🛠 Débogage pas à pas : CMSIS-DAP et Arduino Debug Launcher

Objectives

Move beyond Serial.print for debugging: the R4 is the first UNO with an embedded hardware debugger. The embedded USB bridge (HIC) exposes a CMSIS-DAP interface connected to the RA4M1's SWD, and the official Arduino Debug Launcher extension (VS Code / PlatformIO or IDE 2.x) enables breakpoints, stepping, and inspection of variables and memory — exactly like on STM32.

Hardware

• 1 Arduino UNO R4 WiFi + USB-C data cable (connected to the USB-C port on the HIC side, the one used for uploading)
• VS Code with the Arduino Debug Launcher extension, or Arduino IDE 2.x with the debugging feature enabled
• A non-trivial sketch: a function that calculates, a loop, an array.

Procedure

1. Install the Arduino Debug Launcher extension and the proposed debugging component (OpenOCD/CMSIS-DAP managed by the extension).
2. Open the sketch, place a breakpoint on a line in loop.
3. Launch "Start Debugging": the board is flashed and then stopped at the breakpoint.
4. Explore: local variables, global variables, call stack, Cortex-M4 registers, memory; step through with step-over/step-into (F10/F11).

// Croquis de test : le bug classique du débordement d'index.
int mesures[8];
int n = 0;

void setup { Serial.begin(115200); }

void loop {
  mesures[n] = analogRead(A0);   // point d'arrêt ici : surveillez n
  n++;
  if (n > 8) n = 0;              // bug volontaire : devrait etre n >= 8
  delay(200);
}

Expected Verification

Execution freezes at the breakpoint; the Variables window shows n increasing. By monitoring the array, you see the out-of-bounds write when n == 8 — the kind of bug invisible with Serial.print. Resume (F5), modify the condition, reflash, and recheck: this is the complete debugging loop.

Common Errors

• Compiler optimization: with -O2, variables may "disappear" or stepping may skip lines; switch optimization to -O0 (IDE compilation option) for debugging, then restore it for production.
• delay during debugging: the debugger interrupts the core; the watchdog and WiFi continue running on the ESP32-S3 side — the system state is not frozen as in a simple pause.
• Plugging the cable into the wrong USB-C port: debugging goes through the port connected to the HIC; the other port (RA4M1 native USB) does not provide CMSIS-DAP.
• Confusing the debugger and the serial monitor: during a debugging session, the serial port is often occupied by the session; use the other USB port for a separate CDC if needed.
• Expecting to debug the ESP32-S3: the debugger only controls the RA4M1; the ESP32-S3 is not accessible for stepping via this path.