STM8T143 datasheet(11/65 Pages) STMICROELECTRONICS

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STM8T143

STM8T ProxSense technology

4.1

Capacitive sensing overview

A capacitance exists between any reference point and ground as long as they are

electrically isolated. If this reference point is a sensing electrode, it can help to think of it as

a capacitor. The positive electrode of the capacitor is the sensing electrode, and the

negative electrode is formed by the surrounding area (virtual ground reference in Figure 4).

Figure 4. Coupling with hand increases the capacitance of the sensing electrode

When a conductive object is brought into proximity of the sensing electrode, coupling

appears between them, and the capacitance of the sensing electrode relative to ground

increases. For example, a human hand raises the capacitance of the sensing electrode as it

approaches it. Touching the dielectric panel that protects the electrode increases its

capacitance significantly.

4.2

Charge-transfer acquisition principle

To measure changes in the electrode capacitance, STM8T devices employ bursts of

charge-transfer cycles.

The measuring circuitry is connected to the CX pin. It is composed of a serial resistor RX

plus the sensing electrode itself of equivalent capacitance CX (see Figure 5). The sensing

electrode can be made of any electrically conductive material, such as copper on PCBs, or

transparent conductive material like Indium Tin Oxide (ITO) deposited on glass or Plexiglas.

The dielectric panel usually provides a high degree of isolation to prevent electrostatic

discharge (ESD) from reaching the STM8T touch sensing controller. Connecting the serial

resistor (RX) to the CX pin improves ESD immunity even more.

Lower capacitance

Higher capacitance

Sensing electrode

STM8T143 Datasheet(PDF) 11 Page - STMicroelectronics