ATMega328P Eigene Spannungsversorgung messen

int getVoltagefromVCCPin()

{       

    ADMUX = _BV(MUX3) | _BV(MUX2) | _BV(MUX1);

    delay(5); // wait for Vref to settle

    ADCSRA |= _BV(ADSC); // reading ADC

    while (bit_is_set(ADCSRA, ADSC)); //ADSC will be set to 0 wen conversion is finished

    uint8_t low = ADCL;

    unsigned int adcVal = (ADCH << 8) | low;

    // discard previous result

    ADCSRA |= _BV(ADSC); // Convert

    while (bit_is_set(ADCSRA, ADSC));

    low = ADCL;

    adcVal = (ADCH << 8) | low;

    //return adcVal;

    return ((long)1024 * 1100) / adcVal;

}

void setup() 

{

}

void loop() 

{

  Serial.println( getVoltagefromVCCPin() );

  delay(100);

}