Introduction to the basic principles of touch screens

Introduction to Touchscreens

Characteristics of Touchscreens

(1) Touchscreens are closely related to people, especially capacitive touchscreens.

(2) Touchscreens are closely related to displays.

(3) Typical applications: mobile phones, tablets, cash registers, industrial applications.

Classification of Touchscreens

(1) Common touchscreens are divided into two types: resistive touchscreens and capacitive touchscreens. Resistive touchscreens were used in the early days, and capacitive touchscreens were later invented.

(2) These two types have different characteristics, interfaces, programming methods, and principles.

Relationship and Differences between Touchscreens and Displays

(1) First, it’s important to understand: a touchscreen is a touchscreen used to respond to human touch events; a display is a display used to show images. Currently, most displays are LCDs.

(2) The reason many people confuse these two concepts is mainly because touchscreens and displays are usually integrated in products. Generally, the outer layer is a thin, transparent touchscreen; underneath is the display used to show images. The images we see are displayed on the display and seen through the touchscreen.

Introduction to Input Devices

I/O (Input/Output) is a concept in computer systems

The main function of a computer is to acquire data from the outside world, process it, and output the resulting data back to the outside world (a computer can be viewed as a data processor). Computers interact with the outside world through I/O. Every computer has a standard input and a standard output.

Common Input Devices

Keyboard, mouse, touchscreen, game joystick, sensor (a camera is not a typical input device).

Principle of Capacitive Touchscreens

Human Body Current Sensing

Utilizing the phenomenon of human body current sensing, a capacitor is formed between the finger and the screen. When the finger touches the screen, it draws a tiny current, which causes current to flow across the four electrodes on the touchpad. The controller calculates the ratio of these four currents to determine the coordinates of the touch point (this calculation involves AD conversion).

Dedicated Circuit for Coordinate Calculation (Hardware Interface)

(1) The principle of a resistive touchscreen sensor is very simple, and coordinate calculation is also straightforward. Therefore, it can be directly connected to the touchpad sensor via the SoC’s resistive touchscreen controller. The SoC’s internal controller can handle the coordinate calculation and AD conversion without much burden. However, capacitive touchscreens are different. They require a built-in IC for coordinate calculation, thus eliminating the need for the host SoC controller. Therefore, this interface for capacitive touchscreens is essentially the second type of hardware interface for resistive touchscreens, and currently, capacitive touchscreens can only be implemented with this second interface.

(2) Why this design? The main reason is that coordinate calculation for capacitive touchscreen sensors is too complex, and ordinary programmers cannot write suitable code to solve this problem. Therefore, in addition to the touchpad, a dedicated IC is added to the capacitive touchscreen for coordinate point calculation and statistics. This IC is fully responsible for controlling the touchpad to obtain touch operation information, and then communicating with the host SoC through a digital interface (usually I2C).

Multi-touch Support in Multiple Blocks

(1) Resistive touchscreens do not support multi-touch; this is limited by their inherent principle and cannot be changed or improved.

(2) Capacitive touchscreens can support multi-touch (and single-touch). Following the principle of capacitive touchscreens, a single capacitive touchscreen panel cannot support multi-touch, but a large touchscreen panel can be divided into multiple small blocks, each of which is essentially an independent small capacitive touchscreen panel.

(3) Multi-touch support in multiple blocks complicates the coordinate calculation of capacitive touchscreens, but this complexity is absorbed by the capacitive touch IC, which still communicates with the host SoC via a digital interface to report touch information (number of touch points, coordinates of each touch point, etc.).

External I2C Access Interface

(1) The entire capacitive touchscreen consists of two parts: the touchpad sensor and the capacitive touch IC. A touchpad sensor is a physical device. Capacitive touch ICs are typically integrated onto the touchscreen’s flexible printed circuit board (FPC, such as the chip on the FPC in the image below). The capacitive touch IC controls the touchpad, obtains information such as the number of touch points and touch coordinates through AD conversion and analysis, and then communicates with the SoC via a specific digital interface. This digital interface is I2C.

