BMA255 datasheet(40/119 Pages) ETC2

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BMA255

Data sheet

Page 40

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.

4.7.9 Low-g interrupt

This interrupt is based on the comparison of acceleration data against a low-g threshold, which

is most useful for free-fall detection.

The interrupt is enabled (disabled) by writing ´1´ (´0´) to the (0x17) low_en bit. There are two

modes available, ‘single’ mode and ‘sum’ mode. In ‘single’ mode, the acceleration of each axis

is compared with the threshold; in ‘sum’ mode, the sum of absolute values of all accelerations

contents of the (0x24) low_mode bit: ´0´ means ‘single’ mode, ´1´ means ‘sum’ mode.

The low-g threshold is set through the (0x23) low_th register. 1 LSB of (0x23) low_th always

corresponds to an acceleration of 7.81 mg (i.e. increment is independent from g-range setting).

A hysteresis can be selected by setting the (0x24) low_hy bits. 1 LSB of (0x24) low_hy always

corresponds to an acceleration difference of 125 mg in any g-range (as well, increment is

independent from g-range setting).

The low-g interrupt is generated if the absolute values of the acceleration of all axes (´and´

relation, in case of single mode) or their sum (in case of sum mode) are lower than the

threshold for at least the time defined by the (0x22) low_dur register. The interrupt is reset if the

absolute value of the acceleration of at least one axis (´or´ relation, in case of single mode) or

the sum of absolute values (in case of sum mode) is higher than the threshold plus the

hysteresis for at least one data acquisition. In bit (0x09) low_int the interrupt status is stored.

The relation between the content of (0x22) low_dur and the actual delay of the interrupt

generation is: delay [ms] = [(0x22) low_dur + 1] • 2 ms. Therefore, possible delay times range

from 2 ms to 512 ms.

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