Basic working principle of LCD and How it interac with STM32
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Basic of LCD
Understanding Liquid Crystal Displays (LCDs)
Liquid Crystal Displays, commonly known as LCDs, have largely replaced the older CRT (Cathode Ray Tube) technology due to their numerous advantages. Compared to CRTs, LCDs offer lower power consumption, compact size, high information density, and reduced eye strain, making them the mainstream display choice in modern electronics. Today, LCDs are widely used in televisions, computer monitors, mobile phone screens, and various embedded systems.
Liquid crystal is a unique organic compound that exhibits properties of both solids and liquids. In its natural state, it behaves like a liquid, but its molecules are arranged in an orderly pattern similar to a solid crystal—hence the name liquid crystal. When an electric field is applied, the molecular alignment changes, which affects how light passes through it. Combined with polarizers and color filters, this property allows control of light transmission and color intensity.
By adjusting the voltage across the liquid crystal, the display can regulate the brightness of red, green, and blue (RGB) light components. When these three basic colors are combined at varying intensities, they produce different colors—forming the pixels that make up an image on the screen.
Fig 1 Liquid Crystal Technology Layer
LCD vs LED vs OLED Displays: Understanding the Differences
LCDIt’s important to note that liquid crystal displays (LCDs) do not generate light on their own. Instead, they rely on a backlight to provide illumination. The light passes through several layers—such as polarizers, color filters, and liquid crystal cells—before reaching the screen surface. Because of this complex process, the output brightness of an LCD is much lower than that of the original light source. Additionally, these layers cause a narrow viewing angle, meaning the display content becomes difficult to see clearly from the side. Another limitation of LCDs is their slow response time, as the rotation of liquid crystal molecules takes a short but noticeable moment when changing colors or images.
LED dot matrix displayIn contrast, the LED dot matrix display overcomes many of these drawbacks. Each pixel in a color LED display consists of three small LEDs—red, green, and blue—that combine to produce full-color images. Because each pixel emits its own light, LED displays remain bright and clear even under strong daylight, making them ideal for outdoor or large-screen applications like billboards and public information displays. However, due to the larger size of individual LEDs, these displays have lower pixel density, which limits their use for close-up or high-resolution visuals.
OLED (Organic Light Emitting Diode)A more advanced technology, the OLED (Organic Light Emitting Diode) display, works on a similar principle but uses organic materials as the light-emitting elements. This allows for much higher pixel density, resulting in sharper images and richer colors. OLED displays do not require a backlight, provide high contrast ratios, fast response times, wide viewing angles, and are both thin and lightweight.
Liquid crystal control principle
📘 Structure of an LCD Display Module
A complete LCD display module is typically made up of three main parts — the LCD panel, a capacitive touch panel, and a PCB backplane. The touch panel includes a dedicated touch control chip, which detects and processes touch inputs, then communicates with the external controller through signal lines. The center area of the touch panel is transparent and is laminated onto the LCD panel, forming the main body of the display screen.
Both the touch panel and LCD panel are connected to the PCB backplane via flexible cables. Depending on the application, the PCB backplane may include an LCD controller chip. For example, some modules use an RA8875 controller to manage complex display operations. Since directly controlling an LCD panel requires substantial processing power and memory, most low-cost microcontrollers (MCUs) cannot handle this task efficiently on their own.
The LCD controller acts as an intermediary between the microcontroller and the display. The external MCU simply sends the display data to the controller, which then handles the detailed timing and refresh operations of the LCD.
In some simpler display modules, there may be no dedicated controller on the PCB backplane—only a power management circuit. In this case, the LCD signal lines are connected directly to the external microcontroller, which must then control the display through software or peripheral interfaces such as SPI, I²C, or parallel GPIO.
📘LCD panel control signal
We mainly explain how to control the LCD panel. The control signal lines of the LCD panel are the FPC cables drawn from the LCD panel , and the description is shown in Table 1. The LCD panel communicates with the LCD controller through these signal lines. This communication signal is called RGB Interface (RGB Interface).
RGB signal line:
There are 8 RGB signal lines each, which are used to represent the red, green, and blue color components of a pixel of the LCD screen. It is a common practice to use red, green and blue color components to represent colors. Open the artboard color palette tool that comes with the Windows system, and you can see the red, green and blue component values of the color. The common color representation will be followed by "RGB" with the data bits of each color component value. For example, RGB565 indicates that the number of data lines for red, green and blue is 5, 6, and 5 respectively. There are 16 data bits in total, which can represent 2 16 There are 8 data lines for each color component of this LCD screen, so it supports RGB888 format, a total of 24 data lines, and 224 colors can be represented.