(2) For our host SoC, a capacitive touchscreen is essentially an I2C slave device. The host only needs to access this slave device via the I2C bus (the slave device has its own specific slave address). From this perspective, capacitive touchscreens are no different from other sensors (gsensors, etc.).

IV. The Principle of Resistive Touchscreens

A resistive touchscreen is essentially a sensor. Although it’s not widely used anymore, many LCD modules still employ resistive touchscreens. These screens can use four, five, seven, or eight wires to generate screen bias voltage and simultaneously read back the voltage of the touch points. Here, we will mainly use a four-wire example for explanation.

Thin Film + Glass (Requires a Sharp, Hard Object to Press)

(1) Key points: thin and transparent. The front panel is slightly less hard and can be bent by pressing with a hard object, while the back panel is very hard and will not bend.

(2) The front and back panels are not normally touching. Under external pressure, the front panel undergoes (local) deformation, and at this point, the front and back panels will touch. See the left image below:

ITO (Conductive + Transparent + Uniform Voltage Drop)

(1) ITO is a material, essentially a coating, characterized by its transparency, conductivity, and uniform application. (See the metal coating in the right image above.)

(2) Glass and plastic are normally non-conductive, but after being coated with ITO, they become conductive (while retaining their original transparent properties).

(3) ITO is not only conductive but also resistive. Therefore, uniformly applying ITO in the middle is equivalent to connecting a resistor between the two sides of the same layer. Because the equivalent resistance formed by ITO is uniformly distributed across the entire board, the voltage value at a certain point on the board is proportional to the voltage value at that point.

(4) After the touchscreen is operated, what is needed after a press is the coordinate of the pressed point. The coordinate is essentially position information, which is proportional to the voltage. This voltage can be obtained through AD conversion. This is the working principle of the entire resistive touchscreen.

X/Y Axis Time-Division AD Conversion

(1) Next, we need to study how to obtain the voltage at the pressed point.

(2) Apply voltage to a pair of electrodes on the first panel, and then measure the voltage between an electrode on the other panel and ground on the first panel. There is no result when no press is made, but when someone presses the touchscreen, the two panels contact at the pressed point. This contact causes the overall voltage value on the second panel to be equal to the voltage value at the contact point. Therefore, the voltage measured at this time is the voltage value at the contact point on the first panel.

(3) Performing the above process once in one direction will obtain the coordinate value in that direction. After completing the process, remove the voltage and apply voltage to the electrodes in the other direction, repeating the process to obtain the coordinates in the other direction. This completes one touch event.

For example, as shown in the diagram below: We first apply a voltage between X+ and X-. When someone presses the touchscreen, a contact point is formed at the corresponding location. Then, we measure the voltage between Y+ and GND (or Y- and GND). The obtained voltage value is the voltage value at the contact point. Because the resistance is uniformly distributed, we can calculate the position of that point in the x-direction. The same principle applies to measuring the Y-axis.

Calibration of Resistive Touchscreens

(1) The voltage value is proportional to the coordinate value, so it needs to be calibrated. Calibration involves calculating the voltage value at the (0,0) coordinate point.

Hardware Interface of Resistive Touchscreens

(1) For resistive touchscreens, there are two main types of hardware interfaces: one is the SoC’s built-in resistive touchscreen controller, and the other is an external dedicated touchscreen control chip. Connecting the touchpad sensor to this control chip allows the chip’s internal logic circuitry or built-in program code to calculate the contact point coordinates based on the principles described above and convert them into digital values, which are then sent to the host SoC via the I2C interface.

(2) For the first type of interface, the resistive touch screen controller of the SoC needs to be able to complete the tasks mentioned above and needs to convert the analog signal of the sensor into a digital signal. Therefore, this is usually associated with the ADC. In the S5PV210 SoC, the ADC module and the touch screen module are actually integrated together.

Hangzhou LEEHON Technology Co., Ltd., as a provider of LCD display driver solutions for the industrial field, has established in-depth cooperative relationships with many leading global LCD panel manufacturers such as BOE, TIANMA, IVO, AUO, Innolux, and Kyocera, and professionally supplies multi-brand, full-series industrial-grade LCD displays and customized solutions.