STM8T141 datasheet(13/50 Pages) STMICROELECTRONICS

13 / 50 page

STM8T141

STM8T processing

Doc ID 15699 Rev 7

13/50

5.3

Environment compensation filter (ECF)

5.3.1

ECF principle

The capacitive sensing channel reference value increases or decreases according to

environmental conditions such as temperature, power supply, moisture, and surrounding

conductive objects. The STM8T141 includes a built-in digital infinite impulse response (IIR)

filter capable of tracking slow changes in the environment called the Environment

Compensation Filter (ECF). This is a first order digital low pass filter with a gain of one. The

filter makes the reference follow slow changes of the signal while fast changes are

recognized as a touch or proximity.

When a touch or proximity condition is detected, the corresponding capacitive sensing

channel reference is frozen.

Figure 7.

Environmental compensation filter (ECF) example 1

Table 3.

Explanation of ECF example 1

Zone 1

Zone 2

Zone 3

Zone 4

Event description

The object (finger) is

outside the

electrode field

range.

Electrode

environment is

stable

The object, is inside the

electrode field range. It

induces a signal change

but, not large enough to

cross the detection

threshold (Dth).

The reference adapts

slowly to the object

proximity.

The object comes inside the

detection range before the

reference compensates for

its presence.

A touch or proximity event is

triggered because the

signal level falls below the

reference - DTh.

The object exits

from the

electrode’s

detection range.

Detection state

No detection

Detection

No detection

ECF operation

Active

Halt

Active

Reference

Adapting

Frozen

Adapting

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