5. Analog-to-digital Converter (ADC)

This tutorial is to introduce ADC and explain how it works

The ADC pins read the analog signal and convert the signal to an 8-bit or 10-bit number. For example, if the top value is 5V and an 8-bit ADC pin reads 2.5V, then the MCU will convert 2.5V to 127 since the total count of 8-bit is 255 (counts from 0).

Example 1: Start the ADC function

ADCSRA |= 1<<ADEN; //Enable ADC

By default, the ADC input clock frequency should be between 50 kHz and 200 kHz to get maximum resolution. The input clock frequency can be higher than 200 kHz if the resolution is lower than 10-bit.

Example 2: Enable the prescaler to determine the ADC input clock when the MCU clock frequency is 1MHz

ADCSRA |= 1<<ADPS2; //Set Prescaler to 16, 1,000,000/16 = 62.5 kHz

The reference voltage needs to be set to determine the highest reading voltage. Here are 4 options:

REFS1 REFS0 Voltage Reference Selection

0 0 AREF, Internal Vref turned OFF

0 1 AVCC with external capacitor at AREF pin

1 0 Reserved

1 1 Internal 1.1V voltage reference with external capacitor at AREF pin

Example 3: Set the reference voltage

ADMUX |= 1<<REFS0; //Set the reference voltage at AVcc (input voltage)

ADCH and ADCL registers are used to store the 10-bit value. When ADLAR = 0 (default), the result is right adjusted; when ADLAR = 1, the result is left adjusted.

ADLAR = 0 (right adjusted)

ADCH: - - - - - - ADC9 ADC8

ADCL: ADC7 ADC6 ADC5 ADC4 ADC3 ADC2 ADC1 ADC0

ADLAR = 1 (left adjusted)

ADCH: ADC9 ADC8 ADC7 ADC6 ADC5 ADC4 ADC3 ADC2

ADCL: ADC1 ADC0 - - - - - -

If the result is left adjusted and no more than 8-bit, it is sufficient to read ADCH. Otherwise, ADCL must be read first, then ADCH.

Example 4: Set the result left adjusted

ADMUX |= 1<<ADLAR; //Left adjust the result

Example 5: Enable the ADC conversion complete interrupt and global interrupt

ADCSRA |= 1<<ADIE; //Enable the interrupt

sei(); //Enable the global interrupt

Example 6: Start the ADC conversion

ADCSRA |= 1<<ADSC; //Start the ADC conversion

Example 7: Start an Interrupt Service Routing

ISR(ADC_vect) //Interrupt Service Routing is call after 1<<ADSC

{

// Analog values will be automatically stored in ADCH

….

ADCSRA |= 1<<ADSC; //Start another ADC conversion after the previous one is done

}

The ADC input pin is determined by setting MUX[3:0] in the ADMUS register. If the pin is not specified, the ADC input pin is ADC0 by default.

MUX[3:0] Single Ended Input

0000 ADC0

0001 ADC1

0010 ADC2

0011 ADC3

0100 ADC4

0101 ADC5

0110 ADC6

0111 ADC7

1000 Temperature sensor

Example 8: Full code to have PD0 pin output HIGH when ACD0 reads 2.5V (8-bit) or higher when the reference voltage is 5V at AVcc

#include <avr/io.h>

#include <avr/interrupt.h>

int main(void)

{

DDRD |= 1 << 0; //Set PD0 as an output

PORTD &=~ (1<<0); //Set PD0 to LOW

ADCSRA |= 1<<ADPS2; //Set Prescaler to 16, 1,000,000/16 = 62.5 kHz

ADMUX |= 1<<REFS0; //Set the reference voltage at AVcc (input voltage)

ADMUX |= 1<<ADLAR; //Left adjust the result

ADCSRA |= 1<<ADIE; //Enable the interrupt

ADCSRA |= 1<<ADEN; //Enable ADC

sei(); //Enable the global interrupt

ADCSRA |= 1<<ADSC; //Start the ADC conversion

while (1) {}

}

ISR(ADC_vect) //Interrupt Service Routing is called after 1<<ADSC

{

if (ADCH >= 127) //127 represents 2.5V in 8-bit data

PORTD |= 1<<0;

else

PORTD &=~ (1<<0);

