void init_port();
void init_timer();

void setup() {
  volatile long i;
  int j=0;
  init_port();
  init_timer();
  Serial.begin(9600);
  while(1)
  {
    for(i=0; i<500000; i++);
    Serial.println(j,DEC);
    j++;
  }
}

void init_timer()
{
  volatile char *timer_TCCR1A = (volatile char *) 0x80;
  volatile char *timer_TCCR1B = (volatile char *) 0x81;
  volatile short *timer_TCNT1 = (volatile short*) 0x84;
  volatile short *timer_OCR1A = (volatile short*) 0x88;
  volatile char *timer_TIMSK1 = (volatile char *) 0x6F;

  *timer_TCCR1A = 0;
  *timer_TCCR1B = 0;
  *timer_TCNT1  = 0;

  *timer_OCR1A   = 60000;     // ~1second
  *timer_TCCR1B = 0x0C;
  *timer_TIMSK1  = 0x02;
}

void init_port()
{
  volatile char *dir_F = (volatile char *) 0x30;
  *dir_F = 0x01;
}

ISR(TIMER1_COMPA_vect)
{
  volatile char *dir_F = (volatile char *) 0x31;
  *dir_F ^= 0x01;
}


void loop() {
  // put your main code here, to run repeatedly:

}