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Adjust time calculation after Timer0 frequency change

I have an Arduino Nano with an 328P and need all 6 PWM pins.

Thus, I had to adjust the prescaler and WGM Mode of Timer0.

It is now in phase correct PWM mode with a prescaler of 1.

TCCR0A = _BV(COM0A1) | _BV(COM0B1) | _BV(WGM00);
TCCR0B = _BV(CS00);

Now I need a working time calculation for other libraries, but since Timer0 had that duty everything is out of order now.

I tried adjusting the wiring.c

// the prescaler is set so that timer0 ticks every 64 clock cycles, and the
// the overflow handler is called every 256 ticks.
#define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(64 * 256))

to this

#define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(1 * 510))

But it's like I didn't change anything. (tested other settings that were changed so it was compiled anew )

Whole Code:

void setup() {
 // Set Timer 0, 1 and 2
 // Register A: Output A and B to non-inverted PWM and PWM mode to phase correct.
 // Register B: Pre Scaler to 1.
 TCCR0A = _BV(COM0A1) | _BV(COM0B1) | _BV(WGM00);
 TCCR0B = _BV(CS00);
 TCCR1A = _BV(COM1A1) | _BV(COM1B1) | _BV(WGM10);
 TCCR1B = _BV(CS10);
 TCCR2A = _BV(COM2A1) | _BV(COM2B1) | _BV(WGM20);
 TCCR2B = _BV(CS20);
 pinMode(8, OUTPUT);
}
void loop() {
 digitalWrite(8, LOW);
 delay(65000);
 digitalWrite(8, HIGH);
 delay(65000);
}

Answer*

Fixing the timekeeping functions with your PWM settings is not so
simple. You should at least try to rewrite `ISR(TIMER0_OVF_vect)`,
`micros()`, and probably `delay()`. Here is why:

First, there is a rounding problem. Time is kept using two global
variables:



 volatile unsigned long timer0_millis;
 static unsigned char timer0_fract;

The first one is what `millis()` returns. The second one keeps track of
how much time has passed since the last full millisecond, and it does so
in units of 8&nbsp;µs. The two variables are incremented by
`ISR(TIMER0_OVF_vect)` like this:

	m += MILLIS_INC; // temporary copy of timer0_millis
	f += FRACT_INC; // temporary copy of timer0_fract

On a normal Uno configuration, the ISR is called every 1024&nbsp;µs.
Then `MILLIS_INC` is 1 and `FRACT_INC` is 3. With your timer
configuration, the ISR is called every 31.875&nbsp;µs (510 cycles), then
`MILLIS_INC` should be 0 and `FRACT_INC` should be 3.984375. But since
we are dealing with integers, it will be rounded down to 3, and your
`millis()` will tick about 25% too slow.

A simple fix would be to

 #define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(1 * 512))

in order for `FRACT_INC` to be 4 and `millis()` to be 0.4% too fast. Or
you could make `timer0_fract` a 16-bit variable and have it count clock
cycles, just to avoid this error. Either option should fix `millis()`,
but you still have a problem with `micros()`.

`micros()` works by reading both `timer0_overflow_count` (incremented by
1 in the ISR) and the actual counter value. Since your counter is now
going alternatively up and down, it will be harder to compute a
microsecond count from these readings. Maybe you could take two
consecutive readings of the counter, just to know whether it is going up
or down...

And then there is `delay()`, which relies on `micros()`. If you fix
`micros()`, `delay()` should work fine. If not, You could rewrite
`delay()` to use `millis()` instead, which should be easy but you will
loose some accuracy.

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Edit Summary*

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Thanks for the sophisticated answer! At this point also thanks to @jakec for discussing the problem with me!

Splitframe
– Splitframe

07/17/2015 08:51:01
Commented Jul 17, 2015 at 8:51
Since your counter is now going alternatively up and down Why is the TCNT0 going up or down? It's going up and every time it overflow the ISR is called. Am I missing something? I tried to solve this by increasing the timer0_overflow_count once every 64 times only, and on the micros() function return the time as return ((m << 8) + (t % 64)) * (64 / clockCyclesPerMicrosecond()); So I do a mod on the TCNT0 counter as my Timer is ticking 64 times faster. But although the micros() seem to be ok delay() is still running faster and I'm not understanding why...

Lefteris
– Lefteris

11/20/2017 08:07:46
Commented Nov 20, 2017 at 8:07
@Lefteris: The counter is going up and down because the OP configured it in "phase correct PWM" mode.

Edgar Bonet
– Edgar Bonet

11/20/2017 08:23:02
Commented Nov 20, 2017 at 8:23
Correction on the above, modulo was wrong, I'm dividing by 64 on the return function now: return ((m << 8) + (t/64) ) * (64 / clockCyclesPerMicrosecond()); Still the problem remains with delays function which I don't understand

Lefteris
– Lefteris

11/20/2017 08:25:46
Commented Nov 20, 2017 at 8:25
@EdgarBonet If I have understood correct, the TCTN0 register increases every CLK/64 tick. So it is only going up. It is reset every time it overflows and TIMER0_OVF_vect ISR is called. So still not understanding what phase correct PWM has to do with this.

Lefteris
– Lefteris

11/20/2017 08:29:41
Commented Nov 20, 2017 at 8:29

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