Introduction to capacitive touch screen system and related solutions

Touch screens are widely used in various fields of our daily life, such as mobile phones, media players, navigation systems, digital cameras, digital cameras, digital photo frames, PDAs, game devices, displays, electrical appliance controls, medical equipment and so on.

Common touch screen technologies include resistive and projected capacitive touch screens for mobile devices and consumer electronics, and surface capacitive, surface acoustic wave (SAW) and infrared touch screens for other applications.

Resistive touch screen

The most widely used resistive touch screen (Figure 1) has two layers of ITO (Indium Tin Oxide) with an air gap and a spacer layer. Resistive touch screens are a proven, low-cost technology for high-volume applications. Their disadvantages are: weak mechanical properties; thick stacking and relatively complex; inability to detect multiple finger movements; fragile front panel implementation; limited industrial design options; poor optical performance; and user calibration required.

Figure 1 Resistive touch screen

Projected capacitive touch screen

Capacitive touch control for touch screens uses a surface coated with a conductive material (such as ITO) to store charge. The conductive material conducts current along the X and Y axes of the screen. When a conductive material (such as a finger) touches, the control electric field changes, and the position of the touch along the horizontal and vertical axes can be determined. In applications with button touch location, discrete sensors are placed under specific button locations, and the system records the touch and position when the sensor's electric field is disturbed. A projected capacitive touch screen is shown in Figure 2.

Figure 2 Projected capacitive touch screen

The advantages of projected capacitive touch screens over other touch screen technologies are:

Excellent signal-to-noise ratio;

The entire touch screen surface has high precision;

Ability to support multiple touches;

Sensing through “thick” dielectric materials;

No user calibration required.

QTouch Technology

QTouch technology is patented by Quantum, the predecessor of Atmel's touch technology department. The integrated circuit technology developed is based on charge-transfer capacitive sensing. The QTouch IC detects touch using a single connection between the sensor chip and a simple button electrode (Figure 3). The QTouch device charges the sensing electrode of unknown capacitance to a known potential. The electrode is usually a copper area on a printed circuit board. By measuring the charge after one or more charge-transfer cycles, the capacitance of the sensing plate can be determined. Pressing a finger on the touch surface causes external capacitance to affect the charge flow at that point. This is recorded as a touch. The QTouch microcontroller can also be determined to detect the proximity of the finger, rather than the absolute touch. Signal processing in the decision logic makes QTouch robust and reliable. False triggers caused by electrostatic pulses or momentary unintentional touch or proximity can be eliminated.

Figure 3 Qtouch technology diagram

QTouch sensors can drive single or multiple buttons. When using multiple buttons, a separate sensitivity level can be set for each button. Buttons of different sizes and shapes can be used to meet functional and aesthetic requirements.

QTouch technology can be used in two modes: normal or "touch" mode and high sensitivity or "proximity" mode. Highly sensitive charge transfer proximity sensing is used to detect the end user's approaching finger, and the user interface interrupts the electronic device or electrical device to initiate system functions.

For excellent electromagnetic compatibility, QTouch sensors use spread spectrum modulation and sparse, random charging pulses (with long delays between pulses). A single pulse can be 5% or less shorter than the internal string pulse interval. The advantages of this approach are lower cross-sensor interference, reduced RF radiation and polarization rate, and low power consumption.

QTouch devices have automatic drift compensation for slow changes (due to aging or changing environmental conditions). These devices have a dynamic range of tens of degrees, and they do not require coils, oscillators, RF components, specialized cables, RC networks or a large number of discrete components. QTouch is an engineering solution that is simple, durable and elegant.

When several touch keys are close to each other, the approaching fingers will cause the capacitance of multiple keys to change. Atmel's patented adjacent key suppression (AKS) uses an iterative technique to repeatedly measure the capacitance change on each key, compare the results and determine which key the user wants. AKS suppresses or ignores the signals from all other keys and provides the signal of the selected key. This prevents false touch detection on adjacent keys.

Touch screen system design

A touch screen system includes: front panel, sensor film, display unit, controller board and integration support (Figure 4).

Figure 4 Touch screen system

Atmel provides touch controller ICs, control board reference designs, sensor reference designs, integration support and sensor test equipment designs, while partners provide sensor sheets, integration of sensors and front panels and controller boards.

Atmel offers devices for 1 to 10 buttons and/or sliders/wheels - the QTouch series; devices for up to 48 buttons and/or sliders/wheels - the QMatrix series; and devices for Single Touch and Two Touch touch screens - the QFild and QTwo series. A simple system using the QT5320/5480 device is shown in Figure 5, where the touch screen, slider, wheel and/or button can be selected by the host through an I2C compatible interface.

Figure 5 Simple touch screen system