundreds of kilobytes or with real time requirements e Wikipedia DSP BIOS a royalty free real time multi tasking kernel mini operating system created by Texas Instruments Embedded Systems Common RTOS 57 e Wikipedia QNX e Wikipedia VxWorks A small footprint scalable high performance RTOS Wikipedia Windows CE e Wikipedia Palm OS e pico OS 5 has been ported to the Atmel AVR the ARM and the 80x86 Wikipedia OSEK is not an OS b
180. that interrupt is completely handled interrupts are turned back on at the end of the first level interrupt handler as soon as possible Does not put off the main thread e Allows you to define the importance of IO by changing the priority of the thread Very efficient only makes context changes when needed and does not poll Very clean solution architecturally allows you to be very flexible in how you handle IO The second level interrupt handler can wait for a lock to be released Embedded Systems Locks and Critical Sections Cons e Requires an OS capable of threading Most complex solution This solution is the most flexible and one of the most efficient It also minimizes the risk of starving more important tasks Its probably the most common method used today Embedded Systems Microprocessor Architectures 17 DMA Direct Memory Access Controller In some specialised situations such as where a set of data must be transfered to a communications IO device a DMA controller may be present that can automatically detect when the IO device is ready for more data and transfer that data This technique may be used in conjuction with many of the other techniques for instance an interrupt may be used when the data transfer is complete Pros This provides the best performance since the I O can happen in parallel with other code execution Cons Only applicable to a limited range of problems e Not all syst
181. that were once common PIC model the C541 18 pin DIP with 12 1 0 pins AW R WN Many programs written for the PIC16x family are available for free on the Internet Flash based chips such as the PIC16F88 are far more convenient to develop on and can run code written for the above chips with little or no changes PIC 12x The PIC 12 series are all very small chips with 8 pins and 4 available I O pins These are used only when space is a huge factor and the PIC doesn t have many responsibilities PIC 18x The PIC 18x series are available in a 28 and 40 pin DIP package They have more ports more ADC etc PIC 18s are generally considered to be very high end microcontrollers and are even sometimes called full fledged CPUs Microchip is currently as of 2007 producing 6 Flash microcontrollers with a USB interface All are in the PIC18Fx family The 28 pin PIC18F2450 PIC18F2455 PIC18F2550 and the 40 44 pin PIC18F4450 PIC18F4455 PIC18F4550 The PIC Stack The PIC stack is a dedicated bank of registers separate from programmer accessible registers that can only be used to store return addresses during a function call or interrupt e 12 bit A PIC microcontroller with a 12 bit core the first generation of PIC microcontrollers including most PIC10 some PIC12 a few PIC16 only has 2 registers in its hardware stack Subroutines in a 12 bit PIC program may only be nested 2 deep before the
182. the address of the data to be accessed and then will carry the value of the data Putting both signals on the same bus at different times is a technique known as time division multiplexing or just multiplexing for short The effect of a multiplexed memory bus is that reading or writing to memory actually takes twice as long half the time to send the address to the RAM module and half the time to access the data at that address This means that on a multiplexed bus moving data to and from the memory is a very expensive in terms of time process and therefore memory read write operations should be minimized It also makes it important to ensure algorithms which work on large datasets are cache efficient Demultiplexed Bus The opposite of a multiplexed bus is a demultiplexed bus A demultiplexed bus has the address on one set of wires and the data on another set This scheme is twice as fast as a multiplexed system and therefore memory read write operations can occur much faster Bus Speed In modern high speed microprocessors the internal CPU clock may move much faster than the clock that synchronizes the rest of the microprocessor system This means that operations that need to access resources outside the processor the RAM for instance are restricted to the speed of the bus and cannot go as fast as possible In these situations microprocessors have 2 options They can wait for the memory access to complete slow or they can perfo
183. ther Reading e Windows Programming Embedded Systems Common RTOS DOS DOS Operating systems based off of MS DOS still enjoy a huge market segment within the development community of embedded systems design There are many reasons for this most importantly is that MS DOS can hardly even be called an operating system by many modern measurements Almost all DOS based software usually has exclusive control over the computer while it is running and a major bonus is that the footprint for the operating system is usually very small You can put a stripped down version of FreeDOS a current MS DOS clone that is still being updated that fits in just 100K of hard drive space Even less is required within the memory of the computer You can even still purchase MS DOS 6 22 but it must be from specialized software distributors who are under license from Microsoft and it is no longer supported by Microsoft in terms of any software updates even for known bugs Strengths and Weaknesses The major advantage of DOS is also its largest drawback By having so little actual operating in the computer the software developer for DOS must perform many tasks traditionally thought of as something in the operating system For instance DOS doesn t have built in capability for scheduling or multithreading You must also install interrupt handlers directly into the software application and API calls tend to be through software interrupts rather than some other more di
