empfange keine Daten über uart

#define F_CPU 16000000

#include <avr/io.h>

#include <avr/interrupt.h>

#include <util/delay.h>

#define USART_BAUDRATE 9600

#define MYUBRR F_CPU/16/USART_BAUDRATE-1

void USART_Init(unsigned int ubrr);

void USART_Transmit(unsigned int uart_data);

void TIMER_Init();

volatile uint8_t uart_data;

void USART_Init(unsigned int ubrr)

{

  /*Set baud rate*/

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

  UBRR0L = (unsigned char)(ubrr);

  /* Enable receiver and transmitter  & Interrupts for RX & TX*/

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

  /* Set frame format: 8data, 1stop bit */

  UCSR0C = (1 << UCSZ00) | (1 << UCSZ01);

}

void TIMER_Init(){

  /*Set the Timer Mode to CTC*/

  TCCR0A = (1 << WGM01);

  /* Set the value to count*/

  OCR0A = 0xF9;

  /*Set the ISR COMPA vect*/

  TIMSK0 |= (1 << OCIE0A);

  /*set prescaler to 256 and start the timer*/

  TCCR0B = (1 << CS02);

}

void USART_Transmit(unsigned int uart_data)

{

  /* Wait for empty transmit buffer */

  while (!( UCSR0A & (1<<UDRE0)));

  /* Put data into buffer, sends teh data*/

  UDR0 = uart_data;

}

int main(void)

{

  /*(LED)*/

  DDRB |= (1 << PORTB5);

  PORTB |= (1 << PORTB5);

  USART_Init(MYUBRR);

  _delay_ms(10);

  TIMER_Init();

  /*enable Interrupts*/

  sei();

  while (1){

    if(uart_data) USART_Transmit(uart_data);

  }

}

ISR(USART_RX_vect) //Receive Interrupt

{

  PORTB ^= (1 << PORTB5); //LED

  /* Wait for data to be received */

  while(!(UCSR0A & (1<<RXC0)));

  uart_data = UDR0;

}

ISR(USART_TX_vect) //Transmit Interrupt

{

  //PORTB ^= (1 << PORTB5);  // LED

  uart_data = 0x00;

}

ISR(TIMER0_COMPA_vect)

{

  static unsigned int timer_counter = 0;

  if(++timer_counter >= 250){//1000ms

    timer_counter = 0;

  } 

}