#define ADMUX *(volatile char*)0x7C 
#define ADCSRB *(volatile char*)0x7B // ADC Control and Status Register B
#define ADCSRA *(volatile char*)0x7A
#define ADCL *(volatile short*)0x78
#define ADCH *(volatile char*)0x79
#define DIDR0 *(volatile char*)0x7E //digital input disable register 0
//#define DIDR2 *(volatile char*)0x7D

void adcInit();
short adc_fun();
volatile long res;

void setup() {
  Serial.begin(9600);
  adcInit();
}

void loop() {
  res = adc_fun();
  Serial.print(res*0.0048875855,3);
  Serial.print("V   ");
  Serial.println(res, BIN);
  delay(1000);
}

void adcInit(){
  ADMUX = 0x00; //AREF, Rigth Adjust, ADC0
  ADCSRA = 0x87; // ADC EN, Prescalar 128
  ADCSRB = 0x00; // Free running mode
  DIDR0 = 0X01; //Digital Input Disable on ADC channel 0
}

short adc_fun(){
  ADCSRA |= 0x40; //Start conversion
  while(!(ADCSRA & 0x10)); 
  ADCSRA |= 0x10; //clear ADCIF by writing 1 to it
  //return(ADCH<<8);
  return(ADCL); //return ADCL itself will be 10-bit...no need to include ADCH
}