Table of Contents

Mpu6886 - accelerometer and gyroscope

The MPU-6886 is a 6-axis motion tracking sensor that combines a 3-axis gyroscope and a 3-axis accelerometer including the following features:

  • gyroscope programmable FSR of ±250 dps, ±500 dps, ±1000 dps, and ±2000 dps
  • accelerometer with programmable FSR of ±2g, ±4g, ±8g, and ±16g

Documentation

Usage

Create the Mpu6886 class and pass the I2C device. By default the I2C address for this sensor is 0x68.

// I2C pins need to be configured, for example for pin 22 & 21 for 
// the M5StickC Plus. These pins might be different for other boards.
Configuration.SetPinFunction(22, DeviceFunction.I2C1_CLOCK);
Configuration.SetPinFunction(21, DeviceFunction.I2C1_DATA);

I2cConnectionSettings settings = new(1, Mpu6886AccelerometerGyroscope.DefaultI2cAddress);

using (Mpu6886AccelerometerGyroscope ag = new(I2cDevice.Create(settings)))
{
    Debug.WriteLine("Start calibration ...");
    var offset = ag.Calibrate(1000);
    Debug.WriteLine($"Calibration done, calculated offsets {offset.X} {offset.Y} {offset.Y}");

    Debug.WriteLine($"Internal temperature: {ag.GetInternalTemperature().DegreesCelsius} C");

    while (true)
    {
        var acc = ag.GetAccelerometer();
        var gyr = ag.GetGyroscope();
        Debug.WriteLine($"Accelerometer data x:{acc.X} y:{acc.Y} z:{acc.Z}");
        Debug.WriteLine($"Gyroscope data x:{gyr.X} y:{gyr.Y} z:{gyr.Z}\n");
        Thread.Sleep(100);
    }
}

Sample output

Start calibration ...
Calibration done, calculated offsets 49.189 -86.21099999 -86.21099999
Internal temperature: 64.21664626 C
Accelerometer data x:-0.041503906 y:0 z:1.056884765
Gyroscope data x:4.94384765 y:-8.60595703 z:-15.68603515

Accelerometer data x:-0.040771484 y:-0.0051269531 z:1.062988281
Gyroscope data x:4.94384765 y:-7.56835937 z:-15.014648437

Accelerometer data x:-0.046630859 y:-0.0068359375 z:1.055175781
Gyroscope data x:3.60107421 y:-7.62939453 z:-15.1977539

Accelerometer data x:-0.049560546 y:-0 z:1.061279296
Gyroscope data x:4.39453125 y:-7.32421875 z:-14.28222656

See https://github.com/nanoFramework/nanoFramework.IoT.Device/tree/develop/devices/Mpu6886/samples for a complete sample application.

Calibration

The gyroscope can be calibrated using the Calibrate function. With the iterations parameter you can specify how many values should be read from the sensor to calculate an average (1000 iterations seems to be a good number). During the calibration you want to keep the sensor still and in a fixed position (e.g. lying flat on a table). The Calibrate function will write the values to the corresponding compensation registers of the sensor, values are corrected when retrieved automatically.

The return value of the Calibrate method gives a vector containing the 3 (for the X,Y and Z axes) calculated compensation values.

var offset = ag.Calibrate(1000);
Debug.WriteLine($"Calibration done, calculated offsets {offset.X} {offset.Y} {offset.Y}");

It is possible to write directly to the compensation registers using the SetGyroscopeOffset function. You might want to do this when you create your own custom calibration method, or you retrieve existing calibration data from a persisted data store or memory location.

⚠ When the devices reboots, the offset registers are cleared. So if you don't persist the calibration data yourself you will have to run the calibration method every time the device boots.

Sleep mode

The sensor can be put in sleep mode by calling the Sleep function. After that the WakeUp function should be called.

Setting scales

Both for the gyroscope and accelerometer you can set the desired scales using the AccelerometerScale and GyroscopeScale properties.

// Set the scale of the accelerometer to 2G
ag.AccelerometerScale = AccelerometerScale.Scale2G;

Setting enabled axes

By default all gyroscope and accelerometer axes are enabled. If desired you can specify explicitly which axes should be enabled. In that case, all other axes will be disabled. When an axis is disabled, data can still be read for that axis but the values will not change anymore.

// Enable only the X axis of the gyroscope and accelerometer
ag.EnabledAxes = EnabledAxis.GyroscopeX | EnabledAxis.AccelerometerX;

Features not implemented

The following features are enabled in the MPU6886 but not yet implemented in this driver:

  • registers 0x05 to 0x0b: low noise to low power offset
  • registers 0x0d to 0x0F: self test
  • register 0x1a: FIFO mode, DLPF (digital low pass filter)
  • register 0x1d bit 0 to 3: DLPF
  • register 0x1e bit 4 to 6: averaging filter for low power mode
  • registers 0x20 to 0x22: threshold for wake-on motion
  • register 0x36: fsync interrupt status
  • register 0x37: INT/DRDY pin config
  • register 0x39 and 0x3a: FIFO watermark interrupt
  • register 0x60 and 0x61: FIFO watermark interrupt
  • register 0x68: signal path reset
  • register 0x69: acc. intelligence control
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