🛠 Périphérique composite CDC + HID et bootloader Caterina

Objectives

Build a composite USB device: the Leonardo simultaneously exposes a CDC serial port (debugging) and a HID keyboard (output execution), thanks to the open HID class in the Arduino core (HID.cpp in ArduinoCore-avr). You will then master the Caterina bootloader: AVR109 protocol over USB CDC, detection via a 1200 baud "touch", an 8-second window, and recovering from a sketch that monopolizes the USB bus at every startup (runaway).

Hardware

• 1 Arduino Leonardo + micro-USB cable
• A second USB port or hub useful for recovery in case of a runaway sketch

Complete Code

// Peripherique composite : CDC (Serial) + clavier HID simultanes
#include <Keyboard.h>

void setup {
  Serial.begin(9600);   // interface CDC : le port serie visible par le PC
  Keyboard.begin;     // interface HID : la carte devient clavier
  delay(3000);          // garde-foc : laisser le temps de placer le curseur
  Serial.println(F("Tapez une ligne : elle sera frappee au clavier."));
}

void loop {
  if (Serial.available > 0) {
    String ligne = Serial.readStringUntil('\n');
    ligne.trim;
    if (ligne.length > 0) {
      Keyboard.print(ligne);   // sortie HID vers l'hote
      Keyboard.releaseAll;
      Serial.println(F("Frappe terminee.")); // echo CDC
    }
  }
}

Expected Verification

After uploading, the computer sees both a keyboard device and a serial port. Place the cursor in an editor, type a line in the serial monitor: the Leonardo "retypes" it into the focused application. The composite descriptor is automatically selected by the core as soon as a USB library (Keyboard/Mouse) is included in addition to Serial.

Common Errors

• Upload runaway: A sketch that crashes or hangs at startup can reclaim the USB bus at every reconnection, preventing any new upload. Recovery: press the reset button and start the upload within the bootloader's ~8-second window, or hold reset while plugging in the cable. The previous sketch continues to run upon reconnection because the bootloader is skipped at power-on.
• Manually triggering the bootloader: Opening the port at 1200 baud (the "touch") forces Caterina mode (PID 0x0036, versus 0x8036 in normal operation); the IDE does this automatically, a standard serial monitor does not.
• Modified fuses: The original values are low 0xFF, high 0xD8, extended 0xCB; erasing them removes Caterina — you will need to reprogram via ICSP.
• Ignored enumeration sequence: The COM port disappears/reappears during upload; scripting a reconnection is necessary for automation.
• Runaway HID: Never flood the host with keystrokes without condition; keep the CDC open to cut off remotely.