AVR Timer/Counter. Prof Prabhat Ranjan DA-IICT, Gandhinagar

Transcription

1 AVR Timer/Counter Prof Prabhat Ranjan DA-IICT, Gandhinagar

2 8-bit Timer/Counter0 with PWM Single Compare Unit Counter Clear Timer on Compare Match (Auto Reload) Glitch-free, Phase Correct Pulse Width Modulator (PWM) Frequency Generator External Event Counter 10-bit Clock Prescaler Overflow and Compare Match Interrupt Sources (TOV0 and OCF0)

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5 16-bit Timer/Counter The 16-bit Timer/Counter unit allows accurate program execution timing (event management), wave generation, and signal timing measurement. The main features are: True 16-bit Design (i.e., Allows 16-bit PWM) Two Independent Output Compare Units Double Buffered Output Compare Registers One Input Capture Unit Input Capture Noise Canceler Clear Timer on Compare Match (Auto Reload)

6 16-bit Timer/Counter Glitch-free, Phase Correct Pulse Width Modulator (PWM) Variable PWM Period Frequency Generator External Event Counter Four Independent Interrupt Sources (TOV1, OCF1A, OCF1B, and ICF1)

7 8-bit Timer/Counter2 General purpose, single compare unit, 8-bit Timer/Counter module Single Compare unit Counter Clear Timer on Compare Match (Auto Reload Glitch-free, Phase Correct Pulse Width Modulator (PWM) Frequency Generator 10-bit Clock Prescaler Overflow and Compare Match Interrupt Sources (TOV2 and OCF2) Allows clocking from External 32 khz Watch

8 Based on Newbie's Guide to AVR Timers - Dean Camera

9 Introduction AVR timers are very versatile but at the same time complex Asynchronous to man AVR core Separate circuits on the AVR chip which can run independent of the main program, interacting via the control and count registers, and the timer interrupts Timers can be configured to produce outputs directly to pre-determined pins, reducing the processing load on the AVR core

10 Like all digital systems, the timer requires a clock in order to function As each clock pulse increments the timer's counter by one, the timer measures intervals in periods of one on the input frequency: Code: Timer Resolution = (1 / Input Frequency) Smallest amount of time - one period of the incoming clock signal. If we supply a 100Hz signal : Code: Timer Resolution = (1 / Input Frequency) Timer Resolution = (1 / 100) Timer Resolution =.01 seconds

11 MHz Assume the target to be a MEGA16, running at at 1MHz clock. Modern AVRs come running off their internal ~1MHz RC oscillator by default (will be slightly incorrect due to the RC frequency tolerance). In the sections dealing with toggling a LED, it is assumed to be connected to PORTB

12 int main (void) { Simple Example (Flash LED every 1/20 s) DDRB = (1 << 0); // Set LED as output TCCR1B = (1 << CS10); // Set up timer for (;;) { // Check timer value in if statement, true when count matches 1/20 of a second if (TCNT1 >= 49999) { PORTB ^= (1 << 0); // Toggle the LED } TCNT1=0 : //Reset timer value } }

13 Timer/Counter1 Control Register B TCCR1B Bit 2:0 CS12:0: Clock Select Bit 4:3 WGM13:2: Waveform Generation Mode Bit 5 Reserved Bit Bit 6 ICES1: Input Capture Edge Select Bit 7 ICNC1: Input Capture Noise Canceler

14 Clock Select Bit (CSxx)

15 Prescaler Value 1MHz 1 1uS 8 8uS 64 64uS uS uS

16 Value to count 1/20 s = 0.05 s Clock minimum interval = 1 µs = 1.e-06 s Count = 0.05 s/ (1.e-06 s) = = 49999

17 Pre-scaled Timer Need to measure longer interval, you can prescale clock before bring fed to timer If you want to create exact 1 second Prescaler Value Target Timer Count

18 int main (void) { DDRB = (1 << 0); // Set LED as output TCCR1B = ((1 << CS10) (1 << CS11)); // Set up timer at Fcpu/64 for (;;) { // Check timer value in if statement, true when count matches 1 second if (TCNT1 >= 15624) { PORTB ^= (1 << 0); // Toggle the LED TCNT1 = 0; // Reset timer value } } }

19 CTC Timer Mode CTC Clear on Timer Compare Compares in hardware the current timer value against the wanted value, and when the wanted value is reached a flag in a status register is set and the timer's value reset Handy - comparing is done in hardware checking status register much faster than comparing bytes

20 Pseudo Code Set up LED hardware Set up timer in CTC mode Set timer compare value to one second WHILE forever IF CTC flag IS EQUAL TO 1 END IF THEN Toggle LED Clear CTC flag END WHILE

21 DDRB = (1 << 0); // Set LED as output TCCR1B = (1 << WGM12); // Configure timer 1 for CTC mode OCR1A = 15624; // Set CTC compare value to 1 Hz at 1 MHz AVR clock, with a prescaler of 64 TCCR1B = ((1 << CS10) (1 << CS11)); // Start timer at Fcpu/64

22 for (;;) { if (TIFR & (1 << OCF1A)) { PORTB ^= (1 << 0); // Toggle the LED TIFR = (1 << OCF1A); // clear the CTC flag (writing a logic one to the set flag clears it) } }

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24 CTC Mode using Interrupts AVR timers can have several different Interrupts Overflow Compare Capture Consider only the Compare interrupt, which occurs in CTC mode when the compare value is reached

25 Pseudocode Set up LED hardware Set up timer in CTC mode Enable CTC interrupt Enable global interrupts Set timer compare value to one second WHILE forever END WHILE ISR Timer Compare Toggle LED END ISR

26 Setting Registers DDRB = (1 << 0); // Set LED as output TCCR1B = (1 << WGM12); // Configure timer 1 for CTC mode TIMSK = (1 << OCIE1A); // Enable CTC interrupt sei(); // Enable global interrupts OCR1A = 15624; // Set CTC compare value to 1Hz at 1MHz AVR clock, with a prescaler of 64 TCCR1B = ((1 << CS10) (1 << CS11)); // Start timer at Fcpu/64 for (;;) { }

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28 ISR ISR(TIMER1_COMPA_vect) { } PORTB ^= (1 << 0); // Toggle the LED

29 PURE HARDWARE CTC Look at COM1A1/COM1A0 and COM1B1/COM1B0 Allow us to control the hardware behaviour when a compare occurs Configured to set, clear or toggle the OCxy (where "x" - timer number, "y" - channel letter for timers with more than one channel) hardware pins when a compare occurs.

30 Output Compare Pins OC0 -> PB3 OC1B -> PD4 OC1A -> PD5 OC2 -> PD7

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32 Code Fragment DDRD = (1 << 5); // Set LED as output TCCR1B = (1 << WGM12); // Configure timer 1 for CTC mode TCCR1A = (1 << COM1A0); // Enable timer 1 Compare Output channel A in toggle mode OCR1A = 15624; // Set CTC compare value to 1Hz at 1MHz AVR clock, with a prescaler of 64 TCCR1B = ((1 << CS10) (1 << CS11)); // Start timer at Fcpu/64 for (;;) { }

33 Pulse Width Modulation Ref :

34 PWM Waveform Terms

35 12.5% Duty Cycle

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37 AVR Timer/Counter Sequence for fast PWM

38 Output Compare Register - OCR0

39 PWM Generation

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