File content as of revision 0:b502ea2d6ebb:

#include "MPU9250.h"


MPU9250::MPU9250(PinName sda, PinName scl, PinName tx, PinName rx, int address) : i2c(sda, scl), pc(tx,rx)
{
    if(address == 0)
        MPU9250_ADDRESS = MPU9250_ADDRESS_68;
    else if(address == 1) MPU9250_ADDRESS = MPU9250_ADDRESS_69;
    else {
        printf("Wrong Address\n");
        while(1);
    }

    i2c.frequency(400000);

    for(int i=0; i<=3; i++) {
        magCalibration[i] = 0;
        magbias[i] = 0;
        gyroBias[i] = 0;
        accelBias[i] = 0;
    }
    Mmode = 0x06;        // Either 8 Hz 0x02) or 100 Hz (0x06) magnetometer data ODR
}

void MPU9250::Start()
{
    whoami = readByte(MPU9250_ADDRESS, WHO_AM_I_MPU9250);  // Read WHO_AM_I register for MPU-9250
    pc.printf("I AM 0x%x\n\r", whoami);
    pc.printf("I SHOULD BE 0x71\n\r");

    if (whoami == 0x71) { // WHO_AM_I should always be 0x68
        pc.printf("MPU9250 WHO_AM_I is 0x%x\n\r", whoami);
        pc.printf("MPU9250 is online...\n\r");
        wait(1);

        resetMPU9250(); // Reset registers to default in preparation for device calibration
        MPU9250SelfTest(); // Start by performing self test and reporting values
        /*pc.printf("x-axis self test: acceleration trim within : %f % of factory value\n\r", SelfTest[0]);
        pc.printf("y-axis self test: acceleration trim within : %f % of factory value\n\r", SelfTest[1]);
        pc.printf("z-axis self test: acceleration trim within : %f % of factory value\n\r", SelfTest[2]);
        pc.printf("x-axis self test: gyration trim within : %f % of factory value\n\r", SelfTest[3]);
        pc.printf("y-axis self test: gyration trim within : %f % of factory value\n\r", SelfTest[4]);
        pc.printf("z-axis self test: gyration trim within : %f % of factory value\n\r", SelfTest[5]);*/
        calibrateMPU9250(); // Calibrate gyro and accelerometers, load biases in bias registers
        /*pc.printf("x gyro bias = %f\n\r", gyroBias[0]);
        pc.printf("y gyro bias = %f\n\r", gyroBias[1]);
        pc.printf("z gyro bias = %f\n\r", gyroBias[2]);
        pc.printf("x accel bias = %f\n\r", accelBias[0]);
        pc.printf("y accel bias = %f\n\r", accelBias[1]);
        pc.printf("z accel bias = %f\n\r", accelBias[2]);*/
        wait(2);
        initMPU9250();
        pc.printf("MPU9250 initialized for active data mode....\n\r"); // Initialize device for active mode read of acclerometer, gyroscope, and temperature
        initAK8963();
        pc.printf("AK8963 initialized for active data mode....\n\r"); // Initialize device for active mode read of magnetometer

        whoami = readByte(AK8963_ADDRESS, WHO_AM_I_AK8963);  // Read WHO_AM_I register for MPU-9250
        pc.printf("I AM 0x%x\n\r", whoami);
        pc.printf("I SHOULD BE 0x48\n\r");
        if(whoami != 0x48) {
            while(1);
        }
        /*pc.printf("Accelerometer full-scale range = %f  g\n\r", 2.0f*(float)(1<<Ascale));
        pc.printf("Gyroscope full-scale range = %f  deg/s\n\r", 250.0f*(float)(1<<Gscale));
        if(Mscale == 0) pc.printf("Magnetometer resolution = 14  bits\n\r");
        if(Mscale == 1) pc.printf("Magnetometer resolution = 16  bits\n\r");
        if(Mmode == 2) pc.printf("Magnetometer ODR = 8 Hz\n\r");
        if(Mmode == 6) pc.printf("Magnetometer ODR = 100 Hz\n\r");*/
        wait(1);
    } else {
        pc.printf("Could not connect to MPU9250: \n\r");
        pc.printf("%#x \n",  whoami);

        while(1) ; // Loop forever if communication doesn't happen
    }


    getAres(); // Get accelerometer sensitivity
    getGres(); // Get gyro sensitivity
    getMres(); // Get magnetometer sensitivity
    /*pc.printf("Accelerometer sensitivity is %f LSB/g \n\r", 1.0f/aRes);
    pc.printf("Gyroscope sensitivity is %f LSB/deg/s \n\r", 1.0f/gRes);
    pc.printf("Magnetometer sensitivity is %f LSB/G \n\r", 1.0f/mRes);*/