184. this way are that a both the assembler and the compiler need to be run and b those files need to be manually linked together by the programmer These extra steps are comparatively easy although it does mean that the programmer needs to learn the command line syntax of the compiler the assembler and the linker Inline Assembly vs linked assembly Advantages of inline assembly Short assembly routines can be embedded directly in C function in a C code file The mixed language file then can be completely compiled with a single command to the C compiler as opposed to compiling the assembly code with an assembler compiling the C code with the C Compiler and then linking them together This method is fast and easy If the in line assembly is embedded in a function then the programmer doesn t need to worry about Calling_Conventions even when changing compiler switches to a different calling convention Advantages of linked assembly If a new microprocessor is selected all the assembly commands are isolated in a asm file The programmer can update just that one file there is no need to change any of the c files if they are portably written Calling Conventions When writing separate C and Assembly modules and linking them with your linker it is important to remember that a number of high level C constructs are very precisely defined and need to be handled correctly by the assembly portions of your program Perhaps the biggest
185. ton architecture microcontrollers use exactly the same instructions to access RAM as program Flash C compilers for such architectures typically put all data declared as const into program Flash Functions neither know nor care whether they are dealing with data from RAM or program Flash the same read instructions work correctly whether the function is given a pointer to RAM or a pointer to program Flash storing data in ROM on a Harvard architecture microcontrollers Unfortunately Harvard architecture microcontrollers use completely different instructions to access RAM than program Flash Often they also have yet another set of instructions to access EEPROM and another to access external memory chips This makes it difficult to write a subroutine such as puts that can be called from one part of the program to print out a constant string such as November from ROM and called from another part of the program to print out a variable string in RAM Unfortunately different C compilers even for the same chip require different incompatible techniques for a C programmer to tell a C compiler to put data in ROM There are at least 3 ways for a C programmer to tell a C compiler to put data in ROM 1 Some people claim that using the const modifier to indicate that some data is intended to be stored in ROM is an abuse of notation gt Such people typically propose using some non standard attribute or storage specifier such as P
186. toring data in ROM Another reason to use const is specific to embedded systems On many embedded systems there is much more program Flash or ROM than RAM A c file that uses a definition such as ceni mMmonecasl 4 i geiawieuey y inelorueiey Mene oh Aora ILA ities MunoraeY July August September October November December I forces the compiler to store a all those strings in program Flash then on boot up to copy those values to a location in RAM That wastes precious RAM if as is often the case the program never actually modifies those strings By modifying the declaration to Chace Coasc ECONS TAMMONTA sa He we inform the compiler that we promise to never modify those strings or their order in the array and so the compiler is free to store all those strings in program Flash and fetch the original value from Flash whenever it is needed That saves RAM for variables that really do change Some compilers if you use definitions such as ScAcLe Clagic imomcias 4 osc He are smart enough to work out for themselves whether or not that the program ever actually modifies those strings If the program does modify those strings then of course the compiler must put them in RAM But if not the compiler Embedded Systems C Programming 29 is free to store those strings only once in program Flash storing data in ROM on a Princeton architecture microcontrollers Prince
