Design of Color Liquid Crystal Display System

The popularity of high-performance embedded portable devices has put forward new requirements for the design of embedded display systems: high performance, low power consumption, small size, good portability and the ability to work in various lighting environments. The embedded display system designed in this paper provides a solution for the development of display systems for such portable devices. It not only meets the high performance required by high-end embedded devices, but also maintains low power consumption under high-brightness display conditions. It is suitable for high-end PDAs, portable media players, handheld navigators, portable medical and test equipment and other fields.

The proper selection of the main components is the key to achieve the above design goals. The embedded microprocessor Samsung S3C2440A (also the LCD controller of this display system) selected in this paper is mainly for high-end embedded devices. It uses the ARM920T core, with a maximum operating frequency of 533MHz, and contains 3-channel asynchronous serial ports, SDRAM controller, I2C bus interface, USB host and slave device interface, camera interface, AC97 audio codec interface, touch screen interface and LCD controller and many other on-chip peripherals, low power consumption, high cost performance. In addition, Sharp's 3.5-inch transflective TFT-LCD LQ035Q7DH01 adopts Advanced-TFT technology, which uses the metal film of the conductive traces of interconnected thin film transistors as a reflector to return the incident light through the LCD matrix, thereby improving the brightness under strong ambient light while maintaining low power consumption; it has HR-TFT (high reflective liquid crystal) function in bright places, and has a built-in backlight transmissive liquid crystal function in dim environments, and can work in strong sunlight and dark environments. Its screen resolution is 320×240, and it can display 262144 colors using an 18-bit data signal.

This paper selects embedded Linux as the operating system because of its good stability and platform portability, open source code and low cost. The software development environment uses Linux 2.4.20 platform and the cross compiler is arm-linux-gcc 2.95.3, and the driver development for the LQ035Q7DH01 display is completed.

Display system hardware circuit

1 LCD controller circuit

The LCD controller is used to transmit image data and generate corresponding control signals. The S3C2440A LCD controller can support up to 4K color STN screens and 256K color TFT screens, support various LCD screens at 1024×768 resolution, and has LCD-specific DMA. The control signals generated by the S3C2440A mainly include the frame synchronization signal VFRAME, the line synchronization signal VLINE, the pixel clock signal VCLK, and the data output enable signal VM. The S3C2440A has a total of 24 RGB data lines VD[0:23]. Different data formats have different wiring methods. This article uses the RGB565 method. The timing of the control signal and data signal is shown in Figure 1.

The LCD controller circuit is shown in Figure 2.

2 Timing and Data Matching Circuits

Since the LCD controller of S3C2440A and the LCD screen LQ035Q7DH01 cannot match in data format and display timing, it is necessary to select a timing control IC or use CPLD to map the data interface of different data formats. Since CPLD has a large area and high cost, it is usually only used when the circuit needs to be flexibly configured. The timing control IC of this article uses Sharp's LZ9FC22, which is small in size, stable in performance, and dedicated to QVGA screen TFT-LCD. This is an 18-bit (R6G6B6) controller. Since this article uses the RGB565 16-bit working mode, the chip input pins R0 and B0 are grounded. Pin SIZE is used to select the LCD screen type. When connected to a low level, it is a 320×240 screen. When connected to a high level, it is a 240×320 screen. The LCD in this article is a 240×320 screen, so this pin is connected to a high level. Pin VRVE is used to select the LCD frame scanning direction. When connected to a low level, the scanning direction is from top to bottom, and when connected to a high level, it is the opposite. Pin HRVE is used to select the LCD row scanning direction. When connected to a low level, the scanning direction is from right to left, and when connected to a high level, it is the opposite. In order to facilitate different users to flexibly configure according to their respective needs, this paper designs an LCD scanning direction configuration circuit, as shown in Figure 3. Each of the two pairs of resistors R32, R34 and R33, R35 selects a connection circuit as needed to set the high and low levels of pins VRVE and HRVE. If you want to configure the LCD frame scanning direction from top to bottom, pin VRVE should be low level. At this time, only resistor R35 needs to be connected to the circuit, and resistor R33 does not need to be connected; if you want to configure the LCD frame scanning direction from bottom to top, pin VRVE should be high level. At this time, only resistor R33 needs to be connected to the circuit, and resistor R35 does not need to be connected. The configuration of LCD row scanning direction is similar. The LCD in this article adopts the scanning direction from top to bottom and from left to right, so the resistors R32 and R35 are connected to the circuit and the positions of R33 and R34 are left empty.

Figure 3 Timing and data matching circuit

3 Multi-channel voltage generation circuit

LCD screens integrate digital circuits and analog circuits, and require external digital voltage and analog voltage. In addition, in order to complete data scanning, TFTs need to be turned on/off in turn. When the TFT is turned on, data is loaded to the display electrode through the source driver, and the voltage difference between the display electrode and the common electrode acts on the liquid crystal to realize display. Therefore, it is necessary to control the LCD's turn-on voltage, turn-off voltage, and voltage applied to the common electrode.

This article uses the low-dropout voltage linear regulator LM1117DT-3.3 chip produced by Panasonic to generate the digital voltage required by the timing control IC and LCD. The active matrix liquid crystal display power supply chip MAX1664 launched by MAXIM of the United States is used to generate other voltages. The +5V input voltage required by the two chips is provided by 220V AC after being converted by an AC/DC switching power supply. MAX1664 has two integrated DC-DC converters, of which DC-DC1 provides an output voltage ranging from the input voltage value to +5.5V, and DC-DC2 is a positive and negative voltage dual output, one of which can provide an output voltage from the input voltage value to +28V, and the other can provide an output voltage of 0 to -10V. For small TFT LCDs such as LQ035Q7DH01, MAX1664 can provide it with efficient regulation voltage. In addition, MAX1664 is a high-power switching power supply. Pay attention to the connection of the power supply circuit and the connection of the bypass capacitor. A 33Ω resistor is used to isolate the chip bypass terminals IN and INP, as shown in Figure 4. D4~D6 in Figure 4 should use high-speed Schottky diodes. At the same time, since the DC equivalent resistance of the inductor has a greater impact on the conversion efficiency, L3~L4 should use inductors with low equivalent resistance. In order to reduce noise radiation, shielded inductors should be used. The wiring of the circuit board should be carefully operated, and the connection of the grounding point should be handled carefully, otherwise it will affect the stability of each output voltage.

Figure 4 Multi-channel voltage generation circuit

4 Display system overall structure block diagram

The LCD controller first reads the image data from the display buffer of the memory SDRAM and converts it into the data format of RGB565, and then sends the data signal and the control signal generated by the LCD controller to the timing control IC LZ9FC22. The timing control IC converts the data signal and the control signal into a format that matches the LCD and sends them to the LCD. Finally, the LCD displays the image on the screen. It should be noted here that when wiring the circuit board, the connection distance from the LCD controller to the LCD screen should not be too long, preferably not more than 50cm, otherwise display errors are likely to occur. The overall structure block diagram of the display system is shown in Figure 5.

Figure 5 shows the system structure diagram

Conclusion

The embedded display system designed in this paper has a display brightness of 100 nits. When the LCD is at high brightness, the power consumption of the module is less than 365mW, which overcomes the contradiction that high brightness of general TFT-LCD is accompanied by high power consumption. At the same time, since the designed hardware drive circuit only requires the LCD controller to provide frame synchronization signal, line synchronization signal, pixel clock, data enable signal and RGB data signal, it brings greater flexibility for porting to different platforms and is very practical.