Danke für die schnelle Antwort!
Der Code ist der oben erwähnte Standardcode:

Code:
main.c
#include <stdlib.h>
#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>

#include "uart.h"


/* define CPU frequency in Mhz here if not defined in Makefile */
#ifndef F_CPU
#define F_CPU 4000000UL
#endif

/* 9600 baud */
#define UART_BAUD_RATE      9600      


int main(void)
{
    unsigned int c;
    char buffer[7];
    int  num=134;

    
    /*
     *  Initialize UART library, pass baudrate and AVR cpu clock
     *  with the macro 
     *  UART_BAUD_SELECT() (normal speed mode )
     *  or 
     *  UART_BAUD_SELECT_DOUBLE_SPEED() ( double speed mode)
     */
    uart_init( UART_BAUD_SELECT(UART_BAUD_RATE,F_CPU) ); 
    
    /*
     * now enable interrupt, since UART library is interrupt controlled
     */
    sei();
    
    /*
     *  Transmit string to UART
     *  The string is buffered by the UART library in a circular buffer
     *  and one character at a time is transmitted to the UART using interrupts.
     *  uart_puts() blocks if it can not write the whole string to the circular 
     *  buffer
     */
    uart_puts("String stored in SRAM\n");
    
    /*
     * Transmit string from program memory to UART
     */
    uart_puts_P("String stored in FLASH\n");
    
        
    /* 
     * Use standard avr-libc functions to convert numbers into string
     * before transmitting via UART
     */     
    itoa( num, buffer, 10);   // convert interger into string (decimal format)         
    uart_puts(buffer);        // and transmit string to UART

    
    /*
     * Transmit single character to UART
     */
    uart_putc('\r');
    
    for(;;)
    {
        /*
         * Get received character from ringbuffer
         * uart_getc() returns in the lower byte the received character and 
         * in the higher byte (bitmask) the last receive error
         * UART_NO_DATA is returned when no data is available.
         *
         */
        c = uart_getc();
        if ( c & UART_NO_DATA )
        {
            /* 
             * no data available from UART 
             */
        }
        else
        {
            /*
             * new data available from UART
             * check for Frame or Overrun error
             */
            if ( c & UART_FRAME_ERROR )
            {
                /* Framing Error detected, i.e no stop bit detected */
                uart_puts_P("UART Frame Error: ");
            }
            if ( c & UART_OVERRUN_ERROR )
            {
                /* 
                 * Overrun, a character already present in the UART UDR register was 
                 * not read by the interrupt handler before the next character arrived,
                 * one or more received characters have been dropped
                 */
                uart_puts_P("UART Overrun Error: ");
            }
            if ( c & UART_BUFFER_OVERFLOW )
            {
                /* 
                 * We are not reading the receive buffer fast enough,
                 * one or more received character have been dropped 
                 */
                uart_puts_P("Buffer overflow error: ");
            }
            /* 
             * send received character back
             */
            uart_putc( (unsigned char)c );
        }
    }
    
}
Code:
vorimplemntierer Code
/*************************************************************************
Title:    Interrupt UART library with receive/transmit circular buffers
Author:   Peter Fleury <pfleury@gmx.ch>   http://jump.to/fleury
File:     $Id: uart.c,v 1.12 2014/01/08 21:58:12 peter Exp $
Software: AVR-GCC 4.1, AVR Libc 1.4.6 or higher
Hardware: any AVR with built-in UART, 
License:  GNU General Public License 
          
DESCRIPTION:
    An interrupt is generated when the UART has finished transmitting or
    receiving a byte. The interrupt handling routines use circular buffers
    for buffering received and transmitted data.
    
    The UART_RX_BUFFER_SIZE and UART_TX_BUFFER_SIZE variables define
    the buffer size in bytes. Note that these variables must be a 
    power of 2.
    
USAGE:
    Refere to the header file uart.h for a description of the routines. 
    See also example test_uart.c.