    MagCal();
}

void MPU9250::ReadRawAccGyroMag()
{
    // If intPin goes high, all data registers have new data
    if(readByte(MPU9250_ADDRESS, INT_STATUS) & 0x01) {  // On interrupt, check if data ready interrupt

        readAccelData();  // Read the x/y/z adc values
        AccelXYZCal();
        // Now we'll calculate the accleration value into actual g's
        /*ax = (float)accelCount[0]*aRes - accelBias[0];  // get actual g value, this depends on scale being set
        ay = (float)accelCount[1]*aRes - accelBias[1];
        az = (float)accelCount[2]*aRes - accelBias[2];*/

        readGyroData();  // Read the x/y/z adc values
        GyroXYZCal();
        // Calculate the gyro value into actual degrees per second
        /*gx = (float)gyroCount[0]*gRes - gyroBias[0];  // get actual gyro value, this depends on scale being set
        gy = (float)gyroCount[1]*gRes - gyroBias[1];
        gz = (float)gyroCount[2]*gRes - gyroBias[2];*/

        readMagData();  // Read the x/y/z adc values
        MagXYZCal();
        /*mx = ((float)magCount[0]-xmin)*magCalibration[0] + magbias[0];  // get actual magnetometer value, this depends on scale being set
        my = ((float)magCount[1]-ymin)*magCalibration[1] + magbias[1];
        mz = ((float)magCount[2]-zmin)*magCalibration[2] + magbias[2];*/
    }
}

void MPU9250::writeByte(uint8_t address, uint8_t subAddress, uint8_t data)
{
    char data_write[2];
    data_write[0] = subAddress;
    data_write[1] = data;
    i2c.write(address, data_write, 2, 0);
}

char MPU9250::readByte(uint8_t address, uint8_t subAddress)
{
    char data[1]; // `data` will store the register data
    char data_write[1];
    data_write[0] = subAddress;
    i2c.write(address, data_write, 1, 1); // no stop
    i2c.read(address, data, 1, 0);
    return data[0];
}

void MPU9250::readBytes(uint8_t address, uint8_t subAddress, uint8_t count, uint8_t * dest)
{
    char data[14];
    char data_write[1];
    data_write[0] = subAddress;
    i2c.write(address, data_write, 1, 1); // no stop
    i2c.read(address, data, count, 0);
    for(int ii = 0; ii < count; ii++) {
        dest[ii] = data[ii];
    }
}


void MPU9250::setMres()
{
    getMres();
    switch (Mscale) {
            // Possible magnetometer scales (and their register bit settings) are:
            // 14 bit resolution (0) and 16 bit resolution (1)
        case MFS_14BITS:
            mRes = 10.0*4219.0/8190.0; // Proper scale to return milliGauss
            break;
        case MFS_16BITS:
            mRes = 10.0*4219.0/32760.0; // Proper scale to return milliGauss
            break;
    }
}


void MPU9250::setGres()
{
    getGres();
    switch (Gscale) {
            // Possible gyro scales (and their register bit settings) are:
            // 250 DPS (00), 500 DPS (01), 1000 DPS (10), and 2000 DPS  (11).
            // Here's a bit of an algorith to calculate DPS/(ADC tick) based on that 2-bit value:
        case GFS_250DPS:
            gRes = 250.0/32768.0;
            break;
        case GFS_500DPS:
            gRes = 500.0/32768.0;
            break;
        case GFS_1000DPS:
            gRes = 1000.0/32768.0;
            break;
        case GFS_2000DPS:
            gRes = 2000.0/32768.0;
            break;
    }
}

void MPU9250::setAres()
{
    getAres();
    switch (Ascale) {
            // Possible accelerometer scales (and their register bit settings) are:
            // 2 Gs (00), 4 Gs (01), 8 Gs (10), and 16 Gs  (11).
            // Here's a bit of an algorith to calculate DPS/(ADC tick) based on that 2-bit value:
        case AFS_2G:
            aRes = 2.0/32768.0;
            break;
        case AFS_4G:
            aRes = 4.0/32768.0;
            break;
        case AFS_8G:
            aRes = 8.0/32768.0;
            break;
        case AFS_16G:
            aRes = 16.0/32768.0;
            break;
    }
}

void MPU9250::getMres()
{
    Mscale = MFS_16BITS; // MFS_14BITS or MFS_16BITS, 14-bit or 16-bit magnetometer resolution
}


void MPU9250::getGres()
{
    Gscale = GFS_250DPS; // GFS_250DPS, GFS_500DPS, GFS_1000DPS, GFS_2000DPS
}

void MPU9250::getAres()
{
    Ascale = AFS_2G;     // AFS_2G, AFS_4G, AFS_8G, AFS_16G
}

void MPU9250::MagCal()
{
    printf("START scan mag\n\r\n\r\n\r");

