MMA8452Q Self Test

[stanfordhudson]

Hello,

We are using an MMA8452Q and are utilizing the Self Test feature during production test. 

The datasheet indicates that when self test is activated, the XYZ axes will be deflected by +45, +64, and +420 LSBs off of the current acceleration acting on the sensor respectively.

Our procedure is as follows:

  Set to ACTIVE mode

  Read X,Y,Z

  Set to ST mode

  Read X,Y,Z

  Compute absolute differences in XYZ for ACTIVE and ST mode.

(We are doing quite a bit of averaging along the way to account for any noise that may exist in the system.)

We routinely get values that are 380 and below for the Z axis.  The XY are usually pretty marginal, but sometimes are dead-on.

Each accel is different.

Can you please give me some insight into why the Z axis is not even close to the +420 specified in the datasheet?

(Note that we do set the accel range to +/-4G.  The Active bit in CTRL_REG1 is cleared before we set ST.  The accel is still during the test).

Thanks,

Stanford Hudson

[TomasVaverka]

Hello Stanford,

The reason why the Z axis self-test output does not correspond to the typical value specified in the datasheet is that the self-test is not factory trimmed. Generally, the test is considered successful if the minimum magnitude of change (50 counts for all axes at ±4g mode) is achieved.

Regards,

Tomas

PS: If my answer helps to solve your question, please mark it as "Correct" or “Helpful”. Thank you.

[danielwerder]

Hi Tomas,

I have a question about Self Test range.
About datasheet are XYZ will be deflected by +45, +64 and +420.

Are there no tolerance (min, max) values?

BR,
Daniel

[TomasVaverka]

Hi Daniel,

We do not specify the limits as the self-test output is not trimmed.

Regards,

Tomas

[stanfordhudson]

Thank you Tomas for the response.  I am assuming by magnitude you mean the magnitude of the XYZ vector?

(sqrt(x^2+y2+z^2)).

Would this then be the correct procedure:

1. Measure xyz in Active mode.

2. Compute magnitude (mag0=sqrt(x^2+y2+z^2))

3. Measure xyz in Self Test mode.

4. Compute magnitude (mag1=sqrt(x^2+y2+z^2)).

5. Test pass if abs(mag1-mag2)>=50.

?

Thanks, Stan

[TomasVaverka]

Hi Stan,

Sorry for not being clear enough, the procedure does not require calculating the XYZ vector, just simply compute the difference between the acceleration output of all axes with self-test enabled and disabled as follows:

X_ST = X_{ST_ON} − X_{ST_OFF}

Y_ST = Y_{ST_ON} − Y_{ST_OFF}

Z_ST = Z_{ST_ON} − Z_{ST_OFF}

and if these self-test changes are higher than 50 counts at ±4g mode, the test is considered to pass.

As you are doing correctly, it is recommended to take several samples (e.g. 10) for each axis and average them.

I hope it helps.

Regards,

Tomas

[chrislin_wpi]

Dear Tomas,

Do you mean each axis (Xst,Yst and Zst) higher than 50 counts at ±4g mode, it's pass?

_{And how about ±2g mode?}

_{Best Regards}

_{Chris Lin}

[TomasVaverka]

Dear Chris,

It is 100 counts at ±2g mode.

Best regards,

Tomas

[bobpaddock]

How well does this work out if the device is moving, in uncontrolled ways,  while self-test is being done?