Low Power and Embedded Systems
Contents
1. Low Power and Embedded Systems Workbook 2 Introduction In this workbook we will cover Serial communications and the use of the Analogue to Digital Converter Supporting material atmega1 68P pdf http www cl cam ac uk teaching 0910 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 Lead http www cl cam ac uk teaching 0910 P31 docs usbseriallead pdf Specification for the Serial USB lead used in Exercise 1 USART_functions c http Awww cl cam ac uk teaching 0910 P31 code USART_functions c Library USART functions for exercise 3 connect_pot pdf http www cl cam ac uk teaching 0910 P31 docs connect_pot pdf Circuit diagram for connecting a potentiometer 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 workbook2 html http www cl cam ac uk teaching 0910 P31 workbook2 html You should check this page regularly for announcements and errata You might find it useful to refer to the on line version of this guide in order to follow any provided web links or to cut and paste example code A note on Register an2. so bit 7 of Ifuse is 1 inactive We still want CLKOUT active so bit 6 of Ifuse is 0 Work out new values for the Ifuse check them with the demonstrator then program the device fuses in the same way as you did for exercise 1 i e just using the make fuses command Programming the fuses needs to be followed by amake program a power cycle is not sufficient You previously copied workbook1 exercise2 exercise2 c to workbook2 exercise1 If no change has been made to the program then make program should cause the LED to flash at 14 746 Hz Change the F_CPU setting defined in your program you have copied from workbook1 exercise 2 recompile re program and verify that you have a 1Hz LED flash Remember from workbook 1 that the F_CPU setting must be correct for the _delay_ms and _delay_us functions to give the correct delays Update the comments in the file exercise1 c to reflect the new clock arrangements and how the fuse bits are now set Low Power and Embedded Systems Workbook 2 Once this has been achieved the first of the two problems has been solved the device is running from an accurate clock source USB connection As part of the equipment available to you there is an RS232 to USB converter lead This takes the form of a cable with USB type A plug on one end and a 6 way socket on the other It contains a voltage level converter and carries out the USB negotiation with the host A datasheet for this device is avai
3. caeser cipher part of the serial receive Interrupt routine ISR USART_RX_vect so that any byte received is transmitted straight back with minimal delay This will help with testing Add interrupt handling code for the ADC interrupt ISR ADC_vect so that the value in register ACDH is captured to a global variable We only require 8 bit precision and we chose a Left adjusted result so it is sufficient to read only ADCH In the main function start an ADC conversion just after setting the LED output high by writing a 1 to the ADSc bit in the register ADCSRA Add the USART_Transmit and USART_TransmitUint8 functions to your program by cut and paste from the section below or by following the link at USART_functions c http www cl cam ac uk teaching 0910 P31 code USART_functions c Inthe main function just after setting the LED output low add code to send the value captured from ADCH to the serial output using the function USART_TransmitUint8 Listing for USART_Transmit and USART_TransmitUint8 Low Power and Embedded Systems Workbook 2 Transmit an 8 bit value as is via serial void USART_Transmit unsigned char data while UCSROA amp 1 lt lt UDREO Wait for empty transmit buffer i UCSROA 1 lt lt TXCO clear txc flag UDRO data Put data into buffer sends the data Transmit an unsigned 8 bit value as a series of up to three decimal characters void USART_Tran
4. atasheet UBRROH somevalue UBRROL somevalue UCSROA 0 Turn off double speed add a value to Enable receiver and transmitter all other bits can be left 0 See section 19 10 of the datasheet UCSROB somevalue add a value to Set Asynchronous mode Disable parity 1 stop bit 8 data bits Clock polarity 0 i e not used UCSROC somevalue UCSROA 1 lt lt TXCO clear any existing transmits Low Power and Embedded Systems Workbook 2 Section 17 of the datasheet also has a function to transmit a byte which you can incorporate in your program Now write a function to transmit a null terminated string i e a string terminated with the byte 0x00 not the character 0 Finally add this to your LED flashing program so it both flashes the LED and transmits the string Hello world each second Program your microcontroller and test Debugging The circuit being used does not employ flow control The microcontroller will transmit data even if there is no device connected or there is no program on the PC that is listening to the data stream e You can monitor the TxD output of the microcontroller with an oscilloscope To select a suitable timebase consider that the Baud rate refers to the number of bits transmitted per second The microcontroller should be transmitting at 19200 baud which is roughly 50us per bit Test for the presence of the 14 746MHz clock on PBO e Is