187. ts use no operating system at all presumably using a Embedded Systems Super Loop Architecture instead See Embedded Systems Common RTOS for a list of common real time operating systems xxxx Embedded Systems Real Time Operating Systems 47 References 1 http www embedded com showArticle jhtml articleID 187203732 Embedded Systems Threading and Synchronization One of the most useful developments in the history of computing is multitasking and multithreading This technique isn t always available to an embedded system engineer but some embedded systems and RTOS have multithreading MT capability The chapters in this section will talk about some of the uses of MT and will discuss some of the common pitfalls associated with MT programming This page is only going to serve as a brief reference to multi threaded programming Preemptive Multithreading When the first multi tasking systems were established they did not have a central controller Multi tasking was established by having programs voluntarily give up control to the system and the system would then give control to another process This system worked reasonably well except that any program that was misbehaving would slow down the entire system For instance if a program got stuck in an infinite loop it would never give up control and the system would freeze The solution to this problem is preemptive multithreading In a preemptive environment control could
188. ual oscillator frequency of 4 X External Oscillator For example with a maximum allowable oscillator of 16MHz the virtual oscillator runs at 64MHz Thus the PIC will perform 64 4 Embedded Systems PIC Microcontroller 76 16 MIPS million instructions per second Certain pics also have built in oscillators usually 4Mhz for precisely IMIPS or a low power imprecise 48Khz This frees up to two I O pins for other purposes The pins can also be used to produce a frequency if you want to syncrhonize other hardware to the same clock as one PIC s internal one programming Continue with Embedded Systems PIC Programming Further reading There is a lot of information about using PIC microcontrollers and electronics design in general in the PICList archives If you are really stumped you might consider subscribing to the PICList asking your question and answering someone else s question in return The PICList archives are hosted at MassMind 15 16 17 18 19 20 21 22 23 RC Airplane RCAP discusses a project that uses a PIC16F876A the Parallax SX FAQ 16 by Guenther Daubach Microchip PIC 171 the original manufacturer s web site Getting Starting with PICmicro controllers 6 by Wouter van Ooijen The PIC 16F628A Why the PIC 16F84 is now obsolete t 1 The PIC 16F88 Why the PIC 16F84 is now Really obsolete 7 Free PIC resources and projects with descriptions schematics and source code
189. urces Wikimedia Resources Robotics Communication Systems Digital Signal Processing e List of emulators e Wikiversity Embedded System Engineering Serial Programming Book e Programmable Logic e Parallel Computing and Computer Clusters e Electric Circuits e Analog and Digital Conversion e Embedded Control Systems Design Related Programming Resources Ada Programming Assembly Language e C Programming e C Programming e Forth Software Engineers Handbook Web Resources e AN887 Microcontrollers made easy 15902 a gentle introduction to microcontrollers in general and some of the things they do with lots of pictures Knowledge and concepts behind VLSI chip design 2 e Embedded System description in simple words BI e Microcontrollers Forum e http www EmbeddedRelated com e Embedded White Papers Downloads Companies News Articles 5 e Microcontroller based free projects 6 e Embedded Systems Design Magazine 7 has articles such as The basics of programming embedded processors Part 1 3 by Wayne Wolf e Embedded Systems Resources DI e Electronics Components Tutorials for Robotics 10 e Dedicated Systems Magazine 1 e RTC Magazine 12 e COTS Journal H e Portable Design Magazine e PKG Magazine 15 e www embeddedcommunity com 14 16 Tools for Embedded Developers 17 Embedded System News 18 recommended by the Ganssle Group Embedded Systems
190. ut an open standard for automotive real time operating systems e MaRTE OS Minimal Real Time Operating System for Embedded Applications 6 Is this related to Wikipedia MARTE 2 Wikipedia TinyOS is an open source operating system designed for wireless embedded sensor networks networked sensors e Wikipedia ChibiOS RT is an open source real time operating system that supports LPC214x AT91SAM7X STM32F103x and AVRmega processors e Wikipedia Fusion RTOS is a license free embedded operating system that supports ARM Analog Devices Blackfin Motorola StarCore and Motorola DSP 56800E Wikipedia FreeRTOS is an open source embedded operating system kernel that supports ARM Atmel AVR AVR32 HCS12 MicroBlaze MSP430 PIC 18 dsPIC Renesas H8 S x86 8052 processors FreeRTOS can be configured for both preemptive or cooperative operation FreeRTOS SafeRTOS and OpenRTOS are based on the same code base Wikipedia RTEMS Real Time Executive for Multiprocessor Systems is a free open source real time operating system RTOS designed for embedded systems Wikipedia MicroC OS II is an embedded RTOS intended for safety critical embedded systems such as aviation medical systems and nuclear installations it supports a wide variety of embedded processors The Real time Operating system Nucleus Wikipedia TRON Project Wikipedia DSPnano RTOS Ultra Tiny Embedded Linux and POSIX compatible RTOS for 8 16 Bit MCUs with Dual Licensing
