Design of asynchronous LED display control system

Large-screen displays based on LED technology have been widely used in public places such as squares, airports, stadiums and stations to display multimedia information such as text, animation, images and videos on the large screen. When displaying information, the LED large-screen control system does not need to process the data through a computer. It directly reads the information stored in the display buffer to display it. When the information needs to be updated, the data acquisition module of the display sends new data to the display module. This paper uses the technical method of high-speed data storage and processing to design a color asynchronous LED large-screen display control system.

1. System Design

1.1 Overall framework of the system

The overall framework of the LED large-screen display control system designed in this paper is shown in Figure 1.

Figure 1 LED large screen control system structure diagram

As 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 image data and modify display information;

2) The screen communication interface based on ARM is used to communicate with the host computer and receive image data and control signals from the host computer;

3) The large screen main controller based on CPLD and ARM realizes image grayscale modulation and reconstruction;

4) The large-screen display driving circuit part provides a constant driving current for the LED lamps of the large screen.

Figure 2 shows the data flow diagram of the system.

Figure 2 LED large screen control system data flow diagram

1.2 System Hardware Design

There are currently two mainstream main processors for display screen control systems, one is ARM, and the other is 89055 produced by ATMEL. Because ARM not only has a fast computing speed, but can also process various digital signals in real time, it is very suitable for the main processor of the LED large-screen control system, which needs to move a large amount of data during operation, and also leaves ample time and space for software programming in the later stage of design. Therefore, we selected ARM as the main processor of this system.

1.2.1 Communication module design

In the asynchronous LED large screen control system, different information can be displayed in an offline state. Therefore, the real-time update and adjustment of the large screen display content is an important link that must be carried out in the design of the LED large screen control system. The communication function between the LED display and the host computer is designed and studied in order to quickly and reliably obtain the adjusted display content. Therefore, the system designed in this paper provides three communication methods, namely Ethernet communication, serial communication and wireless radio frequency communication. These three communication methods not only enhance the performance of communication, but also facilitate user use.

1.2.2 Display control module design

The screen control circuit is an important part of the LED large screen control system. The display screen control logic circuit generates read/write control signals and addresses for accessing display buffer data. The read display data is sent to the LED display screen through the display driver. The display control module is mainly composed of a read/write address generator, a counter, a read/write address selector, a read/write selector, a read/write signal generator, a shift latch, and a synchronous controller.

To display a complete graphic image on the LED large screen, CPLD and ARM must work in unison to complete the functions of reading and decompressing the graphic image data in the display memory, making image display effects, grayscale modulation, and generating the driving logic of the display screen. The hardware structure diagram of the display control module is shown in Figure 3.

Figure 3 LED display main controller circuit diagram

In the control circuit, the data bus connects the CPLD and ARM. S3C44B0X selects the data buffer in the CPLD through chip 2, so the base address of the ARM output data is 0x40000000. ARM first copies the image data saved in FLASH to SDRAM. After relevant processing, it calls different dynamic display effect programs to write the image data in SDRAM into the CPLD buffer in a loop. The CPLD generates the corresponding timing conversion and outputs it serially to the LED drive circuit.

2LED large screen control system software design and implementation

Taking into account the operating efficiency of the system hardware driver and the software development cycle, this system uses C language as the software development tool.

The designed application needs to realize the connection function with the display hardware. First, it needs to communicate with the underlying hardware driver of the system, and second, it needs to provide users with a control interface and receive various instructions input by users. For this reason, Visual C++ is used as the development tool for system applications. The communication application is written using the MSComm (Microsoft Communications Control) control. MSComm is an ActiveX control provided by Microsoft to simplify serial communication programming under Windows. The MSComm control transmits and receives data through the serial port of the external device, and can provide serial communication functions for the application. It is very fast and convenient to use the MSComm control to program the serial port. The MSComm control is event-driven. Under normal circumstances, 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 that occur during the program's operation. The biggest advantage of the 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, multiple MSComm controls must be used. The behavior of the MSComm control is determined by its various properties. In VC++, these properties 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++, the MSComm control appears as a derived class of the CWND class. This window must be created when the system is initialized, and its properties must be set using member functions so that it uses the same communication format as the lower computer, otherwise correct serial communication cannot be established.

3 Conclusion

At present, many existing LED large-screen control systems still use 8-bit or 16-bit single-chip microcomputers as the main processor of the system. The system has the disadvantages of slow operation speed, small storage space, single communication method and slow data transmission speed. In view of the above problems, this paper designs a new LED large-screen control system. The system uses ARM as the main processor and combines programmable logic devices to design the system's drive circuit, realizing the display of color images, text and videos on asynchronous LED large screens.