#include <avr/interrupt.h>

#include <avr/io.h>

#define INIT_TIMER_COUNT 6

#define RESET_TIMER2 TCNT2 = INIT_TIMER_COUNT

int ledPin = 13;

int int_counter = 0;

volatile int second = 0;

int oldSecond = 0;

long starttime = 0;

// Aruino runs at 16 Mhz, so we have 1000 Overflows per second…

// 1/ ((16000000 / 64) / 256) = 1 / 1000

ISR(TIMER2_OVF_vect) {

RESET_TIMER2;

int_counter += 1;

if (int_counter == 1000) {

second+=1;

int_counter = 0;

}

if( int_counter < 500)

digitalWrite( ledPin, HIGH);

if( int_counter >500)

digitalWrite( ledPin, LOW);

};

void setup() {

//Timer2 Settings: Timer Prescaler /64,

TCCR2A |= ((1<<CS22) | (0<<CS21) | (0<<CS20));

// Use normal mode

TCCR2A |= (0<<WGM21) | (0<<WGM20);

// Use internal clock – external clock not used in Arduino

ASSR |= (0<<AS2);

TIMSK2 |= (1<<TOIE2) | (0<<OCIE2A);  //Timer2 Overflow Interrupt Enable

RESET_TIMER2;

sei();

starttime = millis();

}

void loop() {

}

Referencias:

http://gonium.net/md/2006/12/27/i-will-think-before-i-code/