Introduction to QTouch® Peripheral Touch Controller (PTC)

The QTouch® Peripheral Touch Controller (PTC) offers built-in hardware for capacitive touch measurement on sensors that function as buttons, sliders, and wheels. The PTC has been designed to perform capacitive touch acquisition on sensors independently from the CPU, resulting in low CPU utilization and reduced power consumption.

Capacitive Measurement Techniques

The PTC supports two methods of capacitance measurement:

  • Self-capacitance
  • Mutual capacitance

Self-capacitance Method

The self-capacitance method involves charging a sense electrode (Y line) of unknown capacitance to a known potential. The resulting charge is transferred into a measurement circuit. By measuring the charge with one or more charge-and-transfer cycles, the capacitance of a sense plate can be determined.

A self-capacitance button is formed using one Y channel, whereas a slider or a wheel requires 3 Y channels.

SCMethod.png

To get a position on sliders or wheels, the relative signal strength of the sensors is measured. Below is an illustration of signal versus finger position on a slider.

SignalVsFinger.png

Mutual Capacitance Method

The mutual capacitance method uses a pair of sensing electrodes (X-Y lines). One electrode (X line) acts as an emitter into which a charge consisting of logic pulses is driven in burst mode. The other electrode (Y line) acts as a receiver that couples to an emitter using the overlying panel dielectric. When a finger touches the panel, field coupling is reduced, and touch is detected.

Mutual capacitance buttons use one X-Y channel whereas sliders and wheels can be configured to use 3 to 8 X-Y channels, depending on the sensor size.

MCMethod.png

While the mutual capacitance method does require two electrodes per sensor surface, it is possible to have many sensors connected to the same X line and Y line in a matrix of sensors. When the layout is done correctly, the mutual capacitance method also offers a better signal to noise ratio on sensors. This makes it possible to get longer tracks from the sensor to the device. In addition, the sensing field of a mutual capacitance sensor does not extend as far away from the sensor, which again can reduce noise compared to the self-capacitance method.

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