    //Assign random value before calibrate
    /*xmax = -4914.0f;
    xmin = 4914.0f;

    ymax = -4914.0;
    ymin = 4914.0f;

    zmax = -4914.0;
    zmin = 4914.0f;

    change=false;

    while(1) {
        readMagData(magCount);

        if(magCount[0]<xmin) {
            xmin = magCount[0];
            change = true;
        }
        if(magCount[0]>xmax) {
            xmax = magCount[0];
            change = true;
        }

        if(magCount[1]<ymin) {
            ymin = magCount[1];
            change = true;
        }
        if(magCount[1]>ymax) {
            ymax = magCount[1];
            change = true;
        }


        if(magCount[2]<zmin) {
            zmin = magCount[2];
            change = true;
        }
        if(magCount[2]>zmax) {
            zmax = magCount[2];
            change = true;
        }

        if(change==true) {
            printf("Mx Max= %f Min= %f\n\r",xmax,xmin);
            printf("My Max= %f Min= %f\n\r",ymax,ymin);
            printf("Mz Max= %f Min= %f\n\r",zmax,zmin);
            change=false;
        }*/

    //Out of Calibration loop
    /*if(button==1) {
        while(button==1);
        break;
    }*/
    //}


    xmax = 188.000000;
    xmin = -316.000000;
    ymax = 485.000000;
    ymin = -26.000000;
    zmax = 165.000000;
    xmin = -230.000000;

    magbias[0] = -1.0;
    magbias[1] = -1.0;
    magbias[2] = -1.0;

    magCalibration[0] = 2.0f / (xmax -xmin);
    magCalibration[1] = 2.0f / (ymax -ymin);
    magCalibration[2] = 2.0f / (zmax -zmin);

    printf("mag[0] %f",magbias[0]);
    printf("mag[1] %f",magbias[1]);
    printf("mag[2] %f\n\r",magbias[2]);
}

void MPU9250::AccelXYZCal()
{
    ax = (float)accelCount[0]*aRes - accelBias[0];  // get actual g value, this depends on scale being set
    ay = (float)accelCount[1]*aRes - accelBias[1];
    az = (float)accelCount[2]*aRes - accelBias[2];
}

void MPU9250::GyroXYZCal()
{
    gx = (float)gyroCount[0]*gRes - gyroBias[0];  // get actual gyro value, this depends on scale being set
    gy = (float)gyroCount[1]*gRes - gyroBias[1];
    gz = (float)gyroCount[2]*gRes - gyroBias[2];
}

void MPU9250::MagXYZCal()
{
    mx = ((float)magCount[0]-xmin)*magCalibration[0] + magbias[0];  // get actual magnetometer value, this depends on scale being set
    my = ((float)magCount[1]-ymin)*magCalibration[1] + magbias[1];
    mz = ((float)magCount[2]-zmin)*magCalibration[2] + magbias[2];
}


void MPU9250::readAccelData()
{
    float destination[3] = {0,0,0};
    uint8_t rawData[6];  // x/y/z accel register data stored here
    readBytes(MPU9250_ADDRESS, ACCEL_XOUT_H, 6, &rawData[0]);  // Read the six raw data registers into data array
    destination[0] = (int16_t)(((int16_t)rawData[0] << 8) | rawData[1]) ;  // Turn the MSB and LSB into a signed 16-bit value
    destination[1] = (int16_t)(((int16_t)rawData[2] << 8) | rawData[3]) ;
    destination[2] = (int16_t)(((int16_t)rawData[4] << 8) | rawData[5]) ;

    for(int i=0; i<=2; i++)
        accelCount[i] = (float)destination[i];
}

void MPU9250::readGyroData()
{
    float destination[3] = {0,0,0};
    uint8_t rawData[6];  // x/y/z gyro register data stored here
    readBytes(MPU9250_ADDRESS, GYRO_XOUT_H, 6, &rawData[0]);  // Read the six raw data registers sequentially into data array
    destination[0] = (int16_t)(((int16_t)rawData[0] << 8) | rawData[1]) ;  // Turn the MSB and LSB into a signed 16-bit value
    destination[1] = (int16_t)(((int16_t)rawData[2] << 8) | rawData[3]) ;
    destination[2] = (int16_t)(((int16_t)rawData[4] << 8) | rawData[5]) ;

    for(int i=0; i<=2; i++)
        gyroCount[i] = (float)destination[i];
}

void MPU9250::readMagData()
{
    float destination[3] = {0,0,0};
    uint8_t rawData[7];  // x/y/z gyro register data, ST2 register stored here, must read ST2 at end of data acquisition
    if(readByte(AK8963_ADDRESS, AK8963_ST1) & 0x01) { // wait for magnetometer data ready bit to be set
        readBytes(AK8963_ADDRESS, AK8963_XOUT_L, 7, &rawData[0]);  // Read the six raw data and ST2 registers sequentially into data array
        uint8_t c = rawData[6]; // End data read by reading ST2 register
        if(!(c & 0x08)) { // Check if magnetic sensor overflow set, if not then report data
            destination[0] = (int16_t)(((int16_t)rawData[1] << 8) | rawData[0]);  // Turn the MSB and LSB into a signed 16-bit value
            destination[1] = (int16_t)(((int16_t)rawData[3] << 8) | rawData[2]) ;  // Data stored as little Endian
            destination[2] = (int16_t)(((int16_t)rawData[5] << 8) | rawData[4]) ;
        }
    }

