ATMega8: AD-Wandlung wird nicht fertig

int main(void)

{

  uint16_t temp;

  char text[40];

  uart_Init();

    ADMUX = 0x00;  //ex reference, no channel selected

    ADCSRA = 0xA7;  //0b1010 0111, division factor: 128, free running

    ADMUX |= ((1<<MUX0)|(1<<MUX1));

    ADMUX &= ~(1<<ADLAR);

    uart_puts("STARTED\n");

    while(1)

    {

         ADCSRA |= (1<<ADSC);

         while(ADCSRA & (1<<ADSC))

           ;

         temp = ADCW;

    sprintf(text,"ADC: %d\n",temp);

    uart_puts(text);

    _delay_ms(100);

    }

}