Electronic

Mainstream Display Types in Embedded Hardware Design

Flexible screens will subvert the presentation of traditional screens, making society enter an era where even a bottle of Coke can be displayed. Before this huge change comes, it is necessary for us to make a summary of the current screens that will be in the past.

Flexible screens will subvert the presentation of traditional screens, making society enter an era where even a bottle of Coke can be displayed. Before this huge change comes, it is necessary for us to make a summary of the current screens that will be in the past.

Types and basic principles of touch screen

For the convenience of operation, people use a touch screen instead of a mouse or a keyboard. As shown in Figure 1, the touch screen is composed of a touch detection part and a touch screen controller; the touch detection part is installed in front of the Display screen to detect the user’s touch position and receive the touch screen controller; the main function of the touch screen controller is to control the touch The point detection device receives the touch information, converts it into contact coordinates, and then sends it to the CPU. It can also receive the commands sent by the CPU and execute them.

Mainstream Display Types in Embedded Hardware Design
Figure 1 touch screen system block diagram

According to the working principle of the touch screen and the difference of the information transmission medium, we divide the touch screen into four types, namely resistive type, capacitive induction type, infrared type and surface acoustic wave type. Each type of touch screen has its own advantages and disadvantages and applicable occasions. The following is a brief introduction to the various types of touch screens mentioned above:

Resistive touch screen

This touchscreen utilizes pressure sensing for control. As shown in Figure 2, the main part of the resistive touch screen is a resistive film screen that fits well with the display surface. This is a multi-layer composite film. It uses a glass or hard plastic plate as the base layer and is coated with a transparent layer on the surface. Oxidized metal (transparent conductive resistance) conductive layer, covered with a hardened outer surface, smooth and scratch-resistant plastic layer, its inner surface is also coated with a layer, between them there are many small (less than 1 /1000 inches) of transparent isolation points to separate the two conductive layers. When the finger touches the screen, the two conductive layers are in contact at the touch point position, the resistance changes, and signals are generated in the two directions of X and Y coordinates, and then sent to the touch screen controller. The controller detects this contact and calculates the (X,Y) position. This is the most basic principle of the resistive technology touch screen.

The key to resistive touch screens lies in material technology. Commonly used transparent conductive coating materials include indium oxide (ITO) and nickel-gold coatings.

Mainstream Display Types in Embedded Hardware Design
Figure 2 Structure diagram of four-wire resistive touch screen

capacitive touch screen

The capacitive touch screen works by using the current sensing of the human body. When the finger touches the metal layer, due to the electric field of the human body, the user and the touch screen surface form a coupling capacitance. For high-frequency current, the capacitance is a direct conductor, so the finger draws a small current from the contact point. This current flows out from the electrodes on the four corners of the touch screen respectively, and the current flowing through these four electrodes is proportional to the distance from the finger to the four corners. The controller obtains the position of the touch point by accurately calculating the ratio of the four currents. .

Mainstream Display Types in Embedded Hardware Design
Figure 3 Working principle of capacitive touch screen

Infrared touch screen

The infrared touch screen uses a densely distributed infrared matrix in the X and Y directions to detect and locate the user’s touch. As shown in Figure 4, the infrared touch screen is installed with a circuit board frame in front of the display, and the circuit board arranges infrared emission tubes and infrared reception tubes on the four sides of the screen, forming a horizontal and vertical infrared matrix corresponding to each other. When the user touches the screen, the finger will block the horizontal and vertical infrared rays passing through the position, so the position of the touch point on the screen can be determined. Any touch object can change the infrared rays on the touch point to realize touch screen operation.

Mainstream Display Types in Embedded Hardware Design
Figure 4 Principle of infrared touch screen

Surface Acoustic Wave Touch Screen

A surface acoustic wave is a type of ultrasonic wave, a mechanical energy wave that propagates shallowly on the surface of a medium (such as a rigid material such as glass or metal). As shown in FIG. 5 , the upper left corner and the lower right corner of the touch screen are respectively fixed with vertical and horizontal ultrasonic transmitting transducers, and the upper right corner is fixed with two corresponding ultrasonic receiving transducers. The four peripheries of the glass screen are engraved with very precise reflective stripes at 45° angles from sparse to dense. Take the X-axis transmitting transducer in the lower right corner as an example: the controller controls the transmitting transducer to emit sound wave energy and transmit it to the left surface, and then the reflection stripes at the bottom of the screen reflect the sound wave energy into an upward uniform surface for transmission; the sound wave energy passes through On the surface of the screen, the reflection stripes on the top of the screen are gathered into a rightward line and propagated to the receiving transducer on the X-axis. The receiving transducer converts the returned surface acoustic wave energy into an electrical signal. When there is no touch, the waveform of the signal received by the receiving transducer is exactly the same as the reference waveform. When a finger or other object that can absorb or block sound wave energy touches the screen, the sound wave energy that goes up through the finger on the X-axis is partially absorbed, and it is reflected in the received waveform that there is an attenuation gap in the waveform at a certain moment. According to the position of the attenuation gap in the waveform, the X-axis coordinate of the touch can be calculated, and the Y-axis coordinate can be obtained in the same way.

Mainstream Display Types in Embedded Hardware Design
Figure 5 Surface Acoustic Wave Touch Screen

What are the specific applications of touch screen? How should the XY coordinates be calculated during design? What are the commonly used resistive screen driver chips? And see the next analysis.

This article only briefly introduces the current mainstream display types in embedded hardware design. With the development of Industry 4.0, embedded devices have spread across various industries. ZLG Zhiyuan Electronics’ embedded products have accumulated nearly 20 years of design experience, and fully guarantee the stability of products from the aspects of RTC clock, power management, ESD protection circuit, various communication interfaces and so on. Zhiyuan Electronics started from the design of 8-bit single-chip microcomputer in 2001, and gradually mastered the processor application technology of ARM7, ARM9, Cortex-A7, A8, A9, M7 and the most cutting-edge A53 processor, and has a full range of industrial-grade ARM cores board and industrial computer. At the same time, based on the understanding and accumulation of embedded technology, we independently develop the next-generation software development platform-Aworks real-time operating system, which helps users to quickly realize product development based on a stable software and hardware platform. The products have been widely used in electric power, rail transit, industrial sites, medical and other occasions with strict requirements on product reliability, and continue to provide a complete set of industry application solutions for various industries.

Mainstream Display Types in Embedded Hardware Design

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