#define TIM0_OVF __vector_7
#define SIGNAL __attribute__ ((signal))

unsigned char *led;
unsigned char *leddir;
unsigned int int_cntr; //contador de interrupções

void print_led(unsigned char value);
void enable_interrupt();
void leds_off(void);
void leds_on(void);

void TIM0_OVF(void) SIGNAL;

int main(void) {
  led = (unsigned char*) 0x38;
  leddir = (unsigned char*) 0x37;
  *leddir = 0xff;

  int int_cntr = 0;
  
  enable_interrupt();
  
  while(1);
  return 0;
}

void TIM0_OVF(void) {
  /* A cada 16s, inverte o estado dos leds. */
  if (int_cntr == 244) {
    int_cntr = 0;
    *led = ~*led;
  }
  else {
    int_cntr++;
  }
}


/* acende os leds de acordo com o parâmetro "value". */
void print_led(unsigned char value) {
  *led = value;
}

/* apaga todos os leds. */
void leds_off(void) {
  *led = 0x0;
}

/* acende todos os leds. */
void leds_on(void) {
  *led = 0xff;
}

/* ativa o uso de interrupções. */
void enable_interrupt(void) {
  unsigned char *sreg;
  sreg = (unsigned char*) 0x5F;
  *sreg |= 0x80;

  /* habilita interrupções do timer. */
  unsigned char *timsk;
  timsk = (unsigned char*) 0x59;
  *timsk |= 0x02;

  /* divisor do contador (ck/1024).  */
  unsigned char *tccr0;
  tccr0 = (unsigned char*) 0x53;
  *tccr0 |= 0x05;
}