BMM150 Z value not correct

[Myzhar]

Hi,
I'm using a BMM150 as auxiliary sensor of a BMI270.
The communication works correctly, but I get weird data for the Z axis.

Debugging the sensor API I noticed that the `compensate_z` function always returns `BMM150_OVERFLOW_OUTPUT` because `dev->trim_data.dig_z2` is always ZERO.

How is this possible?

Thank you
Walter

[Myzhar]

We finally found the problem using X-Ray: a simply short circuit under the sensor due to a wrong way of placing the soldering paste.

Now the sensors seems to work correctly, even if the Z axis is showing a very low working range, but I expect to solve this with hard and soft iron calibration.

[Myzhar]

Hi o_o,
the Z2 value to zero is a possible value, but in the "compensate_z" function it is checked and generates a `BMM150_OVERFLOW_OUTPUT` error.

This is the function I'm referring to:

/*!
 * @brief This internal API is used to obtain the compensated
 * magnetometer Z axis data(micro-tesla) in int16_t.
 */
int16_t compensate_z(int16_t mag_data_z, uint16_t data_rhall, const struct bmm150_dev *dev)
{
	int32_t retval;
	int16_t process_comp_z0;
	int32_t process_comp_z1;
	int32_t process_comp_z2;
	int32_t process_comp_z3;
	int16_t process_comp_z4;

	if (mag_data_z != BMM150_ZAXIS_HALL_OVERFLOW_ADCVAL) {
		if ((dev->trim_data.dig_z2 != 0) && (dev->trim_data.dig_z1 != 0)
		&& (data_rhall != 0) && (dev->trim_data.dig_xyz1 != 0)) {
			/*Processing compensation equations*/
			process_comp_z0 = ((int16_t)data_rhall) - ((int16_t) dev->trim_data.dig_xyz1);
			process_comp_z1 = (((int32_t)dev->trim_data.dig_z3) * ((int32_t)(process_comp_z0))) / 4;
			process_comp_z2 = (((int32_t)(mag_data_z - dev->trim_data.dig_z4)) * 32768);
			process_comp_z3 = ((int32_t)dev->trim_data.dig_z1) * (((int16_t)data_rhall) * 2);
			process_comp_z4 = (int16_t)((process_comp_z3 + (32768)) / 65536);
			retval = ((process_comp_z2 - process_comp_z1) / (dev->trim_data.dig_z2 + process_comp_z4));

			/* saturate result to +/- 2 micro-tesla */
			if (retval > BMM150_POSITIVE_SATURATION_Z) {
				retval =  BMM150_POSITIVE_SATURATION_Z;
			} else {
				if (retval < BMM150_NEGATIVE_SATURATION_Z)
					retval = BMM150_NEGATIVE_SATURATION_Z;
			}
			/* Conversion of LSB to micro-tesla*/
			retval = retval / 16;
		} else {
			retval = BMM150_OVERFLOW_OUTPUT;

		}
	} else {
		/* Overflow condition*/
		retval = BMM150_OVERFLOW_OUTPUT;
	}

	return (int16_t)retval;
}

[o_o]

---

@Myzhar wrote:

We finally found the problem using X-Ray: a simply short circuit under the sensor due to a wrong way of placing the soldering paste.

Now the sensors seems to work correctly, even if the Z axis is showing a very low working range, but I expect to solve this with hard and soft iron calibration.

---

Glad you found your issue. If the part has been short-circuited, I cannot guarantee that will work with full performance, and would advise to replace it to be on the safe side.

On the topic of compensation formula, I will double-check to confirm whether zero is a possible value, and therefore a bug in the API.

o_o