I. TFT Structure
TFT-LED LCD Panel Manufacturing Process 3D Simulation Animation
The main function of the TFT-LCD circuit is to control the voltage values on both surfaces of the liquid crystal to control the deflection angle of the liquid crystal, change the twist angle of the liquid crystal molecules on linearly polarized light, and ultimately control the intensity of the transmitted light through the orientation effect of the polarizer. Each sub-pixel is controlled by a thin-film transistor (TFT). TFTs are switching devices, and their conduction and cutoff states are close to ideal switches. Taking a 1024×768 pixel resolution display as an example, each pixel consists of sub-pixels of the three primary colors R, G, and B, totaling 1024×768×3 TFT transistors.
II. Classification of LCD Displays
(1) Viewing Angle
--TN Screen: Twisted Nematic (TN) LCD. Faster response speed and less prone to ghosting; lower viewing angle (maximum viewing angle around 70 degrees); fewer gray levels output, resulting in a slightly whiter color. TN screens have good dynamic performance and short response time, but poor color, making them suitable for high-speed computer games. VA Panel: Uses a vertically aligned liquid crystal molecule arrangement. It offers a well-balanced performance across the board, making it suitable for home use. A major advantage of VA panels is their contrast ratio, reaching up to 3000:1. However, due to this high contrast, pixel switching takes longer, leading to issues like ghosting and blurring. VA panels are also soft screens; a light touch with a finger will produce a ripple effect, while a TN panel will show a water ripple effect. Therefore, VA panels offer the best contrast ratio, resulting in superior detail in both dark and bright areas, but have the worst response time, making them prone to ghosting.
IPS Panel: Short for In-Plane Switching, also known as "Super TFT." IPS panels use a horizontally aligned liquid crystal molecule arrangement. They have a slower response time, making them prone to ghosting; wide viewing angles (up to 85 degrees per angle); and good color reproduction. They offer good color and static display, but a long response time, making them suitable for general games like League of Legends, movies, and design work.
(2) Resolution
Resolution is the number of pixels. The number of pixels and the size of each pixel determine the size of the display area.
III. LCD Communication Interfaces
TFT-LCD signal interfaces include SPI, MCU, RGB, LVDS, MIPI, eDP, HDMI, and many others. Today, we will discuss the characteristics and signal composition of the SPI interface.
(1) SPI Interface
As the name suggests, the SPI interface is based on the SPI bus protocol to realize data interaction between the CPU and the LCD. This interface has multiple operating modes, and the pin definitions are usually: SDI (sometimes written as SDA) + SDO (if needed) + SPI_CLK (sometimes written as SCK) + CS + power line. The advantages and disadvantages of this interface definition are also very obvious. Because the SPI communication protocol uses fewer lines, the number of pins required for an LCD using this interface is relatively smaller. However, because data transmission is serial, only a single data line is used for input, so the software is more complex, and the transmission speed is slow, making it impossible to achieve fast screen refresh operations such as video playback. However, it is better for single-screen display, occupies fewer resources, and is simpler to control. (2) MCU Interface
The MCU interface, also known as a parallel communication panel, can be divided into 8080 and 6800 interfaces. The principles of these two are the same; the only difference is the timing. Multiple data lines are needed to achieve data interaction between the CPU and the LCD. This interface has multiple operating modes, including 8-bit, 9-bit, 16-bit, 18-bit, and 24-bit modes. The pin definitions are typically: CS (alias: chip select pin, usually used to select the IC) + RS (alias: register select pin, usually used to set the storage location of the transmitted content; high level selects the data register, low level selects the instruction register) + RD (sometimes written as SCK) + CS + power line.
The advantages and disadvantages of this interface are also very obvious. Because the MCU interface consumes more resources, the LCD refresh rate using this interface is faster than that of the SPI interface, and the software control is also simpler compared to SPI.
Advantages: Faster refresh rate than SPI interface; supports both landscape and portrait orientations; simple and convenient operation.
However, because data transmission needs to be temporarily stored in memory for processing, the refresh rate is not very fast. Different products and different resolutions will have significantly different refresh rates. Furthermore, it consumes more resources; higher resolutions require more GRAM, making it difficult to support large screen sizes.
