Uno millis() falls behind over long periods (hours/days)

chrisknightley · August 25, 2021, 11:47am

I support to @alto777 and agree that millis() provides the correct accuracy in 16MHz Arduino.

This is because I know that the ISR that controls the Arduino millis has a process that counts the number of extra 0.024ms and makes an additional increment when it reaches 1ms.

github.com

arduino/ArduinoCore-avr/blob/master/cores/arduino/wiring.c#L32


      
          
          
#include "wiring_private.h"
          
          
// 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))
          
          
// the whole number of milliseconds per timer0 overflow
          #define MILLIS_INC (MICROSECONDS_PER_TIMER0_OVERFLOW / 1000)
          
          
// the fractional number of milliseconds per timer0 overflow. we shift right
          // by three to fit these numbers into a byte. (for the clock speeds we care
          // about - 8 and 16 MHz - this doesn't lose precision.)
          #define FRACT_INC ((MICROSECONDS_PER_TIMER0_OVERFLOW % 1000) >> 3)
          #define FRACT_MAX (1000 >> 3)
          
          
volatile unsigned long timer0_overflow_count = 0;
          volatile unsigned long timer0_millis = 0;
          static unsigned char timer0_fract = 0;
          
          
#if defined(TIM0_OVF_vect)

In other words, millis() sometimes has a number that is skipped by one.

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Uno millis() falls behind over long periods (hours/days)

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