NOTES:
    Based on Atmel Application Note AVR306
                    
LICENSE:
    Copyright (C) 2006 Peter Fleury

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
                        
*************************************************************************/
#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/pgmspace.h>
#include "uart.h"


/*
 *  constants and macros
 */

/* size of RX/TX buffers */
#define UART_RX_BUFFER_MASK ( UART_RX_BUFFER_SIZE - 1)
#define UART_TX_BUFFER_MASK ( UART_TX_BUFFER_SIZE - 1)

#if ( UART_RX_BUFFER_SIZE & UART_RX_BUFFER_MASK )
#error RX buffer size is not a power of 2
#endif
#if ( UART_TX_BUFFER_SIZE & UART_TX_BUFFER_MASK )
#error TX buffer size is not a power of 2
#endif

#if defined(__AVR_AT90S2313__) \
 || defined(__AVR_AT90S4414__) || defined(__AVR_AT90S4434__) \
 || defined(__AVR_AT90S8515__) || defined(__AVR_AT90S8535__) \
 || defined(__AVR_ATmega103__)
 /* old AVR classic or ATmega103 with one UART */
 #define AT90_UART
 #define UART0_RECEIVE_INTERRUPT   UART_RX_vect 
 #define UART0_TRANSMIT_INTERRUPT  UART_UDRE_vect
 #define UART0_STATUS   USR
 #define UART0_CONTROL  UCR
 #define UART0_DATA     UDR  
 #define UART0_UDRIE    UDRIE
#elif defined(__AVR_AT90S2333__) || defined(__AVR_AT90S4433__)
 /* old AVR classic with one UART */
 #define AT90_UART
 #define UART0_RECEIVE_INTERRUPT   UART_RX_vect 
 #define UART0_TRANSMIT_INTERRUPT  UART_UDRE_vect
 #define UART0_STATUS   UCSRA
 #define UART0_CONTROL  UCSRB
 #define UART0_DATA     UDR 
 #define UART0_UDRIE    UDRIE
#elif  defined(__AVR_ATmega8__) || defined(__AVR_ATmega16__) || defined(__AVR_ATmega32__) \
  || defined(__AVR_ATmega323__)
  /* ATmega with one USART */
 #define ATMEGA_USART
 #define UART0_RECEIVE_INTERRUPT   USART_RXC_vect
 #define UART0_TRANSMIT_INTERRUPT  USART_UDRE_vect
 #define UART0_STATUS   UCSRA
 #define UART0_CONTROL  UCSRB
 #define UART0_DATA     UDR
 #define UART0_UDRIE    UDRIE
#elif defined (__AVR_ATmega8515__) || defined(__AVR_ATmega8535__)
 #define ATMEGA_USART
 #define UART0_RECEIVE_INTERRUPT   USART_RX_vect
 #define UART0_TRANSMIT_INTERRUPT  USART_UDRE_vect
 #define UART0_STATUS   UCSRA
 #define UART0_CONTROL  UCSRB
 #define UART0_DATA     UDR
 #define UART0_UDRIE    UDRIE
#elif defined(__AVR_ATmega163__)
  /* ATmega163 with one UART */
 #define ATMEGA_UART
 #define UART0_RECEIVE_INTERRUPT   UART_RX_vect
 #define UART0_TRANSMIT_INTERRUPT  UART_UDRE_vect
 #define UART0_STATUS   UCSRA
 #define UART0_CONTROL  UCSRB
 #define UART0_DATA     UDR
 #define UART0_UDRIE    UDRIE
#elif defined(__AVR_ATmega162__) 
 /* ATmega with two USART */
 #define ATMEGA_USART0
 #define ATMEGA_USART1
 #define UART0_RECEIVE_INTERRUPT   USART0_RXC_vect
 #define UART1_RECEIVE_INTERRUPT   USART1_RXC_vect
 #define UART0_TRANSMIT_INTERRUPT  USART0_UDRE_vect
 #define UART1_TRANSMIT_INTERRUPT  USART1_UDRE_vect
 #define UART0_STATUS   UCSR0A
 #define UART0_CONTROL  UCSR0B
 #define UART0_DATA     UDR0
 #define UART0_UDRIE    UDRIE0
 #define UART1_STATUS   UCSR1A
 #define UART1_CONTROL  UCSR1B
 #define UART1_DATA     UDR1
 #define UART1_UDRIE    UDRIE1
#elif defined(__AVR_ATmega64__) || defined(__AVR_ATmega128__) 
 /* ATmega with two USART */
 #define ATMEGA_USART0
 #define ATMEGA_USART1
 #define UART0_RECEIVE_INTERRUPT   USART0_RX_vect
 #define UART1_RECEIVE_INTERRUPT   USART1_RX_vect
 #define UART0_TRANSMIT_INTERRUPT  USART0_UDRE_vect
 #define UART1_TRANSMIT_INTERRUPT  USART1_UDRE_vect
 #define UART0_STATUS   UCSR0A
 #define UART0_CONTROL  UCSR0B
 #define UART0_DATA     UDR0
 #define UART0_UDRIE    UDRIE0
 #define UART1_STATUS   UCSR1A
 #define UART1_CONTROL  UCSR1B
 #define UART1_DATA     UDR1
 #define UART1_UDRIE    UDRIE1
#elif defined(__AVR_ATmega161__)
 /* ATmega with UART */
