AN4312 datasheet(6/36 Pages) STMICROELECTRONICS

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Capacitive sensing technology in ST

AN4312

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Capacitive sensing technology in ST

STMicroelectronics offers different capacitive sensing technologies for STM8 and STM32

families of products. The technology covered by this application note is based on the charge

transfer acquisition principle and is supported by all the microcontroller lines listed in

Table 1.

1.1

Charge transfer acquisition principle

The surface charge transfer acquisition is a proven, robust and efficient way to measure a

capacitance. It uses a minimum number of external components to operate with a single

ended electrode type. This acquisition is designed around an analog I/O group which is

composed of four GPIOs. Several analog I/O groups are available to allow the acquisition of

several capacitive sensing channels simultaneously and to support a larger number of

capacitive sensing channels. Within a same analog I/O group, the acquisition of the

capacitive sensing channels is sequential.

One of the GPIOs is dedicated to the sampling capacitor (CS). Only one sampling capacitor

I/O per analog I/O group must be enabled at a time.

The remaining GPIOs are dedicated to the electrodes and are commonly called channels.

For some specific needs (such as proximity detection), it is possible to simultaneously

enable more than one channel per analog I/O group.

The surface charge transfer acquisition principle consists of charging an electrode

capacitance (CX) and transferring a part of the accumulated charge into a sampling

capacitor (CS). This sequence is repeated until the voltage across CS reaches a given

threshold (VIH in our case). The number of charge transfers required to reach the threshold

is a direct representation of the size of the electrode capacitance. When the electrode is

“touched”, the charge stored on the electrode is higher and the number of cycles needed to

charge the sampling capacitor decreases.

1.2

Surface capacitance

A capacitance is modified when a finger gets close to a sensing electrode.

The return path goes either through:

•

a capacitor through the user’s feet

•

a capacitor between the user’s hand and the device

•

a capacitor between the user’s body and the application board through the air (like an

antenna)

AN4312 Datasheet(PDF) 6 Page - STMicroelectronics