Simultaneously start multiple timers

[emiel.h]

For a project which requires precise timings, I'm trying to align three PWM signals from three different timers; one 12 MHz, one 1.5 MHz and one 0.75 MHz signal.
As far as I understand this would require all three timers to start counting at the same time from 0.
The only problem is that I can't seem to find a way to start the timers simultaneously; when I run FTM2_SC = FTM1_SC = FTM3_SC = FTM_SC_CLKS(1) | FTM_SC_PS(0); every register is set after each other, which already creates a time offset between the timers.
What I tried in the code below is to give the timers an specific offset before they start, which would make them align when they get started sequentially.

For Teensy 3.6:

// For Teensy 3.6
// 12 MHz on pin 3
// 0.75 MHz on pin 29
// 1.5 MHz on pin 35

void setup() {
  FTM1_SC = 0;
  FTM1_CNT = 0;
  FTM1_MOD = 4;
  FTM1_C0SC = 0x28;
  FTM1_C0V = 2;
  CORE_PIN3_CONFIG = PORT_PCR_IRQC(1) | PORT_PCR_MUX(3);


  FTM2_SC = 0;
  FTM2_CNT = 0;
  FTM2_MOD = 79;
  FTM2_C0SC = 0x28;
  FTM2_C0V = 40;
  CORE_PIN29_CONFIG = PORT_PCR_IRQC(1) | PORT_PCR_MUX(3);


  FTM3_SC = 0;
  FTM3_CNT = 0;
  FTM3_MOD = 39;
  FTM3_C4SC = 0x28;
  FTM3_C4V = 5;
  CORE_PIN35_CONFIG = PORT_PCR_IRQC(1) | PORT_PCR_MUX(3);
}

void loop() {
  syncStart();
  delayMicroseconds(100);
  stop();
  delay(10);
}

void stop() {
  FTM1_SC = 0;
  FTM2_SC = 0;
  FTM3_SC = 0;
}

void syncStart() {
  noInterrupts();

  FTM2_SC = 0;
  FTM2_CNT = 0;

  FTM1_SC = 0;
  FTM3_SC = 0;

  FTM1_CNTIN = 3;
  FTM1_CNT = 0;
  FTM1_CNTIN = 0;

  FTM3_CNTIN = 36;
  FTM3_CNT = 0;
  FTM3_CNTIN = 0;

  FTM2_SC = FTM1_SC = FTM3_SC =  FTM_SC_CLKS(1) | FTM_SC_PS(0);  // start the timers

  interrupts();
}

This seems to work ok, but it produces a small error at the beginning; when starting the timers the output of the channel will always go to HIGH; the following two screenshots explain more than a thousand words:
0.75 MHz & 12 MHz: View attachment SDS00002.BMP
0.75 MHz & 1.5 MHz: View attachment SDS00003.BMP

I assume there is a better way of doing this, one that also doesn't depend on the delay between the execution of timer start commands.
I would love to hear any suggestions!
Thanks!

 

[PaulStoffregen]

You could try configuring the FTM timers to use an external clock, instead of F_BUS. I believe pins 0 and 1 (PTB16 & PTB17) can be used if their config mux is set to ALT4. Look at the SIM_SOPT4 register for configuring how the timers get their clocks.

There's also a hardware mechanism for sync'd trigger documented on section 45.1.6 on pages 1128-1129.

I have not personally tried either of these. If you do, I hope you'll share anything you learn?

 

[emiel.h]

Thanks! I'll look into both options and report back

 

[Theremingenieur]

I'd use the Teensy just to generate the 12MHz clock and use an external 74HC binary counter as a divider by 8 and by 16 to get 1.5 and 0.75 Mhz.