Low Power and Embedded Systems
Contents
1. Low Power and Embedded Systems Workbook 3 Introduction In this workbook we will make a precise timer and we will attach a Liquid Crystal Display LCD to the microcontroller via an interface board Supporting material atmega1 68P pdf http www cl cam ac uk teaching 1011 P31 docs atmega168P pdf Data sheet for the Atmel ATMEGA168P used in these exercises You will need to refer to this frequently Within these workbooks this will be referred to as the datasheet The section numbers referred to in these workbooks refer to revision 8161C of the datasheet dated 05 09 serial_lcd_schematic pdf http Awww cl cam ac uk teaching 1011 P31 docs serial_Icd_schematic pdf Schematic for LCD interface PCB serial_Icd_pcb pdf http Awww cl cam ac uk teaching 1 01 1 P31 docs serial_lcd_pcb pdf Printed Circuit Board layout for LCD interface PCB for reference only Icd_interface_micro c http www cl cam ac uk teaching 1011 P31 code Icd_interface_micro c Microcontroller implementation for the LCD interface PCB for reference only Interrupt Vector Table http Awww nongnu org avr lipc user manual group avr__interrupts html Table of interrupt vector names for ATMEL AVR series microcontrollers An on line version of this guide is available at workbook3 html http www cl cam ac uk teaching 101 1 P31 workbook3 html Exercise 1 Precise timing The _delay_ms function used in workbook 1 is useful especially during testi2. change the code to transmit the ADC reading as a 16 bit value Exercise 2 connecting an LCD So far the only output from the microcontroller has been via a serial data connection or by flashing an LED In this exercise you will add an interface to an LCD controller and display There are a wide range of these alphanumeric displays on the market with different size and backlight options but nearly all use either a Hitachi or Seiko controller These controllers are almost identical in operation but the initialisation is fiddly requiring a set of commands with minimum periods between them The LCDs themselves run from a 5V supply To speed up development for you and to reduce the number of IO pins required a small PCB is provided which takes a clock and data signal and does the interfacing to the LCD for you There is also a supporting library to make the LCD easier to use For your interest links to the schematic diagram Printed Circuit Board and associated C code for the library are available in the supporting material section at the start of the worksheet 1 Copy your C code and Makefile from workbook2 exercise2 the Caeser cipher into the directory called workbook3 exercise2 2 Power up reprogram the device and check that the LED flashes once per second Hardware connections 1 You will need to fit a 4 way pin header to the prototyping board Make sure that the thin pins on the header are the ones fitted into the breadb
3. ine F_CPU 8E6 Copy both serial_Icd h and serial_Icd c from http www cl cam ac uk teaching 1011 P31 code to the workbook3 exercise2 directory Change your port direction initialisation function to define LCD_DATA and LCD CLK as outputs Remember to use not or you risk overriding previous definitions Use the generic names not the pin names to make code maintenance easier in future for example this works independently of which pins on PORTD are used INCI IDIDIR l AL lt s lt 1Ije 1p DYNAN CALS IIe CII amp Add the following to your C program include lt config h gt include lt serial_lcd h gt Add the following to your copy of the serial_Icd h file include lt config h gt Modify the Makefile so that serial_Icd c is compiled and linked by changing the line Kengeuseve Cult me Xe theuUSeumne to xerc ise2 ellk ESCSICOILSIeE GO srenciielll_ilerclse In main within the while 1 loop add a test function something similar to LCD_display_char j Low Power and Embedded Systems Workbook 3 Test your modified code 1 Usemake clean to remove any legacy o files and to force a recompile If you change config h then make will not see a change to the timestamp of the c files and will not recompile them You need to run make clean to remove the o files which forces a complete recompile in this case Program your device and check it works as expected You
4. may need to press the reset switch on the LCD interface board to give it a clean start Ifyou encounter problems check that the LED is flashing and that the Caeser cipher is working If those work fine look at LCD_CLK and LCD_DATA with an oscilloscope set to use a 10uS per division timebase Refer to serial_Icd h and try out the other functions which the library provides for example LCD_display_bin scroll_lcd_left The following section will prove very useful in the future but can be skipped if you are behind schedule Once you have the LCD working make your own 4 way cable using the crimp tool and parts provided You should find this straightforward Try your cable with the LCD Now have a go at using the Insulation Displacement IDC Connectors and large blue machine to make a 16 way lead and test that The key to a successful connection is keeping the ribbon cable square to the connector whilst avoiding squashed fingers Before the class next week create a display of your crsid which scrolls smoothly and readably in one of the four directions Show it to one of the demonstrators at the beginning of next weeks session You will need to think about your choice of mini project in particular if you want to try something which isn t on the suggestions list then talk to lan Wassell or Brian Jones as soon as possible to check that what you propose is suitable and also that it is possible to get the parts required in time f
5. ng but it does not take account of time spent servicing interrupts To get precise timing it is necessary to use a timer The ATMEGA168 has three timers TimerO and Timer2 are 8 bit and Timer1 is 16 bit Have a look at section 14 in the datasheet to get an overview of what the timers can do The requirement of this exercise is to take our ADC samples at exactly 1 second intervals One mode which can be selected for a timer is to count from zero up to a value held in an 8 bit register OCROA for TimerO OCR2A for Timer2 or a 16 bit register OCR1A for Timer1 When the timer reaches Low Power and Embedded Systems Workbook 3 the value held in the compare register it immediately resets the counter to zero issues an interrupt and continues counting from zero This mode is referred to as CTC mode and is the one we will use in this exercise One second is rather a long period with a master clock of 8 MHz so the 16 bit timer will be the appropriate one to use Even so at full clock rate this would only give a maximum period of 2 16 8 x 106 or approx 8 1 milliseconds A divider prescaler can be introduced between the 8 MHz master clock and the timer input which makes it possible to greatly increase the maximum time period achievable by the 16 bit counter The prescaler divider value P is selectable in a number of powers of 2 up to a maximum value of 2 16 As we have seen with a 16 bit timer the maximum period we can create is abou
