NiusIMU

A unified, friendly IMU library for the ArduinoNRF board package. One common API across every IMU, with chip-specific extras where a sensor offers something unique — so your code reads the same whether the board carries an MPU-9250 today or something else tomorrow.

It doubles as a bring-up and debugging tool for the ArduinoNRF core: all sensor traffic flows through one small bus layer, so the I2C (Wire) and SPI drivers get exercised by real devices, not just loopback tests.

Highlights

• Same API for every sensor. update(), then accelG(), gyroDps(), magUT(), temperatureC() — identical no matter the chip.
• Engineering units, always. g, deg/s, µT, °C. Conversions to m/s² and rad/s are one call away.
• Calibrate once, restore forever. Gyro, accel and magnetometer calibration produce a struct you print and paste back — no waving the board at every boot.
• Whole boards in one object. <GY91.h> brings up the MPU + BMP280 together (accelG(), gyroDps(), pressureHpa(), altitudeM()); <GY9250.h> is the 9-axis board. Or use the bare chips directly.
• Pluggable sensors as folders. Drop a driver in src/sensors/<NAME>/, add a one-line forwarding header, and #include <NAME.h> works. See docs/ADDING_A_SENSOR.md.
• Chip-specific power kept available. The MPU-9250 driver still exposes its magnetometer control, raw DLPF codes, data-ready interrupt and raw registers.

Supported sensors and boards

Chips live in src/sensors/; multi-chip breakout boards in src/boards/. Pick whichever name matches what you bought — <GY91.h> for the board, or the individual chip headers if you wired chips up yourself.

Include	Device	Accel	Gyro	Mag	Pressure
<GY91.h>	GY-91 board (MPU-9250/6500 + BMP280)	✅	✅	✅†	✅
<GY9250.h>	GY-9250 board (MPU-9250)	✅	✅	✅ (AK8963)	—
<MPU9250.h>	MPU-9250 chip (9-axis)	✅	✅	✅ (AK8963)	—
<MPU6500.h>	MPU-6500 chip (6-axis)	✅	✅	—	—
<BMP280.h>	BMP280 barometer	—	—	—	✅ (+ temp)

† Many GY-91 boards actually carry a 6-axis MPU-6500 (WHO_AM_I 0x70, no magnetometer). The MPU-9250 driver auto-detects this and runs as a 6-axis part; hasMagnetometer() tells you which one you have.

The MPU-9250 is an MPU-6500 plus an AK8963, so its driver simply extends MPU6500. Over I2C the AK8963 is reached through the MPU's internal I2C master (not bypass), so a dead/absent magnetometer can never jam the accel/gyro bus. On SPI only accel + gyro are read.

Install

This library lives next to the ArduinoNRF package. Either copy this folder into your Arduino libraries/ directory, or compile an example directly:

In Arduino Library Manager it is published as NiusIMU. The GitHub repository keeps the historical ArduinoNRF-IMU name because it is developed alongside the ArduinoNRF core.

arduino-cli compile \
  --fqbn arduinonrf:nrf52:promicro_nrf52840 \
  --library <path-to>/ArduinoNRF-IMU \
  <path-to>/ArduinoNRF-IMU/examples/GY91/GY91_Basic

Wiring (ProMicro nRF52840, default I2C)

Sensor	Board pad	nRF52 pin
SDA	SDA (D6)	P1.00
SCL	SCL (D7)	P0.11
VCC	3V3	—
GND	GND	—

Quick start

#include <MPU9250.h>
MPU9250 imu;

void setup() {
  Serial.begin(115200);
  imu.begin();            // Wire @ 0x68, sensible defaults
  imu.calibrateGyro();    // hold still ~1 s
}

void loop() {
  imu.update();
  Vec3 a = imu.accelG();  // g
  Vec3 g = imu.gyroDps(); // deg/s
  Vec3 m = imu.magUT();   // µT
  delay(50);
}

Examples

Example	What it shows
I2C_Scanner	Find devices on the bus; first thing to run on a new board
GY91/GY91_Basic	Whole GY-91 (IMU + barometer) in one quick read loop
GY91/GY91_Advanced	Full config + guided calibration of every sensor, with on-screen hints
GY9250/GY9250_Basic	Minimal 9-axis read loop
GY9250/GY9250_Calibration	Guided gyro/accel/mag calibration that prints paste-ready code
GY9250/GY9250_Advanced	Custom ranges, DLPF, sample rate, data-ready, raw registers
GY9250/GY9250_SPI	Read over SPI: configure at 1 MHz, then burst data at 8 MHz
GY9250/GY9250_DataReadyInterrupt	Read on the INT pin (D0) — one read per fresh sample, no polling
GY9250/GY9250_FSYNC	Time-stamp an external event on FSYNC (D1) into the sample stream
MPU6500/MPU6500_Basic	Bare 6-axis accelerometer + gyroscope
BMP280/BMP280_Basic	Pressure, temperature and altitude, with sea-level reference

Interfaces: SPI, INT and FSYNC (MPU-6500 / MPU-9250)

The IMU drivers run over I2C or SPI and expose the chip's interrupt and external-sync pins. All three were hardware-verified on a GY-9250 over SPI:

• SPI. beginSPI(SPI, SS) (the part is pinned to SPI via I2C_IF_DIS). Configure at the default 1 MHz, then call setBusClockHz() higher to burst data — verified 100% reliable from 0.5 MHz up to the ArduinoNRF SPI ceiling of 8 MHz, for both register and data reads. The magnetometer is I2C-only, so SPI is a 6-axis stream.
• Data-ready INT. setDataReadyInterrupt(true) pulses the INT pin once per sample; attach an interrupt and read only when a fresh sample exists. Verified a steady 100 Hz with every sample serviced across the whole SPI speed range.
• FSYNC. setExternalSync(MPU6500::FSYNC_TEMP) latches the FSYNC pin level into a sample LSB so an external event is time-stamped against the IMU data; read it back with fsyncLevel(). Verified the captured level tracks the pin exactly.

See the GY9250_SPI, GY9250_DataReadyInterrupt and GY9250_FSYNC examples.

The common API (every sensor)

bool   begin();                       // default bus + sensible defaults
bool   beginI2C(TwoWire&, uint8_t);   // pick bus / address
bool   beginSPI(SPIClass&, uint8_t);  // SPI with a CS pin
uint8_t whoAmI();  bool isConnected();

bool   update();                      // one fresh snapshot
Vec3   accelG();  Vec3 accelMs2();
Vec3   gyroDps(); Vec3 gyroRps();
Vec3   magUT();   float temperatureC();
bool   hasMagnetometer();

bool   setAccelRangeG(uint16_t);      // 2/4/8/16 …
bool   setGyroRangeDps(uint16_t);     // 250/500/1000/2000 …
bool   setLowPassFilterHz(uint16_t);
bool   setSampleRateHz(uint16_t);

bool   calibrateGyro();  calibrateAccel();  calibrateMag();
IMUCalibration getCalibration();  void setCalibration(IMUCalibration);

See docs/ARCHITECTURE.md for how the pieces fit together.

License

Apache-2.0. See LICENSE.