Question

I’m writing a program for an ATMega328P that will take readings from several ADC channels, combine them into a single signal and output this signal through PWM.

I’ve successfully backed off my ADC polling to 50Hz per channel using Single Conversion mode. I’m using Timer/Counter2 for PWM generation, and Timer/Counter1 for doing the calculations I need to do to set compare values for Timer/Counter2. This is the ISR for Timer/Counter1:

// Interrupt service routine called to generate PWM compare values
ISR(TIMER1_COMPA_vect)
{
    // Grab most recent ADC reading for ADC0
    uint32_t sensor_value_0 = adc_readings[0];

    // Get current value for base waveform from wavetable stored in sinewave_data
    uint32_t sample_value_0 = pgm_read_byte(&sinewave_data[sample_0]);

    // Multiply these two values together
    // In other words, use the ADC reading to modulate the amplitude of base wave
    uint32_t sine_0 = (sample_value_0 * sensor_value_0) >> 10;

    // Do the same thing for ADC2    
    uint32_t sensor_value_1 = adc_readings[1];
    uint32_t sample_value_1 = pgm_read_byte(&sinewave_data[sample_1]);
    uint32_t sine_1 = (sample_value_1 * sensor_value_1) >> 10;

    // Add channels together, divide by two, set compare register for PWM
    OCR2A = (sine_0 + sine_1) >> 1;

    // Move successive ADC base waves through wavetable at integral increments
    // i.e., ADC0 is carried by a 200Hz sine wave, ADC1 at 300Hz, etc.
    sample_0 += 2;
    sample_1 += 3;

    // Wrap back to front of wavetable, if necessary
    if (sample_0 >= sinewave_length) {
        sample_0 = 0;
    }

    if (sample_1 >= sinewave_length) {
        sample_1 = 0;
    }
} // END - Interrupt service routine called to generate PWM compare values

My problem is that that I get no PWM output. If I set either sensor_value_0 or sensor_value_1 to 1024 and leave the other sensor_value_ set to read from the ADC, I do get one full-amplitude component wave, and an amplitude-modulated component wave. If however, I choose a different value for the hardcoded, mock amplitude, I am not so lucky (1023, for instance). Any other values give me no PWM output. If I set both sensor_value_s to look at the same ADC channel, I would expect two component waves whose amplitudes are modulated identically. Instead, I get no PWM output. What is most confusing of all to me is that if I choose a value for the hardcoded amplitude that is an exact power of two, all is well.

The whole power-of-two part makes this seem to me to be a bit-twiddling issue that I’m not seeing. Can you see what I must have clearly missed? I’d appreciate any tips at all!

(I’ve posted my entire source here to keep things as neat as possible on SO.)

Answer 1

@Devrin, I appreciate the response, but just manipulating types didn’t do it for me. Here’s what I ended up doing:

uint8_t sine_0 = (pgm_read_byte(&sinewave_data[sample_0]) >> 5) * (adc_readings[1] >> 5);
uint8_t sine_1 = (pgm_read_byte(&sinewave_data[sample_1]) >> 5) * (adc_readings[2] >> 5);
OCR2A = (sine_0 >> 1) + (sine_1 >> 1);

Essentially, I’ve done all my shifting immediately, instead of waiting until the last minute. Unfortunately, I lose a lot of precision, but at least the code works as expected. Now, I wil begin cranking things back up to find the initial cause of my issues.