Generating a short pulse after a delay on ATmega 328/P

Question

I am trying to use one of the hardware timers on the ATmega 328/p (arduino uno) to generate a short pulse some number of microseconds after receiving a pulse on an input.

Currently my code looks like this:

uint16_t pulse_delay = 12000; //half-microseconds
uint16_t pulse_length = 20;

void setup(){
  pinMode(8, INPUT);
  pinMode(9, OUTPUT);
  TCCR1A = 0;
  TCCR1B = _BV(ICNC1)  //input capture noise cancel
      | _BV(ICES1) //positive edge
      | _BV(CS11); // /8 prescaler
  TIMSK1 = _BV(ICIE1); //enable input capture interrupt
}
void loop(){}
ISR(TIMER1_CAPT_vect){
  TCCR1A = _BV(COM1A0) | _BV(COM1A1); //set OC1A on match
  TIMSK1 |= _BV(OCIE1A); //enable match interrupt
  OCR1A = pulse_delay; //pulse begin time

  TCNT1 = TCNT1 - ICR1; //TCNT1 now contains time since input pulse, even if 
                        //the interrupt isn't run immediately
}
ISR(TIMER1_COMPA_vect){
  TIMSK1 &=~ _BV(OCIE1A); //disable match interrupt
  TCCR1A = _BV(COM1A1); //clear OC1A on match

  OCR1A = pulse_delay + pulse_length;
}

This code should theoretically do the task, but it doesn't produce any output at all - to my oscilloscope it looks like the output pin just stays low.

However, if I replace the last line (OCR1A = pulse_delay + pulse_length;) of the compare match interrupt with the following two lines, it outputs a pulse just fine. The issue with this is that it uses significantly more CPU time, and it can only count time from when the interrupt starts so if the interrupt is executed late the pulse will be longer.

  delayMicroseconds(pulse_length);
  TCCR1C = _BV(FOC1A); //manually trigger match event

All that the first version is doing differently is triggering the match event via an 'alarm' set on the timer, rather than waiting to trigger the match manually.

Why does the first version not work, and how can I make it work??

Answer 1

I didn't read the codes that closely but it shouldn't this complex. Assuming the desired duration is reasonable large vs. Isr overhead, all you need is to use the signal to trigger an Isr (a timer cannot be set up as an external interrupt) and in that Isr ttriigger a series of delayed pin actions.

The situation gets tricky if the desired duration is very short.