TFT-LCD Display

1. Principle of Segment LCD

LCD drive is not like LED, add voltage (LED is actually current drive) can display for a long time. LCD driver must use AC voltage drive to maintain a stable display. If a stable DC voltage is applied to the LCD, not only will it fail to display normally, it will also damage the LCD over time. A segment of the LCD consists of a back electrode and a segment electrode. When a display is needed, an appropriate AC voltage (usually a square wave) is applied between the back electrode and the segment electrode. In order to adjust the contrast, you can adjust the display time (ie duty cycle) in each half cycle of the square wave. Generally, in order to save the driving port, multiple back electrodes are connected together to form a common back electrode terminal: COM. In addition, segment electrodes belonging to different COMs are connected together to form a common segment electrode terminal: SEG. When sufficient AC voltage is applied between a COM and a SEG, the corresponding segment will be lit.

2. The organization format of the data on the display

An image is called a frame, each frame is composed of multiple lines, each line is composed of multiple pixels, and the color of each pixel is represented by several bits of data. The display starts from the upper left of the screen, and displays the data of each pixel line by line. When the display reaches the rightmost side of a line, it jumps to the leftmost point of the next line and starts to display the next line; when all lines are displayed, jump Go to the upper left and start the next frame. These jumps are controlled by signals. HSYNC indicates that it is time to jump to the far left, and VSYNC indicates that it is time to jump to the top.

On the monitor at work, a black border can be seen all around. The upper black frame is because when the VSYNC signal is sent, the first row of valid data will appear after several lines; the lower black frame is because the display has not scanned to the bottom (VSYNC signal has not been sent), and the signal is invalid at this time ; The black frame on the left is because when the HSYNC signal is sent out, the first column of valid data needs to pass through several pixels; the black frame on the right is because the display has not sent out the scan to the right (HSYNC) when one row of data is displayed. The signal has not been sent), at this time the data is invalid. The monitor only obtains and displays data according to the VSYNC and HSYNC signals, and does not care whether the data is valid or not. The appropriate data to be sent is determined by the graphics card.

3. How SoC controls LCD display principle:

LCD driver: The LCD driver is generally integrated with the LCD panel. The panel needs a certain analog electrical signal to control the liquid crystal molecules. The LCD driver chip is responsible for providing the panel with analog electrical signals for controlling the liquid crystal molecules. The interface provided by the LCD controller. LCD controller: The LCD controller is integrated in the SoC, and it is responsible for providing the pixel digital signal to be displayed to the external LCD driver through the digital interface. It must communicate with the LCD driver according to a certain timing. The LCD controller is controlled by the SoC, and the SoC will take out the pixel data from the memory to the LCD controller and finally to the LCD driver. Video memory: SoC selects a section of memory in the memory to store color data, and then connects the LCD controller and this section of memory through configuration to form a mapping relationship. Once this relationship is established, the LCD controller will automatically read from the video memory Take the pixel data and send it to the LCD driver. The LCD driver will automatically control the liquid crystal molecules of each pixel to form the final image. After the mapping is established, there is no need for SoC to participate in any behavior.