191. ut this makes interrupt latency much worse Use a lock associated with each data structure When the reader sees that the writer is updating the data structure have the reader tell the task scheduler to run some other process until the writer is finished There are many kinds of locks Use a monitor associated with every routine that uses a data structure Whenever a lock or CS mechanism is called it is important that the RTOS disable the scheduler to prevent the atomic operations from being preempted and executed incorrectly Remember that embedded systems need to be stable and robust so we cannot risk having the operating system itself being preempted while it is trying to create a lock or critical section If we have a function called DisableScheduler we can call that function to disable the scheduler before any atomic operation is attempted and we can then have a function called EnableScheduler to restore the scheduler and continue with normal operation Let us now create a general function for entering a critical section Embedded Systems Locks and Critical Sections 54 EnterCsS DisableScheduler IgE hein and one for exiting a critical section ExitCs EnableScheduler RES By disabling the scheduler during the critical section we have guaranteed that no preemptive task switching will occur during the critical section This method has a disadvantage that it slows
192. values of A and B 65 and 66 respectively onto the stack before the function foo is called You can see that there is no mention of variables a or b in the function foo So any changes that you make to those two values in foo will not affect the values of a and b in the calling function Embedded Systems Mixed C and Assembly Programming 36 Pass by Reference Imagine a situation where you have to pass a large amount of data to a function and apply the modifications done in that function to the original variables An example of such a situation might be a function that converts a string with lower case alphabets to upper case It would be an unwise decision to pass the entire string particularly if it is a big one to the function and when the conversion is complete pass the entire result back to the calling function Here we pass the address of the variable to the function This has two advantages one you don t have to pass huge data therby saving execution time and two you can work on the data right away so that by the end of the function the data in the calling function is already modified But remember any change you make to the variable passed by reference will result in the original variable getting modified If that s not what you wanted then you must manually copy the variable before calling the function 80x86 Pentium do I need to say anything about compact smali large huge here AVR WAR
193. ve by hobbyist standards at around US 300 although much more affordable than many other microntroller emulation systems The JTAGICE has been EOL ed though it is still supported in AVR Studio and other tools 2 The JTAGICE mkII replaces the JTAGICE and is similarly priced The JTAGICE mkII interfaces to the PC via USB and supports both JTAG and the newer debug WIRE interface 3 The AVR Dragon 181 is a low cost approximately 50 substitute for the JTAGICE mkII for certain target parts The AVR Dragon provides in system serial programming high voltage serial programming and parallel programming as well as JTAG or debugWIRE emulation for parts with 32KB of program memory or less There are also several third party JTAG debuggers reprogrammers for around 40 such as those from Ecros and Olimex as well as DIY projects including Evertool 1 and Aquaticus 20 These are clones of the original JTAGICE and do not support the debugWire interface JTAG can also be used to perform a Boundary Scan test 21 which tests the electrical connections between AVRs and other Boundary Scan capable chips in a system Boundary scan is well suited for a production line the hobbyist is probably better off testing with a multimeter or oscilloscope Embedded Systems Atmel AVR 93 debugWIRE debugWIRE M is Atmel s solution for providing on chip debug capabilities via a single microcontroller pin It is particularly useful for lower pin count pa
194. where the compiler assigns them for argument passing Embedded Systems Mixed C and Assembly Programming 37 Fixed registers r0 r1 Never allocated by gcc for local data but often used for fixed purposes rO temporary register can be clobbered by any C code except interrupt handlers which save it may be used to remember something for a while within one piece of assembler code rl assumed to be always zero in any C code may be used to remember something for a while within one piece of assembler code but must then be cleared after use clr rl This includes any use of the f mul s u instructions which return their result in r1 r0 Interrupt handlers save and clear r1 on entry and restore rl on exit in case it was non zero Function call conventions Arguments allocated left to right r25 to r8 All arguments are aligned to start in even numbered registers odd sized arguments including char have one free register above them This allows making better use of the movw instruction on the enhanced core If too many those that don t fit are passed on the stack Return values 8 bit in r24 not r25 16 bit in r25 r24 up to 32 bits in r22 r25 up to 64 bits in r18 r25 8 bit return values are zero sign extended to 16 bits by the caller unsigned char is more efficient than signed char just clr r25 Arguments to functions with variable argument lists printf etc are all passed on stack and char is e
