Episode 02 · Pico Learning Series

// CONCEPT: EXTERNAL INTERRUPTS

InterruptHandling

Raspberry Pi Pico · GPIO IRQ · Rising Edge · ISR Callback

// What you'll learn

What an interrupt is and why it's better than polling

How rising/falling edge detection works in hardware

How to register an ISR (Interrupt Service Routine) callback

How the CPU context-switches to handle an interrupt

GitHub Repo ↗ ← All Projects

// Live Interrupt Simulator

BUTTON

IRQ LINE

CPU

// CPU State

MAIN LOOP

while(true) { } — waiting for events

LED (GPIO 25 — onboard)

Toggle count: 0 / 5

// 01 — Core Concepts

What is an Interrupt?

🔄

Polling vs Interrupts

Polling means the CPU constantly checks a pin: "Is the button pressed? Is the button pressed?" This wastes cycles. An interrupt lets the CPU do real work and only react when the hardware signals a change — much more efficient.

Efficiency

📈

Rising Edge Detection

A rising edge is the moment a signal transitions from LOW (0V) to HIGH (3.3V). The RP2040's GPIO hardware detects this transition in silicon — without software polling — and fires an interrupt request (IRQ) to the CPU.

GPIO_IRQ_EDGE_RISE

⚡

ISR — Interrupt Service Routine

When an IRQ fires, the CPU saves its current state (registers, program counter), jumps to your ISR callback function, executes it, then returns to exactly where it left off. The main loop never even knows it was interrupted.

interrupt_handler()

// 02 — Execution Flow

Step-by-Step Flow

// Interrupt Execution Sequence

GPIO Pin Init

LED pin → OUTPUT. Button pin (GPIO 11) → INPUT.

gpio_init() + gpio_set_dir()

Register IRQ Callback

Tell the hardware: on rising edge at GPIO 11, call interrupt_handler().

gpio_set_irq_enabled_with_callback()

Main Loop Runs

CPU enters while(true) — doing nothing, or any other task. The interrupt is armed and watching.

while(true) { }

Button Pressed → Rising Edge

Hardware detects LOW→HIGH transition on GPIO 11. Fires IRQ. CPU saves context, jumps to ISR.

GPIO_IRQ_EDGE_RISE detected ⚡

ISR Executes

interrupt_handler() runs: toggles LED 5× at 1s intervals using busy_wait_ms().

gpio_put(LED, 1/0) × 5

Return to Main Loop

ISR returns. CPU restores context. Main loop continues exactly where it paused.

context restore → resume

interrupt.c

#include <stdio.h>
#include "pico/stdlib.h"
#include "hardware/gpio.h"

#define LED_OUT     PICO_DEFAULT_LED_PIN
#define EXT_INT_PIN 11

// ISR — called automatically when button is pressed
// CPU suspends main loop, runs this, then returns
void interrupt_handler() {
    // Toggle LED 5 times — visual proof interrupt fired
    for (int i = 0; i < 5; i++) {
        gpio_put(LED_OUT, 1);
        busy_wait_ms(1000);
        gpio_put(LED_OUT, 0);
        busy_wait_ms(1000);
    }
}

int main() {
    stdio_init_all();

    // Setup LED as output
    gpio_init(LED_OUT);
    gpio_set_dir(LED_OUT, GPIO_OUT);

    // Setup interrupt pin as input
    gpio_init(EXT_INT_PIN);
    gpio_set_dir(EXT_INT_PIN, GPIO_IN);

    // Register ISR: fire on rising edge (button press)
    gpio_set_irq_enabled_with_callback(
        EXT_INT_PIN,
        GPIO_IRQ_EDGE_RISE,
        true,
        &interrupt_handler
    );

    // Main loop — CPU free to do other work
    while (true);

    return 0;
}

// Key API

gpio_set_irq_enabled_with_callback(

          pin,         // which GPIO to watch

          trigger,     // EDGE_RISE / EDGE_FALL / LEVEL

          enabled,     // true to arm the interrupt

          callback     // your ISR function pointer

);

// 03 — Hardware

Pin Connections

Connection Table

Component          GPIO Pin    Direction
─────────────────────────────────────────
LED (onboard)     GPIO 25     OUTPUT
Button (EXT)      GPIO 11     INPUT
─────────────────────────────────────────
// EXT_INT_PIN watches for rising edge
// LED_OUT toggles 5× on each interrupt