AN2869 datasheet(5/43 Pages) STMICROELECTRONICS

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AN2869

Capacitive sensing technology in ST

Doc ID 15298 Rev 6

5/43

Capacitive sensing technology in ST

STMicroelectronics offers different capacitive sensing technologies for STM8 and STM32

family products. These technologies are based on:

●

The RC acquisition principle for STM8A, STM8S and STM8L microcontrollers.

●

The charge transfer acquisition principle for STM8L, STM32 F0 and STM32 L1

microcontrollers.

●

The surface ProxSenseTM acquisition principle for STM8T14x microcontrollers.

●

The projected ProxSenseTM acquisition principle for STM8TL5x microcontrollers.

Note:

ProxSense™ is a trademark of Azoteq.

1.1

RC acquisition principle

The RC acquisition principle is based on the charging/discharging time measurement of an

electrode capacitance through a resistor. When the electrode is touched, the

charging/discharging time increases and the variation is used to detect the finger proximity.

The RC acquisition principle is detailed in AN2927.

1.2

Charge transfer acquisition principle

The charge transfer acquisition principle uses the electrical properties of the capacitor

charge (Q). The electrode capacitance is repeatedly charged and then discharged in a

sampling capacitor until the voltage on the sampling capacitor reaches a given threshold.

The number of transfers required to reach the threshold is a representation of the size of the

electrode’s 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.

1.3

Surface ProxSenseTM acquisition principle

The surface ProxSenseTM acquisition principle is similar to the charge transfer one, except

that the acquisition is fully managed by a dedicated hardware IP providing improved

performance. For more information, please refer to the application note AN2970.

1.4

Projected ProxSenseTM acquisition principle

The projected ProxSenseTM acquisition principle is a measurement of a charge transferred

by a driven electrode to another one. Like the charge transfer, there is also a sampling

capacitor which stores the charges coming from the electrodes which form a coupling

capacitor with less capacitance than the sample one. When a finger approaches, the

dielectric (between the two electrodes) is modified and so the capacitance decreases. As a

consequence, the time taken to load the sample capacitor will increase and this difference is

used to detect if a finger is present or not.

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