5. avr_interrupts http www nongnu org avr libc user manual group__avr__interrupts html The compiler takes case of the housekeeping part of the interrupt call e g saving hardware register contents and return addresses As an example the Interrupt Service Routine run when the USART receives a complete byte is coded as Low Power and Embedded Systems Workbook 2 ISR USART_RX_vect TODO add code to process the byte received in UDRO Note The naming isn t entirely consistent you might expect the ISR name to be USARTO_RX_Vect and it is for later devices in the family 2 Changing the register setup so that an interrupt is generated when the correct condition occurs in this case a byte is received on the serial input 3 Clearing any conditions which would cause a spurious interrupt in this case clear any received data then Enable interrupts they are disabled at power up Now carry out these steps as follows Firstly cd to workbook2 exercise2 and copy the Makefile and exercise1 c to the working directory Modify exercise1 to exercise2 in the Makefile 4 places This way you retain a working program in exercise 1 which you can revert to if things stop working 1 Copy or type the skeleton ISR from the section above into your program Replace the TODO code with a function which does the following expressed in pseudo code read the received data from UDRO implement the caeser cipher wait unti
6. d other names In the examples which follow you will see references to register names such as UCSROA and bit position names such as RXCIEO These names and more importantly which ones are relevant depend upon the particular Atmel device being used They are defined in the library headers included at the top of the program for example include lt avr io h gt The angle brackets lt avr io h gt instruct the compiler to search for the header files in the standard library header location usr lib avr include which means the file io h will be found at usr lib avr include avr io h An instruction such as include config h note quotes instead of angle brackets instructs the compiler to search for the file config h in the current directory and its sub directories first then in the Low Power and Embedded Systems Workbook 2 standard library header location A later workbook will explain the use of a configuration file such as config hin more detail Variable definitions The ATMEGA168 microcontroller is an 8 bit device Unless you specifically need a 16 bit or wider variable you should define variables as 8 bits wide to save space The compiler accepts the following definitions and the workbook examples will usually use the form which specifies the width eg uint8_t e uint8_t is used to define an unsigned 8 bit variable identical to char int8_t is used for a signed 8 bit variable identical to signed char e ui
7. d reprogram the device to verify that the toolchain is in place and that the C code is in a working state If make responds make Nothing to be done for all then all output files hex elf o are up to date make will do nothing until a source file c changes or until you type make clean which removes the existing output files A subsequent make will recreate everything from scratch Low Power and Embedded Systems Workbook 2 Exercise 1 Hello World via a serial data link Notation The ATMEGA168 microcontroller has a single USART Universal Synchronous and Asynchronous Receiver and Transmitter For a full description of USART operation see section 17 of the datasheet Older ATMEL devices only ever had 1 USART and the data sheets and example code referred to The USART and its registers for example UCSRA Newer devices in the family may have more than one USART so for reasons of compatibility these devices which include the ATMEGA168 refer to the USART by number even if there is only one so the USART would be referred to as USARTO and the register names also include the number for example UCSROA The Exercise Transmitting data via a serial data link either using the RS232 standard or by USB is a common requirement There are two complicating factors that we will need to consider in this part of the exercise 1 Serial communication relies on a reasonably accurate clock for the Baud rate The internal oscilla
8. e potentiometer you may be reading the wrong input One test option would be to read the temperature sensor instead see section 23 8 but note that the voltage reference in this case should be the 1 1V internal one 10 Low Power and Embedded Systems Workbook 2 5 If the result is always very low 0 3 you might have the result right adjusted and only be seeing the highest 2 bits of a 10 bit result See section 23 9 for a description of the ADLAR bit in the register ADMUX 12
9. e will use the 5V supply to the device as the reference voltage To make the code implementation easier we will use an interrupt generated when the ADC conversion is complete Low Power and Embedded Systems Workbook 2 Software first Take a copy of your code from workbook2 exercise 2 to use as a Starting point for this exercise Check your IO port initialisation code The ADCO input which is also PCO PORTC bit 0 must be defined as an input Add a function to your code called init_adc and within this function Set the ADMUX register to a suitable value so that AVcc is used as the reference with a capacitor on ARef the ADLAR bit is set high so the result is left adjusted and select ADCO as the input Set the ADCSRA register to a suitable value to Enable the ADC Don t start conversion don t auto trigger switch on Interrupt Enable choose suitable values for the prescaler The prescaler value must be chosen to divide the 14 746 MHz clock by a suitable value to run the ADC clock at no more than 200kHz Set the ADCSRB register to a suitable value so that the free running mode the default mode is selected Disable the digital input to save power Section 23 9 5 shows details Note that bits must be set to 1 in DIDRO to disable the relevant digital input which is what we want to do for ADCO Make sure that this new function init_adc is called before the global interrupt enable flag is set Comment out the
