Power-electronicsHow-ToBeginner · 3 min read

How to Read ADC Value in Embedded C: Simple Guide

To read an ADC value in embedded C, first initialize the ADC hardware, start the conversion by setting the proper control bits, then wait for the conversion to complete and read the result from the ADC data register using ADC_Read() or direct register access. This process involves configuring ADC registers, starting conversion, and reading the digital value representing the analog input.

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Syntax

The basic steps to read an ADC value in embedded C usually involve these parts:

Initialize ADC: Set up ADC control registers to configure reference voltage, input channel, and prescaler.
Start Conversion: Trigger the ADC to begin converting the analog signal to digital.
Wait for Completion: Check a flag or status bit to know when conversion is done.
Read Result: Read the converted digital value from ADC data registers.

embedded_c

void ADC_Init() {
    ADMUX = (1 << REFS0); // Use AVcc as reference voltage
    ADCSRA = (1 << ADEN) | (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0); // Enable ADC and set prescaler to 128
}

uint16_t ADC_Read(uint8_t channel) {
    ADMUX = (ADMUX & 0xF8) | (channel & 0x07); // Select ADC channel
    ADCSRA |= (1 << ADSC); // Start conversion
    while (ADCSRA & (1 << ADSC)); // Wait until conversion finishes
    return ADC; // Return ADC value
}

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Example

This example shows how to initialize the ADC, read a value from channel 0, and store it in a variable.

embedded_c

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

void ADC_Init() {
    ADMUX = (1 << REFS0); // AVcc reference
    ADCSRA = (1 << ADEN) | (1 << ADPS2) | (1 << ADPS1) | (1 << ADPS0); // Enable ADC, prescaler 128
}

uint16_t ADC_Read(uint8_t channel) {
    ADMUX = (ADMUX & 0xF8) | (channel & 0x07); // Select channel
    ADCSRA |= (1 << ADSC); // Start conversion
    while (ADCSRA & (1 << ADSC)); // Wait for conversion
    return ADC; // Read ADC value
}

int main(void) {
    uint16_t adc_value;
    ADC_Init();
    while (1) {
        adc_value = ADC_Read(0); // Read ADC channel 0
        _delay_ms(500); // Delay for stability
    }
    return 0;
}

Output

No console output; adc_value variable holds the ADC reading (0-1023).

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Common Pitfalls

Not enabling the ADC before reading causes no conversion.
Forgetting to select the correct ADC channel leads to wrong input readings.
Not waiting for conversion to finish before reading ADC data results in invalid values.
Incorrect prescaler settings can cause inaccurate ADC timing.
Not configuring reference voltage properly affects ADC accuracy.

embedded_c

/* Wrong: Reading ADC without starting conversion */
uint16_t wrong_read() {
    return ADC; // Returns old or invalid data
}

/* Correct: Start conversion and wait */
uint16_t correct_read() {
    ADCSRA |= (1 << ADSC); // Start
    while (ADCSRA & (1 << ADSC)); // Wait
    return ADC; // Read
}

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Quick Reference

Remember these quick tips when reading ADC values:

Always initialize ADC before use.
Select the correct input channel before starting conversion.
Start conversion and wait until it finishes before reading.
Use proper reference voltage for accurate results.
Check your microcontroller datasheet for specific ADC register details.

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Key Takeaways

Initialize ADC hardware and configure reference voltage before reading values.

Start ADC conversion and wait for it to complete before reading the result.

Select the correct ADC input channel to get accurate readings.

Always check ADC status flags to avoid reading invalid data.

Consult your microcontroller datasheet for exact ADC register details.