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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*

THIS IS AN INCOMPLETE ANSWER - Edgar knows what he's talking about, listen to him

I am currently working with this same thing but in an ATMega2560. [This website](https://ucexperiment.wordpress.com/2012/03/16/examination-of-the-arduino-millis-function/) helped me understand the millis() function better. The timer overflows every 510 counts (see datasheet, pg 123 for the atmega2560). I believe it is 510 and not 512 because it counts up and down, but does not repeat a count at the top or bottom - for example if you count starting from 1 up to 10 and then back down (without repeating 10) you will have counted 19 times, not 20. This counter starts at 0, counts to 255, and then back to 0, triggering an overflow before hitting 0 again. This is is 256 (count up including 0 and 255) + 254 (count down, not including 255 or 0) = 510.
I wrote a python script to visualize the effect of these changes and trying to account for them. This counts to 10 million, so it takes time to run, but by the end, even with scaling, the error if simply adjusting millis() after the fact could be ~7 seconds.

 #! /usr/bin/env python
 import numpy as np

 realT = np.zeros(10000000)
 timer0_millis = np.zeros(10000000)
 timer0_fract = np.zeros(10000000)
 i=0

 for i in range (1,10000000):
	 timer0_millis[i] = timer0_millis[i-1] + 1
	 timer0_fract[i] = timer0_fract[i-1] + 3
	 realT[i] = i*31875
	 if timer0_fract[i-1] >= 125:
		 timer0_fract[i] = timer0_fract[i-1] - 125
		 timer0_millis[i] = timer0_millis[i-1] + 1

 adjusted = timer0_millis*510/(256*64)

 print "after 100 timer interrupts millis() will read %d, actual millis is %.6f, adjusted millis is %.6f" % (timer0_millis[99], realT[99]/1000000, adjusted[99])
 print "after 1000 timer interrupts millis() will read %d, actual millis is %.6f, adjusted millis is %.6f" % (timer0_millis[999], realT[999]/1000000, adjusted[999])
 print "after 10000 timer interrupts millis() will read %d, actual millis is %.6f, adjusted millis is %.6f" % (timer0_millis[9999], realT[9999]/1000000, adjusted[9999])
 print "after 100000 timer interrupts millis() will read %d, actual millis is %.6f, adjusted millis is %.6f" % (timer0_millis[99999], realT[99999]/1000000, adjusted[99999])
 print "after 1000000 timer interrupts millis() will read %d, actual millis is %.6f, adjusted millis is %.6f" % (timer0_millis[999999], realT[999999]/1000000, adjusted[999999])
 print "after 10000000 timer interrupts millis() will read %d, actual millis is %.6f, adjusted millis is %.6f" % (timer0_millis[9999999], realT[9999999]/1000000, adjusted[9999999])

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You wrote: "I believe in phase correct mode the timer overflows every 510 counts, not 512 as indicated by Edgar". Please, read my answer again, and do not misquote me.

Edgar Bonet
– Edgar Bonet

10/12/2016 14:27:12
Commented Oct 12, 2016 at 14:27
Sorry, I was refering to this line "#define MICROSECONDS_PER_TIMER0_OVERFLOW (clockCyclesToMicroseconds(1 * 512))" - I have updated my comment, correct me if I am wrong here, I didn't intend to misquote

Nerbsie
– Nerbsie

10/13/2016 12:13:14
Commented Oct 13, 2016 at 12:13
There is a very good reason for having 512 instead of 510 in this line. Read my answer and you will understand.

Edgar Bonet
– Edgar Bonet

10/13/2016 12:28:25
Commented Oct 13, 2016 at 12:28
Okay, so you are adding two timer counts, each being 62.5ns, to account for the error caused by the forced round-down?

Nerbsie
– Nerbsie

10/13/2016 12:38:42
Commented Oct 13, 2016 at 12:38
1

Edgar, if you would like to continue helping me I decided that instead of answering a question with questions of my own and potential misinformation I would ask a question

Nerbsie
– Nerbsie

10/13/2016 14:48:24
Commented Oct 13, 2016 at 14:48

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