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micro:bit V2 LSM303AGR motion sensor

The micro:bit V2 carries an ST LSM303AGR on the internal I²C bus (TWIM0, SCL P0.08 / SDA P0.16). It is two I²C slaves, and LabWired models each as a declarative device:

Component id Type Address Descriptor
accelerometer lsm303agr_accel 0x19 lsm303agr_accel.yaml
magnetometer lsm303agr_mag 0x1E lsm303agr_mag.yaml

Both are attached in configs/systems/microbit-v2.yaml and examples/microbit-v2/system.yaml. The Foundation I²C inventory also lists an FXOS8700 board variant; that variant is not selected here.

Inputs

Each component takes live x, y, z stimulus: acceleration in g and magnetic field in µT. They default to zero; no gravity or heading is invented.

What is modelled

  • Register maps, reset values and WHO_AM_I (0x33 / 0x40) per the ST LSM303AGR datasheet, Rev 11.
  • Sub-address bit 7 (auto-increment) on both halves.
  • Accelerometer output: left-justified 16-bit two's complement at the full scale CTRL_REG4_A FS[1:0] selects.

  • Magnetometer output: 16-bit two's complement at 0.15 µT/LSB.

  • Data-ready: each half raises its XYZ data-ready bits 10 ms (100 Hz) after it starts measuring or after its last output read, through the engine's data_ready rule. The accelerometer measures while CTRL_REG1_A ODR is non-zero; the magnetometer while CFG_REG_A_M MD[1] is clear (continuous or single). Polling firmware therefore reads about 100 samples a second, as on silicon, instead of one per poll.

Not modelled

The configured ODR value (every rate paces at 100 Hz), a fixed sampling grid (the period restarts from each output read), single-shot drop to idle, overrun bits, block-data-update latching, resolution truncation of the low bits, FIFO, interrupts (the shared P0.25 line is unconnected), click/orientation detection, filters, self-test, temperature and hard-iron offset subtraction. Each descriptor header lists these.

Executable proof

microbit_v2_motion_io_guest assembles an original polled driver (motion-polled.inc) with arm-none-eabi-gcc, runs it on the simulated nRF52833, and drives both halves through TWIM0 EasyDMA. Two held poses must produce exact raw samples:

cargo test -p labwired-core --features microbit-board-io-test --test microbit_v2_motion_io_guest