Hardware

Datasheets

Not vendored in the repo (third-party copyright) — download from the vendor:

MGM210L module — datasheet PDF · product family (Silicon Labs)
Sensirion SHT40 — SHT4x datasheet · product page

Module

Silicon Labs MGM210LA22JNF2 — Mighty Gecko wireless module, 18-pin.

SoC: EFR32MG21A (ARM Cortex-M33 @ up to 80 MHz), 2.4 GHz radio.
Protocols: Zigbee 3.0 / Thread / Bluetooth LE (this project uses Zigbee).
Integrated PCB antenna — ready-to-use module, no external RF parts. Place it at the edge of the board and keep the antenna area clear of copper/ground (datasheet §7). Mount horizontally for access to all pins.
Pinout used here (datasheet Table 6.1): VDD = pin 13, GND = pins 6 & 14, RESETn = pin 11 (internal pull-up, no external R needed). Leave DEC (pin 5, internal decouple) and DNC (pin 12) unconnected.
I²C to the SHT40 on PC00 (pin 7) = SCL and PC01 (pin 8) = SDA — adjacent pads for short routing, no series resistor (unlike PC02–PC05), and this keeps PA01–PA04 for SWD and PD00/PD01 free for an optional 32.768 kHz LFXO crystal.

The MG21 has only an on-die temperature sensor (measures silicon temp, not ambient). An external I²C sensor is required for real room temperature — and an RH-capable part (SHT4x / Si7021) also gives relative humidity, which the firmware reads from the same transaction and exposes as a second HA entity.

Bill of materials

Part	Purpose	Notes
MGM210LA22JNF2	MCU + radio	LGA module, needs a carrier/PCB
Sensirion SHT40	Temp + humidity	I²C addr 0x44 (SHT40-AD1B); ±0.2 °C, ±1.8 %RH; ~0.4 µA idle
CR2032 holder	Power	~220 mAh, high internal resistance
4.7 kΩ ×2	I²C pull-ups	SDA + SCL to VDD
10 µF + 100 nF	Module decoupling	near module VDD (pin 13); bulk cap absorbs TX pulses
100 nF	SHT40 decoupling	across SHT40 VDD–VSS, close to the sensor (per datasheet)
—	Antenna	none — integrated in the module

Schematic

Power rail (CR2032 → VDD)

   CR2032
   +3 V nominal
     (+) ●───┬────────┬────────┬─────────────────►  VDD  (module, SHT40, pull-ups)
             │        │        │
            ═╧═10µF   ═╧═100nF  (bulk + HF decoupling, placed close to module VDD —
             │        │         the bulk cap sources the radio TX current pulse the
     (–) ●───┴────────┴──┐      coin cell's high internal R cannot)
                         GND ───────────────────►  GND

No battery divider: the cell sits directly on VDD and firmware reads VDD through the IADC's internal AVDD/4 input (see Battery sensing below).

I²C sensor bus — Sensirion SHT40 (addr 0x44)

                     VDD
                      │
                ┌─────┴─────┐
              4.7kΩ       4.7kΩ         external pull-ups — the module GPIOs
                │(SDA)      │(SCL)       cannot drive the open-drain bus alone
   MGM210L      │           │             SHT40  (Sensirion)
   ┌─────────┐  │           │           ┌────────────┐
   │  PC01 ●─┼──┴───────────┼───────────┤ SDA ●      │
   │  PC00 ●─┼──────────────┴───────────┤ SCL ●      │
   │  VDD  ●─┼──────────────────────────┤ VDD ●─┐    │
   │         │                          │       ═╧═ 100 nF  (decouple AT the SHT40)
   │  GND  ●─┼──────────────────────────┤ VSS ●─┘    │
   └─────────┘                          └────────────┘
   PC00 (pin 7) = I2C0_SCL, PC01 (pin 8) = I2C0_SDA  (matches
   config/sl_i2cspm_sensor_config.h; adjacent module pads = short routing)
   The SHT40 returns temperature AND humidity in one read (cmd 0xFD, ~8 ms).

SWD / debug

   MGM210L               J-Link / debug probe
   ┌───────────────┐
   │ PA01/SWCLK ●──┼── SWCLK
   │ PA02/SWDIO ●──┼── SWDIO
   │ RESETn(11) ●──┼── RESET
   │ VDD / GND  ●──┼── VDD / GND
   └───────────────┘  (SWD carries both flashing and SEGGER RTT logs —
                       no separate SWO pin needed for `commander rtt`)
   RF: none — the module has an integrated antenna (place module at board edge,
   keep the antenna area clear of copper/ground).

Battery sensing (no external parts)

The firmware measures the supply rail with the on-chip IADC: input iadcPosInputAvdd samples AVDD ÷ 4 against the internal 1.21 V reference, and app_read_battery_mv() scales it back to millivolts. That value feeds the ZCL Power Configuration battery voltage/percentage attributes. Because the CR2032 is wired straight to VDD, no divider, extra pin, or GPIO is required.

Power design for CR2032

A coin cell has ~10–40 Ω internal resistance, so a full-power radio TX pulse sags the rail and can brown-out the MCU. To stay reliable for years:

Cap TX power at 0–8 dBm (see SL_..._RADIO_TX_POWER in the .slcp), not the module's +20 dBm max.
Keep the 10 µF bulk + 100 nF decoupling close to VDD to source the pulse.
Sleep in EM2 between the 5-min reads (Power Manager handles this); the sensor's own sleep current (~80 nA) lets it stay powered.
Budget: ~1–3 µA average sleep + a few ms of radio per 5 min ⇒ multi-year life, dominated by the CR2032's self-discharge, not the firmware.

Debug / flash header

SWD only: SWCLK, SWDIO, RESET, GND to a J-Link. Add the module's VCOM/RTT pins if you want live commander rtt logs. Pull the segger_rtt / iostream_rtt components from the .slcp for production builds to shave idle current.

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