Using timer2's ISR instead of timer1's in Arduino wiring.c breaks Serial object

Question

I want to change timer0 frequency because I need to use 4 PWM pins at a frequency lower than 500Hz and the SPI on an ATmega328. So I changed timer2 to Fast PWM with 64 divisor, enabled the overflow interrupt for timer2 and disabled the overflow interrupt for timer0:

 TCCR2A = _BV(WGM21) | _BV(WGM20);
 TCCR2B = _BV(CS22);
 TIMSK2 |= (1 << TOIE2); // enable overflow interrupt for timer2
 TIMSK0 &= ~(1 << TOIE0); // disable overflow interrupt for timer0

and changed the ISR vector in wiring.c from ISR(TIMER0_OVF_vect) to ISR(TIMER2_OVF_vect).

But delay doesn't work. What can be the problem?

• Code Gorilla: I would guess Serial and delay rely on timer 2 and you have just changed it.
• Nick Gammon: HardwareSerial does not use timers. delay uses Timer 0.
• Nick Gammon: Please post a Minimal, Complete, and Verifiable example.
• Nick Gammon: I don't know if millis() and micros() work - why not? Write a "blink" sketch and find out.

Answer 1

Now everything works except for delay()

delay() uses micros() and micros() reads the hardware registers, like this:

unsigned long micros() {
 unsigned long m;
 uint8_t oldSREG = SREG, t;
 cli();
 m = timer0_overflow_count;
#if defined(TCNT0)
 t = TCNT0;
#elif defined(TCNT0L)
 t = TCNT0L;
#else
 #error TIMER 0 not defined
#endif
#ifdef TIFR0
 if ((TIFR0 & _BV(TOV0)) && (t < 255))
 m++;
#else
 if ((TIFR & _BV(TOV0)) && (t < 255))
 m++;
#endif
 SREG = oldSREG;
 return ((m << 8) + t) * (64 / clockCyclesPerMicrosecond());
}

You would need to change all those references (TCNT0, TIFR0, TOV0 etc.) to use the Timer 2 equivalents.

• Vasil Kalchev: This fixed it. Strange that micros() works fine without the fix?
• Nick Gammon: Appeared to work. I'm sure over a longer interval you would have seen anomalies.