AVRFreaks.net

uint16_t readADC(int pin)
{
   uint8_t low, high;
   uint16_t sample;
ADMUX =(1<<ADLAR)|(0<<REFS1)|(0<<REFS0)|(pin & 0x0F);
 ADCSRA|=(1<<ADEN)| (1<<ADIF)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
        //  start conversion
        ADCSRA|= (1<<ADSC);
   // ADSC is cleared when the conversion finishes
   while (bit_is_set(ADCSRA, ADSC));
low = ADCL;
   high = ADCH;

   // combine the two bytes
   sample = (high << 8) | (low & 0xFF);
   return sample;
}