Echo-Test zwischen PC und Atmega644p

#include <avr/io.h>

#include <avr/interrupt.h> 

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <avr/pgmspace.h>

#define F_CPU 3686400UL

#define FOSC 7372000 // Clock Speed

#define BAUD 9600UL

// Berechnungen zur Baudrate:

#define UBRR_VAL ((FOSC+BAUD*8)/(BAUD*16)-1)   // clever runden

#define usart0_rx_buffer_max_size 50 

/***************************************************/

char usart0_rx_buffer[usart0_rx_buffer_max_size];

volatile uint8_t usart0_rx_buffer_size = 0;

/****************************************************/

void USART_Init (unsigned int ubrr)

{

  UBRR0H = (unsigned char)(ubrr>>8);

  UBRR0L = (unsigned char) ubrr;

//  UBRR1H = (unsigned char)(ubrr>>8);

//  UBRR1L = (unsigned char) ubrr;

  UCSR0B = (1<<RXEN0)|(1<<TXEN0)|(1<<RXCIE0)|(0<<TXCIE0)|(0<<UCSZ02)|(0<<UDRIE0);

  UCSR0C = (0<<USBS0) | (1<<UCSZ01)|(1<<UCSZ00);// 1 Stopbit, 8 Bits

  DDRB = 0b11111111; // Setting PB7-PB0 as output

  PORTB = (0<<PB7); // USART0 receive data from pc 

} 

ISR(USART0_RX_vect)

{

  char data = UDR0;

  if (usart0_rx_buffer_size < usart0_rx_buffer_max_size)

  {

    usart0_rx_buffer[usart0_rx_buffer_size] = data;

    ++usart0_rx_buffer_size;

   /* Activation transmission of data, then we have data in the buffer */

    UCSR0B |= (1<<TXCIE0);

  }

}

ISR(USART0_TX_vect)

{

  int i;

  if (usart0_rx_buffer_size == 0) // If we have no data to send

    UCSR0B &= ~(1<<TXCIE0);// we stop transmission

  else

  {

    UDR0 = usart0_rx_buffer[0];

    --usart0_rx_buffer_size;

    if (usart0_rx_buffer_size == 0)

      UCSR0B &= ~(1<<TXCIE0);

    else

      // it can be a circular buffer 

      for (i = 0; i < usart0_rx_buffer_size; ++i)

        usart0_rx_buffer[i] = usart0_rx_buffer[i + 1];

  } 

}

int main (void)

{ 

  USART_Init(UBRR_VAL) ;  // USART Initialisation

  sei();

  while(1)

  {   

  }

}