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

// seconds counter
volatile uint8_t sec;
// minutes counter
volatile uint8_t min;

////////////////////////////////////////////////////////
//
// clock frequency = 4MHz
//
// 64 = prescaler clock
//
////////////////////////////////////////////////////////

int main(void){
	sec = 0; min = 0;
	// Set up CTC mode
	// Timer1 Control Register A
	TCCR1A 	|= 0;
	// Timer1 Control Register B
	// Prescaler = 64
	TCCR1B 	|= _BV(WGM12) | _BV(CS11) | _BV(CS10);

	// Initialize Compare Value
	OCR1A 	 = 62499;

	// Timer Interrupt Mask Register (common)
	// enable T1 compare interrupts
	TIMSK 	|= _BV(OCIE1A) | _BV(OCIE1B);

	sei();	// enable global interrupts

	while (1) {
		// just sleep while waiting
		sleep_enable();
		sleep_cpu();
		sleep_disable();
	}

	return 0;
}

ISR (TIMER1_COMPA_vect) {
	if(++sec >= 60) {
		sec = 0;
		min++;
	}
}