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BMA255

Data sheet

Page 42

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.

4.7.11 No-motion / slow motion detection

The slow-motion/no-motion interrupt engine can be configured in two modes.

In slow-motion mode an interrupt is triggered when the measured slope of at least one enabled

axis exceeds the programmable slope threshold for a programmable number of samples.

Hence the engine behaves similar to the any-motion interrupt, but with a different set of

parameters. 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 (0x27) slo_no_mot_dur<1:0>. The number is N = (0x27) slo_no_mot_dur<1:0> + 1.

In no-motion mode an interrupt is generated if the slope on all selected axes remains smaller

than a programmable threshold for a programmable delay time. Figure 11 shows the timing

diagram for the no-motion interrupt. The scaling of the threshold value is identical to that of the

slow-motion interrupt. However, in no-motion mode register (0x27) slo_no_mot_dur defines the

delay time before the no-motion interrupt is triggered. Table 19 lists the delay times adjustable

with register (0x27) slo_no_mot_dur. The timer tick period is 1 second. Hence using short delay

times can result in considerable timing uncertainty.

If bit (0x18) slo_no_mot_sel is set to ‘1’ (‘0’) the no-motion/slow-motion interrupt engine is

configured in the no-motion (slow-motion) mode. Common to both modes, the engine monitors

the slopes of the axes that have been enabled with bits (0x18) slo_no_mot_en_x, (0x18)

slo_no_mot_en_y, and (0x18) slo_no_mot_en_z for the x-axis, y-axis and z-axis, respectively.

The measured slope values are continuously compared against the threshold value defined in

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

samples depends on the selected bandwidth and equates to 1/(2 * bw).

Table 19: No-motion time-out periods

(0x27)

slo_no_mot_dur

Delay

time

(0x27)

slo_no_mot_dur

Delay

time

(0x27)

slo_no_mot_dur

Delay

Time

1 s

40 s

88 s

2 s

48 s

96 s

3 s

56 s

104 s

...

64 s.

...

15 s

72 s

328 s

16 s

80 s

336 s

Note: slo_no_mot_dur values 22 to 31 are not specified

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