BMA255 datasheet(20/119 Pages) ETC2

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BMA255

Data sheet

Page 20

BST-BMA255-DS004-04 | Revision 1.1 | December 2013

Bosch Sensortec

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

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

4.4 Self-test

This feature permits to check the sensor functionality 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 4 g. The self-test is activated

individually for each axis by writing the proper value to the (0x32) self_test_axis bits (´01b´ for x-

axis, ´10b´ for y-axis, ´11b´ for z-axis, ´00b´ to deactivate self-test). It is possible to control the

direction of the deflection through bit (0x32) self_test_sign. The excitation occurs in negative

(positive) direction if (0x32) self_test_sign = ´0b´ (´1b´). 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. Table 6 shows the minimum differences for each axis. The

actually measured signal differences can be significantly larger.

Table 6: Self-test difference values

x-axis signal

y-axis signal

z-axis signal

resulting minimum

difference signal

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, enable

desired interrupts.

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