1 Basic principles of touch screen The working part of a typical touch screen is generally composed of three parts, two transparent resistive conductor layers, an isolation layer between the two conductors, and electrodes. The resistive conductor layer is made of resistive materials, such as indium tin oxide (ITO) coated on the substrate, the upper substrate is made of plastic, and the lower substrate is made of glass. The isolation layer is a viscous insulating liquid material, such as polyester film. The electrode is made of materials with excellent conductivity (such as silver powder ink), and its conductivity is about 1000 times that of ITO.
When the touch screen is working, the upper and lower conductor layers are equivalent to a resistance network. When a voltage is applied to a certain layer of electrodes, a voltage gradient will be formed on the network. If there is an external force that makes the upper and lower layers contact at a certain point, the voltage at the contact point can be measured on the other layer where no voltage is applied to the electrode, so that the coordinates of the contact point can be known. For example, if a voltage is applied to the top electrode (X+, X-), a voltage gradient is formed on the top conductor layer. When an external force makes the upper and lower layers contact at a certain point, the voltage at the contact point can be measured on the bottom layer , And then according to the distance relationship between the voltage and the electrode (X+), the X coordinate of the place is known. Then, switch the voltage to the bottom electrode (Y+, Y-), and measure the voltage at the contact point on the top layer to know the Y coordinate.
2 The realization of the control of the touch screen In many PDA applications, the touch screen is used as an input device, and there are special chips for the control of the touch screen. Obviously, the control chip of the touch screen has to complete two things: one is to complete the switching of the electrode voltage; the other is to collect the voltage value at the contact point (ie A/D). In many embedded systems, the CPU provides special The module supports the input of LCD and touch screen, making the interface very simple. For example, MOTOROLA's MC68VZ328 (called Dragon Ball) provides special pins to support 8-bit and 4-bit liquid crystal displays, and the support for touch screens is also easily accomplished through SPI2 with the aid of ADS7843.