6. oard Looking at the LCD interface board with the push switch at the top the connections are 5V CLK DATA GND top to bottom You need to connect PD7 from your microcontroller to the DATA line and PD6 to the CLOCK line so you can now choose a suitable place on the prototyping board to make the wiring up easy Wire up the connections to the 4 pin LCD interface board Connect the LCD interface board to your microcontroller using one of the 4 way leads supplied and to the LCD using the 16 way ribbon cable Check the supply to the LCD is correct and power up the board The LCD display should show TEST You may need to adjust the contrast potentiometer to see it clearly Software To use the LCD it is necessary to incorporate a library of low level drivers which take care of communicating via the LCD interface board For flexibility this library allows you to connect the LCD Low Power and Embedded Systems Workbook 3 via any two lines on the same port of your microcontroller For this exercise we will use PD6 and PD7 on PORTD For code portability the library uses generic names for controlling these pins and these need to be defined and the definitions made visible to both the library function and the main C program This is done as follows 1 Define the following in a file config h inthe workbook3 exercise2 directory define LCD_DDR DDRD define LCD_PORT PORTD define LCD_DATA PD7 define LCD_CLK PD6 def
7. or you to start in week 5
8. order Within the initialisation block set TCNT1H 0 TCNT1L 0 to clear Timer1 That completes the initialisation of Timer1 Low Power and Embedded Systems Workbook 3 1 Add a timeri interrupt service routine to start an ADC conversion when the timer reaches the count value Look up the appropriate ISR vector name from Interrupt Vector Table http www nongnu org avr libc user manual group__avr__interrupts html Make sure you choose Timer Counter1 Compare Match A for the ATMEGA168P Start the ADC conversion by setting the ADSC bit in ADCSRA see datasheet section 23 9 2 in the timer ISR Do not set the auto trigger bit ADATE You do not need to clear the timer counter register or take any action to clear the timer interrupt Create 3 global 8 bit variables that is declared outside main and outside any ISR or functions It is normal to declare these right at the top of your code adch will hold the hight byte of the latest adc reading adcl will hold the low byte of the latest adc reading gotadcvalue willbe used as a flag to mark whether there is an ADC value ready to be sent Declare gotadcvalue as volatile It must be declared volatile because it will be changed in one section of code the interrupt routine but tested in a loop in main Without the keyword volatile the compiler would optimise the loop test and never see that the variable had changed When the ADC conversion completes in the ADC in
9. t 8 1 ms We need to choose a prescaler so the maximum count 2 16 gives a period greater than 1 second The best match from the prescaler values available see section 15 11 3 is 256 Dividing by a higher number would reduce the resolution of the timer The period of the 16 bit timer input clock is now 0 125uS 256 32us To get a 16 bit timer period of 1s the 16 bit timer register has to be set to 1 32us 31250 or in hexadecimal 0x7A12 These can be written as a 16 bit value OCRIA 0x7A12 or if written as two 8 bit values they must be written high byte OCR1AH first then Low byte OCR1AL Take a copy of your code from workbook2 exercises to use as a Starting point for this exercise Add an initialisation block for the timer just as you did for the ADC Since interrupts are being used this block must be executed before interrupts are enabled Within the initialisation block set TCCR1A 0 ie normal port operation WGM11 WGM10 both zero Within the initialisation block set TCCR1B binary 00001100 i e CTC mode with the prescaler set to divide by 256 Within the initialisation block set TCCR1C 0 Force Output Compare is not being used Within the initialisation block set TIMSK1 as appropriate to generate an interrupt on Timer Output Compare Match A Within the initialisation block set OCR1A 0x7A12 A 16 bit assignment will work correctly the compiler will make sure the bytes are written in the correct
10. terrupt handler change the state of gotadcvalue and capture the readings into adcl and adch adcl first Note that we do not try to send the ADC value from within the interrupt routine it would take too long Since only one interrupt can be active at a time they must be kept as short as possible Calculate the time needed to transmit the largest ADC value possible via serial at 19200 baud including any trailing space or return characters What happens if this exceeds 1 second It is easy to remove an ISR but forget to disable the interrupt or alternatively get the wrong ISR name Without a suitable ISR the microcontroller will execute random code To avoid this problem add the following to your program catch all interrupt handler Note double underscore in the name ISR vector ceiraule 4 or ISR BADISR_vect Remove the _delay_ms functions from the main loop Move the LED on and LED off to the timer interrupt routine Change the LED state each interrupt giving a 0 5Hz flash rate The main loop will reduce to something like while 1 if gotadcvalue 1 USART_transmit_uint8 adch USART_transmit 0x0A USART_transmit 0x0D gotadcvalue 0 r Low Power and Embedded Systems Workbook 3 Program the device with your revised code and power up the device It should work exactly as Workbook 2 Exercise 3 except the 0 5 Hz should be precise Check using an oscilloscope Optionally
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