Rewriting a bare millis() clock on an AVR without the Arduino environment

This is a short stand-alone C source code that I wrote to simulate the Arduino millis() function on an Atmel AVR 328p microcontroller. It is part of a growing collection of AVR routines.




''
#include <avr/interrupt.h>

volatile uint64_t millis_prv = 0;

void millis_init()
{
 TCCR0A = 0;
 // set timer0 with CLKio/8 prescaler
 TCCR0B = _BV(CS01) | _BV(CS00);
 // clear any TOV1 Flag set when the timer overflowed
 TIFR0 &= ~TOV0;
 // set timer0 counter initial value to 0
 TCNT0 = 0x0;
 // enable timer overflow interrupt for Timer0
 TIMSK0 = _BV(TOIE0);
 // clear the Power Reduction Timer/Counter0
 PRR &= ~PRTIM0;
}

// TIMER0 interrupt handler
ISR(TIMER0_OVF_vect)
{
 // reset the counter (overflow is cleared automatically)
 TCNT0 = (uint8_t)(0xFF - ((F_CPU/8)/1000)); // use CLKio/8 prescaler (set CS0n accordingly above)
 millis_prv++;
}

// return elapsed time in milliseconds
uint64_t millis()
{
 return millis_prv;
}
''