Sine wave frequency vs. PWM frequency (AVR)

Question

I'm using an AVR Mega 168 to generate a sine wave via pulse width modulation. My goal is to generate a sine wave with a frequency of approximately 50 Hz. I understand that the PWM frequency needs to be as fast possible so its easier to attenuate in the filter.

So far, my µC is running at 8 MHz using the internal RC oscillator (I plan on moving to a crystal later on - taking this one step a time). This provides me with a maximum PWM base frequency of 8x10⁶/256 (fast PWM mode) = 31.25 kHz.

My program updates the duty cycle whenever timer0 overflows i.e. every 32 µS (I think... since 1/31250 = 32 µS).

My question now is, how can I 'control' the frequency of the actual sine wave? I can easily slow down by prescaling but that would obviously make the base frequency quite low.

I have included my code below:

uint8_t i=0;


int main(void)
{
     int ipwm;

     DDRD = 0xFF;

     TCCR0A = 0b10000011;
     TCCR0B = (1 << WGM12);

     TIMSK0 |= (1 << TOIE0);
     TIFR1 |= (1 << OCF0A);
     TCCR0B = (1 << CS10);
     TCNT0 = 0;

     OCR0A = 128;

     sei();

     for(;;)
     {

     }

     return 1;
}

 ISR(TIMER0_OVF_vect)
 {
    OCR0A=pgm_read_byte(&sinewave[i]); // update duty cycle
    i++;
 }

Answer 1

The 31.25kHz is 625x your 50Hz. So all you have to do is build a 625 entries sine lookup table, and at each interrupt set your output compare value to the next table entry. If you have a 32\$\mu\$s interrupt you'll have ran over the full table (=full sine cycle) every 625 x 32\$\mu\$s = 20ms, so that will be your 50Hz.