BMA020 datasheet(14/50 Pages) BOSCH

14 / 50 page

Data sheet

BMA020

Digital, triaxial acceleration sensor

Bosch Sensortec

Rev. 1.2

Page 14

30 May 2008

as copying and passing on to 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.

3.2.6 Latch_INT

If this bit (address 15h, bit 4) is at 1, interrupts are latched. The INT pad stays high until

microprocessor detects it and writes reset_INT control bit to 1 (section 3.3.1). When this bit is at

0, interrupts are set and reset directly by BMA020 according to programmable criteria (sections

3.2.7 and 3.2.8).

3.2.7 LG_thres, LG_hyst, LG_dur, counter_LG

LG_thres (address 0C, bits 7-0 / low-g threshold) and LG_hyst (address 11h, bits 2-0 / low-g

threshold hysteresis) can basically (although not recommended with BMA020) be used to detect

a free fall. The threshold and duration codes define one criterion for interrupt generation when

absolute value of acceleration is low for long enough duration.

Data format is unsigned integer.

LG_thres criterion_x is true if

|acc_x | ≤ LG_thres / 255 * range

LG_thres interrupt is set if

(LG_thres criterion_x AND LG_thres criterion_y AND

LG_thres criterion_z) AND interrupt counter = (LG_dur+1)

LG_thres criterion_x is false if

|acc_x | > (LG_thres + 32*LG_hyst) / 255 * range

LG_thres interrupt is reset if

NOT(LG_thres criterion_x AND LG_thres criterion_y AND

LG_thres criterion_z)

LG_thres and LG_hyst codes must be chosen to have (LG_thres + 32*LG_hyst) < 511.

When LG_thres criterion becomes active, an interrupt counter is incremented by 1 LSB/ms.

When the low-g interrupt counter value equals (LG_dur+1), an interrupt is generated.

Depending on counter_LG (address 0Bh, bit 3 and 2) register, the counter could also be reset

or count down when LG_thres criterion is false.

Table 6: Description of debouncing counter counter_LG

counter_LG<1:0>

low acceleration interrupt counter status when

LG_thres criteria is false

reset

Count down by 1 LSB/ms

Count down by 2 LSB/ms

Count down by 3 LSB/ms

If latch_INT=0, the interrupt is not a latched interrupt and then it is reset as soon as LG_thres

criteria becomes false. When interrupt occurs, the interrupt counter is reset.

Remark: The LG_thres criteria is set with an AND condition on all three axes to be used for free

fall detection. However, please note due to the relatively wide sensitivity tolerance of the

BMA020 the absolute threshold values for low-g and high-g interrupt can only provide a rough

estimation of the motion profile.

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