ADCSRA |= 1<<ADSC; //Start another ADC conversion after the previous one is done

}

Example 9 (extended): Full code to have PD0 pin output HIGH when ADC0 reads 2.5V (10-bit) or higher (reference voltage is 5V at AVcc)

#include <avr/io.h>

#include <avr/interrupt.h>

int main(void)

{

DDRD |= 1 << 0; //Set PD0 as an output

PORTD &=~ (1<<0); //Set PD0 to LOW

ADCSRA |= 1<<ADPS2; //Set Prescaler to 16, 1,000,000/16 = 62.5 kHz

ADMUX |= 1<<REFS0; //Set the reference voltage at AVcc (input voltage)

ADCSRA |= 1<<ADIE; //Enable the interrupt

ADCSRA |= 1<<ADEN; //Enable ADC

sei(); //Enable the global interrupt

ADCSRA |= 1<<ADSC; //Start the ADC conversion

while (1) {}

}

ISR(ADC_vect) //Interrupt Service Routing is called after 1<<ADSC

{

uint8_t LowBits = ADCL; //Need to create this variable because ADCL is read first

uint16_t TenBitResult = ADCH<<8 | LowBits; //Shift ADCH to the right by 8 bits and put ADCL to lower bits

if (TenBitResult >= 511) //511 represents 2.5V in 10-bit data

PORTD |= 1<<0;

else

PORTD &=~ (1<<0);

ADCSRA |= 1<<ADSC; //Start another ADC conversion after the previous one is done

}

Example 10 (extended): Full code to have PD0 pin output HIGH when ADC0 reads 2.5V (10-bit) or higher, and to have PD1 pin output LOW when ACD1 reads 2.5V (10-bit) or higher (reference voltage is 5V at AVcc)

#include <avr/io.h>

#include <avr/interrupt.h>

int main(void)

{

DDRD |= 0b00000011; //Set PD0 and PD1 as outputs

ADCSRA |= 1<<ADPS2; //Set Prescaler to 16, 1,000,000/16 = 62.5 kHz

ADMUX = 0b01000000; //Set the reference voltage at AVcc, right shifted, and ADC0 as an input

ADCSRA |= 1<<ADIE; //Enable the interrupt

ADCSRA |= 1<<ADEN; //Enable ADC

sei(); //Enable the global interrupt

ADCSRA |= 1<<ADSC; //Start the ADC conversion

while (1) {}

}

ISR(ADC_vect) //Interrupt Service Routing is called after 1<<ADSC

{

uint8_t LowBits = ADCL; //Need to create this variable because ADCL is read first

uint16_t TenBitResult = ADCH<<8 | LowBits; //Shift ADCH to the right by 8 bits and put ADCL to lower bits

switch (ADMUX) //switch the ADC ports

{

case 0b01000000: //when ADC0 is reading, PD0 is HIGH when ADC0 >= 2.5V, PD0 is LOW when ADC0 < 2.5V

if (TenBitResult >= 511) //511 represents 2.5V in 10-bit data

PORTD |= 1<<0;

else

PORTD &=~ (1<<0);

ADMUX = 0b01000001; //switch to ADC1 next time;

break;

case 0b01000001: //when ADC1 is reading, PD1 is LOW when ADC1 >= 2.5V, PD1 is HIGH when ADC1 < 2.5V

if (TenBitResult >= 511)

PORTD &=~ (1<<1);

else

PORTD |= (1<<1);

ADMUX = 0b01000000; //switch to ADC0 next time

break;

default: //no default condition

break;

}

ADCSRA |= 1<<ADSC; //Start another ADC conversion after the previous one is done

}

Based on the test, the ADC0 and ADC1 has to constantly read signals to ensure the corresponding outputs show correct values. In other words, ADC0 and ADC1 cannot be in tri state.