Category Archives: DCC

Hacking DCC, Part 1

So, this is interesting.  I finally got a chance to use my oscilloscope for actual scientific purposes!

I figured it would be possible, but I wasn’t quite sure how, to use the ATMEGA328P’s timers to create two opposing square waves.  Turns out, it’s pretty easy to make one waveform, but to output both…

Actually, it turns out that’s pretty easy too.  All it takes is a FOC1B to make OC1B spin on a different cycle, 180° in opposition to OC1A.  OC1A and OC1B output on PB1 and PB2, respectively.

Square waves in opposition

Cool.  The pink signal is OC1A, and the green one is OC1B.  This picture is actually from a little bit later in the game, when I was manipulating the signal to send pulses of varying length.  The longer pulses represent 0, and the shorter ones represent 1.

What on Earth will happen when I hook up PB1 and PB2 to an H-bridge, and give it 15 volts or so?  It becomes a modulated AC signal, that can be used to transmit power and data down the same two wires! 🙂

But I did notice, after all this, that OC1B is a little bit delayed.  But I think I can live with a delta of 569 nanoseconds.

OC1B follows OC1A slowly

Next, I need to sort out how to disrupt the signal entirely, for approximately 454 microseconds.

Not for the faint of heart: here’s some code.  I do the JavaScript thing in my day job, Bitwise AVR register manipulation is a little bit… weird… if you ask me.

void startDcc() {
    // Toggle both OC1A and OC1B on compare match
    TCCR1A &= ~(_BV(COM1A1) | _BV(COM1B1));
    TCCR1A |= (_BV(COM1A0) | _BV(COM1B0));

    // Set waveform generation mode to CTC
    // Using OCR1A for comparison
    TCCR1A &= ~(_BV(WGM11) | _BV(WGM10));
    TCCR1B &= ~(_BV(WGM13));
    TCCR1B |= (_BV(WGM12));
    
    // Set clock source to 8x prescaler
    TCCR1B &= ~(_BV(CS12) | _BV(CS10));
    TCCR1B |= (_BV(CS11));

    // Force output compare on OC1B
    TCCR1C |= (_BV(FOC1B));
    
    // Set default waveform (string of 1's)
    OCR1A = BIT_1_LENGTH;
    
    // Set pin modes to output
    // PB2, PB1
    DDRB |= (_BV(DDB2) | _BV(DDB1));
    PORTB &= ~(_BV(PORTB2) | _BV(PORTB1));

    //Interrupts.
    TIMSK1 |= (_BV(ICIE1) | _BV(OCIE1A));
}