Analog-to-Digital Conversion (ADC)

Real-world signals — light, temperature, sound — are analog. An ADC is the peripheral that translates a continuous voltage into a number your code can actually use.

A GPIO pin configured as a digital input can only tell you high or low. Many sensors, though — a light-dependent resistor, a temperature sensor, a microphone — produce a continuously varying voltage, not a clean on/off signal. An analog-to-digital converter (ADC) is the peripheral that bridges the two worlds: it samples a voltage at a given instant and reports back the closest matching number.

That number's precision depends on the ADC's resolution, measured in bits. A common 10-bit ADC divides its full input voltage range into 1,024 discrete steps (2 to the power of 10), numbered 0 through 1023. A reading of 0 means the input was at (or below) the ADC's reference low voltage; a reading of 1023 means it was at (or above) the reference high voltage; anything in between is proportional.

Using an ADC typically involves selecting which pin to sample (many PICs share one ADC among several pins), starting a conversion, waiting a short time for it to finish, and then reading the result out of a pair of registers (since a 10-bit value doesn't fit in a single 8-bit register).

unsigned int read_adc(void) {
    ADCON0bits.CHS = 0;      // select channel AN0
    ADCON0bits.GO = 1;       // start conversion
    while (ADCON0bits.GO);   // wait for it to finish
    return ((ADRESH << 8) + ADRESL); // combine into one 10-bit value
}

void main(void) {
    TRISA0 = 1;              // AN0 as input
    ADCON1 = 0b00001110;     // AN0 analog, rest digital
    ADCON0 = 0b00000001;     // enable the ADC module

    while(1) {
        unsigned int value = read_adc();
        // use "value", 0–1023, however your project needs
        __delay_ms(200);
    }
}