Absturz beim RS-232 empfang (mega8)

#define   F_CPU 8000000UL          // MP-Takt

#include  <avr/io.h>

#include  <stdio.h>

#include  <stdlib.h>

#include   <inttypes.h>

#include   <avr/interrupt.h>

#include   <avr/wdt.h>

#define BAUD     19200UL          //Baundrate definieren

#define UBRR_BAUD  ((F_CPU)/(16*(BAUD))-1)  //Autoberechnung bei Änderung des MP-Takt

char         out[15];

int     in     =  1;

int     Zahl   = 15;

long int   i    =  1;

int     n     =  1;

int     Daten   =100;

// Ausgaberoutinen

int uart_putc(unsigned char c)

{

  while (!(UCSRA & (1<<UDRE)));

  UDR = c;

  return 0;

}

int uart_puts( char* str )

{

  while( *str )

    uart_putc( *str++ );

  return 0;

}

int wait()

{

for (i=0;i<200000;)

  {

  i=i+1;

  }

}

//  Main-Funktion

int main (void) 

{

//  Programm

// RS232 aktivieren  

  UCSRB   |= (1 << TXEN) | ( 1 << RXEN ) | ( 1 << RXCIE ) ;          // UART TX, RX einschalten

   UCSRC   |= ( 1 << URSEL ) | ( 1<<UCSZ1 ) | ( 1<<UCSZ0 ) | ( 1<<UPM1 );    // Asynchron 8N1, Parity even

   UBRRH  = (uint8_t) (UBRR_BAUD>>8);        // USART Baud

    UBRRL  = (uint8_t) UBRR_BAUD;

  TCCR1A = (1 << WGM11) | (1 << WGM10) | ( 1 << COM1A1);

  TCCR1B =  ( 1 << CS11) ;

  OCR1A = 0;

  sei();   

  DDRB  = 0xFF;

  for (;;) 

  {

    while (!(UCSRA & (1<<UDRE)));

      UDR   = Daten;

    OCR1A   = Daten;

    wait();

    Daten=Daten+20;

  }      

}

ISR(SIG_USART_RECV)

{

Daten = UDR;

}