The development of information technology promotes the continuous progress of flat panel display (FPD) technology. New display technologies such as LCD (liquid crystal), PDP (plasma) and LED (light emitting diode) have been widely used in the field of display screen. Among them, LCD (liquid crystal) technology has become the mainstream, with many products and mature market mechanism. Although LCD technology has achieved great success, people still do not stop exploring and studying new technologies. LED display technology is a new technology in recent years. LED technology has attracted more and more attention because of its unique characteristics of high brightness, low power consumption and long service life. Large screen display based on LED technology has been widely used in public places such as squares, airports, sports venues and stations to display multimedia information such as text, animation, image and video on the large screen. When displaying information, the LED large screen control system does not need to process the data by the computer. It directly reads the information stored in the display buffer to display. When the information needs to be updated, the data acquisition module of the display sends new data to the display module. In this paper, a color asynchronous LED large screen display control system is designed by using the technical method of high-speed data storage and processing.
1 system design1.1 overall framework of the systemThe overall framework of the LED large screen display control system designed in this paper is shown in Figure 1.
Figure 1 structure diagram of LED large screen control systemAs can be seen from the figure, the system mainly includes the following four components:1) The upper computer user control platform is mainly used to compress the image data and modify the display information;
2) The screen communication interface part based on arm is used to communicate with the host computer and receive the image data and control signal of the host computer;3) The large screen main controller based on CPLD and arm realizes image gray modulation and reconstruction;4) The large screen display driving circuit provides a constant driving current for the LED tube of the large screen.
Figure 2 shows the data flow diagram of the system.Figure 2 data flow diagram of LED large screen control system1.2 system hardware design
At present, there are two mainstream main processors for display control system, one is arm, the other is 89055 produced by ATMEL company. Because arm not only has fast operation speed, but also can process all kinds of digital signals in real time, it is very suitable for the characteristics that the main processor of LED large screen control system needs to move a large amount of data during operation. At the same time, it also leaves plenty of time and space for software programming in the later stage of design. Therefore, we choose arm as the main processor of the system.
1.2.1 communication module design
In asynchronous LED large screen control system, different information can be displayed in offline state. Therefore, to realize the real-time update and adjustment of large screen display content is an important link in the design of LED large screen control system. The communication function between LED display and host computer is designed and studied in order to obtain the adjusted display content quickly and reliably. Therefore, the system designed in this paper provides three communication modes, namely Ethernet communication, serial communication and wireless RF communication. These three communication modes not only enhance the performance of communication, but also facilitate the use of users.
1.2.2 design of display control moduleThe screen control circuit is an important part of the LED large screen control system. The display control logic circuit generates the read-write control signal and the address to access the display buffer data. The read display data is sent to the LED display through the display driver. The display control module is mainly composed of read / write address generator, counter, read / write address selector, read / write selector, read / write signal generator, shift latch and synchronization controller.To display a complete graphic image on the LED large screen, CPLD and arm must work together to complete the functions of reading and decompressing the graphic image data in the display memory, making the image display effect, gray modulation and generating the driving logic of the display screen. The hardware structure diagram of the display control module is shown in Figure 3.
Fig. 3 circuit diagram of LED display main controllerIn the control circuit, the data bus connects CPLD and arm, and S3C44B0X gates the data buffer in CPLD through chip 2, so the base address of arm output data is 0x4000000. ARM first copies the image data saved in FLASH to SDRAM. After related processing, it calls different dynamic display program to write the image data in SDRAM into CPLD buffer, CPLD generates the corresponding timing conversion, and outputs it to LED drive circuit serial.2 software design and implementation of LED large screen control system
Considering the operating efficiency of the system hardware driver and the software development cycle, the system uses C language as the software development tool.
The designed application program should realize the connection function with the display hardware. First, it should communicate with the hardware driver at the bottom of the system. Second, it should provide the user with a control interface and receive various instructions input by the user. Therefore, Visual C is used as the development tool of system application. The communication application program is written using MSComm (Microsoft communications control). MSComm is an ActiveX control for simplified window and serial communication programming provided by Microsoft for users. MSComm control transmits and receives data through the serial port of external devices, which can provide serial communication function for applications. Using MSComm control for serial port programming is very fast and convenient. MSComm control is event driven. In general, it must be notified when an event occurs. In actual programming, you can add your own processing code to the OnComm event processing function. The OnComm event processing function can also check and handle communication errors in the running process of the program. The biggest advantage of OnComm control is that the program response time is very short and the reliability is high. Each MSComm control corresponds to a serial port. If the application needs to access multiple serial ports, use multiple MSComm controls. The behavior of MSComm controls is determined by their various properties. In VC , these attributes can be set through a series of member functions to control the data format, baud rate and other characteristics of the serial port. In addition, in VC , MSComm control appears as a derived class of CWnd class. This window must be created during system initialization, and its properties must be set with member functions to make it use the same communication format as the lower computer, otherwise correct serial communication cannot be established.