Programmable Logic In LCD Touch Screen Control

- Sep 25, 2019-

 

Touch displays are now available in a variety of handheld consumer electronics, medical applications, vending machines/ticket machines/ATM machines, point-of-sale (POS), industrial and process control equipment. Touch screen displays are gradually entering the field of office automation, automotive and marine instrumentation, home appliances and gaming consoles.


Various factors that influence the choice of touch screen technology can be implemented in a variety of ways. In addition to cost, the choice of technology depends on several factors: Performance: Performance includes stability such as speed, sensitivity, accuracy, resolution, drag, Z-axis, dual/multi-touch, parallax angle and calibration stability . Input Flexibility: Input flexibility parameters affect the way human-computer interactions, such as gloves, glove materials, nails, stylus, handwriting recognition, and acquisition signatures. Environment: Environmental factors are temperature, humidity, chemical resistance, scratch resistance, splash/droplet, height, interior installation, impact, vibration, fracture and anti-break safety. Electrical and mechanical properties: Electrical and mechanical properties need to cover power, floating ground, electrostatic discharge (ESD), electromagnetic interference (EMI), size, curvature, etc. Optical properties that affect technology choices include light transmission, clarity, color Purity and reflection

Types of Touch Screen Technology According to various factors described above, the main touch screen technologies can be classified into the following types: Resistive type: From the current popularization and application, the resistive touch screen is the dominant touch technology. It consists of a glass panel, an antimony tin oxide (ITO) resistive coating with a conductive coating and a silver bus bar along the edge. The two layers are separated by insulated dots. When the screen is touched, the shield is bent to contact the coating film on the glass.

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The controller can choose to drive the glass layer and the +5 V shield and read the voltage generated from the shield and glass layers to determine the X and Y coordinates based on the pressure drop in the layer being measured. This technology requires four wires – the aforementioned bus bar, which is called 4-wire resistive touch screen technology. Due to the continuous bending of the shield, there are minute cracks in the ITO coating film. The linearity and accuracy of the 4-wire resistive touch screen technology will be degraded, and environmental changes will also cause drift in accuracy. These effects have been eliminated with the ever-improving 5, 6, 7 and 8-wire resistive touch screens. Acoustic Pulse Recognition (APR): The APR consists of a glass display coating or other hard substrate with four piezoelectric sensors mounted on the back. The sensor is mounted on two diagonal corners of the visible area and connected to the control card via a bent cable. When the user touches the screen, the drag between the finger or the stylus and the glass collides or rubs, and thus a sound wave is generated. The wave radiation leaves the contact point and is transmitted to the sensor to generate an electrical signal in proportion to the sound wave. These signals are amplified in the control card and then converted to a digital data stream. The data is compared with a list of previously stored sounds to determine the location of the touch. APR is designed to eliminate environmental influences and external sounds because these factors do not match the stored sound list. Surface Acoustic Wave (SAW): The SAW touch screen is a glass coating with a transmitting and receiving piezoelectric sensor for the X and Y axes. The controller sends an electrical signal to the transmitting sensor and converts the signal into ultrasonic waves within the surface of the glass. These waves cover the entire touch screen through the array of reflectors. The opposite reflector collects and controls these waves to the receiving sensor, converting them into electrical signals. Repeat this process for each axis. A portion of the propagating wave is absorbed by the user when touched. The received signals corresponding to the X and Y coordinates are compared to the stored digital map to identify the changes and calculate the coordinates. Capacitive: Capacitive touch screen technology can be further subdivided into surface capacitive and projected capacitive. The surface capacitive technique is to apply the same conductor to the glass panel. The electrodes around the edge of the panel distribute a low voltage across the entire conductive layer, creating an identical electric field. When you touch it, you get current from each corner. The controller measures the current ratio obtained from each corner to calculate the position of the touch. The projected capacitive touch screen consists of a sensor grid of fine lines between two glass protective layers. The parts can be placed behind the user-installed material, including riot-proof glass up to 18 mm thick. When touched, a capacitance is formed between the finger and the sensor. The touch position can be calculated from the electrical characteristics of the changed sensor grid. Infrared/optical: High-resolution infrared (IR) technology uses a small frame around the display with surface-mounted LEDs, photoreceptors on opposite sides, and infrared transparent borders hidden behind. The controller continuously sends LEDs to build an infrared scanning grid. Touching blocks one or more of the infrared light on each axis, so you can determine the corresponding X, Y coordinates. The salient advantages and typical applications of the above-mentioned main touch screen technologies are summarized in  


Programmable logic in LCD touch screen control For the type of touch technology, the type of display, and the display manufacturer, the interface of the liquid crystal display is often different. It is often difficult for designers of equipment to select a display controller chip on their product line to accommodate all of the different displays. More and more designers working on human-machine interface (HMI) system integration with touch-screen LCD panels have turned to programmable logic devices to achieve the flexibility they need. Field Programmable Gate Array (FPGA) technology enables system architects to determine the architecture of the HMI controller at a time, while extending to the entire product family, using different microcontrollers, CPUs, and LCD panels to meet a variety of applications. . FPGA technology also makes it easy to implement high-performance vector graphics and interface to the real world with a single chip. Lattice's LCD-Pro is specifically designed for FPGA-based advanced touch-screen video graphics controllers, providing system designers with a single human-machine interface structure that accelerates time-to-market and saves development costs. Combined with existing IP, LCD-Pro simplifies design and enables designers to launch new products faster to meet emerging market requirements without having to redesign the platform.