🛠 Lire un potentiomètre avec l'ADC 12 bits

Objectives

Use the Pico's analog-to-digital converter: a 12-bit SAR, 4 channels output at 500 ksps (GP26 = ADC0, GP27 = ADC1, GP28 = ADC2; ADC3 = VSYS/3 and ADC4 = internal temperature sensor). You will read a potentiometer and display the values on the USB serial console. Note: the Pico has no DAC — the ADC is the only analog path, read-only.

Hardware

• 1 × Raspberry Pi Pico
• 1 × potentiometer (e.g., 10 kΩ)
• Cables: one end of the potentiometer to 3V3, the other to GND, the wiper (middle pin) to GP26
• Serial console at 115200 baud (PuTTY, picocom, etc.) on the Pico's USB port

Complete code

// adc_pot.c — ADC0 (GP26), moyenne sur 16 échantillons
#include <stdio.h>
#include "pico/stdlib.h"
#include "hardware/adc.h"

int main(void) {
    stdio_init_all;               // console sur USB (stdio_usb)
    adc_init;
    adc_gpio_init(26);              // GP26 en mode ADC
    adc_select_input(0);            // canal ADC0
    while (true) {
        uint32_t sum = 0;
        for (int i = 0; i < 16; i++) sum += adc_read;
        uint16_t raw = sum / 16;
        float mv = raw * 3300.0f / 4095.0f;
        printf("raw=%u  ~%.0f mV\n", raw, mv);
        sleep_ms(200);
    }
}

Expected verification

Turning the potentiometer, the values scroll from ~0 to ~4095 (12 bits) and the calculated voltage follows 0 → 3.3 V. The values may be slightly noisy: this is normal, the Pico's ADC uses its own filtered 3.3 V supply as a reference, with an inherent offset of approximately 30 mV documented in the datasheet.

Common errors

• Forgetting adc_gpio_init(26): the pin remains in GPIO mode, the reading is incorrect or zero.
• Wrong channel: adc_select_input(3) reads VSYS/3, 4 reads the temperature — not your potentiometer!
• Measuring more than 3.3 V: the ADC cannot handle more than its supply voltage; and GP26-GP29 have a diode to IOVDD (beyond ~3.6 V it conducts, and powering a pin on an unpowered board powers the entire 3V3 rail).
• Expecting precision: for noise, average multiple samples; for accuracy, subtract the offset (reading a grounded channel) or force the regulator into PWM mode (GPIO23).
• stdio_init_all over UART in older examples: connect to the correct port (USB for stdio_usb).