 #error "AVR ATmega161 currently not supported by this libaray !"
#elif defined(__AVR_ATmega169__) 
 /* ATmega with one USART */
 #define ATMEGA_USART
 #define UART0_RECEIVE_INTERRUPT   USART0_RX_vect
 #define UART0_TRANSMIT_INTERRUPT  USART0_UDRE_vect
 #define UART0_STATUS   UCSRA
 #define UART0_CONTROL  UCSRB
 #define UART0_DATA     UDR
 #define UART0_UDRIE    UDRIE
#elif defined(__AVR_ATmega48__) || defined(__AVR_ATmega88__) || defined(__AVR_ATmega168__) || defined(__AVR_ATmega48P__) || defined(__AVR_ATmega88P__) || defined(__AVR_ATmega168P__) || defined(__AVR_ATmega328P__) \
 || defined(__AVR_ATmega3250__) || defined(__AVR_ATmega3290__) ||defined(__AVR_ATmega6450__) || defined(__AVR_ATmega6490__)
 /* ATmega with one USART */
 #define ATMEGA_USART0
 #define UART0_RECEIVE_INTERRUPT   USART_RX_vect
 #define UART0_TRANSMIT_INTERRUPT  USART_UDRE_vect
 #define UART0_STATUS   UCSR0A
 #define UART0_CONTROL  UCSR0B
 #define UART0_DATA     UDR0
 #define UART0_UDRIE    UDRIE0
#elif defined(__AVR_ATtiny2313__) 
 #define ATMEGA_USART
 #define UART0_RECEIVE_INTERRUPT   USART_RX_vect
 #define UART0_TRANSMIT_INTERRUPT  USART_UDRE_vect
 #define UART0_STATUS   UCSRA
 #define UART0_CONTROL  UCSRB
 #define UART0_DATA     UDR
 #define UART0_UDRIE    UDRIE
#elif defined(__AVR_ATmega329__) || \
      defined(__AVR_ATmega649__) || \
      defined(__AVR_ATmega325__) || \
      defined(__AVR_ATmega645__) 
  /* ATmega with one USART */
  #define ATMEGA_USART0
  #define UART0_RECEIVE_INTERRUPT   USART0_RX_vect
  #define UART0_TRANSMIT_INTERRUPT  USART0_UDRE_vect
  #define UART0_STATUS   UCSR0A
  #define UART0_CONTROL  UCSR0B
  #define UART0_DATA     UDR0
  #define UART0_UDRIE    UDRIE0
#elif defined(__AVR_ATmega2560__) || defined(__AVR_ATmega2561__) || defined(__AVR_ATmega1280__)  || defined(__AVR_ATmega1281__) || defined(__AVR_ATmega640__)
/* ATmega with two USART */
  #define ATMEGA_USART0
  #define ATMEGA_USART1
  #define UART0_RECEIVE_INTERRUPT   USART0_RX_vect
  #define UART1_RECEIVE_INTERRUPT   USART1_RX_vect
  #define UART0_TRANSMIT_INTERRUPT  USART0_UDRE_vect
  #define UART1_TRANSMIT_INTERRUPT  USART1_UDRE_vect
  #define UART0_STATUS   UCSR0A
  #define UART0_CONTROL  UCSR0B
  #define UART0_DATA     UDR0
  #define UART0_UDRIE    UDRIE0
  #define UART1_STATUS   UCSR1A
  #define UART1_CONTROL  UCSR1B
  #define UART1_DATA     UDR1
  #define UART1_UDRIE    UDRIE1  
#elif defined(__AVR_ATmega644__)
 /* ATmega with one USART */
 #define ATMEGA_USART0
 #define UART0_RECEIVE_INTERRUPT   USART0_RX_vect
 #define UART0_TRANSMIT_INTERRUPT  USART0_UDRE_vect
 #define UART0_STATUS   UCSR0A
 #define UART0_CONTROL  UCSR0B
 #define UART0_DATA     UDR0
 #define UART0_UDRIE    UDRIE0
#elif defined(__AVR_ATmega164P__) || defined(__AVR_ATmega324P__) || defined(__AVR_ATmega644P__)
 /* ATmega with two USART */
 #define ATMEGA_USART0
 #define ATMEGA_USART1
 #define UART0_RECEIVE_INTERRUPT   USART0_RX_vect
 #define UART1_RECEIVE_INTERRUPT   USART1_RX_vect
 #define UART0_TRANSMIT_INTERRUPT  USART0_UDRE_vect
 #define UART1_TRANSMIT_INTERRUPT  USART1_UDRE_vect
 #define UART0_STATUS   UCSR0A
 #define UART0_CONTROL  UCSR0B
 #define UART0_DATA     UDR0
 #define UART0_UDRIE    UDRIE0
 #define UART1_STATUS   UCSR1A
 #define UART1_CONTROL  UCSR1B
 #define UART1_DATA     UDR1
 #define UART1_UDRIE    UDRIE1
#elif defined(__AVR_AT90USB646__) || defined(__AVR_AT90USB1286__) || defined(__AVR_AT90USB647__) || defined(__AVR_AT90USB1287__)
 /* AT90USBxx with one USART */
 #define AT90USB_USART
 #define UART0_RECEIVE_INTERRUPT   USART1_RX_vect
 #define UART0_TRANSMIT_INTERRUPT  USART1_UDRE_vect
 #define UART0_STATUS   UCSR1A
 #define UART0_CONTROL  UCSR1B
 #define UART0_DATA     UDR1
 #define UART0_UDRIE    UDRIE1
#else
 #error "no UART definition for MCU available"
#endif


