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                                                                                       Principle of touch screen

The touch screen consists of two transparent layers stacked one above the other. The four-wire and eight-wire touch screens consist of two layers of transparent resistive material with the same surface resistance. The five-wire and seven-wire touch screens consist of a resistive layer and a conductive layer. An elastic material is used to separate the two layers. When sufficient pressure is exerted on the surface of the touch screen, such as by the tip of a stylus or a finger, contact occurs between the top layer and the bottom layer. All resistive touch screens use a voltage divider principle to generate voltages that represent the X and Y coordinates.the voltage divider is realized by connecting two resistors in series. The upper resistor (R1) connects to the positive reference voltage (VREF), and the lower resistor (R2) connects to ground. The voltage measured at the junction of two resistors is proportional to the resistance of the lower resistor.

To measure a coordinate in a specific direction on a resistive touch screen, a resistive layer needs to be biased: connect one side to VREF and the other to ground. Also, connect the unbiased layer to the high-impedance input of an ADC. The resistive surface is separated into two resistors when pressure on the touchscreen is strong enough to make contact between the two layers. Their resistance is proportional to the distance from the touch point to the offset edge. The resistance between the touch point and ground is equivalent to the lower resistor in the voltage divider. Therefore, the voltage measured across the unbiased plane is proportional to the distance from the touch point to ground.

All touch screens can detect whether a touch has occurred by pulling one layer up with a weak pull-up resistor and pulling the other layer down with a strong pull-down resistor. If the measured voltage of the pull-up layer is greater than a certain logic threshold, it indicates that there is no touch, otherwise, there is a touch. The problem with this approach is that the touch screen is a huge capacitor, and you may need to increase the capacitance of the touch screen leads in order to filter out the noise introduced by the LCD. A weak pull-up resistor connected to a large capacitor will make the rise time longer, which may cause spurious touches to be detected.
Figure 6 The four-wire and eight-wire touch screens can measure the contact resistance, that is, RTOUCH in Figure 5. RTOUCH is approximately proportional to touch pressure. To measure touch pressure, the resistance of one or both layers in the touchscreen needs to be known. The formula in Figure 6 gives the calculation method. It should be noted that if the measured value of Z1 is close to or equal to 0 (when the touch point is close to the grounded X bus during the measurement), there will be some problems in the calculation, which can be effectively improved by using a weak pull-up method.
Advantages and disadvantages of resistive touch screen
The advantage of resistive touch screen is that its screen and control system are relatively cheap, and the response sensitivity is also very good, and whether it is a four-wire resistive touch screen or a five-wire resistive touch screen, they are a completely isolated working environment from the outside world, not afraid of dust And water vapor, can adapt to various harsh environments. It can be touched with any object and has better stability. The disadvantage is that the outer film of the resistive touch screen is easily scratched and the touch screen is unusable. The multi-layer structure will cause a lot of light loss. For handheld devices, it is usually necessary to increase the backlight to make up for the problem of poor light transmission, but this is also Will increase battery consumption.