L9177A datasheet(38/55 Pages) STMICROELECTRONICS

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Figure 32. Input-output behaviour of VRS interface

0.1

0.15

VRS voltage [V]

threshold

Squared signal

hysteresis

0.05

0.0

-0.05

-0.1

0.8

0.6

0.4

0.2

0.53

0.535

DT1

DT2

DT2<Tf

Tf = Filtering time

DT2>Tf

0.54

0.545

0.55

0.555

0.56

0.53

0.535

0.54

0.545

0.55

0.555

0.56

GAPGPS00615

3.6.3

Auto-adaptative time filter

This characteristic is useful to set the best internal filter time as function of the input signal

frequency.

The Tfilter time depends on the previous period duration Tn according to the following formula:

Tfilter(n+1) = 1/32 * Tn if Tn > Tfilter(n)

The filtering time purpose is to filter very short spikes.

The digital filtering time is applied to internal squared signal (int_vrs), obtained by voltage comparators.

The output of time filtering block is out_vrs signal.

The filtering time Tfilter is applied to int_vrs signal in two different ways:

•

Rising edge: if int_vrs high level lasts less than Tfilter, out_vrs is not set to high level.

•

In absence of any spikes during input signal rising edge out_vrs signal is expected with a delay of Tfilter

time.

•

Falling edge: the falling edge of int_vrs is not delayed through time filtering block: after falling edge for a time

Tfilter any other transition on int_vrs signal is ignored

The initial value (Default) and maximum for Tfilter must be considered at RPM_min = 20 e.g. Tmax filter = 180 µs.

The minimum available value is Tmin filter = 2.8 µs.

L9177A

VRS interface

DS12175 - Rev 5

page 38/55

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