/*
 *  module global variables
 */
static volatile unsigned char UART_TxBuf[UART_TX_BUFFER_SIZE];
static volatile unsigned char UART_RxBuf[UART_RX_BUFFER_SIZE];
static volatile unsigned char UART_TxHead;
static volatile unsigned char UART_TxTail;
static volatile unsigned char UART_RxHead;
static volatile unsigned char UART_RxTail;
static volatile unsigned char UART_LastRxError;

#if defined( ATMEGA_USART1 )
static volatile unsigned char UART1_TxBuf[UART_TX_BUFFER_SIZE];
static volatile unsigned char UART1_RxBuf[UART_RX_BUFFER_SIZE];
static volatile unsigned char UART1_TxHead;
static volatile unsigned char UART1_TxTail;
static volatile unsigned char UART1_RxHead;
static volatile unsigned char UART1_RxTail;
static volatile unsigned char UART1_LastRxError;
#endif



ISR (UART0_RECEIVE_INTERRUPT)	
/*************************************************************************
Function: UART Receive Complete interrupt
Purpose:  called when the UART has received a character
**************************************************************************/
{
    unsigned char tmphead;
    unsigned char data;
    unsigned char usr;
    unsigned char lastRxError;
 
 
    /* read UART status register and UART data register */ 
    usr  = UART0_STATUS;
    data = UART0_DATA;
    
    /* */
#if defined( AT90_UART )
    lastRxError = (usr & (_BV(FE)|_BV(DOR)) );
#elif defined( ATMEGA_USART )
    lastRxError = (usr & (_BV(FE)|_BV(DOR)) );
#elif defined( ATMEGA_USART0 )
    lastRxError = (usr & (_BV(FE0)|_BV(DOR0)) );
#elif defined ( ATMEGA_UART )
    lastRxError = (usr & (_BV(FE)|_BV(DOR)) );
#elif defined( AT90USB_USART )
    lastRxError = (usr & (_BV(FE1)|_BV(DOR1)) );
#endif
        
    /* calculate buffer index */ 
    tmphead = ( UART_RxHead + 1) & UART_RX_BUFFER_MASK;
    
    if ( tmphead == UART_RxTail ) {
        /* error: receive buffer overflow */
        lastRxError = UART_BUFFER_OVERFLOW >> 8;
    }else{
        /* store new index */
        UART_RxHead = tmphead;
        /* store received data in buffer */
        UART_RxBuf[tmphead] = data;
    }
    UART_LastRxError |= lastRxError;   
}


ISR (UART0_TRANSMIT_INTERRUPT)
/*************************************************************************
Function: UART Data Register Empty interrupt
Purpose:  called when the UART is ready to transmit the next byte
**************************************************************************/
{
    unsigned char tmptail;

    
    if ( UART_TxHead != UART_TxTail) {
        /* calculate and store new buffer index */
        tmptail = (UART_TxTail + 1) & UART_TX_BUFFER_MASK;
        UART_TxTail = tmptail;
        /* get one byte from buffer and write it to UART */
        UART0_DATA = UART_TxBuf[tmptail];  /* start transmission */
    }else{
        /* tx buffer empty, disable UDRE interrupt */
        UART0_CONTROL &= ~_BV(UART0_UDRIE);
    }
}


