Corrupted accelerometer data

[joyxoy]

Hi everyone.

I have a very strange behaviour of MPU9150 accelerometer. I have five sensors from the same batch. Every accelerometer sensor from this batch has wrong value on y-axis (say 5000, for example). My first guess was the wrong Y-offset value for accelerometer, but each sensor has reasonable value (within 1000).  The wrong value on the axis does not depend on scaling, clock source, or digital filtering (on accelerometer side) - i checked all these things. Due to this, the freefall detection doesn`t work at all, the only one way to make it work - to disable Y-axis of accelerometer (always returns 0).

So my assumption is - may be there are something with MPU internals?

 

Just to check - my code of accelerometer initialization and reading data from sensors:

// wake up sensor and disable temperature sensor
tmp = 0x08;
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_PWR_MGMT_1, tmp);


//put gyroscope into standby mode
tmp = 0x07;
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_PWR_MGMT_2, tmp);


//set scale for accelerometer, disable self-test, enable HPF
tmp = 0x12;
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_ACCEL_CONFIG, tmp);


//set DLPF, 188 Hz
tmp = 0x01;
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_CONFIG, tmp);


//set sensor sampling rate to 50Hz
tmp = 0x13;
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_SMPLRT_DIV, tmp);


// set motion and freefall detection
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_MOT_THR, 0x06);
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_MOT_DUR, 0x0A);
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_MOT_DETECT_CTRL, 0x0F);
i2c->writeb(MPU6050_DEFAULT_ADDRESS, 0x1d, 0xBF);
i2c->writeb(MPU6050_DEFAULT_ADDRESS, 0x1e, 0xFF);


// set interrupt config, active high, push-pull, int high until cleared by reading 58 register
tmp = 0x20;
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_INT_PIN_CFG, tmp);


// enable FF interrupt and motion detection interrupt
tmp = 0xC0;
i2c->writeb(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_INT_ENABLE, tmp);

And reading from the sensor:

i2c->read(MPU6050_DEFAULT_ADDRESS, MPU6050_RA_ACCEL_XOUT_H, &buffer[0], 6);
*x = (((int16_t)buffer[0]) << 8) | buffer[1];
*y = (((int16_t)buffer[2]) << 8) | buffer[3];
*z = (((int16_t)buffer[4]) << 8) | buffer[5];

On different sensors i am getting different values on Y-axis, which changes when i set up offset value to ennormous values, for example, reading on y-axis when offset register are factory-defined:

x = 806 y = -32768 z = 4112
x = 808 y = -32768 z = 4109
x = 827 y = -32768 z = 4109
/* second sensor */
x = 91 y = 9677 z = 3912
x = 145 y = 9569 z = 3872
x = 214 y = 9562 z = 3910
x = 213 y = 9552 z = 3910
/* third sensor */
x = 516 y = 12547 z = 4098
x = 523 y = 12481 z = 4070
x = 482 y = 12499 z = 4113
x = 503 y = 12525 z = 4087

Does anyone have any thoughts regarding this subject? Thanks a lot! Hope to find a valid solution for this...

 

The same behaviour is observed even if i initialize sensor only by writing 0x00 value to PWR_MGM_1 register.