MMA8652 Z axis MSB is showing as being maxed out

[jonathanbirch]

Hi,

Our design includes a MMA8652 for which the Z axis MSB is showing as being maxed out (-128) even though I can read the other two axis fine. Any suggestions as to what this may be? We developed the code using the sensors development board and the Z-axis can be read fine on that. The only hardware difference I can see is the marking between the chips the one that works has the marking 6522XZ the two that don't are 6524XY. Thanks in advance.

Jonathan

[TomasVaverka · Accepted solution]

Hi Jonathan,

It sounds to me more like a quality problem known as Z-axis stiction failure. Devices with this failure usually have a positive (0x7FF0) or negative (0x8000) railed output.

In fact, sticking can arise during manufacturing or in the use of the device. As you can see in the PCN15581, there have been made some design improvements to eliminate this failure. In‐use sticking may occur after a high acceleration event causes its movable elements to contact its fixed surfaces. Another mechanical shock usually bring them back to work properly.

Customers that experience an issue similar to this can contact our quality team and customer service team. The NXP Quality team will enter the information into the NXP CQC system to route to the correct quality team and business line contacts for support.

Regards,

Tomas

[jonathanbirch]

Thanks Tomas,

I gave it a small intense wack with the end of a pen which freed up the Z-axis. is this likely to re-occur or should it be ok assuming no extreme acceleration is incurred?

Best regards

Jonathan

[david_diaz]

Hello Jonathan,

Despite the accelerometers are “trimmed” for offset and sensitivity in the factory by adjusting the offset trim codes and gain, some conditions can cause further offset or shifts.

For example, when the accelerometers is soldered onto a PCB upon which various mechanical strains are produced:

• On the package from mounting the part to the PCB
• From the PCB mount holes or screws
• From other components placed close to the sensor.

Besides, sources of shifts can also occur due to temperature changes and aging.

That is the reason why I recommend to recalibrate your device.

Please refer to the Application Note AN4069 in order to get more information about the Offset Calibration.

Besides, you can find a MMA8652FC - Bare metal example project  that includes the calibration process in case it is useful for you.

I hope this information will be useful for you.

If I misunderstood your question, feel free to let me know.  I will be glad to help.

Have a great day.

David Diaz.

Note: If this post answers your question, please click the Correct Answer button. Thank you!

[jonathanbirch]

Thanks for the info David. I will refer to this to calibrate to zero. (the actual issue turned out to be a sticky axis)

Regards

Jonathan