    for(int i=0; i<=2; i++)
        magCount[i] = (float)destination[i];
}

void MPU9250::readTempData()
{
    int16_t destination;
    uint8_t rawData[2];  // x/y/z gyro register data stored here
    readBytes(MPU9250_ADDRESS, TEMP_OUT_H, 2, &rawData[0]);  // Read the two raw data registers sequentially into data array
    destination = (int16_t)(((int16_t)rawData[0]) << 8 | rawData[1]) ;  // Turn the MSB and LSB into a 16-bit value
    destination = ((float) destination) / 333.87f + 21.0f;
    temperature = destination;
}


void MPU9250::resetMPU9250()
{
    // reset device
    writeByte(MPU9250_ADDRESS, PWR_MGMT_1, 0x80); // Write a one to bit 7 reset bit; toggle reset device
    wait(0.1);
}

void MPU9250::initAK8963()
{
    float destination[3] = {0,0,0};
    // First extract the factory calibration for each magnetometer axis
    uint8_t rawData[3];  // x/y/z gyro calibration data stored here
    writeByte(AK8963_ADDRESS, AK8963_CNTL, 0x00); // Power down magnetometer
    wait(0.01);
    writeByte(AK8963_ADDRESS, AK8963_CNTL, 0x0F); // Enter Fuse ROM access mode
    wait(0.01);
    readBytes(AK8963_ADDRESS, AK8963_ASAX, 3, &rawData[0]);  // Read the x-, y-, and z-axis calibration values
    destination[0] =  (float)(rawData[0] - 128)/256.0f + 1.0f;   // Return x-axis sensitivity adjustment values, etc.
    destination[1] =  (float)(rawData[1] - 128)/256.0f + 1.0f;
    destination[2] =  (float)(rawData[2] - 128)/256.0f + 1.0f;
    writeByte(AK8963_ADDRESS, AK8963_CNTL, 0x00); // Power down magnetometer
    wait(0.01);
    // Configure the magnetometer for continuous read and highest resolution
    // set Mscale bit 4 to 1 (0) to enable 16 (14) bit resolution in CNTL register,
    // and enable continuous mode data acquisition Mmode (bits [3:0]), 0010 for 8 Hz and 0110 for 100 Hz sample rates
    writeByte(AK8963_ADDRESS, AK8963_CNTL, Mscale << 4 | Mmode); // Set magnetometer data resolution and sample ODR
    wait(0.01);

    for(int i=0; i<=2; i++)
        magCalibration[i] = destination[i];
}


void MPU9250::initMPU9250()
{
// Initialize MPU9250 device
// wake up device
    writeByte(MPU9250_ADDRESS, PWR_MGMT_1, 0x00); // Clear sleep mode bit (6), enable all sensors
    wait(0.1); // Delay 100 ms for PLL to get established on x-axis gyro; should check for PLL ready interrupt

// get stable time source
    writeByte(MPU9250_ADDRESS, PWR_MGMT_1, 0x01);  // Set clock source to be PLL with x-axis gyroscope reference, bits 2:0 = 001

// Configure Gyro and Accelerometer
// Disable FSYNC and set accelerometer and gyro bandwidth to 44 and 42 Hz, respectively;
// DLPF_CFG = bits 2:0 = 010; this sets the sample rate at 1 kHz for both
// Maximum delay is 4.9 ms which is just over a 200 Hz maximum rate
    writeByte(MPU9250_ADDRESS, CONFIG, 0x03);

// Set sample rate = gyroscope output rate/(1 + SMPLRT_DIV)
    writeByte(MPU9250_ADDRESS, SMPLRT_DIV, 0x04);  // Use a 200 Hz rate; the same rate set in CONFIG above

// Set gyroscope full scale range
// Range selects FS_SEL and AFS_SEL are 0 - 3, so 2-bit values are left-shifted into positions 4:3
    uint8_t c =  readByte(MPU9250_ADDRESS, GYRO_CONFIG);
    writeByte(MPU9250_ADDRESS, GYRO_CONFIG, c & ~0xE0); // Clear self-test bits [7:5]
    writeByte(MPU9250_ADDRESS, GYRO_CONFIG, c & ~0x18); // Clear AFS bits [4:3]
    writeByte(MPU9250_ADDRESS, GYRO_CONFIG, c | Gscale << 3); // Set full scale range for the gyro