10. erial device has been recognised and can now be referred to as dev ttyUSB1 Low Power and Embedded Systems Workbook 2 Start the minicom program in settings mode by typing minicom s Select serial port setup Select A and select the serial device as tt yUSB1 or whatever name you got from the dmesg command Type E and selecta baud rate of 19200 and a framing format with no parity 8 data bits and 1 stop bit which will be shown at the top of the Comm parameters window as current 19200 8N1 Make sure that both Hardware Flow control and Software Flow control are set to No With flow control enabled the system will freeze waiting for a condition which will never occur Type Save setup as dfl Select Exit to finish the configuration For a list of minicom commands type lt ctrl gt A Z Note to exit minicom type lt ctr1 gt A Q Microcontroller software Refer to section 19 and write a function to initialise the USART You will need to refer to section 19 11 to set the correct values for baud rate and framing format i e 19200 Baud with 8 data bits no parity and 1 stop bit This format is written in the style 19200 N 8 1 When calculating the throughput note that by default there is also 1 start bit so transmitting a byte takes 10 bit times Here is a partial template void USARTO_Init void add a value to Set baud rate to 19200 based on 14 746MHz clock and single speed U2X0 0 See section 19 11 in the d
11. l the transmit buffer is empty put the new value into the transmit buffer UDRO Note that UDRO is the register name used for both reading from the received data register and writing to the transmit register This may seem odd but the read register is read only the write register is write only and they are at the same physical address in the address space of the microcontroller to save address space Change the initialisation code for the USART so that an interrupt is generated when a complete byte of data is received See section 19 10 3 of the data sheet You will need to set the receive complete Interrupt enable bit RxCIEO in the UCSROB register As the final stage of initialisation after the interrupt setup you implemented in the previous step read from UDRO which clears any pending receive complete interrupts Set the global interrupt enable bit in the Status Register SREG see section 6 3 1 of the datasheet Note when testing In minicom use lt ctrl gt A E to turn local echo on or off If you type hello you should see khoor in minicom echo off or hkehloloor echo on Low Power and Embedded Systems Workbook 2 Make sure you also test boundary conditions such as the characters aAxyzZ Also note that the LED flashing is carried out by the main portion of the code and the serial receive and transmit is carried out entirely in the interrupt code The 1Hz flashing will now be even less accu
12. lable from http Avww cl cam ac uk teaching 0910 P31 docs usbseriallead pdf Also available is a home made right angled 6 way adaptor On this adaptor the thin pins fit into the prototyping board and the thicker square pins connect to the USB lead Important Make sure you only fit the small pins in the prototyping board the thick pins will damage the board Refer to the datasheets for the USB lead and the ATMEGA168 and connect the USB serial lead to the microcontroller as described below With all RS232 communications there is the potential for confusion over Transmit vs Receive The microcontroller uses TxD to refer to the pin from which it will transmit The USB lead uses the term TXD to refer to data transmitted by the PC towards the microcontroller Hence you need to connect TxD pin 3 on the microcontroller to Receive on the USB lead the yellow wire A schematic is shown here http Awww cl cam ac uk teaching 0910 P31 docs connect_serial pdf An example is shown here http www cl cam ac uk teaching 0910 P31 docs connect_serial jpg PC serial software Plug the USB serial lead into the PC Type dmesg at the terminal You should see something like usb 2 1 configuration 1 chosen from 1 choice tice Sie Z ilsil Oe WWD WS Seriell Devilee conweruer clerectecl Usisy Zaks DCERE CTCE O 2RN usb 2 1 FTDI USB Serial Device converter now attached to ttyUSB1 The important part to note here is the ttyUSB1 The s
13. nt16_t is used to define an unsigned 16 bit variable identical to unsigned int e int16_t is used for a signed 16 bit variable identical to int uint32_t is used to define an unsigned 32 bit variable identical to unsigned long Avoid using 32 bit values unless there really is no alternative e int32_t is used for a signed 32 bit variable identical to long e uint64_t is used to define an unsigned 64 bit variable identical to unsigned long long You won t need to use it The compiler makes a reasonable job of 16 bit values but 32 bit will cause slow bloated code 64 bit is far worse than that e int64_t is used for a signed 64 bit variable identical to long long Exercises Preparation The exercises this week follow on directly from Exercise 2 of Workbook1 Select embedded_systems workbook2 exercise1 as your working directory Take a copy of the C program for Exercise2 from Workbook1 which you can use as a Starting point for this exercise Also copy the Makefile as follows cd embedded_systems cd workbook2 exercisel cp workbookl exercise2 exercise2 c cp workbook1 exercisel Makefil mv exercise2 c exercisel c We will reuse the LED flashing part from the previous exercise during testing As a first test make sure that at switch on the hardware still runs Exercise 2 of Workbook1 It should run just as soon as power is applied To test the software compile the code for workbook2 exercise2 c an
