🛠 Transformer la carte en clavier USB HID avec TinyUSB

Objectives

The RP2040 integrates a USB 1.1 full-speed device AND host controller (datasheet, key features) — no external chip required. With TinyUSB (the USB stack embedded in the Pico SDK, github.com/hathach/tinyusb), your generic board can impersonate a HID keyboard: pressing the GP2 button sends the "A" key to any PC, without drivers. This is the basis for hardware shortcuts, presentation triggers, and other serial→HID bridges.

Hardware

• 1 × generic RP2040 board with the USB pin connected (the Zero exposes USB via the original connector)
• 1 × button between GP2 and GND
• Pico SDK ≥ 1.5 with the pico_sdk_init lib + TinyUSB: copy the pico_sdk/examples/lib_pico_stdlib template then add pico_sdk_init and tinyusb_board/tinyusb_device via pico_sdk_init + include(../../lib/tinyusb/src/tinyusb.cmake) — the simple way is to copy the official example tinyusb/01_device_hello from the raspberrypi/pico-examples repository and switch to a 00_hid_keyboard-style (HID) setup; TinyUSB installation docs: raspberrypi/pico-sdk lib/tinyusb

Complete Code

// hid_clavier.c — squelette HID clavier (inspiré de la pile TinyUSB)
#include "pico/stdlib.h"
#include "tusb.h"

#define BTN 2

// descripteur de rapport clavier standard (boot protocol, 8 octets)
#define REPORT_ID_KEYBOARD 1
static const uint8_t hid_desc[] = TUD_HID_REPORT_DESC_KEYBOARD(HID_REPORT_ID(REPORT_ID_KEYBOARD));

const uint8_t desc_cfg[TUD_CONFIG_SIZE(1) + TUD_HID_DESC_SIZE(1)] = {
    TUD_CONFIG_DESCRIPTOR(1, 1, 0, TUD_CONFIG_DESC_LEN, 0, 100),
    TUD_HID_DESCRIPTOR(0, 0, HID_ITF_PROTOCOL_KEYBOARD, sizeof(hid_desc),
                       0x81, sizeof(hid_desc), 10),   // EP 0x81, 10 ms
};
uint8_t const *tud_hid_descriptor_report_cb(uint8_t itf) { (void)itf; return hid_desc; }
uint16_t tud_hid_get_report_cb(uint8_t itf, uint8_t report_id, hid_report_type_t t, uint8_t *b, uint16_t len) { (void)itf;(void)report_id;(void)t;(void)b;(void)len; return 0; }
void tud_hid_set_report_cb(uint8_t itf, uint8_t report_id, hid_report_type_t t, uint8_t const *b, uint16_t len) { (void)itf;(void)report_id;(void)t;(void)b;(void)len; }

int main(void) {
    gpio_init(BTN); gpio_set_dir(BTN, GPIO_IN); gpio_pull_up(BTN);
    tusb_init;
    uint8_t prev = 1;
    while (true) {
        tud_task;                       // fait tourner la pile USB
        uint8_t now = gpio_get(BTN);
        if (tuh_mounted(0) || tud_mounted) {
            if (now != prev) {            // front : envoyer touche 'A' puis relâcher
                hid_keyboard_report_t press = {0, 0, {HID_KEY_A, 0, 0, 0, 0, 0}, 0};
                hid_keyboard_report_t rel   = {0, 0, {0, 0, 0, 0, 0, 0, 0}, 0};
                tud_hid_report(REPORT_ID_KEYBOARD, &press, sizeof(press));
                sleep_ms(10);
                tud_hid_report(REPORT_ID_KEYBOARD, &rel, sizeof(rel));
                prev = now;
            }
        }
    }
}

Expected Verification

Plug in the board: the PC sees a keyboard device (no serial port, no driver). Open a text editor, press GP2: an "a" is typed. lsusb (Linux) or Device Manager (Windows) lists a HID device. This proves that the complete USB device enumeration (descriptors, endpoint 0x81, keyboard protocol) is working.

Common Errors

• Coding the application before tud_task runs: without regular calls to the stack, enumeration fails and the PC displays "unknown device".
• Forgetting the release report: the PC sees a key held down (infinite repetition of "a") — always send press then release.
• Report descriptor inconsistent with the sent frame: Windows is strict, the device is rejected — keep the standard descriptor TUD_HID_REPORT_DESC_KEYBOARD.
• Confusing tud_mounted (we are a device, the host opened us) and tuh_mounted (we are a host): in simple device mode, only the first one makes sense.
• The board "disappears" when you also plug in the USB debug: two native USB controllers on the same PC, two enumerations — one cable is enough for testing.
• Boot protocol vs full report: some BIOSes only speak the 8-byte boot protocol — the standard descriptor above is compatible with both.
• Only 12 Mbit/s: the RP2040's USB is full-speed (12 Mbit/s max), more than enough for HID, never for a webcam.