// Set accelerometer configuration
    c =  readByte(MPU9250_ADDRESS, ACCEL_CONFIG);
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG, c & ~0xE0); // Clear self-test bits [7:5]
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG, c & ~0x18); // Clear AFS bits [4:3]
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG, c | Ascale << 3); // Set full scale range for the accelerometer

// Set accelerometer sample rate configuration
// It is possible to get a 4 kHz sample rate from the accelerometer by choosing 1 for
// accel_fchoice_b bit [3]; in this case the bandwidth is 1.13 kHz
    c = readByte(MPU9250_ADDRESS, ACCEL_CONFIG2);
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG2, c & ~0x0F); // Clear accel_fchoice_b (bit 3) and A_DLPFG (bits [2:0])
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG2, c | 0x03); // Set accelerometer rate to 1 kHz and bandwidth to 41 Hz

// The accelerometer, gyro, and thermometer are set to 1 kHz sample rates,
// but all these rates are further reduced by a factor of 5 to 200 Hz because of the SMPLRT_DIV setting

    // Configure Interrupts and Bypass Enable
    // Set interrupt pin active high, push-pull, and clear on read of INT_STATUS, enable I2C_BYPASS_EN so additional chips
    // can join the I2C bus and all can be controlled by the Arduino as master
    writeByte(MPU9250_ADDRESS, INT_PIN_CFG, 0x22);
    writeByte(MPU9250_ADDRESS, INT_ENABLE, 0x01);  // Enable data ready (bit 0) interrupt
}

// Function which accumulates gyro and accelerometer data after device initialization. It calculates the average
// of the at-rest readings and then loads the resulting offsets into accelerometer and gyro bias registers.
void MPU9250::calibrateMPU9250()
{
    uint8_t data[12]; // data array to hold accelerometer and gyro x, y, z, data
    uint16_t ii, packet_count, fifo_count;
    int32_t gyro_bias[3] = {0, 0, 0}, accel_bias[3] = {0, 0, 0};

// reset device, reset all registers, clear gyro and accelerometer bias registers
    writeByte(MPU9250_ADDRESS, PWR_MGMT_1, 0x80); // Write a one to bit 7 reset bit; toggle reset device
    wait(0.1);

// get stable time source
// Set clock source to be PLL with x-axis gyroscope reference, bits 2:0 = 001
    writeByte(MPU9250_ADDRESS, PWR_MGMT_1, 0x01);
    writeByte(MPU9250_ADDRESS, PWR_MGMT_2, 0x00);
    wait(0.2);

// Configure device for bias calculation
    writeByte(MPU9250_ADDRESS, INT_ENABLE, 0x00);   // Disable all interrupts
    writeByte(MPU9250_ADDRESS, FIFO_EN, 0x00);      // Disable FIFO
    writeByte(MPU9250_ADDRESS, PWR_MGMT_1, 0x00);   // Turn on internal clock source
    writeByte(MPU9250_ADDRESS, I2C_MST_CTRL, 0x00); // Disable I2C master
    writeByte(MPU9250_ADDRESS, USER_CTRL, 0x00);    // Disable FIFO and I2C master modes
    writeByte(MPU9250_ADDRESS, USER_CTRL, 0x0C);    // Reset FIFO and DMP
    wait(0.015);

// Configure MPU9250 gyro and accelerometer for bias calculation
    writeByte(MPU9250_ADDRESS, CONFIG, 0x01);      // Set low-pass filter to 188 Hz
    writeByte(MPU9250_ADDRESS, SMPLRT_DIV, 0x00);  // Set sample rate to 1 kHz
    writeByte(MPU9250_ADDRESS, GYRO_CONFIG, 0x00);  // Set gyro full-scale to 250 degrees per second, maximum sensitivity
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG, 0x00); // Set accelerometer full-scale to 2 g, maximum sensitivity

    uint16_t  gyrosensitivity  = 131;   // = 131 LSB/degrees/sec
    uint16_t  accelsensitivity = 16384;  // = 16384 LSB/g

// Configure FIFO to capture accelerometer and gyro data for bias calculation
    writeByte(MPU9250_ADDRESS, USER_CTRL, 0x40);   // Enable FIFO
    writeByte(MPU9250_ADDRESS, FIFO_EN, 0x78);     // Enable gyro and accelerometer sensors for FIFO (max size 512 bytes in MPU-9250)
    wait(0.04); // accumulate 40 samples in 80 milliseconds = 480 bytes

