NAVIO: Getting accelerometer, gyroscope and magnetometer data from MPU9250 [C++]

This document is not updated, please proceed to docs.emlid.com for latest version!

MPU9250 is one of the best in class inertial sensors, which combines a gyroscope, an accelerometer and a magnetometer in one device. The MPU sensor family is not only popular as a part of drone autopilot projects, but is also widely used in devices like cellphones, tablets, etc.

MPU9250 example

If you haven’t already done that, download NAVIO drivers and examples code like this:

git clone www.github.com/emlid/navio

Move to folder Navio/Examples/AccelGyroMag, compile and run the example

cd Navio/C++/Examples/AccelGyroMag
make 
./AccelGyroMag

You should immediately see 9 values, updated in real time. Try to move the device around and see them change. They include Accelerometer, Gyroscope and Magnetometer data, three axis each.

MPU9250 driver

Let’s see the example code:

#include "Navio/MPU9250.h"

int main()
{
   MPU9250 imu;
   imu.initialize();

   float ax, ay, az, gx, gy, gz, mx, my, mz;

   while(1) {
      imu.getMotion9(&ax, &ay, &az, &gx, &gy, &gz, &mx, &my, &mz);

      printf("Acc: %+05.3f %+05.3f %+05.3f ", ax, ay, az);
      printf("Gyr: %+05.3f %+05.3f %+05.3f ", gx, gy, gz);
      printf("Mag: %+05.3f %+05.3f %+05.3fn", mx, my, mz);

      sleep(0.5);
   }
}

The first thing we should pay attention to is the line imu.initialize(), as it’s does an important job of setting internal device parameters:

bool MPU9250::initialize(int sample_rate_div, int low_pass_filter)
{
    uint8_t i = 0;
    uint8_t MPU_Init_Data[MPU_InitRegNum][2] = {
        {0x80, MPUREG_PWR_MGMT_1},     // Reset Device
        {0x01, MPUREG_PWR_MGMT_1},     // Clock Source
        {0x00, MPUREG_PWR_MGMT_2},     // Enable Acc & Gyro
        {low_pass_filter, MPUREG_CONFIG},  // Use DLPF set Gyroscope bandwidth 184Hz, temperature bandwidth     188Hz
        {0x18, MPUREG_GYRO_CONFIG},    // +-2000dps
        {0x08, MPUREG_ACCEL_CONFIG},   // +-4G
        {0x09, MPUREG_ACCEL_CONFIG_2}, // Set Acc Data Rates, Enable Acc LPF , Bandwidth 184Hz
        {0x30, MPUREG_INT_PIN_CFG},    //
        //{0x40, MPUREG_I2C_MST_CTRL},   // I2C Speed 348 kHz
        //{0x20, MPUREG_USER_CTRL},      // Enable AUX
        {0x20, MPUREG_USER_CTRL},       // I2C Master mode
        {0x0D, MPUREG_I2C_MST_CTRL}, //  I2C configuration multi-master  IIC 400KHz

        {AK8963_I2C_ADDR, MPUREG_I2C_SLV0_ADDR},  //Set the I2C slave address of AK8963 and set for write.
        //{0x09, MPUREG_I2C_SLV4_CTRL},
        //{0x81, MPUREG_I2C_MST_DELAY_CTRL}, //Enable I2C delay

        {AK8963_CNTL2, MPUREG_I2C_SLV0_REG}, //I2C slave 0 register address from where to begin data transfer
        {0x01, MPUREG_I2C_SLV0_DO}, // Reset AK8963
        {0x81, MPUREG_I2C_SLV0_CTRL},  //Enable I2C and set 1 byte

        {AK8963_CNTL1, MPUREG_I2C_SLV0_REG}, //I2C slave 0 register address from where to begin data transfer
        {0x12, MPUREG_I2C_SLV0_DO}, // Register value to continuous measurement in 16bit
        {0x81, MPUREG_I2C_SLV0_CTRL}  //Enable I2C and set 1 byte

    };

for(i=0; i < MPU_InitRegNum; i++) {
WriteReg(MPU_Init_Data[i][1], MPU_Init_Data[i][0]);
usleep(10000); //I2C must slow down the write speed, otherwise it won't work
}

set_acc_scale(BITS_FS_16G);
set_gyro_scale(BITS_FS_2000DPS);

calib_mag();
return 0;
}

Note that this function also sets scales for both Accelerometer and Gyroscope:

{0x18, MPUREG_GYRO_CONFIG},    // +-2000dps
{0x08, MPUREG_ACCEL_CONFIG},   // +-4G

The main function loop is pretty straightforward: read the data, print the data.

You can find all the useful information about MPU9250, including the datasheet and the register map here.