FXLS8471Q Datasheet, PDF (52/75 Pages) Freescale Semiconductor, Inc – Linear Accelerometer

10.8.3
A_FFMT_THS (0x17), A_FFMT_ THS_X_MSB (0x73), A_FFMT_THS_X_LSB (0x74),
A_FFMT_THS_Y_MSB (0x75), A_FFMT_THS_Y_LSB (0x76), A_FFMT_THS_Z_MSB
(0x77), A_FFMT_THS_Z_LSB (0x78) registers
Freefall/motion detection threshold registers.
Table 78. A_FFMT_THS (0x17) register
a_ffmt_dbcntm
0
ths[6:0]
7’b0000000
Table 79. A_FFMT_THS (0x17) bit descriptions
Field
a_ffmt_dbcntm
ths[6:0]
Description
The ASIC uses a_ffmt_dbcntm to set the acceleration FFMT debounce counter clear mode independent of the value of
the a_ffmt_ths_xyz_en.
a_ffmt_dbcntm bit configures the way in which the debounce counter is reset when the inertial event of interest is
momentarily not true.
When a_ffmt_dbcntm bit is a logic ‘1’, the debounce counter is cleared to 0 whenever the inertial event of interest is no
longer true (part b, Figure 17) while if the a_ffmt_dbcntm bit is set to logic ‘0’ the debounce counter is decremented by 1
whenever the inertial event of interest in longer true
(part c, Figure 17) until the debounce counter reaches 0 or the inertial event of interest become active.
The decrementing of the debounce counter acts to filter out irregular spurious events which might impede the correct
detection of inertial events.
Freefall/motion detection threshold: default value: 7’b0000000. Resolution is fixed at 63 mg/LSB.
Table 80. A_FFMT_THS_X_MSB (0x73) register
a_ffmt_ths_xyz_en
0
a_ffmt_ths_x[12:6]
7’b0000000
Table 81. A_FFMT_THS_X_MSB (0x73) bit descriptions
Field
a_ffmt_ths_xyz_en
a_ffmt_ths_x[12:6]
Description
For a_ffmt_ths_xyz_en = 0 the ASIC uses the ffmt_ths[6:0] value located in register x17[6:0] as a common threshold
for the X, Y, and Z-axis acceleration detection. The common unsigned 7-bit acceleration threshold has a fixed
resolution of 63 mg/LSB, with a range of 0-127 counts.
For a_ffmt_ths_xyz_en = 1 the ASIC ignores the common 7-bit G_FFMT_THS value located in register x17 when
executing the FFMT function, and the following independent threshold values are used for each axis:
A_FFMT_THS_X_MSB and A_FFMT_THS_X_LSB are used for the X-axis acceleration threshold,
A_FFMT_THS_Y_MSB and A_FFMT_THS_Y_LSB for the Y-axis acceleration threshold,
A_FFMT_THS_Z_MSB and A_FFMT_THS_Z_LSB for the Z-axis acceleration threshold.
The A_FFMT_THS_X/Y/Z thresholds are 13-bit unsigned values that have the same resolution as the accelerometer
output data determined by XYZ_DATA_CFG fs [1:0]. The a_ffmt_ths_xyz_en and a_ffmt_trans_ths_en bits must not
be enabled simultaneously.
7-bit MSB of X-axis acceleration threshold
Table 82. A_FFMT_THS_X_LSB (0x74) register
a_ffmt_ths_x[5:0]
6’b000000
Table 83. A_FFMT_THS_Y_MSB (0x75) register
a_ffmt_trans_ths_en
0
Table 84. A_FFMT_THS_Y_LSB (0x76) register
a_ffmt_ths_y[5:0]
6’b000000
a_ffmt_ths_y[12:6]
7’b0000000
—
—
0
0
—
—
0
0
FXLS8471Q
52
Sensors
Freescale Semiconductor, Inc.