Touch screen principle

Resistive touch screen

The resistive touch screen converts the physical position of the touch point (X, Y) in the rectangular area into voltages representing X and Y coordinates. Then after ADC conversion, the sampled analog voltage signal is converted into a digital signal and stored in a certain register for subsequent processing. The main structure of the resistive screen is: film + glass, with an insulating fulcrum in the middle. The former can be bent by touch, the latter will not. Then the two layers are coated with ITO. ITO is a kind of material, in fact a kind of paint, which is characterized by transparency, conductivity and uniform application. When a voltage is applied to a certain layer of electrodes, a voltage gradient is 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 the electrode is not voltage applied, so that the coordinates of the contact point can be known. If you measure the voltage in the x direction, apply voltage to both sides of the x and not to both ends of the y direction. After touching the two layers of contact, measure the voltage at the contact point, that is, the voltage in the x direction.

Capacitive touch screen

When a finger touches the metal layer, due to the electric field of the human body, a coupling capacitor is formed between the user and the surface of the touch screen. For high-frequency current, the capacitor is a direct conductor, so the finger draws a small current from the contact point. This current flows from the electrodes on the four corners of the touch screen, 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 these four currents. .

The coordinate calculation of the capacitive touch screen sensor is too complicated for programmers to write appropriate codes. Therefore, in addition to the touch panel, an IC is added to the capacitive touch screen for special coordinate point calculation and statistics. This IC is fully responsible for controlling the touch panel to obtain touch operation information, and then communicates with the host SoC through a digital interface (usually I2C).

The resistive touch screen does not support multi-touch. A single capacitive touch screen panel cannot support multi-touch. A large touch panel can be divided into multiple small blocks, and each block is equivalent to an independent small capacitive touch screen panel.