AVR Freaks

I ran an experiment to test using the fast PWM mode of timer0 for duty cycle control.  I learned something.

I wanted to set up the timer so that I could control period and duty cycle of timer0 to generate an output signal.

I set up the OCR0A to be the period (i.e., TOP, value 7) and OCR0B to be the duy cycle (i.e., the "match", value 0-7).

I also set up COM0B to clear on match, set on BOTTOM.  For a 16 MHz clock the period was 0.5 us and the logic high varied from 1 to 8 cycles.

But I wanted 0 to 7 cycles.   That requires using the COM0B to be inverted: set on match, clear on BOTTOM.

The sample program sets up an 8 cycle PWM (from 16 MHz => 0.5 usec period).

Here is the timing diagram:

Here is a scope trace:

and the code.   My intent is to have a controller set d_num and d_den every 100 us.

#include <avr/io.h>
#include <avr/interrupt.h>
#include <util/delay.h>

volatile uint8_t d_num = 2;		/* duty cycle numerator */
volatile uint8_t d_den = 7;		/* duty cycle denominator  */

void timer0_setup(void) {
  /* use Fast PWM mode => WGM0 = 0b111
   * clr OC0B on match, set OC0B on BOTTOM => COM0B = 0b10, COM0A = 0b00
   * slow /256 clock for now => CS0 = 0b100 (fast clock => CS0 = 0b001)
   * => OCR0A is period, OCR0B/OCR0A is duty cycle, output on pin TBD
   * OC0B is board pin D3 PD5 on arduino micro
   * w/ 0x09 get 620.0 ns / 4065.0 ns = 0.1525 vs 1/8 = 0.1250
   */
  TCCR0A = 0x23;	      /* COM0A=0, COM0B=0b10, WGM[1:0]=0b11 */
  TCCR0A = 0x33;	      /* COM0A=0, COM0B=0b11, WGM[1:0]=0b11 */
  TCCR0B = 0x09;	      /* WGM[2]=0b1, CS0=0b001=/1 */
  TIMSK0 = 0;		      /* no interrupts */
  OCR0A = d_den;	      /* default */
  OCR0B = d_num;	      /* default */

  DDRD |= _BV(PIN0);		   /* Pin D0 is DC-DC switch = OC0B */
}

#define CYCLE 2

int main(void) {
  uint8_t i, n;
  uint16_t k = 0;

  /* "The setup of the OC0x should be performed before setting the 
   * Data Direction Register for the port pin to output." - sec 13.4.3 */
  timer0_setup();
  
  /* blink led w/ duty cycle */
  DDRC |= _BV(PIN7);
  while (1) {
    if (k++ >= CYCLE) {
      d_num = (d_num + 1) % 8;
      OCR0B = d_num;
      k = 0;
    }
    n = (d_num % 8) + 1;
    
    PORTC |= _BV(PIN7);
    for (i = 0; i < n; i++) {
      _delay_ms(200);
      PORTC &= ~_BV(PIN7);
      _delay_ms(200);
      PORTC |= _BV(PIN7);
    }

    PORTC &= ~_BV(PIN7);
    for (i = n; i < 10; i++) {
      _delay_ms(200);
      _delay_ms(200);
    }
  }
}