Application of TFT LCD in GPS Navigator

Overview

With the development of my country's automobile industry and the increase of automobile users, the autonomous real-time navigation technology of vehicles has attracted more and more attention and has been widely used. GPS vehicle navigators receive satellite signals and then cooperate with electronic map data to provide timely navigation. It is an embedded product that can master its own position and destination. The autonomous navigation mode does not charge any usage fees. Users can selectively obtain map data according to their own needs.

The display system module is an important module of the GPS navigator. The TFT LCD display has more realistic colors, is smoother and more delicate, has a stronger sense of layering, and has the advantages of small size and light weight. This design uses Samsung's LTV350QV-F05 TFT LCD Screen. The system block diagram is shown in Figure 1.

LCD control interface for XSCAL PXA255 processor

The PXA255 processor is a processor based on the XSCAL core (compatible with the ARM instruction set) launched by Intel. It integrates an LCD controller and can support DSTN and TFT LCD. The interface signals of its LCD controller are:

L_DD[15:0]: color data signal.

L_PCLK: pixel clock signal, used to drive color data into the shift register of the LCD display. In active mode, the pixel clock jumps continuously.

L_LCLK: used for synchronous walking signal.

L_FCLK: used for synchronization frame signal.

L_BIAS: While transmitting color data, the enable signal (OE) is valid to notify the LCD display to receive data.

LTV350QV－F05 TFT LCD screen

In this design, Samsung's LTV350QV-F05 3.5-inch TFT LCD screen is used. This screen has two working modes, DE mode and SYNC mode. The power-on sequence and initialization of the TFT LCT screen are configured through the simulated I2C bus. The maximum resolution of the display is 320×240, and it supports up to 24-bit color signals. The data interface is RGB signal, including 8-bit red (R), 8-bit blue (B), and 8-bit green (G).

Bias circuit and VCOM circuit

The LTV350QV-F05 module has two bias voltages VGL (-10V) and VGH (18V), one analog power supply AVDD (5V) and one digital power supply VDD (3V). In order to save money, the bias circuit is composed of discrete components such as gate circuits, diodes and capacitors. The circuit is shown in Figure 2, in which LCD_5V is the system power supply.

(1) VCOM circuit

The VCOM signal is formed by superimposing a DC voltage on the COM signal generated by LTV350QV-F05. The VCOM signal will affect the display effect of the screen, but the LCD screen can be adjusted by adjusting the DC voltage superimposed on the VCOM signal. To save board space, the VCOM signal can be implemented with an op amp. The circuit schematic diagram is shown in Figure 3.

(2) Backlight circuit

The MP1518 backlight chip can be used to drive the six series-connected light-emitting diodes inside the LTV350QV-F05LCD to achieve backlight display. The circuit diagram is shown in Figure 4.

LCD driver part

The LCD controller is connected to the application program and operating system through FRAME BUFFER. In embedded devices, FRAM BUFFER is usually a virtual display area opened in SDRAM. The user program can achieve the purpose of displaying images by calling the FRAM BUFFER driver through the system. The schematic block diagram of the LCD driver part is shown in Figure 5:

This design uses the Windows CE operating system and needs to configure the initialization function lcd_init() of the LCD screen. This screen can be configured through the analog I2C interface. The main functions used are: LCDClockHigh(), LCDDataHigh(), LCDSCHigh(), LCDCSLow(), LCDClockLow(), LCDDataLow(), SDATA_OUT(), voidLCD_Write(), etc. The above functions also implement the simulation of the I2C bus.

The configuration sequence in the lcd_init() function is shown in Figure 6.

Among them, R01-R19 are the internal configuration registers of LTV350QV-F05. Add appropriate delay functions between the above steps to ensure the completion of the register configuration. After the LCD screen is initialized, the application can make system calls to the FRAM BUFFER. After the operation, the image can be displayed normally.

in conclusion

The LCD module designed by the above scheme has been successfully used in GPS car navigation systems, and actual use has proven that the image is clear and natural.