i have this simple code that i use for generating a square wave signal of 125 KHz on pin 9 using Timer1 in the "Clear Timer on Compare Match (CTC) Mode". I also use Timer1 Compare Match A interrupt for toggling pin 7 of PORTD.
void setup() {
pinMode(9, OUTPUT);
pinMode(7, OUTPUT);
TCNT1 = 0;
TCCR1A = 0;
TCCR1B = 0;
OCR1A = 7;
TIMSK1 = _BV(OCIE1A);
TCCR1A = _BV(COM1A0);
TCCR1B = _BV(CS11) | _BV(WGM12);
}
void loop() {
}
ISR(TIMER1_COMPA_vect)
{
PORTD = PORTD ^ 128;
}
It happens that
The 125KHz waveform on pin 7 is delayed by 1us with respect to the waveform of pin 9. I think that is related to the time that the microcontroller takes to "entering" the interrupt function and toggling the PORTD pin (1us are 16 clock cycle of the 16MHz master clock).
Sometimes the waveform on pin 7 (the one generated inside the interrupt service routine) has some "gaps". For example, this is the waveform acquired with my scope:enter image description hereWhy this strange behaviour happens?
Thank you
1 Answer 1
This isn't the only ISR
in the Arduino, there is at least the one counting millis
. Counter 0 overflow ISR is not as short as yours, so every 1024us
it'll cause some jitter (or as in your case, skipping the edge and basically inverting the phase). It'd be also several clock cycles delayed (against HW generated signal). Pushing and poping the registers into the stack are the deterministic delays, but the latency between setting the ISR
flag and calling the ISR
routine depends on currently executed instruction (some of them takes 2cycles, and there are also longer ones).
There is not much time to do anything else either. Your ISR
might take less than 20
cycles, but you only have 64
cycles (ISR
is called at 250kHz
rate so it generates 125kHz
signal).
So if anything else prevents your ISR
to be called (another ISR
, disabled interrupts), you can miss them, as there is only one interrupt flag.
If you want the same waveform on pin 7, just use a wire and let the pin mode as INPUT
.
BTW: You don't have to use PORTB = PORTB ^ _BV(PB7);
, the PINB = _BV(PB7)
toggles PB7 without read-modify-write access.
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This is a good oportunity to talk about how to keep things in sync (drifting, missing beats).user31481– user3148112/27/2017 14:18:45Commented Dec 27, 2017 at 14:18
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Thank you for the detailed explanation! Also setting TCCR0A and TCCR0B to zero solved the problems with the "gaps".Anacleto– Anacleto12/27/2017 14:46:13Commented Dec 27, 2017 at 14:46
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@Anacleto but be aware of using anything related to
millis
likedelay
. It's exactly the same problem as you can't use thedelay
in another ISR.KIIV– KIIV12/27/2017 14:49:14Commented Dec 27, 2017 at 14:49
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