#define F_CPU   3333333UL
#include <avr/io.h>
#include <avr/interrupt.h>

#define Baudrate 115200

char c;

ISR(USART0_RXC_vect)
{
  c = USART0_RXDATAL;
  USART0_TXDATAL = c;   // und gleich retour
}

int main(void)
{
  USART0.BAUD  = (64*F_CPU/16/Baudrate);
  PORTA.DIR    = 0b00000001;    // A0: TxD0
  USART0.CTRLA = 0b10000000;    // RXCIE
  USART0.CTRLB = 0b11000000;    // RXEN, TXEN
    USART0_TXDATAL = 'S';
    USART0_TXDATAL = 'L';
  sei();
  while (1);
}