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ADC

ADCs turn voltages into numbers. Use them for battery voltage, light sensors, potentiometers, current-shunt amplifiers — anything where the output is "a voltage between 0 and N volts."

info

References: Zephyr ADC overview and the ADC API doxygen.


Enable ADC​

CONFIG_ADC=y

ESP32-S3 has two SADCs (adc1, adc2) — but adc2 is unavailable when WiFi is active, so default to adc1 for any pin you also need during a BLE/WiFi session. nRF52840 has a single 12-bit SAADC with 8 channels.


Devicetree — declare a channel​

&adc1 {
    status = "okay";
    #address-cells = <1>;
    #size-cells = <0>;

    channel@4 {
        reg = <4>;
        zephyr,gain = "ADC_GAIN_1";
        zephyr,reference = "ADC_REF_INTERNAL";
        zephyr,acquisition-time = <ADC_ACQ_TIME_DEFAULT>;
        zephyr,resolution = <12>;
    };
};

/ {
    zephyr,user {
        io-channels = <&adc1 4>;
    };
};

The io-channels property is the standard Zephyr way for an application (or a sensor driver) to reference one or more ADC channels.


C code — single read​

#include <zephyr/drivers/adc.h>

static const struct adc_dt_spec adc_chan =
    ADC_DT_SPEC_GET(DT_PATH(zephyr_user));

int main(void)
{
    int16_t sample;
    struct adc_sequence seq = {
        .buffer      = &sample,
        .buffer_size = sizeof(sample),
    };

    adc_channel_setup_dt(&adc_chan);
    adc_sequence_init_dt(&adc_chan, &seq);

    while (1) {
        adc_read_dt(&adc_chan, &seq);

        int32_t mv = sample;
        adc_raw_to_millivolts_dt(&adc_chan, &mv);

        printk("Raw: %d  mV: %d\n", sample, mv);
        k_msleep(1000);
    }
}

adc_raw_to_millivolts_dt() accounts for gain and reference, so you get a real voltage instead of a raw count.


Battery voltage example​

To read a LiPo battery through a 2:1 divider on GPIO4:

int32_t bat_mv = sample;
adc_raw_to_millivolts_dt(&adc_chan, &bat_mv);
bat_mv *= 2;   /* undo the divider */
LOG_INF("Battery: %d mV", bat_mv);

A 3.7V LiPo reads around 4200 mV fully charged, 3300 mV nearly empty. Below 3000 mV the cell is damaged — cut off and sleep.


Oversampling for cleaner readings​

ADC samples are noisy. Average several:

seq.oversampling = 4;   /* take 2^4 = 16 samples, return mean */

This is a hardware feature on nRF SAADC. On ESP32-S3 it's software in the driver — same API, slightly slower.


Common gotchas​

SymptomCause
Reading is 0 or maxed (4095)Channel disabled, wrong pin, or input out of reference range
Reading drifts with WiFi/BLE activeESP32 adc2 blocked by radio — switch to adc1
Always the same valueForgot adc_channel_setup_dt() before first adc_read_dt()
Off by 2x or 3xVoltage divider on the input — multiply in software
warning

On ESP32-S3, the SAR ADC is not linear at the rails. Calibration accuracy drops below ~150 mV and above ~2450 mV (default attenuation). Design your divider so the expected range falls in the linear region.