STMPE1208S datasheet(17/51 Pages) STMICROELECTRONICS

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STMPE1208S

Capacitive sensors

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Figure 11.

3-stage data filtering system

Integration and filtering unit

TOUCH is sampled across a programmable period of time. The output of the integration

stage would be a "STRENGTH" (in STRENGTH register) that indicates the number of times

a "TOUCH" is seen, across the integration period. The "STRENGTH" is then compared with

the value in "STRENGTH THRESHOLD" register. If STRENGTH exceeds the STRENGTH

THRESHOLD, this is considered a valid TOUCH.

If required, a 2nd stage filtering feature controlled by FILTER_ PERIOD and

FILTER_THRESHOLD registers.

In data filtering stage, 3 modes of operation are supported:

AFS Mode 1: Only the TOUCH channel with highest STRENGTH is taken

AFS Mode 2: All TOUCH channels with STRENGTH > STRENGTH THRESHOLD is taken

AFS Mode 3: The 2 TOUCH channels with the highest STRENGTH are selected.

These modes are selected using the FEATURE SELECTOR register.

The final, filtered data is accessible through the TOUCH_BYTE register.

5.3.2

Noise filtering

When the STMPE1208S is operating in the vicinity of highly emissive circuits (DC-DC

Converter, PWM Controller/Drive etc), the sensor inputs will be affected by high-frequency

noise. In this situation, the 2-stage time-integrating function could be used to distinguish

between real touch, or emission-related false touch.

5.3.3

BEEP output

STMPE1208S is able to drive an external Piezo Buzzer directly with the built-in beep

generator. The BEEP output can be programmed to varies from 1.5 KHz to 400 KHz, with

period of 100 uS to 2.5 S.

Integration

unit

Integration period

Integration threshold

Filter period

Filter threshold

AFS mode

Touch byte

Filter

unit

AFS

unit

Touch/No-touch from

calibration unit

CS00044

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