#define EICRA *((volatile uint8_t *)0x69)
#define EIMSK *((volatile uint8_t *)0x3D)
#define EIFR *((volatile uint8_t *)0x3C)
#define PORTB 0x25
 
void setup() {
  // put your setup code here, to run once:
 EICRA |= (1<<0) | (1<<1);
 EIMSK |= (1<<0);
 //pinMode(13,OUTPUT);
}

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

}
ISR (INT0_vect)
{
 *((volatile uint8_t *)PORTB) ^= (1<<5);

  
}