Flashing Method for Tuya Module CB3S with Color Display (v2.1.8, MCU v1.1.6)

dabrunch wrote:

(...) e, humidity)?
température and humidity sensor with color display. cb3s module. P/N: 2.01.01.10674
AI: Have you tried any specific methods or tools to flash the module so far? If so, please describe them.
not yet. There are 4 pins named P2 with vcc gnd rx tx, and 4 others named p3 with vcc gnd txd rxd.

Initial Assessment of Available Information
You have a battery-powered temperature/humidity (T/H) sensor with a color display, containing a Tuya CB3S module (firmware principal v2.1.8, MCU v1.1.6). The sensor board exposes two sets of pins:
• P2: VCC, GND, RX, TX
• P3: VCC, GND, TXD, RXD

Your goal is to flash custom firmware (or updated Tuya firmware) onto that CB3S module. The CB3S typically uses a BK7231N chip, meaning standard ESP8266-based tools (like esptool.py) will not work. Instead, you must use BK7231-specific methods (e.g., bkWriter, Tuya CloudCutter, or OpenBK7231 tools).

Below is a comprehensive procedure, collating information from available sources and current best practices for flashing CB3S modules.

---

Direct Answer to the Question
To flash your battery-powered T/H sensor (CB3S, BK7231N-based), you will:

1. Provide a stable 3.3 V power source and ground reference to the CB3S.
2. Connect a USB-to-TTL serial adapter at 3.3 V logic levels to either P2 or P3 (test which pins work for flashing, but start with P2).
3. Use BK7231-compatible flashing software (e.g., bkWriter 1.60+, OpenBK7231N firmware tools, or the Tuya “Burning and Authorization” tool).
4. Put the module into programming mode (often by grounding a BOOT pin or a dedicated programming pad at power-up, or by following the specific instructions for your firmware tool).
5. Initiate the flash process with the correct firmware and watch for successful write confirmation.

Because the module is battery powered, ensure your power connections are stable and consider removing or bypassing the battery in favor of a regulated external 3.3 V supply during flashing.

---

Detailed Problem Analysis

1. Identifying the Module (CB3S with BK7231N)
• The CB3S is one of Tuya’s newer Wi-Fi modules based on a non-ESP chip (BK7231N).
• Traditional ESP8266/ESP32 flashing methods (like esptool.py or Tasmotizer) are not compatible. You need specialized tools designed for BK7231.

2. Determining the Correct Pins
You noted two sets of pins (P2 and P3). Each has VCC and GND plus RX/TX or TXD/RXD pairs. One set is typically wired to the main UART interface used for programming. Although specific device documentation may recommend one set over the other, you can test the following approach first:
• Use P2 as the primary programming connector.
• Map pins:
– CB3S TX → USB-to-TTL RX
– CB3S RX → USB-to-TTL TX
– CB3S VCC → USB-to-TTL 3.3 V
– CB3S GND → USB-to-TTL GND

(Some sensor boards swap TX and RX labels; if you cannot initiate a serial connection, try reversing TX/RX or testing the P3 header.)

3. Required Tools and Firmware
• Hardware:
– USB-to-TTL converter (3.3 V only)
– Jumper wires or similar connectors
– Optional external power supply if the battery alone is insufficient or difficult to work with
• Software:
– Tuya “Burning and Authorization” tool, or
– bkWriter (version ≥1.60) for BK7231N, or
– OpenBK7231N Python script tools, or
– Tuya CloudCutter (an offline approach that sometimes works on certain firmware versions)

4. Entering Flash Mode
Unlike ESP-based modules (which often pull GPIO0 low), BK7231 modules may require a different pin or approach. Depending on your board layout, you might have to:
• Hold a dedicated “BOOT” or “PROGRAM” pin low while powering the device.
• Press or hold a button during power-up.
• Use an official Tuya tool that automatically detects and triggers programming mode.

If no official instructions are available, examine the board for labeled pins or test pads, or consult references (including the OpenBK7231 wiki or teardown threads).