// At end of sample accumulation, turn off FIFO sensor read
    writeByte(MPU9250_ADDRESS, FIFO_EN, 0x00);        // Disable gyro and accelerometer sensors for FIFO
    readBytes(MPU9250_ADDRESS, FIFO_COUNTH, 2, &data[0]); // read FIFO sample count
    fifo_count = ((uint16_t)data[0] << 8) | data[1];
    packet_count = fifo_count/12;// How many sets of full gyro and accelerometer data for averaging

    for (ii = 0; ii < packet_count; ii++) {
        int16_t accel_temp[3] = {0, 0, 0}, gyro_temp[3] = {0, 0, 0};
        readBytes(MPU9250_ADDRESS, FIFO_R_W, 12, &data[0]); // read data for averaging
        accel_temp[0] = (int16_t) (((int16_t)data[0] << 8) | data[1]  ) ;  // Form signed 16-bit integer for each sample in FIFO
        accel_temp[1] = (int16_t) (((int16_t)data[2] << 8) | data[3]  ) ;
        accel_temp[2] = (int16_t) (((int16_t)data[4] << 8) | data[5]  ) ;
        gyro_temp[0]  = (int16_t) (((int16_t)data[6] << 8) | data[7]  ) ;
        gyro_temp[1]  = (int16_t) (((int16_t)data[8] << 8) | data[9]  ) ;
        gyro_temp[2]  = (int16_t) (((int16_t)data[10] << 8) | data[11]) ;

        accel_bias[0] += (int32_t) accel_temp[0]; // Sum individual signed 16-bit biases to get accumulated signed 32-bit biases
        accel_bias[1] += (int32_t) accel_temp[1];
        accel_bias[2] += (int32_t) accel_temp[2];
        gyro_bias[0]  += (int32_t) gyro_temp[0];
        gyro_bias[1]  += (int32_t) gyro_temp[1];
        gyro_bias[2]  += (int32_t) gyro_temp[2];

    }
    accel_bias[0] /= (int32_t) packet_count; // Normalize sums to get average count biases
    accel_bias[1] /= (int32_t) packet_count;
    accel_bias[2] /= (int32_t) packet_count;
    gyro_bias[0]  /= (int32_t) packet_count;
    gyro_bias[1]  /= (int32_t) packet_count;
    gyro_bias[2]  /= (int32_t) packet_count;

    if(accel_bias[2] > 0L) {
        accel_bias[2] -= (int32_t) accelsensitivity;   // Remove gravity from the z-axis accelerometer bias calculation
    } else {
        accel_bias[2] += (int32_t) accelsensitivity;
    }

// Construct the gyro biases for push to the hardware gyro bias registers, which are reset to zero upon device startup
    data[0] = (-gyro_bias[0]/4  >> 8) & 0xFF; // Divide by 4 to get 32.9 LSB per deg/s to conform to expected bias input format
    data[1] = (-gyro_bias[0]/4)       & 0xFF; // Biases are additive, so change sign on calculated average gyro biases
    data[2] = (-gyro_bias[1]/4  >> 8) & 0xFF;
    data[3] = (-gyro_bias[1]/4)       & 0xFF;
    data[4] = (-gyro_bias[2]/4  >> 8) & 0xFF;
    data[5] = (-gyro_bias[2]/4)       & 0xFF;

/// Push gyro biases to hardware registers
    /*  writeByte(MPU9250_ADDRESS, XG_OFFSET_H, data[0]);
      writeByte(MPU9250_ADDRESS, XG_OFFSET_L, data[1]);
      writeByte(MPU9250_ADDRESS, YG_OFFSET_H, data[2]);
      writeByte(MPU9250_ADDRESS, YG_OFFSET_L, data[3]);
      writeByte(MPU9250_ADDRESS, ZG_OFFSET_H, data[4]);
      writeByte(MPU9250_ADDRESS, ZG_OFFSET_L, data[5]);
    */
    gyroBias[0] = (float) gyro_bias[0]/(float) gyrosensitivity; // construct gyro bias in deg/s for later manual subtraction
    gyroBias[1] = (float) gyro_bias[1]/(float) gyrosensitivity;
    gyroBias[2] = (float) gyro_bias[2]/(float) gyrosensitivity;

// Construct the accelerometer biases for push to the hardware accelerometer bias registers. These registers contain
// factory trim values which must be added to the calculated accelerometer biases; on boot up these registers will hold
// non-zero values. In addition, bit 0 of the lower byte must be preserved since it is used for temperature
// compensation calculations. Accelerometer bias registers expect bias input as 2048 LSB per g, so that
// the accelerometer biases calculated above must be divided by 8.

    int32_t accel_bias_reg[3] = {0, 0, 0}; // A place to hold the factory accelerometer trim biases
    readBytes(MPU9250_ADDRESS, XA_OFFSET_H, 2, &data[0]); // Read factory accelerometer trim values
    accel_bias_reg[0] = (int16_t) ((int16_t)data[0] << 8) | data[1];
    readBytes(MPU9250_ADDRESS, YA_OFFSET_H, 2, &data[0]);
    accel_bias_reg[1] = (int16_t) ((int16_t)data[0] << 8) | data[1];
    readBytes(MPU9250_ADDRESS, ZA_OFFSET_H, 2, &data[0]);
    accel_bias_reg[2] = (int16_t) ((int16_t)data[0] << 8) | data[1];

    uint32_t mask = 1uL; // Define mask for temperature compensation bit 0 of lower byte of accelerometer bias registers
    uint8_t mask_bit[3] = {0, 0, 0}; // Define array to hold mask bit for each accelerometer bias axis

    for(ii = 0; ii < 3; ii++) {
        if(accel_bias_reg[ii] & mask) mask_bit[ii] = 0x01; // If temperature compensation bit is set, record that fact in mask_bit
    }

