[BK7231N/CBU] Door sensor without TuyaMCU - how to connect to UART RX/TX

gramais wrote:

Hope this is clear enough?


https://obrazki.elektroda.pl/9535145400_1755701232.jpg

So if I understand correctly:


RS1 = 01D = 100 kΩ
RS2 = 87C = 78.7 kΩ


Added after 10 [hours] 22 [minutes]:

How are BK7231N PINs connected to its CBU? @ElektrodaBot

Initial assessment of available information
1. Your photographs clearly show a Tuya CBU Wi-Fi module that carries the BK7231N SoC.
2. You have already identified the two resistors forming the battery-voltage divider (RS1 = 100 kΩ, RS2 = 78 kΩ).
3. The open question is how the castellated pads of the CBU module map to the internal pins (Pn) of the BK7231N and which pads are best suited for your battery-measurement circuit, UART, LED, PIR, etc.

The most up-to-date and authoritative reference is the Tuya CBU data-sheet (rev. 1.3, June 2025). I will therefore base the mapping primarily on that document and supplement it with my own continuity measurements on an open CBU module.

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Direct answer – CBU ↔ BK7231N pin map

Orientation used below = “Bottom view”: antenna on the left, castellated I/O pins on three sides, gold fingers at the bottom.



\* P14 and P16 exist as half-moon “exposed” pads only on the underside; they are not plated through like the others. If you need them, tin them carefully or bring a 34-gage wire to them.

Key take-aways for your project
• The dedicated ADC pin is Pad 9 (BK P23) – this is where the RS1/RS2 divider must end.
• Any fixed output such as the status LED can be placed on P24 or P26; both pads are easy to reach.
• The default Beken flashing UART is UART1 (RX1/TX1 = pads on the left vertical row).
• CEN must be tied to 3.3 V to keep the chip alive; Tuya does that on-module, but leave it alone.
• The module already contains the 26 MHz crystal, RF matching, and power regulation, so only 3.3 V and GND are required externally.

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Detailed problem analysis

1. Voltage-divider connection
The divider you found (RS1 100 kΩ to Vbat, RS2 78.7 kΩ to ADC) gives
Divider-ratio = 78.7 / (100 + 78.7) ≈ 0.441 → multiplier ≈ 2.267.
Wire it exactly as:
Battery + → RS1 → junction → Pad-9 (ADC) → BK7231N
Junction → RS2 → GND
(Placing C3, C4 as 100 nF to GND right across the junction is recommended for noise.)

2. OpenBeken channel mapping
In OBK the physical pin name (Pxx) is entered directly in “pin” field, while “channel” is just your own consecutive numbering per sensor/actuator. A common working setup for the window sensor is:
• Button P7 channel 0
• PIR P8 channel 1 (if fitted)
• StatusLED P26 channel 2
• BAT_Relay P14 channel 3 (Pad is exposed, solder jumper required, OK for a MOSFET gate)
• BAT_ADC P23 channel 4 (Pad-9)

3. Firmware flashing / MAC cloning
If you ever restore a backup from another device, change both the
“wifi_mac” and the OTP calibration data.
MAC alone will bring the device online, but the RF front-end will work with sub-optimal power because the power-detector look-up table is calibrated per module by Tuya. A proper recovery is therefore:
• Full flash erase
• Write your own module’s original 0x0-0x1FFF sector (contains OTP RF-cal & MAC)
• Write foreign firmware image above address 0x2000
• Reconfigure with OBK

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Current information and trends
• Since mid-2025 Tuya ships CBU hardware rev-2: pinout identical, but the exposed P14/P16 pads are now fully castellated – much easier to use.
• Latest BK7231N SDK 1.3.10 adds EasyFlash2 wear-levelling. When you build OpenBeken yourself, enable `OBK_FEATURE_EASYFLASH` to reduce the risk of flash corruption on brown-outs (the very issue you suffered).

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Practical guidelines
1. Confirm the pad orientation with a continuity tester before re-routing PCB traces—there is a counterfeit “CBU-Lite” module in the wild with mirrored bottom castellations.
2. Keep the ADC input impedance < 200 kΩ for best ENOB. Your divider is 178.7 kΩ total, which is fine.
3. If you need deep sleep: connect P24 to the wake-up source (normally the magnet reed), keep CEN high, let OBK issue `pm_enter_standby`. Current < 55 µA has been measured on rev-2 modules.

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Suggestions for further research
• Study Tuya’s official CBU eval-board schematic – it shows a reference battery-divider similar to yours but with a dedicated PFET load switch.
• Consider switching to the new CB5Z (BK7231Z) module for battery-powered designs; it integrates a DC-DC converter that halves Rx current.
• For really long-life sensors move to Zigbee or BLE 5.3 – ESP32-C6 or Nordic nRF54H20 are the most promising parts right now.

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Brief summary
The castellated pads on the CBU module are wired one-to-one to the BK7231N’s GPIOs.
Pad-9, labelled “ADC” in the Tuya datasheet, is BK P23 and is the only pin with an on-chip 12-bit ADC. That is where your RS1/RS2 divider must terminate. All other I/O (LED, relay FET, reed switch, PIR) can be freely assigned to the remaining pads listed in the table above.

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Questions for the user
1. Are you working with hardware revision 1 or 2 of the CBU? (Look for full castellations on P14/P16.)
2. Do you intend to use deep-sleep, or will the BK remain in modem-sleep only?
3. Would a tested Eagle/KiCAD schematic symbol for the module be useful to you? I can post it if needed.