LCD hardware principle

LCD hardware principle

Displays made of liquid crystals are called LCDs (liquid crystal displays), which are divided into static drive, simple matrix drive and active matrix drive according to the driving mode. Among them, the simple matrix type can be subdivided into two types, twisted nematic (TN) and super twisted nematic (STN). The active matrix type is mainly thin film transistor type (TFT).

TN-type liquid crystal display technology is the most basic of LCDs, and other types of LCD are improved based on TN-type. TN-type LCD display quality is very poor, the color is single, the contrast is low, and the reflection speed is very slow. It is mainly used for the display of simple numbers and characters, such as electronic watches and electronic calculators.

The display principle of STN LCD is similar to that of TN. The difference is that TN-type liquid crystal molecules rotate incident light by 90°, while STN can rotate incident light by 180°～270°. STN improves the shortcomings of the narrow viewing angle of TN, and improves the contrast, and the display quality is higher than that of TN.

STN is matched with a color filter to divide any pixel of the monochrome display matrix into 3 sub-pixels. The three primary colors of red, green, and blue are displayed through the color filter, and then the three primary colors are proportionally adjusted to display a full-color mode. color. The color contrast of the screen displayed by STN is still relatively small, and the response speed is relatively slow. It can be used as a general operation display interface.

DSTN is displayed through dual scan mode, and the display effect is greatly improved compared with STN. The response speed of DSTN can reach 100ms, but the recovery process of each pixel is slow when the electric field repeatedly changes the voltage. Therefore, when the screen changes rapidly, a "tailing" phenomenon will occur.

Both TN and STN liquid crystal displays use a field voltage drive method. If the display size is enlarged, the reaction time of the center part to electrode changes will be lengthened, and the display speed cannot keep up. In order to solve this problem, active matrix driving was proposed. The structure of active TFT liquid crystal displays is more complicated, including backlight tubes, light guide plates, polarizing plates, filter plates, glass substrates, alignment films, liquid crystal materials, thin film transistors, etc. .

In TFT LCD, the transistor matrix turns on or off the voltage of the liquid crystal molecules according to the display signal, so that the axis of the liquid crystal molecules is turned into a contrast of "bright" or "dark", avoiding the display's dependence on electric field effects. The display quality of TFT LCD is better than that of TN/STN, the screen display contrast can reach 150:1 or more, and the response speed is close to 30ms or even faster. It is suitable for PDA, notebook computer, digital camera, MP4, etc.

An LCD screen displays images not only requires an LCD driver, but also a corresponding LCD controller. Usually the LCD driver is fabricated with the LCD glass substrate in the form of COF/COG, while the LCD controller is implemented by an external circuit. Many MCUs directly integrate LCD controllers, and STN and TFT screens can be conveniently controlled through LCD controllers.