/*************************************************************************
Function: uart_init()
Purpose:  initialize UART and set baudrate
Input:    baudrate using macro UART_BAUD_SELECT()
Returns:  none
**************************************************************************/
void uart_init(unsigned int baudrate)
{
    UART_TxHead = 0;
    UART_TxTail = 0;
    UART_RxHead = 0;
    UART_RxTail = 0;
    
#if defined( AT90_UART )
    /* set baud rate */
    UBRR = (unsigned char)baudrate; 

    /* enable UART receiver and transmmitter and receive complete interrupt */
    UART0_CONTROL = _BV(RXCIE)|_BV(RXEN)|_BV(TXEN);

#elif defined (ATMEGA_USART)
    /* Set baud rate */
    if ( baudrate & 0x8000 )
    {
    	 UART0_STATUS = (1<<U2X);  //Enable 2x speed 
    	 baudrate &= ~0x8000;
    }
    UBRRH = (unsigned char)(baudrate>>8);
    UBRRL = (unsigned char) baudrate;
   
    /* Enable USART receiver and transmitter and receive complete interrupt */
    UART0_CONTROL = _BV(RXCIE)|(1<<RXEN)|(1<<TXEN);
    
    /* Set frame format: asynchronous, 8data, no parity, 1stop bit */
    #ifdef URSEL
    UCSRC = (1<<URSEL)|(3<<UCSZ0);
    #else
    UCSRC = (3<<UCSZ0);
    #endif 
    
#elif defined (ATMEGA_USART0 )
    /* Set baud rate */
    if ( baudrate & 0x8000 ) 
    {
   		UART0_STATUS = (1<<U2X0);  //Enable 2x speed 
   		baudrate &= ~0x8000;
   	}
    UBRR0H = (unsigned char)(baudrate>>8);
    UBRR0L = (unsigned char) baudrate;

    /* Enable USART receiver and transmitter and receive complete interrupt */
    UART0_CONTROL = _BV(RXCIE0)|(1<<RXEN0)|(1<<TXEN0);
    
    /* Set frame format: asynchronous, 8data, no parity, 1stop bit */
    #ifdef URSEL0
    UCSR0C = (1<<URSEL0)|(3<<UCSZ00);
    #else
    UCSR0C = (3<<UCSZ00);
    #endif 

#elif defined ( ATMEGA_UART )
    /* set baud rate */
    if ( baudrate & 0x8000 ) 
    {
    	UART0_STATUS = (1<<U2X);  //Enable 2x speed 
    	baudrate &= ~0x8000;
    }
    UBRRHI = (unsigned char)(baudrate>>8);
    UBRR   = (unsigned char) baudrate;

    /* Enable UART receiver and transmitter and receive complete interrupt */
    UART0_CONTROL = _BV(RXCIE)|(1<<RXEN)|(1<<TXEN);

#elif defined ( AT90USB_USART )
   /* set baud rate */
    if ( baudrate & 0x8000 ) 
    {
    	UART0_STATUS = (1<<U2X1 );  //Enable 2x speed 
    	baudrate &= ~0x8000;
    }
    UBRR1H = (unsigned char)(baudrate>>8);
    UBRR1L = (unsigned char) baudrate;

    /* Enable UART receiver and transmitter and receive complete interrupt */
    UART0_CONTROL = _BV(RXCIE1)|(1<<RXEN1)|(1<<TXEN1);
    
    /* Set frame format: asynchronous, 8data, no parity, 1stop bit */
    UCSR1C = (1<<UCSZ11)|(1<<UCSZ10);
#endif

}/* uart_init */


/*************************************************************************
Function: uart_getc()
Purpose:  return byte from ringbuffer  
Returns:  lower byte:  received byte from ringbuffer
          higher byte: last receive error
**************************************************************************/
unsigned int uart_getc(void)
{    
    unsigned char tmptail;
    unsigned char data;


    if ( UART_RxHead == UART_RxTail ) {
        return UART_NO_DATA;   /* no data available */
    }
    
    /* calculate /store buffer index */
    tmptail = (UART_RxTail + 1) & UART_RX_BUFFER_MASK;
    UART_RxTail = tmptail; 
    
    /* get data from receive buffer */
    data = UART_RxBuf[tmptail];
    
    data = (UART_LastRxError << 8) + data;
    UART_LastRxError = 0;
    return data;

}/* uart_getc */


/*************************************************************************
Function: uart_putc()
Purpose:  write byte to ringbuffer for transmitting via UART
Input:    byte to be transmitted
Returns:  none          
**************************************************************************/
void uart_putc(unsigned char data)
{
    unsigned char tmphead;

    
    tmphead  = (UART_TxHead + 1) & UART_TX_BUFFER_MASK;
    
    while ( tmphead == UART_TxTail ){
        ;/* wait for free space in buffer */
    }
    
