Recommended I2C Pinout for BME280 Sensor on CBU Module: SDA/SCL Pin Numbers?

qscented wrote:

@ElektrodaBot
Which pins is best for connecting a i2c sensor BME280 to the CBU module

Initial assessment of available information
- “CBU” is a Tuya Wi‑Fi + BLE module based on the BK7231N SoC. The official CBU datasheet lists the GPIOs that are brought out on the module’s castellated pads, but it does not designate any dedicated I²C pins. I²C can be implemented on general‑purpose pins. Notably, TX2/RX2 (P0/P1) are used as a log/boot UART and should be avoided for application wiring. (developer.tuya.com)
- Tuya’s own “CB series wiring” guide shows the common GPIO groups exposed on CBU (P24/P26, P7/P8, P6/P26, P14/P16, etc.), which are suitable pairings for bit‑banged or software I²C. (developer.tuya.com)
- The Bosch BME280 supports I²C at 3.3 V logic and has two possible I²C addresses (0x76 or 0x77). Breakout boards often include pull‑ups and, sometimes, a regulator/level‑shifter, but the bare IC is a 1.71–3.6 V device. (bosch-sensortec.com)

Direct answer to the question
- Best, low‑risk pin pair on a CBU module for an I²C BME280:
- SCL → P24
- SDA → P26
Reasons: both are regular GPIOs (PWM‑capable, no boot/console role), they are adjacent on the CBU edge making short, tidy wiring near 3V3/GND, and they are widely used for I²C on BK7231N designs. (developer.tuya.com)

- Good alternatives (if P24/P26 are already used in your device):
- P7 ↔ P8 (pair)
- P14 ↔ P16 (pair, originally SPI pins but usable as GPIO)
- P6 ↔ P26 (mixed pair shown in Tuya wiring guide)
Avoid using TX2/RX2 (P0/P1) as they are the module’s log/boot UART. (developer.tuya.com)

Detailed problem analysis
- Why P24/P26?
- Electrical neutrality: P24 and P26 are general‑purpose I/O with PWM capability and no special boot‑strapping roles. They are safe for bus signals like I²C. (developer.tuya.com)
- Layout convenience: On the CBU pin map these sit next to the power pins (GND pad 13 and 3V3 pad 14), which shortens the I²C loop area and makes routing and strain‑relief easier. Shorter runs reduce bus capacitance and improve signal integrity. (developer.tuya.com)
- Why avoid P0/P1 (TX2/RX2)?
- The datasheet explicitly labels TX2 as the “print log port,” and these pins are commonly used during boot and debugging. Using them for I²C can conflict with boot messages or flashing workflows. (developer.tuya.com)
- Using alternative pairs:
- P7/P8 and P14/P16 are also clean, general GPIOs (P14/P16 are SPI‑capable but can be repurposed). They’re listed as accessible “groups” in Tuya’s CB‑series wiring guide, which is why many community designs pair them for buses. (developer.tuya.com)
- Electrical interface to BME280:
- Supply the sensor at 3.3 V when interfacing with CBU (BK7231N I/O is 3.3 V). The BME280 IC itself specifies VDD 1.71–3.6 V and VDDIO 1.2–3.6 V; many breakout boards accept 3–5 V due to onboard regulation, but 3.3 V is the safest common denominator with CBU. (bosch-sensortec.com)
- I²C pull‑ups: If your BME280 breakout lacks them, add 2.2–4.7 kΩ pull‑ups from SDA/SCL to 3.3 V. Reputable BME280 guides and libraries recommend these values. (github.com)
- Addressing: default is 0x76 or 0x77 depending on board vendor; SDO tied low selects 0x76, tied high selects 0x77. Confirm with an I²C scan. (learn.adafruit.com)

Practical guidelines
- Wiring (recommended pins):
- CBU 3V3 → BME280 VCC
- CBU GND → BME280 GND
- CBU P24 → BME280 SCL
- CBU P26 → BME280 SDA
- Keep wires short (≤20–30 cm). If you must run longer, lower the I²C clock (e.g., 100 kHz) and/or use stronger pull‑ups (≈2.2 kΩ). (github.com)
- Firmware examples:
- OpenBeken:
- Assign roles in Web UI: set P24 = I2C_SCL, P26 = I2C_SDA; Save/Reboot.
- Console: i2c_scan → verify 0x76/0x77 is detected; then startDriver BME280.
- (OpenBeken exposes an i2c_scan command; typical workflow aligns with OBK docs and practice.) (elektroda.com)
- TuyaOS (custom app development): use any two GPIOs you control for a software I²C implementation; avoid the UART/log pins. Reference the CBU pin table when selecting and initializing pins. (developer.tuya.com)

Troubleshooting checklist
- No device found:
- Confirm 3.3 V at the BME280 VCC and common ground.
- Run an I²C scan to see the address; if your driver expects 0x77 but the board is at 0x76, set it explicitly. (learn.adafruit.com)
- Add/verify pull‑ups on SDA/SCL to 3.3 V (2.2–4.7 kΩ). (github.com)
- Try a different pin pair (e.g., P7/P8) if the first pair is busy in your device. (developer.tuya.com)
- Sporadic reads/wire length issues:
- Reduce speed to 100 kHz and keep leads short; check for solid grounds and clean solder joints. (github.com)

Brief summary
- Use P24 (SCL) and P26 (SDA) on CBU for the BME280’s I²C—clean, adjacent, and conflict‑free with boot/console. Alternatives: P7/P8 or P14/P16. Avoid TX2/RX2 (P0/P1). Power at 3.3 V, ensure pull‑ups, and set the correct I²C address (0x76/0x77). (developer.tuya.com)

Questions for you
- Which firmware are you running on the CBU (TuyaOS, OpenBeken, LibreTiny/ESPHome)?
- Is this a bare CBU module on your own PCB, or is it inside a finished Tuya product (some pins may already be used)?
- If it is inside a product, can you share a photo or tell us which pins are free? I can propose the safest pair accordingly.

References
- Tuya CBU module datasheet (GPIO map and function notes, including TX2 as log port). (developer.tuya.com)
- Tuya CB‑series wiring guidance for CBU GPIO groupings (recommended pin pairs). (developer.tuya.com)
- Bosch BME280 product page (voltages, interface). (bosch-sensortec.com)
- Adafruit BME280 pinouts and address selection (0x76/0x77). (learn.adafruit.com)
- BME280 wiring/pull‑up guidance used by common Arduino library. (github.com)