Abstract

The millis() returns the number of milliseconds since the board has booted.

ソースコード

Source Code

The millis() is defined in hardware/arduino/avr/cores/arduino/wiring.c as below. Only the souce code for Arduino UNO is quoted. The original source code supports many tips using #if’s.

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volatile unsigned long timer0_millis = 0;
 
unsigned long millis()
{
    unsigned long m;
    uint8_t oldSREG = SREG;
 
    // disable interrupts while we read timer0_millis or we might get an
    // inconsistent value (e.g. in the middle of a write to timer0_millis)
    cli();
    m = timer0_millis;
    SREG = oldSREG;
 
    return m;
}

The millis() has no input argument and returns unsigned long value.

First it reserves the status register SREG.

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unsigned long millis()
    {
    unsigned long m;
    uint8_t oldSREG = SREG;

The SREG is a status register of ATMega328P.

Next it disables interrupt, copies the timer0_millis which holds the number of milliseconds to m, restores the reserved status register value and returns m. The timer0_millis is set in a interrupt handler named TIMER0_OVF_vect.

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    // disable interrupts while we read timer0_millis or we might get an
    // inconsistent value (e.g. in the middle of a write to timer0_millis)
    cli();
    m = timer0_millis;
    SREG = oldSREG;
 
    return m;
}

cli() is a macro that executes an assembler macro to disable interrupt.

As SREG includes interrupt flag, it is not need to enable interrupt explicitly.

Version

Arduino 1.8.13

Last Update

January 23, 2021