[Solved] Rewriting Firmware for Wi-Fi DIY Smart Switch (2022.10.15 model) using CB2S (BK7231N)

Hello everyone.
This is my first post on this site. I'm a newbie, please help me out.

Now, I have the following Smart Switch at hand and am trying to rewrite the firmware for the model with [2022.10.15] written on the printed circuit board.





In past posts of this site, I found the post;
https://www.elektroda.com/rtvforum/topic3895572.html
may be helpful, but so far I have been unable to rewrite the firmware.

I have gathered a lot of information and the Wi-Fi module used in this device is CB2S (BK7231N), which matches the information in the datasheet, so I hope to be able to rewrite that.
I have tried to flash the firmware but I can not enter to the flash mode.

So far, I have attempted the following,
(1) Supply 5VDC to AMS1117 as external power supply
(2) Connect RX1 and TX1 of CB2S to TX and RX of USB to UART converter by cross connection
(3) Cut the pattern on the board for connecting RX1 to Button.
(4) Rewrite with BK7231 GUI Flash Tool 1.0.4

Does anyone have specific information on rewriting the firmware for this model?

Added after 23 [minutes]:

Add supplemental information.
The following figure shows actual pictures of (1) external power supply and (3) pattern cutting on the board.
Please feel free to point out any mistakes.

Hello, I would like to suggest you the most reliable approach.
Just desolder CB2S.
First add some flux to the pads and remove solder with soldering wick:

Then solder the wires:

Then power module from 3.3V and connect to USB to TTL:

Then use BK GUI flasher. If it doesn't work for you (there are some rare reports that BK GUI doesn't work with certain USB converters, but hid_download_py does), use hid_download_py method from this video:

Make sure that OBK access point shows up.
Then desolder wires, and clear the module pads:

Then prepare to solder back the module, add some flux to the pads:

And solder back the module:

Hi, p.kaczmarek2
Thank you for your reply.
The information you provided is in the video and is very clear.

It seems that the most common method is to disconnect the Wi-Fi sub-board from the main board (unsolder it) and rewrite it on its own.
If this seems to be the only way, I will try it.

Does anyone have any information on how to rewrite this model (2022.10.15) without removing the sub board?

hga00453 wrote:

Does anyone have any information on how to rewrite this model (2022.10.15) without removing the sub board?

1. Locate AMS1117 3.3V LDO regulator on the board
2. connect 5V to AMS1117 input, connect common ground
3. connect RX and Tx, also make sure that RX and TX is not used for something else on the board (if so, cut the trace)
4. Start flashing in the programmer tool. Try BK GUI flasher, if it fails, you can also try hid_download_py
5. The tool will wait for device reboot (for "getting bus"), then:
OPTION A: disconnected 5V power supply and reconnect it quickly (so module reboots)
OPTION B: use extra CEN wire and short it for 0.25 seconds to ground (so module reboots)
6. flashing should proceed

hid_download_py commands for firmware backup (2MB, please do it first!) and writing are on the video linked above.

This method should work as long as you have no interference on RX/TX lines.
You can also read this topic (I also cut the trace here):
ZN268131 WiFi Smart Switch that allows you to connect a bistable button
You can also watch this CB2S flashing video (I had to cut the trace there):

Hi, @p.kaczmarek2
Thank you for your additional information provided.
I have tried them in the past, but failed to flash the firmware.

It seems that the RX and TX pins are connected to the relay module, as the relay ticks when I run the flashing application.
It seems like there is a missing pattern cut on the board, so I guess I should still follow your advice and separate the CB2S sub-board of the Wi-Fi module before flashing.

There are many things that can go wrong with flashing BK7231. It is not guaranteed that desoldering WiFi module will help, but at least it will tell you that your problem is not the RX/TX tracks interference.

Hi, @p. kaczmarek2
Your advice has been very helpful.
The direction you recommended is still generic for the future, so I will try that approach.
Thanks.

As suggested, I responded by separating the CB2S sub-board from the main board.

Is the flashing working now? @hga00453

p.kaczmarek2 wrote:

Is the flashing working now? @hga00453

YES! I did it.

Just remove the WiFi (CB2S) module from the main board.

Very good job, it's' the most reliable way.