14. rate because the _wait functions take no account of the time spent in the interrupt routine This will be addressed in a later exercise Exercise 3 Reading a value from the Analogue to Digital converter The objective of this exercise is to connect a potentiometer to one of the analogue inputs on the ATMEGA 168 read the resulting analogue voltage using the built in Analogue to Digital Converter and transmit the result as a decimal number using the serial interface once every second This builds upon the code already written for Exercise 2 You will need to refer to section 23 of the datasheet for details of the Analogue to Digital converter ADC The microcontroller has one 10 bit ADC which can be programmed to select its input from one of four IO pins The 10 bit result can be presented left adjusted or right adjusted in a 16 bit word The ADC inputs share pins with IO ports It is necessary to define those pins as inputs It is also possible to reduce power consumption of the device by powering off the circuitry within the device associated with the digital inputs The ADC requires a clock in the frequency range 0 200kHz to function This is derived from the master CPU clock currently 14 7465MHz called clkio The ADC also requires a reference voltage which must be higher than the highest input voltage to be applied As with the master clock to the ATMEGA168 this can be derived internally or externally supplied For this exercise w
15. smitUint8 uint8_t val unsigned char buf 3 int8_t ptr for ptr 0 ptr lt 3 ptr buf ptr val 10 0 val 10 for ptr 2 ptr gt 0 ptr if buf ptr 0 break for ptr gt 0 ptr USART_Transmit buf ptr Hardware We are using an external VCC reference voltage so we must connect AVCC to 5 V Itis safe to connect a potentiometer from 5V to GND with the wiper connected to the analogue input For stable readings AREF should be decoupled to ground with a 47nF or 100nF ceramic capacitor For a circuit diagram see connect_pot pdf http www cl cam ac uk teaching 091 0 P31 docs connect_pot pdf and connect_pot jpg Program the device with your revised code set the potentiometer to a middle position and power up the microcontroller Exercise 3 Debugging Things to consider when testing 1 Is the LED flashing If not consider whether the program ever gets that far or whether the program is executing an endless loop in an interrupt service routine 2 The loopback code is still in place so pressing a key on the terminal should immediately echo it back check that it isn t local echo in Minicom 3 If the result is always zero does the potentiometer have a 5V supply You can check the potentiometer voltage with a scope or multimeter Is there a reference voltage present at Aref Are you reading the correct input 4 lf the result from the ADC doesn t change when you adjust th
16. the LED on your prototyping board flashing at the correct rate of 1Hz e Is the transmit from the microcontroller correctly connected to receive on the USB serial device Exercise 2 Caeser Cipher Loopback In this exercise you will create a loopback that is to receive data from the PC and immediately transmit it back You will change the data to remove any confusion with the local echo function of the serial terminal program during testing The change to be made is to make a Caeser cipher Add 3 to the characters a z mapping x y z back to a b c and similarly for A Z Leave all others characters unchanged However there is an immediate problem If our device is in a loop waiting for characters from the serial input it cannot be doing anything else This is unsatisfactory since all other actions are blocked including flashing the LED The solution is to set up an interrupt so that when a byte is received an appropriate action is taken In this exercise the byte will be modified and sent back to the PC There is a problem with this approach which will be addressed in a later exercise but for the simple case in this exercise the problem can be ignored There are three steps to setting up the interrupt 1 The creation of a chunk of code called an Interrupt Service Routine ISR which will be called when the interrupt occurs The C compiler has a set of names defined in interrupt h for these ISRs There is a list of these ISR names
17. tor would be accurate enough but we will switch to using an external 14 746MHz crystal as the clock source which entails using the Full Swing Crystal Oscillator option for our microcontroller We derive the Baud rate clock for serial communication from this clock source 2 RS232 uses positive and negative voltage levels typically 9V 9 V which will require the use of a voltage level converter When viewing RS232 traffic with an oscilloscope note that a 1 is represented by a negative voltage a 0 by a positive one Not all PCs have serial ports and so an alternative is to use the USB interface on the PC although this requires some additional hardware In this exercise we will use a converter lead which takes care of the voltage level conversion and additionally carries out the USB negotiation when the lead is first connected to the host PC Microcontroller Clock Change the wiring on the prototype board to match that shown on page 31 of the datasheet using a 14 746MHz crystal and 33pF capacitors for C1 and C2 Note the Capacitors and crystal aren t polarised so you can fit them either way round An example photograph is available at crystal_connections jpg http www cl cam ac uk teaching 0910 P31 docs crystal_connections jpg Refer to section 8 2 of the datasheet for Clock sources and tables 8 5 and 8 6 for suitable SUT 1 0 and CLKSEL 3 0 settings for a Full Swing Crystal Oscillator with fast rising power We don t want the divide by 8
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