Synchronous clock signal CLK:
The LCD screen uses the synchronous communication method with the outside, and uses the CLK signal as the synchronous clock. Driven by the synchronous clock, each clock transmits one pixel data.
Horizontal synchronization signal HSYNC:
The horizontal synchronization signal HSYNC (Horizontal Sync) is used to indicate the end of the transmission of a line of pixel data on the LCD screen. When the transmission of a line of pixel data on the LCD screen is completed, the level of HSYNC will jump, such as a display screen with a resolution of 800x480 (800 columns). , 480 lines), the level of HSYNC will jump 480 times when transmitting one frame of image.
Vertical synchronization signal VSYNC:
The vertical synchronization signal VSYNC (Vertical Sync) is used to indicate the end of the transmission of one frame of pixel data on the LCD screen. When the transmission of one frame of pixel data is completed, the level of VSYNC will jump. where "frame" is the unit of the image, a frame
An image is called a frame, and in an LCD screen, a frame refers to a complete screen of LCD pixels. People often use "frames per second" to represent the refresh characteristics of the LCD screen, that is, how many frames of images the LCD screen can display per second. For example, when the LCD screen runs at a rate of 60 frames per second, the level of VSYNC will jump 60 per second. Second-rate.
Data enable signal DE:
The data enable signal DE (Data Enable) is used to indicate the validity of the data. When the DE signal line is at a high level, the data represented by the RGB signal line is valid.
📘LCD data transmission timing
When transmitting pixel data to the LCD screen through the above signal lines, the timing of each signal line is shown in Figure 4. The figure shows the time sequence of transmitting one frame of image data to the LCD screen, omitting multiple lines and multiple pixels in the middle.
The image displayed on the LCD screen can be seen as a rectangle, which can be understood in conjunction with Figure 5. The LCD screen has a display pointer that points to the pixel that will be displayed. The scanning direction of the display pointer is from left to right, from top to bottom, and the graphics are drawn pixel by pixel. The data of these pixels are transmitted to the LCD screen through the RGB data line, and they are transmitted to the LCD screen one by one under the drive of the synchronous clock CLK, and handed over to the display pointer. When the transmission is completed for one line, the level of the horizontal synchronization signal HSYNC jumps once , and the VSYNC level jumps once when a frame is transmitted.
Fig 4 Timing Data transmission
The LCD requires short delays when switching between lines and frames, defined by the HSYNC and VSYNC signals. During these intervals, the Data Enable (DE) signal stays low, making RGB data invalid. When DE goes high, the RGB data becomes valid and updates the visible display area on the screen.
📘video memory
Each pixel in the LCD screen is data. In practical applications, the data of each pixel needs to be cached and then transmitted to the LCD screen. Generally, SRAM or SDRAM memory is used, and these are specially used for storage and display. The memory for data is called video memory. Generally, the video memory should be able to store at least one frame of display data of the LCD screen. For example, if the LCD screen with a resolution of 800x480 is displayed in RGB888 format, the size of one frame of display data is: 3x800x480=1152000 bytes; if it is displayed in RGB565 format, a The frame display data size is: 2x800x480=768000 bytes. Generally speaking, the external LCD controller will have its own video memory, and chips with integrated LCD controller such as STM32F429 can use internal SRAM or external SDRAM for video memory space.
Resistive touch screen
The LCD controller and screen are separate, though some versions include an integrated controller. For example, a 3.2-inch resistive touch LCD with 320×240 resolution consists of two main parts:
- LCD Touch Panel: Combines the LCD screen and resistive touch layer. The ILI9341 controller communicates with the microcontroller via an 8080 parallel interface and stores display data in internal video memory, continuously refreshing the screen.
- PCB BackplaneHouses the XPT2046 touch controller, an ADC that calculates touch coordinates from voltage values. The backplane connects to the touch panel via FPC and outputs signals through pin headers for easy connection to the experiment board.
As another example, a 3.0-inch TFT LCD module with the R61509V3 driver, 240×400 resolution, and 16-bit parallel interface uses 2×17 male pin headers. A 16-bit interface is preferred over 8-bit for faster data transfer, especially when displaying images. Even 8-bit screens can connect to the high or low 8 bits of a 16-bit interface for compatibility. The module also includes a touch interface, which will be explained in later chapters.
MCU Requirement
The STM32F429 series of chips do not require an additional LCD controller, which means that it integrates the functions of the dedicated LCD controller into the STM32F429 chip, which can be understood as the computer's CPU integrated graphics card, which saves additional controller costs. Since the STM32F1 series chip does not integrate the LCD controller into the chip, it can only drive the screen with its own controller, which can be understood as the external graphics card of the computer. In general, the control block diagrams for these two types of screens are shown in Figure.
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