STMicroelectronics AN2927 RC Acquisition Principle Application Note

About this manual

This document explains the RC acquisition principle used to detect human touch in capacitive touch-sensitive controls. It details how the method measures small variations in electrode capacitance by charging and discharging a network formed by a resistor and the touch electrode. The note covers the underlying physics, the hardware implementation using a single resistor, and the firmware implementation within the ST touch sensing library.

What's inside

• RC acquisition principle
• Hardware implementation
• Firmware implementation
• Charge Time measurement principle
• Oversampling
• Input voltage measurement principle
• Touched effect
• Multi-acquisitions and HF noise rejection
• Revision history

Frequently asked questions

What is the typical capacitance variation caused by a human finger touching an electrode?

The capacitance variation is typically a few picofarads, specifically around 5pF.

How many successive measurements are required for STM8 microcontrollers to span all possible input voltage values?

Eight successive measurements are necessary to span all the possible values.

What is the value of the optional resistor R2 used to reduce noise sensitivity?

The optional resistor R2 has a value of 10kQ.