5. Flashing Sequence
1. Ensure the module is unpowered.
2. Connect the USB-to-TTL adapter to the CB3S P2 or P3 pins as outlined above.
3. If required, prepare the “BOOT” or “PROGRAM” pin to enter flash mode.
4. Power up the board with stable 3.3 V.
5. Open your flashing software:
– Select the correct COM port and baud rate (115200 is standard for many BK7231 devices, but the default can vary).
– Load your target firmware (e.g., OpenBK7231N or updated Tuya firmware).
– Start the flash process.
6. Await confirmation of a successful flash. If the process fails, adjust the baud rate or check wiring.

6. Battery-Powered Device Considerations
• If the device remains battery powered during flashing, watch for power drops. A fresh battery is essential, or connect an external supply in parallel, ensuring no conflicts between the supply and battery circuit.
• Some battery-based Tuya devices aggressively enter deep sleep. Keep them awake by toggling or pressing a sensor button if needed.
• Removing the battery altogether and powering from a stable 3.3 V lab supply can simplify flashing and reduce risk.

7. Post-Flash Configuration
After successful flashing:
• Power cycle the device.
• If using custom firmware (e.g., OpenBK7231N), connect to the new access point or serial console.
• Configure Wi-Fi credentials, sensor pin assignments, and calibrations if required.
• Optionally integrate with MQTT, HTTP, or your home automation system.

---

Current Information and Trends
• BK7231-based modules are increasingly replacing ESP8266 in budget Tuya devices. Over-the-air Tuya-Convert methods may no longer work with newer firmware versions (e.g., v2.1.8).
• Active community projects like OpenBeken (for BK7231N/T) regularly update their flashing tools and firmware to improve performance, sensor support, and ease of use.

---

Supporting Explanations and Details
• Backup Original Firmware: If possible, back up the factory firmware before overwriting it. This might require specialized reading commands in bkWriter or a similar tool.
• Overcoming Lockdowns: Some modules ship with locked bootloaders. If standard serial flashing is blocked, advanced hardware debugging interfaces or Tuya’s official tools might be needed.

---

Interdisciplinary Connections
• This process intersects embedded systems programming, hardware design, and IoT privacy concerns (as custom firmware frees you from proprietary cloud dependencies).
• Battery management strategies and power optimization are relevant in low-power devices.

---

Ethical and Legal Aspects
• Check licensing and local regulations: modifying or flashing devices may void warranties or violate local IoT legislation.
• Respect intellectual property agreements, especially with official Tuya firmware.

---

Practical Guidelines
• Always confirm your module’s power requirements (3.3 V!).
• Label or note all pin connections clearly.
• Use a breadboard or stable jig to reduce wire movement.
• Keep a log of any changes in case you need to revert or troubleshoot.

---

Possible Disclaimers or Additional Notes
• Success is not guaranteed if the device has hardware-level locking.
• Make sure to handle the battery safely and avoid short-circuits if you choose to remove or bypass it.

---

Suggestions for Further Research
• Explore the OpenBK7231N GitHub repository for documentation, pin mappings, and the latest community insights.
• Consult teardown guides or other user experiences for your exact T/H sensor model to see known quirks in the display interface or sensor driver.

---

Brief Summary
Flashing a CB3S-based Tuya sensor requires BK7231-compatible tools, correct pin connections (3.3 V logic, stable power), and the right procedure to enter programming mode. Once flashed, you can configure custom firmware features and integrate the sensor into your chosen platform (MQTT, HTTP, etc.). Carefully verify connections, potential boot pins, and battery considerations to avoid bricking the device.

---

Questions for You
1. Can you confirm the exact labeling or part number of the display driver and humidity sensor IC (e.g., AHT20, SHT30, etc.)?
2. Have you been able to identify any dedicated BOOT or reset pin on the PCB?
3. Do you prefer an official Tuya-based firmware update or a full custom firmware (like OpenBK7231N)?

Feel free to provide more details about your hardware setup or any specific stumbling blocks so we can refine the flashing strategy further. Good luck with the process, and stay safe when handling battery-powered boards!