AN2927 Datasheet, PDF (1/12 Pages) STMicroelectronics – RC acquisition principle for touch sensing applications

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
I²C/SPI interface.
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
March 2009
Rev 2
1/12
www.st.com