Advantages of adopting capacitive touch sensors

• No need to consider deterioration due to wear
• High degree of design freedom
• Can be used not only as a switch, but also as a slider or wheel

Although it has many advantages as described above, it also has the following weaknesses, so it was not possible to adopt it for any application.

Weaknesses of capacitive touch sensing

• Susceptible to external noise and electrostatic discharge (ESD)
• It is difficult to use under severe environmental conditions because it malfunctions when water or oil adheres to it.
• power consumption increases

This time, we will introduce the technology incorporated in Texas Instruments (hereafter, TI) microcomputers that overcomes these weaknesses.

Compatible with two capacitive methods

CapTIvate technology supports both self-capacitance and mutual-capacitance methods.

Self-capacitance method with high sensitivity and easy introduction

When the human body (GND) such as a finger approaches the electric field of a single sensor electrode, a pseudo capacitor is formed and the capacitance of the sensor electrode itself increases. The self-capacitance method detects this increase in capacitance and determines whether it has been touched. The self-capacitance method is said to be easy to introduce into a touch panel because of its simple electrode structure and high sensitivity. However, it is known that it is difficult to distinguish between the adhesion of water droplets and the increase in capacitance due to finger touch.

Mutual capacitance method that can distinguish between water droplet adhesion and finger touch

In the mutual capacitance method, touch is detected by the state change of the electromagnetic field between two electrodes.
When water droplets adhere to the surface of the sensor, the inter-electrode capacitance increases, but when a finger approaches, part of the electromagnetic field is blocked by the finger and the inter-electrode capacitance decreases. With such a mechanism, it is possible to distinguish between the adhesion of water droplets and the touch of a finger.

Why can't I detect touches when wearing gloves?

In general, touch detection using capacitance detects a touch based on changes in the capacitance of the sensor electrode, so it is known that wearing gloves reduces the conductivity and makes it difficult to detect a touch. It is

How to detect touch even when wearing gloves?

In order to respond even with gloves, it is realized by detecting the change in capacitance between the two plates. In a self-capacitance capacitive touch system, the sensor pad forms one side of a parallel plate capacitor and the user's finger is the other side, but the finger is replaced by a conductive overlay material and float it above the capacitive touch sensor to form the other plate of the capacitor to be measured.

The mechanism is also capable of penetrating not only gloves, but also through thick glass/plastic/metal protective layers and working in humid/dirty/greasy conditions.

For more information, please refer to page 5 "1.2 Technologies for touch through metal applications" in the link below.
Capacitive touch through metal with MSP430™ MCUs with CapTIvate™ technology

CPU can be stopped until just before the conductor touches the touch sensor

A microcomputer that uses CapTIvate technology is equipped with a dedicated hardware IP (CAPTIVATE-IP) that specializes in touch detection, including basic noise filtering, so that a conductor (hand or finger) approaches the touch sensor. Ultra-low power consumption is achieved by a mechanism (wake-on-touch) that keeps the CPU stopped until just before the touch is detected and operates the CPU only during the period when processing is required after touch detection (wake-on-touch).

For details, please see the illustration here.
Ultra low power consumption with CapTIvate wake-on-touch

What you can do with the CapTIvate Design Center GUI

You can drag and drop capacitive buttons, sliders, wheels, and proximity sensors into your workspace and use the GUI to generate code, configure your design, and tune in real time. Please refer to the following CapTIvate installation instructions for how to use it.

　

This installation guide also includes the following: You can read it with a simple registration, so if you are interested, please download it and use it.

・About evaluation tools

・About the development kit

・Introduction of development environment

・How to install

・Demonstration (how to use GUI, adjust touch sensitivity, etc.)