FXOS8700CQ datasheet(46/86 Pages) FREESCALE

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Freescale Semiconductor, Inc.

9.9

Freefall and Motion detection

The freefall/motion detection block can be configured to detect low-g (freefall) or high-g (motion) events utilizing the

A_FFMT_CFG[a_ffmt_oae] bit.

In low-g detect mode (A_FFMT_CFG[a_ffmt_oae] = 0) a low-g condition will need to occur on all enabled axes (ex. X, Y and Z)

for the A_FFMT_SRC[a_ffmt_ea] bit to be affected. And, in high-g detect mode (A_FFMT_CFG[a_ffmt_oae] = 1) a high-g

condition occurring in any of the enabled axes (ex. X, Y or Z) will suffice to affect the A_FFMT_SRC [a_ffmt_ea] bit.

The detection threshold(s) are programed in register 0x17 (A_FFMT_THS) for common threshold operation, and 0x73-0x78

(A_FFMT_THS_X/Y/Z) for individual axis threshold operation.

A_FFMT_CFG[a_ffmt_ele] bit determines the behavior of A_FFMT_SRC[a_ffmt_ea] bit in response to the desired acceleration

event (low-g/high-g). When A_FFMT_CFG[a_ffmt_ele] = 1, the freefall or motion event is latched and the

A_FFMT_SRC[a_ffmt_ea] flag can only be cleared by reading the A_FFMT_SRC register. When A_FFMT_CFG[a_ffmt_ele] = 0,

freefall or motion events are not latched

, and the A_FFMT_SRC[a_ffmt_ea] bit reflects the real-time status of the event detection.

A_FFMT_THS[a_ffmt_dbcntm] bit determines the debounce filtering behavior of the logic which sets the

A_FFMT_SRC[a_ffmt_ea] bit. See Figure 55 for details.

It is possible to enable/disable each axis used in the freefall/motion detection function by configuring bits

A_FFMT_CFG[a_ffmt_xefe], A_FFMT_CFG[a_ffmt_yefe], and A_FFMT_CFG[a_ffmt_zefe].

The freefall/motion detection function has the option to use a common 7-bit unsigned threshold for each of the X, Y, Z axes, or

individual unsigned 13-bit thresholds for each axis. When A_FFMT_THS_X_MSB[a_ffmt_ths_xyz_en] = 0, the 7-bit threshold

value stored in register 0x17 is used as a common 7-bit threshold for the X, Y, and Z axes. When a_ffmt_ths_xyz_en = 1, each

axis may be programmed with an individual 13-bit threshold (stored in the A_FFMT_X/Y/Z MSB and LSB registers).

9.9.1

A_FFMT_CFG (0x15) register

Freefall/motion configuration register.

Table 46. Trip angles versus hysteresis settings

Hysteresis register value

Landscape to Portrait trip angle

Portrait to Landscape trip angle

045

149

252

356

459

562

666

769

Table 47. Portrait/Landscape orientation definitions

Position

Description

y ~ -1g, x ~ 0

y ~ +1g, x ~ 0

y ~ 0, x ~ +1g

y ~ 0, x ~ -1g

a_ffmt_ele

a_ffmt_oae

a_ffmt_zefe

a_ffmt_yefe

a_ffmt_xefe

—

000000

Figure 44. A_FFMT_CFG register

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