The Problem: A Bricked Uno R4 WiFi from a Bare-Metal Run #
The Arduino Uno R4 WiFi is a robust board, featuring the powerful Renesas RA4M1 microcontroller. However, pushing its limits with low-level programming can sometimes lead to issues. Our specific breakdown occurred while experimenting with bare-metal code, specifically attempting to run the Rust blink.rs example from the arduino-uno-r4-hal GitHub repository.
When working outside of the standard Arduino framework, a program often takes full control of the microcontroller, potentially overwriting or failing to interact correctly with the pre-installed Arduino bootloader. This resulted in a board that was completely useless: it still shows up as a standard Arduino Uno R4 WiFi but you cannot upload any sketch or enter bootloader mode.
1discotool
2- UNO WiFi R4 CMSIS-DAP -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
3 Arduino [SN:DC5475CFC9C0]
4 /dev/cu.usbmodemDC5475CFC9C02 ()
The official recovery method, the double-press of the RESET button, failed to enter the DFU (Device Firmware Upgrade) mode.
https://github.com/arduino/ArduinoCore-renesas/blob/main/bootloaders/UNO_R4/README.md
1/Users/YourUser/Downloads/macos-arm64/rfp-cli -device ra -dtr -port /dev/cu.usbmodemDC5475CFC9C02 -p dfu_wifi.hex
2Renesas Flash Programmer CLI V1.14
3Module Version: V3.21.00.000
4Load: "/Users/YourUser/Library/Arduino15/packages/arduino/hardware/renesas_uno/1.5.1/bootloaders/UNO_R4/dfu_wifi.hex " (Size=43366, CRC=A53F3491)
5
6Connecting the tool (COM port)
7Tool: /dev/cu.usbmodemDC5475CFC9C02
8Interface: 2 wire UART
9
10Connecting the target device
11
12[Error] E3000105: The device is not responding.
The microcontroller’s flash memory, which holds the bootloader, was now running the experimental code, leaving no pathway for the Arduino IDE to upload a new sketch. When standard tools fail, it's time to leverage the hardware's deeper capabilities.
The Clue: CMSIS-DAP is Still Alive #
Even with the Arduino bootloader corrupted, the board wasn't completely dead. A key saving grace of the Uno R4 platform is that the internal CMSIS-DAP debug probe remains functional.
1probe-rs list
2The following debug probes were found:
3[0]: UNO WiFi R4 CMSIS-DAP -- 2341:1002:DC5475CFC9C0 (CMSIS-DAP)
This internal debugger is crucial because it provides low-level access to the RA4M1 microcontroller's flash memory via the SWD (Serial Wire Debug) protocol. This access effectively bypasses the broken bootloader and is the only way to perform a true factory-level recovery.
The Solution: PyOCD to the Rescue #
We need a flexible, cross-platform tool that can communicate with the CMSIS-DAP probe and directly program the flash. The open-source and Python-based PyOCD tool is perfectly suited for this low-level task.
Step 1: Install PyOCD and Dependencies #
First, ensure you have Python and the PyOCD library installed on your system. PyOCD will use the native USB drivers for the internal debug probe.
1# Install the core PyOCD tool
2pip install pyocd
Step 2: Acquire the Official Bootloader Binary #
We need a fresh copy of the official Arduino bootloader. This file is typically named dfu_wifi.hex and is included with your Arduino board package installation.
You will need to search your Arduino installation directory structure for the Renesas core files to locate it: /Users/YourUser/Library/Arduino15/packages/arduino/hardware/renesas_uno/1.5.1/bootloaders/UNO_R4/dfu_wifi.hex
Step 3: Crucially, Install the Target Pack #
If you attempt to run the flash command without this step, you will encounter an error like: Target type r7fa4m1ab not recognized.
This means PyOCD doesn't know the memory map or internal configuration for the specific Renesas RA4M1 chip (r7fa4m1ab). You must install the correct device family pack:
1# Update the list of available packs
2pyocd pack update
3
4# Install the pack for the Renesas RA4M1 chip
5pyocd pack install r7fa4m1ab
Flashing the Bootloader #
With the target pack installed, we can perform the full recovery. We must instruct PyOCD to erase the sectors and write the bootloader starting at the absolute beginning of the flash memory (0x0).
The Recovery Command Connect your Uno R4 WiFi via USB and run the following command, making sure to use the correct absolute path to your dfu_wifi.hex file:
1pyocd flash -t r7fa4m1ab -e sector -a 0x0 /path/to/dfu_wifi.hex
Success! PyOCD will show progress as it verifies and writes the bootloader. Once the command completes, press the physical RESET button on your Uno R4 WiFi to cycle the power. The board should now be recovered!
The double-press RESET functionality will be restored, and the board will once again appear as a standard serial port in the Arduino IDE, ready to upload your sketches.
Next Steps: Back to the Arduino IDE #
We've used a low-level debugger interface to repair a bootloader corrupted by bare-metal code.
Open the Arduino IDE.
Select Tools > Port and verify your Arduino Uno R4 WiFi is correctly listed.
Upload a simple Blink sketch to confirm full functionality.
This experience highlights that even when official recovery paths are blocked by low-level code issues, leveraging community tools like PyOCD and the accessible debug probe can always bring your board back from the brink!