Atmel Studio 6 Problem

#include <avr/interrupt.h>

#include <avr/io.h>

#include <stdio.h>

#define F_CPU 8000000

#include <util/delay.h>

#define DMX_BAUD 250000

#define DMX_BAUD_BREAK 80000

volatile unsigned char dmx_buffer[400];

//############################################################################

//DMX Senderoutine

ISR (USART_TX_vect)

//############################################################################

{

  static unsigned int  dmx_channel_tx_count = 0;

  static unsigned char dmx_tx_state = 0;

  switch (dmx_tx_state)

  {

    case (0):

      UBRR0   = (F_CPU / (DMX_BAUD_BREAK * 16L) - 1);

      UDR0 = 0; //RESET Frame

      dmx_tx_state = 1;

      break;

    case (1):

      UBRR0   = (F_CPU / (DMX_BAUD * 16L) - 1);

      UDR0 = 0; //Start Byte

      dmx_tx_state = 2;

      break;

    case (2):

      _delay_us(10);

      //Ausgabe des Zeichens

      UDR0 = dmx_buffer[dmx_channel_tx_count];

      dmx_channel_tx_count++;

      if(dmx_channel_tx_count == 400)

      {

        dmx_channel_tx_count = 0;

        dmx_tx_state = 0;

      }

      break;

  }

}

//############################################################################

//Hauptprogramm

int main (void) 

//############################################################################

{  

  //Init usart DMX-BUS

  UBRR0   = (F_CPU / (DMX_BAUD * 16L) - 1);

  DDRD |= (1<<PD1); //Output TXD Pin ATmega88

  UCSR0B|=(1<<TXEN0)|(1<<TXCIE0); // TXEN0 Transmitter enable / TXCIE0 TX complete interrupt enable 

  UCSR0C|=(1<<USBS0); //USBS0 2 Stop bits  

  sei();//Globale Interrupts Enable

  UDR0 = 0;//Start DMX

  //Endlosschleife

  while(1)

  {

    dmx_buffer[0]=255;

    dmx_buffer[1]=255;

    dmx_buffer[2]=255;

    dmx_buffer[3]=255;

    dmx_buffer[4]=255;

    dmx_buffer[6]=255;

    dmx_buffer[5]=255;

  }

}