timebase.c

/*
 * timebase.c
 *
 *  Created on: Dec 3, 2012
 *      Author: vic
 */
#include <inttypes.h>
#include <avr/io.h>
#include <avr/interrupt.h>
#include "timebase.h"


// double buffered timestamp
volatile uint32_t timestamp_ms_buf[2] = {0, 1};
volatile uint8_t  timestamp_ms_buf_volatile_idx = 0;

void timebase_init(){
    // set up the timer0 overflow interrupt for 1ms
    TCNT0  = TIMER0_1MS_OVERFLOW_TCNT;
    TIMSK0 = _BV(TOIE0);

    // start timer0
    TCCR0A = TIMER0_1MS_OVERFOW_PRESCALER;
}


// fires once per millisecond, don't block
// can't miss TWI interrupts for anything
ISR(TIMER0_OVF_vect, ISR_NOBLOCK){
    TCNT0 = TIMER0_1MS_OVERFLOW_TCNT;

    // modify the volatile index value
    timestamp_ms_buf[timestamp_ms_buf_volatile_idx] += 2;

    // change the volatile index
    timestamp_ms_buf_volatile_idx = 1 - timestamp_ms_buf_volatile_idx; // always 0 or 1
}


uint32_t timebase_now(){
    uint8_t idx = timestamp_ms_buf_volatile_idx; // copy the current volatile index
    return timestamp_ms_buf[1 - idx];            // return the value from the non-volatile index
}