Design of Timing Control Module for TFT-LCD System

Abstract: This paper explains the relationship between the timing control module and other submodules in the LCD system, and analyzes the timing problems that the timing control module needs to solve. Based on the analysis of the problem, a realization structure of the timing control module suitable for medium and small size LCD display systems is proposed. The function of the timing control module is verified, and the FPGA logic function verification result is given to prove that the design is feasible.

LCD technology has become the mainstream technology for flat panel displays, among which medium and small size LCD products have become the mainstream of development. The application of medium and small size LCD will be more extensive.
The main technologies used for medium and small size LCD displays are:
(1) STN-LCD (Super Twisted Nematic);
(2) TFT-LCD (Thin Film Transistor);
(3) LTPS (Low Temperature Poly Silicon). Among them, TFT-LCD is the most mature technology.
Due to the low current of TFT, it is impossible to design the circuit directly on TFT. Therefore, in order to make TFT-LCD work, an external IC control circuit is required. At present, most of
the materials about TFT-LCD control IC have a very brief introduction to the timing controller, and lack in-depth analysis of its timing control and generation principle. In fact, the synchronous control signal generated by the timing controller TCON (Timing Controller) is the key to determine whether the TFT-LCD can display normally. Therefore, it is one of the core control parts in the composition of TFT-LCD module, that is, the control center.
This paper first briefly introduces the composition of the liquid crystal display system, then focuses on analyzing the working principle of the timing controller (TCON) used in medium and small size TFT-LCDs, and proposes a structural block diagram of the TCON module based on the principle. Finally, the FPGA verification of the TCON module is given.

2 General Structure of TFT-LCD System
The TFT-LCD system consists of two parts: LCD control module and LCD panel module. In practical applications, there are two types of LCD panels, traditional panels and smart integrated panels, as shown in Figure 1.

Figure 1 TFT-LCD System Structure Block Diagram
The TFT-LCD Monitor system includes an analog-to-digital converter to process analog signals output by the PC graphics card, a digital video interface, a video decoder (to process video signals), a TMDS receiver, an on-screen display, a control unit, a timing controller, a backlight Source driver, a Gate driver, etc.

When the TFT-LCD display is working, the main task of the control circuit module in the front part is to convert the signal output by the PC host or audio-visual device (such as DVD Player). For example,
the analog signal output by the PC display card is converted into a digital signal through the ADC component; similarly, the audio-visual signal is converted into the same digital signal through the Video Decoder. These signals are then amplified or reduced by the Scaler IC and digital image processing is performed. The signal is then transmitted to the LCD module through the cable line, and then the required timing control signal is generated by the timing controller to drive the vertical driver IC and the horizontal driver IC. The vertical driver IC is responsible for controlling the writing of data, and the horizontal driver IC controls the on/off of the transistor, and cooperates with other components, such as the power supply module, to correctly display the image. [page]

3 Design Analysis of TCON Module
For large-size LCD panels, the design of TCON module is relatively complex. Because the clock frequency is very high in high-resolution display systems, before generating timing control signals based on clock signals, the synchronous clock needs to be spread spectrum processed to reduce EMI (Electromagnetic Interference) and pass EMC tests. Since the synchronous clock has been spread spectrum processed, the video data must also be processed accordingly to be displayed normally, otherwise data will be lost.

The TCON control of medium and small size LCD is relatively simple. The basic working process is to first determine the resolution, working mode, display mode and other information of the TFT-LCD screen by the input signal, then determine the appropriate parameter value based on this information, and finally generate the required control signal, so that the TFT-LCD can work normally without processing the video data.
This article only discusses the timing control module suitable for medium and small size LCDs.

3.1 Mode Selection
Since different resolutions of LCD and different working modes require different output control signals, when TCON is working, it is necessary to make a judgment and select the correct working mode in order to generate the corresponding appropriate control signal. These selections include LCD resolution selection, external/phase-locked loop (PLL) clock mode selection, separation/composite working mode selection, NTSC/PAL format selection and display mode selection under high resolution.

LCD resolution selection: In the actual display system, TFT-LCD has different sizes and resolutions. Generally, the resolutions of medium and small TFT-LCD are 480*234 (2.5", 3.5"),
960*234 (3.6", 5", 6.4"), 1200*234 (6.5"), 1440*234 (6.2", 7"), 1920*234 (9"), etc. For LCDs with different resolutions, some of the required control signals will be different.

External/PLL clock mode selection: There are two ways to provide the working clock source: phase-locked loop circuit (PLL) mode and external clock mode. In the PLL working mode, the VCO circuit generates oscillations, and after phase modulation by the phase-locked loop circuit, it provides a stable working clock to TCON; in the external working mode, the working clock is provided by the outside.

Separate/composite mode selection: The input synchronization signal can be divided into composite synchronization signal and separate synchronization signal. The synchronization pulse flag bits of these two synchronization signals are different.

NTSC/PAL format selection: The number of lines per frame and the flag bit of the synchronization pulse of the NTSC and PAL formats are completely different, so a judgment should be made before processing the input signal and generating the control signal. Display mode selection under high resolution: There are 8 different display modes under high resolution, including Full Mode, Normal Center Mode, Normal Wide Mode, Normal Left Mode, Normal Right Mode, Zooml Mode, Zoom2 Mode and Zoom3 Mode. Under different display modes, the display area and output control signal pulse width will be different.

3.2 Gate Driver and Source Driver The
equivalent circuit of the array control TFT-LCD panel is shown in Figure 2, in which each TFT and capacitor represents a display point, and a basic display unit pixel requires three such display points to represent the three primary colors of RGB. For a TFT-LCD with a resolution of 480*234, a total of 480*234*3 such points are required. The general structure of the entire panel is shown in Figure 2.