    UART_TxBuf[tmphead] = data;
    UART_TxHead = tmphead;

    /* enable UDRE interrupt */
    UART0_CONTROL    |= _BV(UART0_UDRIE);

}/* uart_putc */


/*************************************************************************
Function: uart_puts()
Purpose:  transmit string to UART
Input:    string to be transmitted
Returns:  none          
**************************************************************************/
void uart_puts(const char *s )
{
    while (*s) 
      uart_putc(*s++);

}/* uart_puts */


/*************************************************************************
Function: uart_puts_p()
Purpose:  transmit string from program memory to UART
Input:    program memory string to be transmitted
Returns:  none
**************************************************************************/
void uart_puts_p(const char *progmem_s )
{
    register char c;
    
    while ( (c = pgm_read_byte(progmem_s++)) ) 
      uart_putc(c);

}/* uart_puts_p */


/*
 * these functions are only for ATmegas with two USART
 */
#if defined( ATMEGA_USART1 )

ISR(UART1_RECEIVE_INTERRUPT)
/*************************************************************************
Function: UART1 Receive Complete interrupt
Purpose:  called when the UART1 has received a character
**************************************************************************/
{
    unsigned char tmphead;
    unsigned char data;
    unsigned char usr;
    unsigned char lastRxError;
 
 
    /* read UART status register and UART data register */ 
    usr  = UART1_STATUS;
    data = UART1_DATA;
    
    /* */
    lastRxError = (usr & (_BV(FE1)|_BV(DOR1)) );
        
    /* calculate buffer index */ 
    tmphead = ( UART1_RxHead + 1) & UART_RX_BUFFER_MASK;
    
    if ( tmphead == UART1_RxTail ) {
        /* error: receive buffer overflow */
        lastRxError = UART_BUFFER_OVERFLOW >> 8;
    }else{
        /* store new index */
        UART1_RxHead = tmphead;
        /* store received data in buffer */
        UART1_RxBuf[tmphead] = data;
    }
    UART1_LastRxError |= lastRxError;   
}


ISR(UART1_TRANSMIT_INTERRUPT)
/*************************************************************************
Function: UART1 Data Register Empty interrupt
Purpose:  called when the UART1 is ready to transmit the next byte
**************************************************************************/
{
    unsigned char tmptail;

    
    if ( UART1_TxHead != UART1_TxTail) {
        /* calculate and store new buffer index */
        tmptail = (UART1_TxTail + 1) & UART_TX_BUFFER_MASK;
        UART1_TxTail = tmptail;
        /* get one byte from buffer and write it to UART */
        UART1_DATA = UART1_TxBuf[tmptail];  /* start transmission */
    }else{
        /* tx buffer empty, disable UDRE interrupt */
        UART1_CONTROL &= ~_BV(UART1_UDRIE);
    }
}


/*************************************************************************
Function: UART1_init()
Purpose:  initialize UART1 and set baudrate
Input:    baudrate using macro UART_BAUD_SELECT()
Returns:  none
**************************************************************************/
void UART1_init(unsigned int baudrate)
{
    UART1_TxHead = 0;
    UART1_TxTail = 0;
    UART1_RxHead = 0;
    UART1_RxTail = 0;
    

    /* Set baud rate */
    if ( baudrate & 0x8000 ) 
    {
    	UART1_STATUS = (1<<U2X1);  //Enable 2x speed 
      baudrate &= ~0x8000;
    }
    UBRR1H = (unsigned char)(baudrate>>8);
    UBRR1L = (unsigned char) baudrate;

    /* Enable USART receiver and transmitter and receive complete interrupt */
    UART1_CONTROL = _BV(RXCIE1)|(1<<RXEN1)|(1<<TXEN1);
    
    /* Set frame format: asynchronous, 8data, no parity, 1stop bit */   
    #ifdef URSEL1
    UCSR1C = (1<<URSEL1)|(3<<UCSZ10);
    #else
    UCSR1C = (3<<UCSZ10);
    #endif 
}/* uart_init */


/*************************************************************************
Function: UART1_getc()
Purpose:  return byte from ringbuffer  
Returns:  lower byte:  received byte from ringbuffer
          higher byte: last receive error
**************************************************************************/
unsigned int UART1_getc(void)
{    
    unsigned char tmptail;
    unsigned char data;


    if ( UART1_RxHead == UART1_RxTail ) {
        return UART_NO_DATA;   /* no data available */
    }
    
