Added README
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# nRF52840 SHT41 BTHome Beacon
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An ultra-low-power Bluetooth LE temperature and humidity beacon for the
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Pro Micro nRF52840 (nice!nano v2 compatible) with a Sensirion SHT41 sensor,
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built on Zephyr / the nRF Connect SDK.
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The device broadcasts [BTHome v2](https://bthome.io/) advertisements, so it
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is discovered automatically by Home Assistant's BTHome integration (and any
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other BTHome-compatible receiver). It never accepts connections: it only
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advertises.
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## How it works
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Every **60 s** the firmware:
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1. Switches on the SHT41's supply from a GPIO (P0.17) and waits 3 ms for it to start.
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2. Takes one high-repeatability temperature / humidity measurement over I2C.
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3. Puts the I2C pins into a disconnected "sleep" state and cuts sensor power.
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4. Measures the supply voltage (VDDH) with the SAADC, right after the sensor load.
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5. Updates the advertising payload.
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Between measurements the CPU sits in System ON idle while the radio sends a
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non-connectable advertisement every **10 s** (the payload therefore carries
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the same values for six consecutive adverts). If a sensor read fails, the
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last good values keep being advertised.
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Power-saving measures, mostly configured in `app.overlay` and `prj.conf`:
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- Sensor supply gated through a `power-domain-gpio` node, driven by Zephyr's
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runtime device power management (`pm_device_runtime_get/put`).
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- I2C bus enabled only during transfers; SDA/SCL disconnected while idle so
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the breakout's pull-ups can't back-feed the unpowered sensor.
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- DC/DC converters enabled on both REG0 (VDDH → VDD) and REG1 (VDD → core).
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- 0 dBm TX power, 10 s advertising interval.
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- UART, USB, console, logging and unused peripherals (I2C1, SPI2, PWM0) disabled.
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## Hardware
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| Signal | nRF52840 pin | SHT41 breakout |
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| -------------- | ------------ | -------------- |
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| Sensor power | P0.17 | VIN / VCC |
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| I2C SDA | P0.20 | SDA |
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| I2C SCL | P0.22 | SCL |
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| Ground | GND | GND |
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The SHT41 is at I2C address `0x44`. Battery voltage is read internally
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from VDDH (VDDH/5 input), so no external divider is needed — power the board
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from a cell connected to the battery / VDDH input.
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The sensor supply pin uses high drive (`NRF_GPIO_DRIVE_H0H1`). Keep the
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breakout's total load small; if your board has large bulk capacitance or a
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power LED, remove it or increase `startup-delay-us` in `app.overlay`.
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## Project layout
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```
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.
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├── CMakeLists.txt
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├── prj.conf # Kconfig: BLE broadcaster, drivers, PM, console off
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├── app.overlay # Devicetree: sensor power domain, I2C, ADC, regulators
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├── README.md
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└── src/
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└── main.c # Measurement loop and BTHome payload
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```
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## Building
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Requires the nRF Connect SDK (or upstream Zephyr) with `west`.
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```sh
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west build -b nice_nano/nrf52840 -p
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```
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On older SDKs that use the pre-hardware-model-v2 board names, use
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`-b nice_nano_v2` instead.
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## Flashing
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The nice!nano ships with a UF2 bootloader:
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1. Double-tap the reset button (or briefly short RST to GND twice). A USB
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drive named `NICENANO` appears.
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2. Copy `build/zephyr/zephyr.uf2` onto the drive. The board reboots into the
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new firmware.
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Because USB and serial are disabled in the application, the board will **not**
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enumerate over USB while running. Use the double-tap reset each time you
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want to reflash. You can also flash over SWD with a J-Link or similar probe
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using `west flash`.
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## Advertising payload
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Advertised name: `SHT41` (set by `CONFIG_BT_DEVICE_NAME`). The device uses its
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identity address, so it appears with a stable MAC.
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Service data (UUID `0xFCD2`):
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| Bytes | Meaning |
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| ------- | ----------------------------------------------- |
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| `D2 FC` | BTHome UUID (little-endian) |
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| `40` | BTHome v2, unencrypted, regular interval |
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| `0C` + 2 | Voltage, uint16 LE, 1 mV |
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| `2E` + 1 | Humidity, uint8, 1 % |
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| `45` + 2 | Temperature, sint16 LE, 0.1 °C |
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Example: `D2 FC 40 0C 1B 0C 2E 2D 45 E7 00` decodes as 3.099 V, 45 % RH,
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23.1 °C. The full advertisement (flags + service data + name) is 23 bytes,
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inside the 31-byte legacy limit.
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## Configuration
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| What | Where |
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| ----------------------- | -------------------------------------------- |
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| Measurement interval | `MEAS_INTERVAL` in `src/main.c` |
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| Advertising interval | `ADV_INTERVAL` in `src/main.c` (0.625 ms units, legacy max 10.24 s) |
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| Device name | `CONFIG_BT_DEVICE_NAME` in `prj.conf` |
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| TX power | `CONFIG_BT_CTLR_TX_PWR_*` in `prj.conf` |
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| Pins | `sens_pwr` and `pinctrl` nodes in `app.overlay` |
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| Sensor power-up delay | `startup-delay-us` in `app.overlay` |
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| ADC resolution / averaging | `&adc` channel in `app.overlay` |
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## Home Assistant
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With a Bluetooth adapter or ESPHome Bluetooth proxy in range, the BTHome
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integration discovers the device automatically and creates temperature,
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humidity and voltage sensors. Expect the first values within about 10 s of
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power-up, and new readings every 60 s.
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## Troubleshooting
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- **Nothing shows up:** check that the board is powered from the battery
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input and wait at least one advertising interval (10 s). The first
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advertisement is sent even if the initial sensor read fails, with zeros
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for temperature and humidity.
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- **Temperature/humidity stuck at 0:** the sensor isn't responding. Check the
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wiring, the I2C address and that P0.17 actually powers the breakout.
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- **Voltage reads 0:** the ADC read failed; check the `&adc` node and that
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`CONFIG_ADC=y`.
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- **Can't flash:** the application disables USB, so you must enter the
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bootloader with a double reset first.
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