volatile int adcReading = 0;
volatile boolean adcDone = false;
byte adcPin;

ISR (ADC_vect)
  {
  adcReading = ADC;
  adcDone = true;
  } 

void begin(uint8_t prescaler, uint8_t)

void setup() {
pinMode(A0,INPUT);
Serial.begin(115200);
adcPin = 0;
ADCSRA = 0;            //Reset the ADCSRA register
ADCSRA =  bit (ADEN);  //Enable ADC

//Select one of the below prescalers (16 and up recommended)

//ADCSRA |= bit (ADPS0);                               //   2  ( 1.6875 µs) 592592 Conversions/sec
//ADCSRA |= bit (ADPS1);                               //   4  ( 3.375  µs) 296296 Conversions/sec
//ADCSRA |= bit (ADPS0) | bit (ADPS1);                 //   8  ( 6.75   µs) 148148 Conversions/sec
//ADCSRA |= bit (ADPS2);                               //  16  (13.5    µs)  74074 Conversions/sec
//ADCSRA |= bit (ADPS0) | bit (ADPS2);                 //  32  (27      µs)  38462 Conversions/sec
//ADCSRA |= bit (ADPS1) | bit (ADPS2);                 //  64  (54      µs)  18518 Conversions/sec
ADCSRA |= bit (ADPS0) | bit (ADPS1) | bit (ADPS2);   // 128  (108      µs)   9259 Conversions/sec

ADMUX  =  bit (REFS0) | (adcPin & 0x07);    // AVcc and select input port
//ADMUX = bit (REFS0) | bit (REFS1) |  (adcPin & 0x07);    // 1.1V and select input port
//ADMUX = (adcPin & 0x07);    // AREF pin ref and select input port
ADCSRA |= bit (ADSC) | bit (ADIE);     //Start
}

void loop() {

  if(adcDone){
  ADCSRA |= bit (ADSC) | bit (ADIE);     //Start
  adcDone = false;
}


}