STM8T141 datasheet(12/50 Pages) STMICROELECTRONICS

12 / 50 page

STM8T processing

STM8T141

12/50

Doc ID 15699 Rev 7

STM8T processing

The STM8T141 device is designed to ensure reliable operation whatever the environment

and operating conditions. To achieve this high level of robustness, dedicated processing

have been implemented:

●

Signal and reference calculation

●

Determining touch/proximity

●

Self-calibration

●

Environmental compensation filter

●

Debounce filter

5.1

Signal and reference calculation

Capacitive touch or proximity sensing is a technique based on detecting the electrode

capacitance change when someone is in proximity of the sensing electrode. The

capacitance change, induced by the presence of a finger or a hand in the device detection

area, is sensed by the variation in the number of charge transfer pulses composing the

burst. The charge transfer pulse number, also called “signal” is compared to a reference to

decide if there is a touch/proximity detection or not.

At power-up, a calibration sequence is performed to compute one reference value per

capacitive sensing channel. The reference is extracted from 32 burst measurements. Then,

the ECF takes care of its slow evolution over time.

To speed up the calibration process, the device is kept in normal mode whatever the low

power mode selected. The device operates in the selected low power mode when the

calibration process is completed.

5.2

Determining touch/proximity

The minimum difference between the reference and the signal necessary to report a

touch/proximity is the detection threshold (DTh). A time filtering, similar to the debouncing of

the mechanical switches, is applied to avoid noise induced detections.

Four different detection threshold settings are available and selectable by option byte. The

touch and sensitive touch levels are relative, which means the actual sensing distance is not

influenced by the Cs capacitor. The two thresholds should be able to adapt to various

surroundings and panel material or thickness. The proximity sensitivity thresholds are

absolute. This implies that the detection distance increases with the Cs capacitor. It provides

an easy way to tune the proximity sensing distance according to the application needs.

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