Figure 2 Equivalent circuit of liquid crystal panel
When TFT-LCD is working, first turn on each row of Gate Driver in a certain order, then the whole row of Source Drivers will charge the whole row of display electrodes to the required voltage at the same time, displaying different grayscales. When this row is charged, the Gate Driver will turn off the voltage, then the Gate Driver of the next row will turn on the voltage, and then the same row of Source Drivers will charge and discharge the next row of display electrodes. This sequence continues, and when the last row of display electrodes is charged, it will go back and start charging from the first row again.

In order to make the above process proceed normally, the TCON module must generate the correct timing signal to control the Gate Driver and Source Driver to open and close at the appropriate time so that the TFT-LCD can
work normally. The control signal is shown in Figure 3.

Figure 3 Driver control signal
As shown in Figure 3 (a), under the control of the TCON output signal, the Gate Driver of each row will be turned on in turn. At the same time, as shown in Figure 3 (b), the enable signal of the Source Driver will sample the RGB data to determine the voltage to be reached, and finally charge the display electrode to display the image. It should be noted that the duration of the Gate Driver enable signal pulse must be long enough to allow the Source Driver to make the display electrode reach the appropriate voltage. In addition, in actual applications, not all data signals are valid, but there are certain row and field start positions and display areas. Therefore, the start of the Gate Driver and the Source Driver must be controlled to ensure that only valid data is displayed.
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3.3 Control of other components
In order for the entire TFT-LCD system to work properly, in addition to controlling the Driver, some other components must also be controlled to cooperate with the Driver to display normally. Such as decoder, Common voltage of TFT-LCD, RGB data output order reversal, control of phase-locked loop circuit in PLL clock mode, etc. Decoder: For the decoder, TCON should control its working timing so that its decoding output working timing matches the working timing of Gate Driver.

Common voltage of TFT-LCD: In TFT-LCD design, in order to reduce the maximum operating voltage of the Source Driver and the complexity and cost of the circuit design, and because the liquid crystal molecules cannot be fixed at the same voltage for a long time, when the TFT-LCD is working, the Common voltage needs to continuously perform polarity conversion under the control of the TCON output signal.

RGB data output sequence inversion: with the Common voltage inversion, the LCD panel is controlled to display the correct color according to the level value as shown in Figure 4.

Figure 4 Common voltage and RGB timing Phase-
locked loop circuit: In the PLL clock mode, the output clock frequency of the external VCO oscillator circuit will drift under the influence of certain unstable factors, directly affecting the normal operation of the TCON module and indirectly affecting the entire TFT-LCD system. In order to prevent this from happening, the TCON module will generate a clock phase comparison signal, and through the phase-locked loop circuit, the working clock frequency of the VCO output is stabilized within an allowable range.

3.4 Structure of the timing control module
Based on the above analysis, a structural block diagram of the timing control module is given, as shown in Figure 5:

Figure 5 Timing control module structural block diagram
The mode selection module is used to determine the LCD resolution and working mode, select the appropriate clock signal, generate the required time parameters (such as the output control signal pulse width, etc.), as well as the horizontal and vertical start positions and display area information, which are used to control the Driver to correctly display valid data. The Source Driver control module limits the starting line position of each field and the number of lines displayed in each field according to the time parameters, vertical starting position and display area generated by the mode selection, and then outputs a control signal to enable the Source Driver to work properly in conjunction with the line and field synchronization signal timing. For display screens with different resolutions, the sampling period of the Source Driver is different. The higher the resolution, the shorter the sampling time of each pixel will be, so that the complete picture can be displayed within a fixed field and line period. Similarly, the Gate Driver control module limits the starting point of each line and the display area of ​​each line according to the time parameters, horizontal starting position and display area generated by the mode selection, and then outputs a control signal to enable the Gate Driver to work properly in conjunction with the line and field synchronization signal timing.

When the Driver is working, the other component control module controls other components to match the Driver working timing through the line and field synchronization signals and time parameters, so that the LCD panel can display normally.

4 FPGA logic function verification
Referring to the structure diagram in Figure 5, the timing control module is described at the behavioral level using the Verilog language, and the FPGA logic function is verified in combination with other components of the TFT-LCD.

This paper selects the Spartan-II series XC2S200 chip of XILINX as the target chip. The chip integrates 200,000 equivalent logic gates, contains 5,292 logic units, and the maximum operating frequency can reach more than 200MHz. Modelsim5.5 is used as the simulation platform, and the entire structure is described using Verilog HDL language, and compiled, simulated and downloaded for verification. Simulation environment: the input clock source is PLL mode, the TFT-LCD resolution is 480×234, NTSC format, and composite synchronization mode.

The driver's control signal is only effectively output in the display area required by the corresponding working mode, matching the timing of other control signals. The TFT-LCD displays normally.

5 Conclusion
The coordination of each module is crucial when the TFT-LCD display system is working. The design of the TCON module structure directly affects the correctness and working efficiency of its output display timing signal. As the central control module of the TFT-LCD display system, it is responsible for controlling the most important display part of the LCD system and is the key to achieving a good display effect. This paper uses Modelsim5.5 as the simulation platform, uses VerilogHDL language to describe the entire structure, and compiles and simulates it. The XILINX Spartan-II series XC2S200 chip is selected as the target chip for verification, and the simulation and verification results are given. Practice has proved that the LCD display effect is good, and the TCON module design proposed in this paper is feasible and can be used as a reference.