🛠 Interruptions GPIO et alarmes matérielles avec le Pico SDK

Objectives

Move away from "polling": on a generic RP2040 board without embedded LEDs or buttons, you want to react instantly to an external event and wake up at a fixed time without blocking the CPU. The RP2040 provides an IO_BANK0 interrupt controller (levels and edges on each GPIO) and a 64-bit timer with 12 hardware alarms for this purpose (RP2040 datasheet). In C (Pico SDK), you will combine both for a clean button implementation: IRQ on the falling edge, debouncing via a delayed alarm, and CPU sleeping in between.

Hardware

• 1 × generic RP2040 board + USB data cable
• 1 × button between GP2 and GND, 1 × LED (with 330 Ω resistor) on GP15
• Pico SDK (C): standard CMake project, target linked with pico_stdlib pico_time hardware_irq

Complete Code

// irq_alarm.c — IRQ front descendant + alarme anti-rebond (Pico SDK)
#include "pico/stdlib.h"
#include "hardware/irq.h"
#include "pico/time.h"

#define BTN 2
#define LED 15

// Exécutée 50 ms après le front : contexte d'interruption, rester bref.
bool confirmer_appui(alarm_id_t id, void *user) {
    (void)id; (void)user;
    if (!gpio_get(BTN)) {              // toujours enfoncé ? vrai appui
        gpio_put(LED, !gpio_get(LED));
    }
    return false;                      // one-shot
}

// Handler IO_BANK0 : au plus court, on planifie juste l'alarme.
void on_gpio(uint gpio, uint32_t events) {
    (void)gpio; (void)events;
    add_alarm_in_ms(50, confirmer_appui, NULL, true);
}

int main(void) {
    stdio_init_all;
    gpio_init(BTN); gpio_set_dir(BTN, GPIO_IN); gpio_pull_up(BTN);
    gpio_init(LED); gpio_set_dir(LED, GPIO_OUT);
    // active l'IRQ du front ET enregistre le callback IO_BANK0 partagé
    gpio_set_irq_enabled_with_callback(BTN, GPIO_IRQ_EDGE_FALL,
                                       true, &on_gpio);
    while (true) {
        __wfi;   // le CPU s'end ; l'IRQ GPIO (ou une alarme) le réveille
    }
}

Expected Verification

Each firm press toggles the LED only once: the IRQ captures the edge, the alarm re-checks the state 50 ms later and ignores bounces (the contact vibrates for ~1 ms). Add a printf in the handler (with stdio_init_all) to confirm that __wfi indeed terminates upon the event. On a µA-meter, WFI sleep causes consumption to drop — the datasheet states SLEEP ≈ 1.3 mA at 25 °C, confirmed by the "dodo-meter".

Common Errors

• sleep_ms or long printf in the IRQ handler: the handler must be minimal — schedule an alarm, set a flag, handle it elsewhere.
• Forgetting gpio_pull_up: the input floats and triggers bursts of IRQs (interrupt storm, the CPU does nothing else).
• Handler not called: gpio_set_irq_enabled_with_callback also enables the IO_BANK0 IRQ at the NVIC — a simple gpio_set_irq_enabled without a callback does nothing.
• Alarm not repeating: the callback must return true to be rescheduled, false for a one-shot.
• Two cores, same alarms: the 12 hardware alarms are shared and the SDK reserves some for sleep_ms — for long-term use, create an alarm_pool.
• Bounce on release ignored: if you also want the rising edge, add GPIO_IRQ_EDGE_RISE to the mask — a single handler manages both via events.