Consecutive BMI160 self-tests fail without delay

[sebmadgwick]

I am using BMI160_driver v3.7.7 to perform a self-test of the BMI160 gyroscope and accelerometer.  The following code makes two consecutive calls of bmi160_perform_self_test, the first is for the gyroscope, and the second is for the accelerometer.  This code is called repeatedly in a loop to log performance.  The result is that the self-tests will fail for 992 of 1000 iterations.

// Perform gyroscope self-test
if (bmi160_perform_self_test(BMI160_GYRO_ONLY, &bmi160) != BMI160_OK) {
    return false;
}

// TODO: Delay here

// Perform accelerometer self-test
uint8_t data[] = {0x2C};
bmi160_set_regs(0x40, data, sizeof (data), &bmi160); // configure for self-test as per page 43 of datasheet
if (bmi160_perform_self_test(BMI160_ACCEL_ONLY, &bmi160) != BMI160_OK) {
    return false;
}

I have found it is necessary to insert a delay between the two function calls.  This results in both self-tests passing 1000 of 1000 iterations.  I have found this to be the case for delays as short as 300 us.

I have not been able to find any documentation for the requirement of this delay.  My test system uses an 10 MHz SPI+DMA interface with a host MCU running at 200 MHz; perhaps the delay is only necessary for faster systems.  I suggest that the API or documentation is updated.

[sebmadgwick]

Update - Tests of >8000 iterations reveal that the above delay alone is not sufficient.

The following code achieves successful self-tests for 100,000 iterations. It includes three delays: A, B, and C. All three delays are necessary. I have not determined if 10 ms is the minimum required delay.

I am sharing this code so that others can replicated the issue. The code should be called in a loop. Removing the delays will demonstrate the fault. Tests were conducted using BMI160_driver v3.7.7 and a 10 MHz SPI interface.

bool SelfTest() {

    // Initialise device
    bmi160_init(&bmi160);

    // Perform soft reset
    bmi160_soft_reset(&bmi160);

    // Configure device
    bmi160.accel_cfg.odr = BMI160_ACCEL_ODR_400HZ;
    bmi160.accel_cfg.range = BMI160_ACCEL_RANGE_16G;
    bmi160.accel_cfg.bw = BMI160_ACCEL_BW_NORMAL_AVG4;
    bmi160.accel_cfg.power = BMI160_ACCEL_NORMAL_MODE;
    bmi160.gyro_cfg.odr = BMI160_ACCEL_ODR_400HZ;
    bmi160.gyro_cfg.range = BMI160_GYRO_RANGE_2000_DPS;
    bmi160.gyro_cfg.bw = BMI160_GYRO_BW_NORMAL_MODE;
    bmi160.gyro_cfg.power = BMI160_GYRO_NORMAL_MODE;
    bmi160_set_sens_conf(&bmi160);

    // Delay A
    TimerDelay(10);

    // Perform gyroscope self-test
    if (bmi160_perform_self_test(BMI160_GYRO_ONLY, &bmi160) != BMI160_OK) {
        return false;
    }

    // Delay B
    TimerDelay(10);

    // Perform accelerometer self-test
    uint8_t data[] = {0x2C};
    bmi160_set_regs(0x40, data, sizeof (data), &bmi160); // configure for self-test as per page 43 of datasheet
    if (bmi160_perform_self_test(BMI160_ACCEL_ONLY, &bmi160) != BMI160_OK) {
        return false;
    }

    // Delay C
    TimerDelay(10);

    // Self-test successful
    return true;
} 

[kgoveas]

Hi sebmadgwick,

Thanks for the hint. We will look into it and get back to you when we have an update.

Regards,
kgoveas