Phase shifted PWM outputs - shift changing unexpectedly?

Official Hardware Nano Family Classic Nano
1

I have been using the code below to create two offset square waves. It works - with one caveat I can't seem to overcome:

The requested phase shift is 90 degrees. However, running the loop below, it will occasionally give me a shift of 270 degrees instead.

I've set the outputs into toggle mode - and if I missed a toggle, then I can see it would cause these symptoms - but I can't for the moment work out why this might be occuring. Any ideas...?

int ttlA = 9;
int ttlB = 10;

void setup() {
  Serial.begin(9600);
  pinMode(ttlA, OUTPUT);  // Arduino Pin  9 = OCR1A
  pinMode(ttlB, OUTPUT);  // Arduino Pin 10 = OCR1B

  // Set both outputs in toggle mode
  TCCR1A = 0x00;
  bitSet(TCCR1A, COM1A0);
  bitSet(TCCR1A, COM1B0);

  TCCR1B = 0x00;
  bitSet(TCCR1B, WGM12);
  bitSet(TCCR1B, WGM13);

  OCR1A = 0;  // First output is the base, it always toggles at 0
}

// prescaler of 1 will get us 8MHz - 122Hz
// At 16Mhz, toggling every tick (clocks per toggle=1), we toggle at 16MHz, which gives us 8Mhz frequency
// ICR1 is a 16 bit register, so a maximum value of 2^16=65536
// if clocks per toggle is 65536, then we get 122Hz

#define CLK 16000000UL  // Clock speed is 16MHz on Arduino Uno

int setWaveforms(unsigned long freq, int shift) {
  //Stop the timer whilst we change things and re-zero the timer registers
  TCCR1B = 0x00;

  unsigned long clocks_per_toggle = (CLK / freq) / 2;  // /2 becuase it takes 2 toggles to make a full wave
  ICR1 = clocks_per_toggle;
  unsigned long offset_clocks = (clocks_per_toggle * shift) / 180UL;  // Do mult first to save precision
 
  OCR1A = 0;              //counts until TCNT = O then toggles
  OCR1B = offset_clocks;  //counts until TCNT = offset_clocks then toggles

  // Turn on timer
  TCNT1H = 0x00;
  TCNT1L = 0x00;
  bitSet(TCCR1B, WGM12);
  bitSet(TCCR1B, WGM13);
  bitSet(TCCR1B, CS10);
}

void loop() {
	setWaveforms(1000, 90);
    delay(4000);
	setWaveforms(1500, 90);
    delay(4000);
	setWaveforms(2000, 90);
    delay(4000);
	setWaveforms(2500, 90);
    delay(4000);
}
2

As a workaround, I'd set both GPIO's to a defined state in setup() as well as setWaveforms. This way you know what the starting position is.

3

Good idea - I added:

digitalWrite(ttlA, 0);
  digitalWrite(ttlB, 0);

to both setup and in setWaveforms (just before reenabling timer1)
However, on a scope I can still see the pase shift is sometimes 90 degrees, and sometimes 270....

4

Setting control bits one by one will result in possibly unwanted timer states. Better set the bits in a temporary variable and only write the final value to a timer register.

1 Like
5

Good idea - I had stopped the timer when editing values to try to avoid that, but have now explicitly used things like:

  byte temp=0x00;
  bitSet(temp, WGM12);
  bitSet(temp, WGM13);
  bitSet(temp, CS10);
  TCCR1B=temp;

However, whilst a bit neater, this doesn't seem to have changed the occasional phase shifts I am seeing

6

Yeah, it's puzzling. I couldn't explain the phase shift from the non-atomic bit operations, so I'm not too surprised that this modification hasn't been effective.

Sorry to be of little help!

7

This repo came to my rescue:

It looks very similar to what I had already, with the key difference that it works reliably!

I'll do some digging to try to work out why that is, but for now, all is good :slight_smile:

8

It sounds like you may be using a Nano, Uno or 2560 in CTC mode (toggle). If so, it will occasionally flip-phase 180 degrees when trying to adjust the phase-shift angle during run-time.

You can get around this using the Forced Output Compare registers to clean things up during the phase-change. The idea is, stop Timer, reset wavegens to LOW and re-start timer.

There is an example project at Runtime Micro that illustrates how to make run-time changes and achieve (no phase-flip) results. The down-side is your O-Scope won't like the Stop-Start action of code during phase-angle changes. The good news is the phase-flip is gone.

hth

1 Like
9

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