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BMA255

Data sheet

Page 29

BST-BMA255-DS004-05 | Revision 1.2 | August 2014

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.

slope_th

INT

slope

acceleration

acc(t0)

acc(t0−1/(2*bw))

slope(t0)=acc(t0)−acc(t0−1/(2*bw))

time

slope_dur

4.7.5 Slope / any-motion detection

Slope / any-motion detection uses the slope between successive acceleration signals to detect

changes in motion. An interrupt is generated when the slope (absolute value of acceleration

difference) exceeds a preset threshold. It is cleared as soon as the slope falls below the

threshold. The principle is made clear in figure 7.

Figure 7: Principle of any-motion detection

The threshold is defined through register (0x28) slope_th. In terms of scaling 1 LSB of (0x28)

slope_th corresponds to 3.91 mg in 2g-range (7.81 mg in 4g-range, 15.6 mg in 8g-range and

31.3 mg in 16g-range). Therefore the maximum value is 996 mg in 2g-range (1.99g in 4g-

range, 3.98g in 8g-range and 7.97g in 16g-range).

The time difference between the successive acceleration signals depends on the selected

bandwidth and equates to 1/(2*bandwidth) (

t=1/(2*bw)). In order to suppress false triggers, the

interrupt is only generated (cleared) if a certain number N of consecutive slope data points is

larger (smaller) than the slope threshold given by (0x28) slope_th. This number is set by the

(0x27) slope_dur bits. It is N = (0x27) slope_dur + 1 for (0x27).

Example: (0x27) slope_dur = 00b, …, 11b = 1decimal, …, 4decimal.

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