AN4310 datasheet(2/8 Pages) STMICROELECTRONICS

2 / 8 page

Charge transfer acquisition principle overview

AN4310

2/8

DocID024789 Rev 2

Charge transfer acquisition principle overview

STM32xx, STM8L and STM8AL touch sensing applications may use charge transfer

acquisition principle(a) to sense changes in capacitance. The electrode capacitance (CX) is

charged to a stable reference voltage (VDD for general purpose STM8/STM32 devices). The

charge is then transferred to a known capacitor referred to as the sampling capacitor (CS).

This sequence is repeated until the voltage on the CS capacitor reaches an internal

reference voltage (VIH for general purpose STM8/STM32 devices). The number of transfers

required to reach the threshold depends on the size of the electrode capacitance and

represents its value.

To ensure stable operation of the solution, the number of transfers needed to reach the

threshold is adjusted by an infinite impulse response (IIR) filter which compensates for

environmental changes such as temperature, power supply, moisture, and surrounding

conductive objects.

Since the CS capacitor is an integral part of the design, it is important to consider the non-

ideal effects of capacitors.

a. Charge transfer acquisition principle is supported by STMTouch touch sensing libraries.

See Introduction for a list of supported microcontrollers and associated STMTouch

libraries.

AN4310 Datasheet(PDF) 2 Page - STMicroelectronics