    /* calculate /store buffer index */
    tmptail = (UART1_RxTail + 1) & UART_RX_BUFFER_MASK;
    UART1_RxTail = tmptail; 
    
    /* get data from receive buffer */
    data = UART1_RxBuf[tmptail];
    
    data = (UART1_LastRxError << 8) + data;
    UART1_LastRxError = 0;
    return data;

}/* UART1_getc */


/*************************************************************************
Function: UART1_putc()
Purpose:  write byte to ringbuffer for transmitting via UART
Input:    byte to be transmitted
Returns:  none          
**************************************************************************/
void UART1_putc(unsigned char data)
{
    unsigned char tmphead;

    
    tmphead  = (UART1_TxHead + 1) & UART_TX_BUFFER_MASK;
    
    while ( tmphead == UART1_TxTail ){
        ;/* wait for free space in buffer */
    }
    
    UART1_TxBuf[tmphead] = data;
    UART1_TxHead = tmphead;

    /* enable UDRE interrupt */
    UART1_CONTROL    |= _BV(UART1_UDRIE);

}/* UART1_putc */


/*************************************************************************
Function: UART1_puts()
Purpose:  transmit string to UART1
Input:    string to be transmitted
Returns:  none          
**************************************************************************/
void UART1_puts(const char *s )
{
    while (*s) 
      UART1_putc(*s++);

}/* UART1_puts */


/*************************************************************************
Function: UART1_puts_p()
Purpose:  transmit string from program memory to UART1
Input:    program memory string to be transmitted
Returns:  none
**************************************************************************/
void UART1_puts_p(const char *progmem_s )
{
    register char c;
    
    while ( (c = pgm_read_byte(progmem_s++)) ) 
      UART1_putc(c);

}/* UART1_puts_p */


#endif
Code:
header
#ifndef UART_H
#define UART_H
/************************************************************************
Title:    Interrupt UART library with receive/transmit circular buffers
Author:   Peter Fleury <pfleury@gmx.ch>   http://jump.to/fleury
File:     $Id: uart.h,v 1.12 2012/11/19 19:52:27 peter Exp $
Software: AVR-GCC 4.1, AVR Libc 1.4
Hardware: any AVR with built-in UART, tested on AT90S8515 & ATmega8 at 4 Mhz
License:  GNU General Public License 
Usage:    see Doxygen manual

LICENSE:
    Copyright (C) 2006 Peter Fleury

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
    
************************************************************************/

/** 
 *  @defgroup pfleury_uart UART Library
 *  @code #include <uart.h> @endcode
 * 
 *  @brief Interrupt UART library using the built-in UART with transmit and receive circular buffers. 
 *
 *  This library can be used to transmit and receive data through the built in UART. 
 *
 *  An interrupt is generated when the UART has finished transmitting or
 *  receiving a byte. The interrupt handling routines use circular buffers
 *  for buffering received and transmitted data.
 *
 *  The UART_RX_BUFFER_SIZE and UART_TX_BUFFER_SIZE constants define
 *  the size of the circular buffers in bytes. Note that these constants must be a power of 2.
 *  You may need to adapt this constants to your target and your application by adding 
 *  CDEFS += -DUART_RX_BUFFER_SIZE=nn -DUART_RX_BUFFER_SIZE=nn to your Makefile.
 *
 *  @note Based on Atmel Application Note AVR306
 *  @author Peter Fleury pfleury@gmx.ch  http://jump.to/fleury
 */
 
/**@{*/


#if (__GNUC__ * 100 + __GNUC_MINOR__) < 304
#error "This library requires AVR-GCC 3.4 or later, update to newer AVR-GCC compiler !"
#endif


/*
** constants and macros
*/

/** @brief  UART Baudrate Expression
 *  @param  xtalcpu  system clock in Mhz, e.g. 4000000UL for 4Mhz          
 *  @param  baudrate baudrate in bps, e.g. 1200, 2400, 9600     
 */
#define UART_BAUD_SELECT(baudRate,xtalCpu)  (((xtalCpu) + 8UL * (baudRate)) / (16UL * (baudRate)) -1UL)

/** @brief  UART Baudrate Expression for ATmega double speed mode
 *  @param  xtalcpu  system clock in Mhz, e.g. 4000000UL for 4Mhz           
 *  @param  baudrate baudrate in bps, e.g. 1200, 2400, 9600     
 */
#define UART_BAUD_SELECT_DOUBLE_SPEED(baudRate,xtalCpu) ( ((((xtalCpu) + 4UL * (baudRate)) / (8UL * (baudRate)) -1UL)) | 0x8000)