    // Construct total accelerometer bias, including calculated average accelerometer bias from above
    accel_bias_reg[0] -= (accel_bias[0]/8); // Subtract calculated averaged accelerometer bias scaled to 2048 LSB/g (16 g full scale)
    accel_bias_reg[1] -= (accel_bias[1]/8);
    accel_bias_reg[2] -= (accel_bias[2]/8);

    data[0] = (accel_bias_reg[0] >> 8) & 0xFF;
    data[1] = (accel_bias_reg[0])      & 0xFF;
    data[1] = data[1] | mask_bit[0]; // preserve temperature compensation bit when writing back to accelerometer bias registers
    data[2] = (accel_bias_reg[1] >> 8) & 0xFF;
    data[3] = (accel_bias_reg[1])      & 0xFF;
    data[3] = data[3] | mask_bit[1]; // preserve temperature compensation bit when writing back to accelerometer bias registers
    data[4] = (accel_bias_reg[2] >> 8) & 0xFF;
    data[5] = (accel_bias_reg[2])      & 0xFF;
    data[5] = data[5] | mask_bit[2]; // preserve temperature compensation bit when writing back to accelerometer bias registers

// Apparently this is not working for the acceleration biases in the MPU-9250
// Are we handling the temperature correction bit properly?
// Push accelerometer biases to hardware registers
    /*  writeByte(MPU9250_ADDRESS, XA_OFFSET_H, data[0]);
      writeByte(MPU9250_ADDRESS, XA_OFFSET_L, data[1]);
      writeByte(MPU9250_ADDRESS, YA_OFFSET_H, data[2]);
      writeByte(MPU9250_ADDRESS, YA_OFFSET_L, data[3]);
      writeByte(MPU9250_ADDRESS, ZA_OFFSET_H, data[4]);
      writeByte(MPU9250_ADDRESS, ZA_OFFSET_L, data[5]);
    */
// Output scaled accelerometer biases for manual subtraction in the main program
    accelBias[0] = (float)accel_bias[0]/(float)accelsensitivity;
    accelBias[1] = (float)accel_bias[1]/(float)accelsensitivity;
    accelBias[2] = (float)accel_bias[2]/(float)accelsensitivity;
}


// Accelerometer and gyroscope self test; check calibration wrt factory settings
void MPU9250::MPU9250SelfTest()   // Should return percent deviation from factory trim values, +/- 14 or less deviation is a pass
{
    //float destination[6] = {0,0,0,0,0,0};
    uint8_t rawData[6] = {0, 0, 0, 0, 0, 0};
    uint8_t selfTest[6];
    int16_t gAvg[3], aAvg[3], aSTAvg[3], gSTAvg[3];
    float factoryTrim[6];
    uint8_t FS = 0;

    writeByte(MPU9250_ADDRESS, SMPLRT_DIV, 0x00); // Set gyro sample rate to 1 kHz
    writeByte(MPU9250_ADDRESS, CONFIG, 0x02); // Set gyro sample rate to 1 kHz and DLPF to 92 Hz
    writeByte(MPU9250_ADDRESS, GYRO_CONFIG, 1<<FS); // Set full scale range for the gyro to 250 dps
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG2, 0x02); // Set accelerometer rate to 1 kHz and bandwidth to 92 Hz
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG, 1<<FS); // Set full scale range for the accelerometer to 2 g

    for( int ii = 0; ii < 200; ii++) { // get average current values of gyro and acclerometer

        readBytes(MPU9250_ADDRESS, ACCEL_XOUT_H, 6, &rawData[0]); // Read the six raw data registers into data array
        aAvg[0] += (int16_t)(((int16_t)rawData[0] << 8) | rawData[1]) ; // Turn the MSB and LSB into a signed 16-bit value
        aAvg[1] += (int16_t)(((int16_t)rawData[2] << 8) | rawData[3]) ;
        aAvg[2] += (int16_t)(((int16_t)rawData[4] << 8) | rawData[5]) ;

        readBytes(MPU9250_ADDRESS, GYRO_XOUT_H, 6, &rawData[0]); // Read the six raw data registers sequentially into data array
        gAvg[0] += (int16_t)(((int16_t)rawData[0] << 8) | rawData[1]) ; // Turn the MSB and LSB into a signed 16-bit value
        gAvg[1] += (int16_t)(((int16_t)rawData[2] << 8) | rawData[3]) ;
        gAvg[2] += (int16_t)(((int16_t)rawData[4] << 8) | rawData[5]) ;
    }

    for (int ii =0; ii < 3; ii++) { // Get average of 200 values and store as average current readings
        aAvg[ii] /= 200;
        gAvg[ii] /= 200;
    }

