AN3393 datasheet(45/103 Pages) STMICROELECTRONICS

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AN3393

State machine

103

The four coefficients can be chosen by using registers 1Bh, 1Ch, 1Dh and 1Eh (vector filter

coefficient registers). In this way, different filter configurations can be implemented. For

example, a band-pass filter can be obtained by choosing the coefficients: 53, 127, 127, 53.

To enable the vector filter, the VFILT bit in the CTRL_REG3 register (23h) must be set to “1”.

5.4

State machine blocks

Output data coming from the signal block are sent to the state machine block composed of

the two state machines. There are some differences in terms of functionality between the

two state machines:

State Machine 2 has decimator functionality, according to DES2 factor (DES2 register,

78h).

State Machine 2 has DIFF functionality/filter. The DIFF filter can be configured in two

different ways:

DIFF filtering with previous data values (X, Y, Z) as diff

DIFF filtering with constant shift register values (X, Y, Z) as cs

When DIFF functionality is selected in State Machine 2, the vector value calculated (V)

is left intact (DIFF is not applied in vector data).

Figure 7. State machine structure

5.4.1

Decimator

The decimation function is a method to reduce the sample rate of the data going to State

Machine 2.

The decimation function is based on the initial value of the DES2 register (78h) and DCC

(decimation counter register) according to the selected ODRx factor.

ODR_SM2 = ODR / (DES + 1)

At startup:

DCC = DES2 (initial decimation value)

when sample clock occurs:

AM14773V1

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