#define DDRB (*(volatile unsigned char *)(0x37))
#define PORTB (*(volatile unsigned char *)(0x38))


#define TIMSK (*(volatile unsigned char *)(0x59))
#define TIFR (*(volatile unsigned char *)(0x58))
#define TCCR0 (*(volatile unsigned char *)(0x53))
#define SREG (*(volatile unsigned char *)(0x5f))

#define ENTRADA 0xff
#define SAIDA 0x00

#define ACENDE 0x00
#define APAGA 0xff

#define IRQ0 __vector_1
#define IRQ7 __vector_7
#define SIGNAL __attribute__ ((signal))


#define VL_PARADA 27
#define NUM_OVERFLOWS 900

void IRQ7(void) SIGNAL;

int main(void) {
    SREG = 0x80;  //habilita as interrupções

    TCCR0 = 0x04; //overflow para 256
    TIMSK = 0x02; //interupção por overflow
    
    DDRB = ENTRADA;
    PORTB = APAGA;

    while(1) {}
    return 0;
}

void IRQ7(void) {
  static int num_overs = 0;
  num_overs++;
  if (num_overs == NUM_OVERFLOWS)
  {
    num_overs = 0;
    PORTB = ~PORTB;
  }
}