#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/sleep.h>

// global variable, # of overflows
volatile uint16_t overflow_cnt;
volatile uint16_t sec_cnt;
volatile uint16_t min_cnt;

// TIMER0 overflow interrupt service routine
ISR(TIMER0_OVF_vect)
{
  	// keep a track of number of overflows
  	overflow_cnt++;
}

void timer_init()
{
    TCCR0 |= (1 << CS01);
	TCCR0 |= (1 << CS00); // set prescaler = 64
    TCNT0 = 132;	      // set TIMER0 = 132
	TIMSK |= (1<<TOIE0);  // enable TIMER0 interrupts
	sei();
}

////////////////////////////////////////////////////////
//
// clock frequency = 8MHz
//
// 64 = prescaler clock source
// 132 = start value (125 clocks up to 257 overflow)
//
// 1 / (8MHz / 64 / 125) = 1ms * 1000 = 1s
//
////////////////////////////////////////////////////////

int main(void)
{
	while(1)
	{
		timer_init();
		sleep_enable();
    	sleep_cpu();
		sleep_disable();
		if (overflow_cnt >= 1000)
		{
			overflow_cnt = 0;
			sec_cnt++;
		}
		if (min_cnt >= 60){
			sec_cnt = 0;
			min_cnt++;
		}
	}

}