AN2381 datasheet(24/47 Pages) STMICROELECTRONICS

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Registers Description

AN2381

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mode) the data used to generate DD interrupt come directly from digital block or

temperature compensation block while if HPDD is set to ‘1’ the interrupt signals are based

on High-Pass filtered data. Complete data flow is represented in Figure 11.

HPFF bit allows to select between filtered or not filtered data to be processed by the Signal

Processing (WakeUp or FreeFall) block as described in Figure 11. If HPFF is set ‘0’ (default

mode) data used to generate WU or FF interrupt come directly from digital block or

temperature compensation block while if HPFF is set ‘1’ the interrupt signals are based on

High-Pass filtered data. Complete data flow is represented in Figure 11.

FDS bit decides whether data stored in output registers are High Pass filtered or not. In

default mode (FDS bit set to ‘0’) signal are not filtered while it is possible to access data

coming from HP filter setting FDS bit to ‘1’. Complete data flow is represented in Figure 11.

CFS<1:0> bit select High-pass filter Cut-off Frequency coefficient. Increasing this number

makes Cut-off Frequency (@-3dB) move to lower values.

The Cut-off Frequency behavior is described by the following equation:

where HPc = 512, 1024, 2048, 4096 when CFS<1:0>= 00, 01, 10, 11 respectively and ODR

represents the Output Data Rate selectable through the DF1, DF0 bits in CTRL_REG1.

The figure below gives a representation of the possible transfer functions obtained

modifying Output Data Rate and High Pass Filter coefficient (HPc).

Figure 10.

HP Filter Transfer Function

Cut

off

–

0.318

HPc

---------------

⎝⎠

⎛⎞ ODR

--------------

⎝⎠

⎛⎞

−2

−1

−4

−3.5

−3

−2.5

−2

−1.5

−1

−0.5

↓ −3dB

Frequence (Hz)

HP Filter frequency responce

ODR = 40Hz; HP coeff.= 512

ODR = 40Hz; HP coeff.= 1024

ODR = 40Hz; HP coeff.= 2048

ODR = 40Hz; HP coeff.= 4096

ODR = 160Hz; HP coeff.= 512

ODR = 160Hz; HP coeff.= 1024

ODR = 160Hz; HP coeff.= 2048

ODR = 160Hz; HP coeff.= 4096

ODR = 640Hz; HP coeff.= 512

ODR = 640Hz; HP coeff.= 1024

ODR = 640Hz; HP coeff.= 2048

ODR = 640Hz; HP coeff.= 4096

ODR = 2560Hz; HP coeff.= 512

ODR = 2560Hz; HP coeff.= 1024

ODR = 2560Hz; HP coeff.= 2048

ODR = 2560Hz; HP coeff.= 4096

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