MMA8652FC datasheet(46/77 Pages) FREESCALE

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MMA8652FC

Sensors

Freescale Semiconductor, Inc.

6.9.4

0x17: FF_MT_THS Freefall and Motion Threshold register

FF_MT_THS is the threshold register used to detect freefall motion events.

•

The unsigned 7-bit FF_MT_THS threshold register holds the threshold for the freefall detection where the magnitude of

the X and Y and Z acceleration values is lower or equal than the threshold value.

•

Conversely, the FF_MT_THS also holds the threshold for the motion detection where the magnitude of the X or Y or Z

acceleration value is higher than the threshold value.

The threshold resolution is 0.063g/LSB and the threshold register has a range of 0 to 127 counts. The maximum range is to 8g.

Note that even when the full scale value is set to 2g or 4g, the motion still detects up to 8g.

The DBCNTM bit configures the way in which the debounce counter is reset when the inertial event of interest is momentarily not

true.

•

When DBCNTM bit is 1, the debounce counter is cleared to 0 whenever the inertial event of interest is no longer true as

shown in Figure 40, (b).

•

While the DBCNTM bit is set to 0, the debounce counter is decremented by 1 whenever the inertial event of interest is no

longer true (Figure 40, (c)) until the debounce counter reaches 0 or until the inertial event of interest becomes active.

Decrementing the debounce counter acts as a median enabling the system to filter out irregular spurious events (which might

impede the detection of inertial events).

Bit 7

Bit 6

Bit 5

Bit 4

Bit 3

Bit 2

Bit 1

Bit 0

DBCNTM

THS6

THS5

THS4

THS3

THS2

THS1

THS0

Figure 38. 0x17 FF_MT_THS register (Read/Write)

Table 37. FF_MT_THS register

Bit(s)

Field

Description

7DBCNTM

Debounce counter mode selection

0 Increments or decrements debounce (default)

1 Increments or clears counter.

6–0

THS[6:0]

Freefall /Motion Threshold

000_0000 (default)

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