195. xtended to int Warning There was no such alignment before 2000 07 01 including the old patches for gcc 2 95 2 Check your old assembler subroutines and adjust them accordingly Microchip PIC Unfortunately several different incompatible calling conventions are used in writing programs for the Microchip 2 PIC And several features of the PIC architecture make most subroutine calls require several instructions much more verbose than the single instruction on many other processors The calling convention must deal with The paged flash program memory architecture limitations on the hardware stack perhaps by simulating a stack in software the paged RAM data memory architecture making sure a subroutine call by an interrupt routine doesn t scramble information needed after the interrupt returns to the main loop Sparc The Sparc has special hardware that supports a nice calling convention A register window further reading Instruction Set Simulation in C P by Robert Gordon 2002 describes gradually converting from a pure C algorithm to a mixed assembly and C language for testing Interfacing Assembly and C Source Files AN2129 ll describes mixing C and assembly language code on a Cypress PSoC processor Inline Assembler Cookbook P describes mixing C and assembly language code on an Atmel AVR processor Embedded Systems Mixed C and Assembly Programming 38 External Links
196. y nor will they have any precompiled C routines to handle port input and output The purpose of this chapter then is to teach the reader how to brew their own input and output routines The lt iohw h gt interface Some C compiler distributions include the lt iohw h gt interface It allows relatively portable hardware device driver 1 code to be written It is used to implement the standard C lt hardware gt interface x86 Output Routines The x86 instruction set contains 2 instructions in and out both functions take 2 arguments a port number and then another parameter to receive the data or to send the data from depending on which command you use we can define 2 functions in assembly using the CDECL calling convention that we can link with our C programs and call from our C programms to handle port output and input Embedded Systems IO Programming 39 Synchronous and Asynchronous Data can be transmitted either synchronously or asynchronously synchronous transmissions are transmissions that are sent with a clock signal This way the receiver knows exactly where each bit begins and ends This way there is less susceptability to noise and jitter Also synchronous transmissions frequently require extensive hand shakeing between the transmitter and receiver to ensure that all timing mechanisms are synchronized together Conversely asynchronous transmissions are sent without a clock signal and often without muc
197. y by modern 32 bit operating systems Entering Protected Mode For instructions to enter protected mode see X86 Assembly Protected Mode Embedded Systems Bootloaders and Bootsectors To simplify many tasks programmers for many systems will often employ a generic piece of software called a bootloader that will set some system settings such as enabling protected mode and then will be used to load the kernel and then transfer control to the kernel for system operation In embedded systems particularly bootloaders are useful when doing work on the kernel the kernel can be altered and tested and the bootloader will automatically load each new version into memory To further simplify the process the programmer can employ a tool called a bootmenu which is essentially a bootloader that allows the user to select which kernel to load from a list of possibilities This is usefull when multiple kernels are being compared or when different versions of the same kernel are being debugged Bootloaders are used on many microcontrollers A bootloader is often the fastest way to update a program in a microcontroller with small changes That makes the edit compile download test cycle a little bit faster The microcontroller can also have minimal hard coded silicon dedicated to a simpler programming interface which needs an expensive programmer socket A vendor can then put a tiny program in Flash which reads the real program through the interface du
198. ystems Resources Embedded Systems Licensing Wikibooks Collections Preface Wikibooks Collections Preface This book was created by volunteers at Wikibooks http en wikibooks org What is Wikibooks Started in 2003 as an offshoot of the popular Wikipedia project Wikibooks is a free collaborative wiki website dedicated to creating high quality textbooks and other educational books for students around the world In addition to English Wikibooks is available in over 130 languages a complete listing of which can be found at http www wikibooks org Wikibooks is a wiki which means anybody can edit the content there at any time If you find an error or omission in this book you can log on to Wikibooks to make corrections and additions as necessary All of your changes go live on the website immediately so your effort can be enjoyed and utilized by other WI KI BOOKS readers and editors without delay Books at Wikibooks are written by volunteers and can be accessed and printed for free from the website Wikibooks is operated entirely by donations and a certain portion of proceeds from sales is returned to the Wikimedia Foundation to help keep Wikibooks running smoothly Because of the low overhead we are able to produce and sell books for much cheaper then proprietary textbook publishers can This book can be edited by anybody at any time including you We don t make you wait two years to get a new edition and we don t stop

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