March 2009

Rev 2

1/12

AN2927

Application note

RC acquisition principle for touch sensing applications

Introduction

In applications requiring user interface, capacitive touch-sensitive controls are becoming the

solution of choice to replace conventional electro-mechanical switches.

STMicroelectronics has developed a complete touch-sensing software library to transform

any 8-bit STM8 microcontroller into a capacitive touchkey controller. For more details,

please go to www.st.com/mcu

This touch sensing software library allows to detect human touch by controlling the

charge/discharge timing cycle of a RC network formed by a single resistor and the touch

electrode capacitance. Any variation in the RC timing due to the electrode capacity change

is detected then filtered and eventually reported to a host system using dedicated I/Os or

The bill of material is low-cost as only one resistor is needed per touch channel to enable

this function.

The scope of this application note is to describe the RC time constant acquisition principle

used in the touch sensing software library.

Abbreviations

Table 1.

List of terms

Acronym

Description

EMI

Electromagnetic interference

RC

Resistor–capacitor

TS Library

ST touch sensing firmware library

www.st.com