Disadvantages: Does not support video playback; cannot be used on large screens; consumes more resources.
(3) RGB Interface
The RGB interface, as the name suggests, uses the three primary color data signals as a basis to realize data interaction between the CPU and the LCD. This interface has many operating modes, but without exception, they all require multiple data lines. Depending on the number of signal lines, the interface can be subdivided into several operating modes such as 6-bit, 16-bit, 18-bit, and 24-bit. The pin definitions are typically: CS (alias: chip select pin, usually used to select the IC) + VSYNC (vertical synchronization signal, selects the effective vertical signal range on the LCD) + HSYNC (horizontal synchronization signal, selects the effective horizontal signal range on the LCD) + DE (data enable signal) + data lines (R0~Rx, G0~Gx, B0~Bx, x = actual data/3-1, for example, in 24-bit, x = 24/3-1 = 7) + power lines (VCC, GND, LEDA, LEDK, etc.).
This interface also has its advantages and disadvantages. Because the RGB interface consumes more resources, LCDs using this interface have a very fast refresh rate, and software control is relatively simple. Furthermore, since the display data of this interface does not need to be written to memory for processing and can be directly written to the LCD for display, the response and refresh rates are much faster than MCU interfaces.
Advantages: Fast refresh rate, can play video, simple and convenient operation and control;
However, because it requires setting vertical and horizontal sync signals, corresponding circuitry must be set up on the motherboard to coordinate with the software to control the frequency and data of the vertical and horizontal sync signals. Some motherboard controller ICs have reserved functions for vertical and horizontal sync, which leads to two scenarios for driving this screen: one requires additional circuitry and software initialization to control the LCD driver; the other only requires additional software initialization to control the LCD driver.
Disadvantages: Requires additional circuitry for control, additional program for software initialization, and consumes more resources;
(4) LVDS Interface
LVDS interface is short for Low-Voltage Differential Signaling interface. Each data line is divided into two, one positive and one negative. Two low-voltage differential signal lines are combined to form a single data line. Data interaction between the CPU and LCD is achieved through the combination of multiple such data lines. This interface has many operating modes. Depending on the number of data lines, the interface can be further subdivided into 1-port and 2-port modes, which can be further subdivided into 3-line and 4-line modes. Pin definitions are usually divided into two types:
First pin definition: Single-channel LVDS: RXINx-, RXINx+ (If it is 3-line, then it is RXIN0-~RXIN2-/RXIN0+~RXIN2+; if it is 4-line, then it is RXIN0-~RXIN3-/RXIN0+~RXIN3+) + (RXCLKIN-, RXCLKIN+) + power line + RESET.
Second pin definition: Dual-channel LVDS (divided into ODD and EVEN channels, meaning twice the number of data lines compared to single-channel LVDS):
RXOINx-, RXOIN+ (if 3-line, then RXOIN0-~RXOIN2-/RXOIN0+~RXOIN2+; if 4-line, then RXOIN0-~RXOIN3-/RXOIN0+~RXOIN3+) + (RXOClockIN-, RXOClockIN+) + RXEINx-, RXEIN+ (if 3-line, then RXEIN0-~RXEIN2-/RXEIN0+~RXEIN2+; if 4-line, then RXEIN0-~RXEIN3-/RXEIN0+~RXEIN3+) + (RXEClockIN-, RXEClockIN+) + Power line + RESET
This interface also has its advantages and disadvantages. Because it uses a very low-amplitude differential line to transmit signals, the noise generated is very low, the power consumption is also very low, and the transmission rate can reach thousands of Mbps. Furthermore, because the polarities of the differential signals are opposite, they can cancel each other out, resulting in very low interference.
Advantages: Fast refresh rate, video playback capability, strong anti-interference ability, low power consumption, etc.
However, because it requires setting vertical and horizontal sync signals, corresponding circuitry must be included in the motherboard to coordinate with software control of the frequency and data of the vertical and horizontal sync signals. Some main control ICs do not support the LVDS interface, requiring the use of a signal conversion chip, which increases the complexity of the motherboard circuitry to some extent. The software also differs somewhat, requiring careful configuration of the vertical and horizontal signal parameters.
Disadvantages: High resource consumption, requires additional signal conversion chips, more complex software, cannot transmit commands.