AVR Tutorial - 6. Analogue to Digital Converter ( ADC )

1.                  ADC

·         The world around us is analogue in nature and the microcontroller usually understands only digitial information .ie. Whether input is there or not.

·         There is a built in Analogue to digital converter inside the microcontroller which could be used to read the input voltage given to the ADC pins of the µC.

·         AVcc and AGND are the pins to power on the ADC peripheral and we use the voltage given in AREF as the analogue reference voltage.

·         There is a 10 bit counter inside the µC for counting the ADC input as the fraction of AREF voltage. So the ADC has got a resolution of 10-bit and it has got an operational speed of 15kSPS (Kilo Samples per Second)

·         We need to set two registers for using ADC:

i.                    ADCSRA – ADC Control and Status Register

ADEN: This bit is set to enable the ADC.

ADSC: This bit is set to start an ADC Conversion

ADIF: ADC Interrupt flag set by the hardware when a conversion is over.

ADIF: ADC Interrupt mask bit.

ADSPx : ADC prescaler bit. ADC Sampling works well at a frequence range of 50kHz to 200kHz. So we need to prescale the system clock value down to a value within that frequency.

ii.                  ADMUX – ADC Multiplexer

This register is being mainly used to set the ADC source, ADC reference source and also the ADC bit alignment selector

·         ADC could be used in 8-bit and 10-bit modes.

·         We will be having ADCH and ADCL as the ADC counter. If we are using the 8-bit mode then we could get the maximum reading as 255 and in 10bit mode we could get it as 1023.

·         In 8 bit mode the input voltage will be (ADC_count * VREF) / 255

·         In 10 bit mode the input voltage will be (ADC_count * VREF) / 1023

#include <avr/io.h>

#include <util/delay.h>

#include<avr/interrupt.h>

void main()

{

     DDRB = 0b00000111; //Green in PB0, Yellow in PB1, Red in PB2

     PORTB = 0x00;

     ADCSRA = (1<<ADEN) | (1<<ADSC) | (1<<ADIE);

ADCSRA|=(1<<ADPS2)|(1<<ADPS1)|(0 << ADPS0); //Setting the prescaler

     ADMUX = (1<<ADLAR)|(1<<MUX0)|(1<<MUX1)|(1<<MUX2); //ADC7 for I/P

    

     sei();

          

     while(1)

     {

    

     }

}

ISR(ADC_vect)

{

     uint8_t x = ADCH; //Using the 8-bit ADC mode

     if( (((float)x*5)/255) > 3.5)

     {

                PORTB = 0x02;

     }

     else if( (((float)x*5)/255) < 1.5)

     {

                PORTB = 0x01;

     }

     else

           PORTB = 0x04;

    

     ADCSRA |= (1<<ADSC);

}