Analog to digital sample code

First i bring a simple sample code on an analog to digital conversion

#include <avr/io.h>
int ADC_Read;        //Variable used to store the value read from the ADC converter
#define PB5 5
int main(void){

  DDRB |= (1<<PB5);    ///PB5/digital 13 is an output

  ADCSRA |= ((1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0));    //Prescaler at 128 so we have an 125Khz clock source
  ADMUX |= (1<<REFS0);
  ADMUX &= ~(1<<REFS1);                //Avcc(+5v) as voltage reference
  ADCSRB &= ~((1<<ADTS2)|(1<<ADTS1)|(1<<ADTS0));    //ADC in free-running mode
  ADCSRA |= (1<<ADATE);                //Signal source, in this case is the free-running
  ADCSRA |= (1<<ADEN);                //Power up the ADC
  ADCSRA |= (1<<ADSC);                //Start converting

  for(;;){            //The infinite loop
    ADC_Read = ADCW;    //Read the ADC value, really that's just it
    if(ADC_Read > 512){
      PORTB |= (1<<PB5);    //If ADC value is above 512 turn led on
    }
    else {
      PORTB &= ~(1<<PB5);    //Else turn led off
    }
  }

  return 0;
}

This example demonstrates the use of the ADC of a ATmega328 
using the internal reference voltage  
To adapt to other AVR and / or other reference voltages 
see comments in this tutorial and in the data sheet

// This example demonstrates the use of the ADC of a ATmega169 
// using the internal reference voltage of nominally 1.1 V 
// To adapt to other AVR and / or other reference voltages 
// see comments in this tutorial and in the data sheet

/* Initialize the ADC */
void ADC_Init ( void )  {

  uint16_t result;

  // Select voltage reference for the ADC=> Avcc(+5v) 
  ADMUX = (1<< REFS0);
  ADMUX &= ~(1<<REFS1);                

  // Bit ADFR ("freerunning") in ADCSRA stands at power 
  // already set to 0, ie single conversion 
  ADCSRA = (1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0) ;      // frequency prescaler 
  ADCSRA |= (1<<ADEN) ;                   // enable ADC

  /* After activating the ADC is a "dummy readout" recommended reading
   So a value and rejects this in order to "warm up" the ADC */

  ADCSRA |= ( 1 << ADSC ) ;                   // an ADC conversion 
  while  ( ADCSRA & ( 1 << ADSC )  )  {          // wait for the conversion is complete 
  } 
  /* ADCW must be read once, otherwise the result of the next
   Conversion is not taken. */
  result = ADCW;
}

/* ADC single measurement */
uint16_t ADC_Read (uint8_t channel) 
{ 
  // channel choose to influence without other bits 
  ADMUX = (ADMUX &~ (0xF0))|(channel &0x0F) ;
  ADCSRA |= (1<<ADSC) ;             // a  "single conversion" 
  while  (ADCSRA & (1<<ADSC) ) {    // wait for the conversion is complete 
  } 
  return ADCW;                     // ADC read and return 
}

/* ADC with multiple measurement  */
/* Note: Range of sum variables */
uint16_t ADC_Read_Avg (uint8_t channel, uint8_t nsamples) 
{ 
  uint32_t sum = 0 ;

  for  (uint8_t i = 0; i< nsamples;  i++ )  { 
    sum += ADC_Read (channel) ;
  }

  return  ( uint16_t ) ( sum/nsamples ) ;
}


/* Example calls: 
 */

int main ( ) 
{
  uint16_t adcval;
  uint16_t adcval1;
  ADC_Init ( ) ;

  while (  1  )  { 
    adcval= ADC_Read(0) ;   // channel 0 
    // do something with adcval

    adcval1= ADC_Read_Avg(2,4) ;   // Channel 2, mean of 4 measurements 
    // do something with adcval 
  } 
}

Reference (Part. 1) : http://dubworks.blogspot.co.uk/2012/08/beyond-gpio-pins-peripherals-intro-to.html