Priyank

#define F_CPU 8000000UL

#include <avr/io.h>
#include <util/delay.h>

// initialize adc
void adcInit()
{
  // AREF = AVcc
  ADMUX = (1<<REFS0);

  // ADC Enable and prescaler of 128
  // 8000000/128 = 62500
  ADCSRA = (1<<ADEN)|(1<<ADPS2)|(1<<ADPS1)|(1<<ADPS0);
}

// read adc value
uint16_t adcRead(uint8_t ch)
{
  // select the corresponding channel 0~7
  // ANDing with '7' will always keep the value
  // of 'ch' between 0 and 7
  ch &= 0b00000111;  // AND operation with 7
  ADMUX = (ADMUX & 0xF8)|ch;   // clears the bottom 3 bits before ORing

  // start single conversion
  // write '1' to ADSC
  ADCSRA |= (1<<ADSC);

  // wait for conversion to complete
  // till then, run loop continuously
  // The loop does nothing while ADIF is set to 0,
  // it exits as soon as ADIF is set to one,
  // i.e. conversion is complete.
  while(!(ADCSRA & (1<<ADIF)));

  //Clear ADIF by writing one to it
  ADCSRA|=(1<<ADIF);

  return (ADC);
}

void pwmInit()
{
   /*
   TCCR0 - Timer Counter Control Register (TIMER0)
   -----------------------------------------------
   BITS DESCRIPTION

   NO:   NAME   DESCRIPTION
   --------------------------
   BIT 7 : FOC0   Force Output Compare [Not used in this example]
   BIT 6 : WGM00  Wave form generation mode  [SET to 1]
   BIT 5 : COM01  Compare Output Mode    [SET to 1]
   BIT 4 : COM00  Compare Output Mode    [SET to 0]

   BIT 3 : WGM01  Wave form generation mode  [SET to 1]
   BIT 2 : CS02   Clock Select         [SET to 0]
   BIT 1 : CS01   Clock Select         [SET to 0]
   BIT 0 : CS00   Clock Select         [SET to 1]

   The above settings are for
   --------------------------

   Timer Clock = CPU Clock (No Prescalling)
   Mode    = Fast PWM
   PWM Output  = Non Inverted

   */

   TCCR0|=(1<<WGM00)|(1<<WGM01)|(1<<COM01)|(1<<CS00);

   //Set OC0 PIN as output. It is  PB3 on ATmega16 ATmega32

   DDRB|=(1<<PB3);
}

/******************************************************************
Sets the duty cycle of output. 

Arguments
---------
duty: Between 0 - 255

0 = 0%

255 = 100%

The Function sets the duty cycle of pwm output generated on OC0 PIN
The average voltage on this output pin will be

         duty
 Vout=  ------ x 5v
         255 

This can be used to control the brightness of LED or Speed of Motor.
*********************************************************************/

void setPWMOutput(uint8_t duty)
{
   OCR0=duty;
}

int main()
{
  uint16_t adcResult;
  uint8_t pwmBrightness;

  // initialize adc and pwm
  adcInit();
  pwmInit();

  _delay_ms(50);

  while(1)
  {
    adcResult = adcRead(0);    // read adc value at PA0

    // Mapping the adc value in the range of 0-1024 (2^10 bits)
    // to 0-255, which can be sent to 8 bit OCR0 register.
    // So the adcResult is divided by 4, to make it in the
    // range of 0-255.
    // After that the value is subtracted from 255, so that if
    // the adc value is less, we want to send high value to the
    // OCR0, for increasing brightness, and if the adc value is
    // high, we want to send lower values for the PWM signal
    // generation, to reduce the brightness of the LED.
    pwmBrightness = (uint8_t)(255 - (adcResult/4));

    //Send the pwm value to the OCR0, to control the LED brightness
    setPWMOutput(pwmBrightness);
   }
}