We having some issues using the self test of the mma8652. During our production we we have three consecutive steps, where we perform the self test of the mma8652 and in all three steps the self test sometimes fails.
Self test is performed the following way:
- Read XYZ in 8-bit mode without ST bit set
- Read XYZ in 8-bit mode with ST bit set
- if deltaZ = abs(z2 - z1) > 24 ==> SUCCESS, otherwise FAILED
according to datasheet on the Z axis a delta of 48 LSB (782 LSB for 12bit mode) should be detected. For many devices this test succeeded.
But we also have some devices, that fail in one of the 3 production steps. Even if they passed the first production step, they fail in the second one. Or sometimes they pass the 1st and 2nd step and fail in the final 3rd step.
I just checked 3 devices, that failed in the first step and they show a change in Z Axis of ~9 LSB, the other axis show almost no change. Although, all three axes show correct absolute values.
Could you give me any feedback on the self-test?
Am I doing something wrong?
Will these devices still work as intended?
Do we have to adopt the limit for the deltaZ?