Data Sheet
Sensors
28 Freescale Semiconductor
MMA8452Q
This register keeps track of the acceleration event which is triggering (or has triggered, in case of ELE bit in FF_MT_CFG
register being set to 1) the event flag. In particular EA is set to a logic ‘1’ when the logical combination of acceleration events
flags specified in FF_MT_CFG register is true. This bit is used in combination with the values in INT_EN_FF_MT and
INT_CFG_FF_MT register bits to generate the freefall/motion interrupts.
An X,Y, or Z motion is true when the acceleration value of the X or Y or Z channel is higher than the preset threshold value
defined in the FF_MT_THS register.
Conversely an X, Y, and Z low event is true when the acceleration value of the X and Y and Z channel is lower than or equal
to the preset threshold value defined in the FF_MT_THS register.
0x17: FF_MT_THS Freefall and Motion Threshold Register
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 detects up to 8g. If the Low Noise bit is set in Register
0x2A then the maximum threshold will be limited to 4g regardless of the full scale range.
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 a logic ‘1’, the debounce counter is cleared to 0 whenever the inertial event of interest is no longer true
as shown in Figure 14, (b). While the DBCNTM bit is set to logic ‘0’ the debounce counter is decremented by 1 whenever the
inertial event of interest is no longer true (Figure 14, (c)) until the debounce counter reaches 0 or 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.
Table 27. Freefall/Motion Source Description
EA
Event Active Flag. Default value: 0.
0: No event flag has been asserted; 1: one or more event flag has been asserted.
See the description of the OAE bit to determine the effect of the 3-axis event flags on the EA bit.
ZHE
Z Motion Flag. Default value: 0.
0: No Z Motion event detected, 1: Z Motion has been detected
This bit reads always zero if the ZEFE control bit is set to zero
ZHP
Z Motion Polarity Flag. Default value: 0.
0: Z event was Positive g, 1: Z event was Negative g
This bit read always zero if the ZEFE control bit is set to zero
YHE
Y Motion Flag. Default value: 0.
0: No Y Motion event detected, 1: Y Motion has been detected
This bit read always zero if the YEFE control bit is set to zero
YHP
Y Motion Polarity Flag. Default value: 0
0: Y event detected was Positive g, 1: Y event was Negative g
This bit reads always zero if the YEFE control bit is set to zero
XHE
X Motion Flag. Default value: 0
0: No X Motion event detected, 1: X Motion has been detected
This bit reads always zero if the XEFE control bit is set to zero
XHP
X Motion Polarity Flag. Default value: 0
0: X event was Positive g, 1: X event was Negative g
This bit reads always zero if the XEFE control bit is set to zero
0x17 FF_MT_THS Register (Read/Write)
Bit
7
Bit
6
Bit
5
Bit
4
Bit
3
Bit
2
Bit
1
Bit
0
DBCNTM THS6 THS5 THS4 THS3 THS2 THS1 THS0
Table 28. FF_MT_THS Description
DBCNTM
Debounce counter mode selection. Default value: 0.
0: increments or decrements debounce, 1: increments or clears counter.
THS[6:0] Freefall /Motion Threshold: Default value: 000_0000.