🛠 Récupérer un Micro bloqué : bootloader Caterina et rituel de l'upload

Objectives

Understand the Micro upload ritual (Caterina bootloader, AVR109 protocol) and learn how to recover a board that no longer accepts any uploads: the 1200 baud window, manual reset, and finally reprogramming the bootloader via ICSP as a last resort.

Hardware

• 1 Arduino Micro (or Pro Micro 32U4) + micro-USB data cable
• As a last resort: an ICSP programmer (for example, another Arduino board set up as Arduino ISP) and a 6-pin ISP cable

Complete code

// Sketch-minimal de controle apres recuperation : clignotement + amorce CDC.
// Apres un upload reussi, cette LED prouve que l'application tourne.
void setup {
  pinMode(LED_BUILTIN, OUTPUT);  // broche 13
  Serial.begin(9600);            // CDC USB
}

void loop {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(200);
  digitalWrite(LED_BUILTIN, LOW);
  delay(200);
}
// Ritual d'upload Caterina :
// 1) L'IDE ouvre le port a 1200 bauds (upload.use_1200bps_touch) : le
//    bootloader reste actif ~8 s puis redemarre sur l'application.
// 2) Si l'ouverture 1200 ne declenche rien : appuyer sur RESET juste
//    apres l'affichage du message de televersement, dans la fenetre de 8 s.
// 3) En secours absolu : graver bootloaders/caterina/Caterina-Micro.hex
//    via ICSP (arduino-cli ou IDE > Graver le bootloader).

Expected verification

After double-pressing reset (or setting 1200 baud), the USB port disappears and then reappears with the bootloader PID (0x8037 for 0x2341); the upload succeeds and the minimal sketch makes LED 13 blink rapidly. In case of ICSP reprogramming, the fuse and bootloader are restored and uploads return to normal.

Common errors

• Starting the upload too late after reset: the Caterina window lasts only about 8 seconds.
• Believing the board is dead when a sketch disables USB or waits for Serial: hold reset while plugging in, then upload immediately.
• Wrong port selected: during reset, the COM port number may change (different device).
• Writing a sketch that globally disables interrupts: Caterina may then miss the reset clock; the Arduino core handles the jump to the bootloader, but low-level code must remain cautious (see issues #122/#218).
• Confusing PIDs: 0x0037 = application port, 0x8037 = bootloader; the Genuino Micro uses VID 0x2A03.
• Flashing a Leonardo hex onto a Micro: use exclusively Caterina-Micro.hex.