// Configure the accelerometer for self-test
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG, 0xE0); // Enable self test on all three axes and set accelerometer range to +/- 2 g
    writeByte(MPU9250_ADDRESS, GYRO_CONFIG, 0xE0); // Enable self test on all three axes and set gyro range to +/- 250 degrees/s
    //delay(25); // Delay a while to let the device stabilize

    for( int ii = 0; ii < 200; ii++) { // get average self-test values of gyro and acclerometer

        readBytes(MPU9250_ADDRESS, ACCEL_XOUT_H, 6, &rawData[0]); // Read the six raw data registers into data array
        aSTAvg[0] += (int16_t)(((int16_t)rawData[0] << 8) | rawData[1]) ; // Turn the MSB and LSB into a signed 16-bit value
        aSTAvg[1] += (int16_t)(((int16_t)rawData[2] << 8) | rawData[3]) ;
        aSTAvg[2] += (int16_t)(((int16_t)rawData[4] << 8) | rawData[5]) ;

        readBytes(MPU9250_ADDRESS, GYRO_XOUT_H, 6, &rawData[0]); // Read the six raw data registers sequentially into data array
        gSTAvg[0] += (int16_t)(((int16_t)rawData[0] << 8) | rawData[1]) ; // Turn the MSB and LSB into a signed 16-bit value
        gSTAvg[1] += (int16_t)(((int16_t)rawData[2] << 8) | rawData[3]) ;
        gSTAvg[2] += (int16_t)(((int16_t)rawData[4] << 8) | rawData[5]) ;
    }

    for (int ii =0; ii < 3; ii++) { // Get average of 200 values and store as average self-test readings
        aSTAvg[ii] /= 200;
        gSTAvg[ii] /= 200;
    }

// Configure the gyro and accelerometer for normal operation
    writeByte(MPU9250_ADDRESS, ACCEL_CONFIG, 0x00);
    writeByte(MPU9250_ADDRESS, GYRO_CONFIG, 0x00);
    //delay(25); // Delay a while to let the device stabilize

    // Retrieve accelerometer and gyro factory Self-Test Code from USR_Reg
    selfTest[0] = readByte(MPU9250_ADDRESS, SELF_TEST_X_ACCEL); // X-axis accel self-test results
    selfTest[1] = readByte(MPU9250_ADDRESS, SELF_TEST_Y_ACCEL); // Y-axis accel self-test results
    selfTest[2] = readByte(MPU9250_ADDRESS, SELF_TEST_Z_ACCEL); // Z-axis accel self-test results
    selfTest[3] = readByte(MPU9250_ADDRESS, SELF_TEST_X_GYRO); // X-axis gyro self-test results
    selfTest[4] = readByte(MPU9250_ADDRESS, SELF_TEST_Y_GYRO); // Y-axis gyro self-test results
    selfTest[5] = readByte(MPU9250_ADDRESS, SELF_TEST_Z_GYRO); // Z-axis gyro self-test results

    // Retrieve factory self-test value from self-test code reads
    factoryTrim[0] = (float)(2620/1<<FS)*(pow( (float)1.01 , ((float)selfTest[0] - (float)1.0) )); // FT[Xa] factory trim calculation
    factoryTrim[1] = (float)(2620/1<<FS)*(pow( (float)1.01 , ((float)selfTest[1] - (float)1.0) )); // FT[Ya] factory trim calculation
    factoryTrim[2] = (float)(2620/1<<FS)*(pow( (float)1.01 , ((float)selfTest[2] - (float)1.0) )); // FT[Za] factory trim calculation
    factoryTrim[3] = (float)(2620/1<<FS)*(pow( (float)1.01 , ((float)selfTest[3] - (float)1.0) )); // FT[Xg] factory trim calculation
    factoryTrim[4] = (float)(2620/1<<FS)*(pow( (float)1.01 , ((float)selfTest[4] - (float)1.0) )); // FT[Yg] factory trim calculation
    factoryTrim[5] = (float)(2620/1<<FS)*(pow( (float)1.01 , ((float)selfTest[5] - (float)1.0) )); // FT[Zg] factory trim calculation

// Report results as a ratio of (STR - FT)/FT; the change from Factory Trim of the Self-Test Response
// To get percent, must multiply by 100
    for (int i = 0; i < 3; i++) {
        SelfTest[i] = (float)100.0*((float)(aSTAvg[i] - aAvg[i]))/factoryTrim[i]; // Report percent differences
        SelfTest[i+3] = (float)100.0*((float)(gSTAvg[i] - gAvg[i]))/factoryTrim[i+3]; // Report percent differences
    }

}