BMA423 datasheet(49/129 Pages) BOSCH

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Data Sheet

BMA423

Page 49

BST-BMA423-DS000-00 | Version 1.0 | August 2017

Bosch Sensortec

BOSCH and the symbol are registered trademarks of Robert Bosch GmbH, Germany.

Note: Specifications within this document are subject to change without notice.

4.9. Sensor Self-Test

The BMA423 has a comprehensive self test function for the MEMS element by applying electrostatic

forces to the sensor core instead of external accelerations. By actually deflecting the seismic mass,

the entire signal path of the sensor can be tested. Activating the self-test results in a static offset of

the acceleration data; any external acceleration or gravitational force applied to the sensor during

active self-test will be observed in the output as a superposition of both acceleration and self-test

signal.

Before the self-test is enabled the g-range should be set to 8g. The self-test is activated for all axes

by writing ACC_SELF_TEST.acc_self_test_en = 1b1. The self-test is disabled by writing

ACC_SELF_TEST.acc_self_test_en = 1b0. It is possible to control the direction of the deflection

through bit ACC_SELF_TEST.acc_self_test_sign. The excitation occurs in positive (negative) direction

if ACC_SELF_TEST.acc_self_test_sign= 1b1 (’b0). The amplitude of the deflection has to be set high

by writing ACC_SELF_TEST.acc_self_test_amp = 1b1. After the self-test is enabled, the user should

wait 50ms before interpreting the acceleration data.

In order to ensure a proper interpretation of the self-test signal it is recommended to perform the self-

test for both (positive and negative) directions and then to calculate the difference of the resulting

acceleration values. The table below shows the minimum differences for each axis in order for the self

test to pass. The actually measured signal differences can be significantly larger.

Self-test: Resulting minimum difference signal for BMA423.

x-axis signal

y-axis signal

z-axis signal

BMA423

400 mg

800 mg

400 mg

It is recommended to perform a reset of the device after a self-test has been performed. If the reset

cannot be performed, the following sequence must be kept to prevent unwanted interrupt generation:

disable interrupts, change parameters of interrupts, wait for at least 50ms, and enable desired

interrupts.

The recommended self test procedure is as follows:

1. Enable accelerometer with register PWR_CTRL.acc_en=1b1.

2. Set ±8g range in register ACC_RANGE.acc_range

3. Set self test amplitude to high by setting ACC_SELF_TEST.acc_self_test_amp = 1b1

4. Set ACC_CONF.acc_odr=1600Hz, Continuous sampling mode,

ACC_CONF.acc_bwp=norm_avg4, ACC_CONF.acc_perf_mode=1b1.

5. Wait for > 2 ms

6. Enable self-test and set positive self-test polarity (ACC_SELF_TEST.acc_self_test_sign=

1b1)

7. Wait for > 50ms

8. Read and store positive acceleration value of each axis from registers DATA_8 to DATA_13

9. Enable self-test and set negative self-test polarity ACC_SELF_TEST.acc_self_test_sign=

1b0)

10. Wait for > 50ms

11. Read and store negative acceleration value of each axis from registers DATA_8 to DATA_13

12. Calculate difference of positive and negative acceleration values and compare against

threshold values

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