/** Size of the circular receive buffer, must be power of 2 */
#ifndef UART_RX_BUFFER_SIZE
#define UART_RX_BUFFER_SIZE 32
#endif
/** Size of the circular transmit buffer, must be power of 2 */
#ifndef UART_TX_BUFFER_SIZE
#define UART_TX_BUFFER_SIZE 32
#endif

/* test if the size of the circular buffers fits into SRAM */
#if ( (UART_RX_BUFFER_SIZE+UART_TX_BUFFER_SIZE) >= (RAMEND-0x60 ) )
#error "size of UART_RX_BUFFER_SIZE + UART_TX_BUFFER_SIZE larger than size of SRAM"
#endif

/* 
** high byte error return code of uart_getc()
*/
#define UART_FRAME_ERROR      0x1000              /* Framing Error by UART       */
#define UART_OVERRUN_ERROR    0x0800              /* Overrun condition by UART   */
#define UART_PARITY_ERROR     0x0400              /* Parity Error by UART        */ 
#define UART_BUFFER_OVERFLOW  0x0200              /* receive ringbuffer overflow */
#define UART_NO_DATA          0x0100              /* no receive data available   */


/*
** function prototypes
*/

/**
   @brief   Initialize UART and set baudrate 
   @param   baudrate Specify baudrate using macro UART_BAUD_SELECT()
   @return  none
*/
extern void uart_init(unsigned int baudrate);


/**
 *  @brief   Get received byte from ringbuffer
 *
 * Returns in the lower byte the received character and in the 
 * higher byte the last receive error.
 * UART_NO_DATA is returned when no data is available.
 *
 *  @param   void
 *  @return  lower byte:  received byte from ringbuffer
 *  @return  higher byte: last receive status
 *           - \b 0 successfully received data from UART
 *           - \b UART_NO_DATA           
 *             <br>no receive data available
 *           - \b UART_BUFFER_OVERFLOW   
 *             <br>Receive ringbuffer overflow.
 *             We are not reading the receive buffer fast enough, 
 *             one or more received character have been dropped 
 *           - \b UART_OVERRUN_ERROR     
 *             <br>Overrun condition by UART.
 *             A character already present in the UART UDR register was 
 *             not read by the interrupt handler before the next character arrived,
 *             one or more received characters have been dropped.
 *           - \b UART_FRAME_ERROR       
 *             <br>Framing Error by UART
 */
extern unsigned int uart_getc(void);


/**
 *  @brief   Put byte to ringbuffer for transmitting via UART
 *  @param   data byte to be transmitted
 *  @return  none
 */
extern void uart_putc(unsigned char data);


/**
 *  @brief   Put string to ringbuffer for transmitting via UART
 *
 *  The string is buffered by the UART library in a circular buffer
 *  and one character at a time is transmitted to the UART using interrupts.
 *  Blocks if it can not write the whole string into the circular buffer.
 * 
 *  @param   s string to be transmitted
 *  @return  none
 */
extern void uart_puts(const char *s );


/**
 * @brief    Put string from program memory to ringbuffer for transmitting via UART.
 *
 * The string is buffered by the UART library in a circular buffer
 * and one character at a time is transmitted to the UART using interrupts.
 * Blocks if it can not write the whole string into the circular buffer.
 *
 * @param    s program memory string to be transmitted
 * @return   none
 * @see      uart_puts_P
 */
extern void uart_puts_p(const char *s );

/**
 * @brief    Macro to automatically put a string constant into program memory
 */
#define uart_puts_P(__s)       uart_puts_p(PSTR(__s))



/** @brief  Initialize USART1 (only available on selected ATmegas) @see uart_init */
extern void UART1_init(unsigned int baudrate);
/** @brief  Get received byte of USART1 from ringbuffer. (only available on selected ATmega) @see uart_getc */
extern unsigned int UART1_getc(void);
/** @brief  Put byte to ringbuffer for transmitting via USART1 (only available on selected ATmega) @see uart_putc */
extern void UART1_putc(unsigned char data);
/** @brief  Put string to ringbuffer for transmitting via USART1 (only available on selected ATmega) @see uart_puts */
extern void UART1_puts(const char *s );
/** @brief  Put string from program memory to ringbuffer for transmitting via USART1 (only available on selected ATmega) @see uart_puts_p */
extern void UART1_puts_p(const char *s );
/** @brief  Macro to automatically put a string constant into program memory */
#define UART1_puts_P(__s)       UART1_puts_p(PSTR(__s))

/**@}*/


#endif // UART_H
Fuse-Bits sind auf 0xC2 und 0xBF