#include <avr/io.h>
#include <avr/interrupt.h>

uint8_t dato = 0x20;

ISR(INT0_vect){

  while(!(UCSR0A & 1 << UDRE0));
  UDR0= dato;

  dato = (dato = 0x7E)? 0x20: dato+1;
}
ISR(USART_RX_vect){
  PORTB = UDR0;
}

int main(){
  DDRD = 0x02;
  PORTD = 0x04;
  DDRB = 0xFF;

  EICRA = 0x02;
  EIMSK = 0x01;

  UBRR0 = 12;
  UCSR0A = 0x00;
  UCSR0B = 0x98;

  UCSR0C = 0x06;

  sei ();

  while (1)
  asm ("NOP");
}