gistfile1.c

// Interrupt service routine called to generate PWM compare values
ISR(TIMER1_COMPA_vect) {
  
  // unsigned long == 32-bit
  unsigned long sine_0, sine_1;
  
  // These are the two ADC 'readings'.  I've added a check that ensures that each remains >= 1023. 
  unsigned long reading_0 = (unsigned long) adc_readings[0] > 1023 ? 1023: (unsigned long) adc_readings[0];
  unsigned long reading_1 = (unsigned long) adc_readings[1] > 1023 ? 1023: (unsigned long) adc_readings[1];
      
  // These are the two samples from the wavetable.  Also added checks that ensure that each remain >= 255.
  unsigned long wave_0 = (unsigned long) pgm_read_byte(&sinewave_data[sample_0]) > 255 ? 255 : (unsigned long) pgm_read_byte(&sinewave_data[sample_0]);
  unsigned long wave_1 = (unsigned long) pgm_read_byte(&sinewave_data[sample_1]) > 255 ? 255 : (unsigned long) pgm_read_byte(&sinewave_data[sample_1]);
      
  // Generate sine wave for each channel.
  sine_0 = reading_0 * wave_0;
  sine_1 = reading_1 * wave_1;

  // Sum sine waves and update compare register.  Have tried shifting further.  This produces no output.
  OCR2A = (sine_0 + sine_1) >> 11;

  // These both work, however, so I'm clearly having an overflow issue:
  // OCR2A = sine_0 >> 10;
  // OCR2A = sine_1 >> 10;
}

// Tested with this in another program:
void setup() {
  // Force maximum value for mock ADC readings.
  unsigned long reading_0 = 1023;
  unsigned long reading_1 = 1023;
  
  // Force maximum value for mock wavetable readings.
  unsigned long wave_0 = 255;
  unsigned long wave_1 = 255;
  
  // Multiply those.
  unsigned long sine_0 = reading_0 * wave_0;
  unsigned long sine_1 = reading_1 * wave_1;
  
  // Look at sums both as unsigned longs and as chars.
  unsigned long compare_ul = (sine_0 + sine_1) >> 11;
  unsigned char compare_char = (sine_0 + sine_1) >> 11;
  
  // Printing out all of the above confirms that the arithmetic is NOT overflowing. I get 254 for both
  // compare_ul and compare_char, as expected.
}