Design of large screen LED dot matrix display system using AT89C51

1 Introduction

The system uses the newly launched AT89S51 of ATMEL as the main control chip. The single-chip microcomputer completes the communication with the PC, and also completes data storage, cyclic display, etc. The system can run offline, and has the characteristics of simple structure, convenient maintenance, fast display refresh speed, and low cost. With the online download program, the displayed content can be updated at any time, and the size of the display area can be changed by cascading, which is very convenient to use.
This paper designs a monochrome LED display control system with equal grayscale. The control system uses AT89S51 as the controller and adopts RS-232 communication standard. It can display Chinese characters, characters, numbers or monochrome pictures.

2 Composition of LED display system

2.1 Principle block diagram and hardware circuit structure

The display screen control system consists of a display controller and an LED dot matrix display screen. The structural block diagram of the control system is shown in Figure 1. It is mainly composed of a microprocessor, a communication circuit, a data storage device, a display circuit, etc.

2.2 AT89S51 chip

This display system uses the newly launched AT89S51 from ATMEL as the main control chip. AT89S51 has the same core and pin arrangement as INTEL's 8051. Table 1 is the internal function table of AT89S51. In addition to all the functions of 8051, AT89S51 also has some practical functional components built in. For example, the internal program memory of AT89S51
is a 4KB erasable flashROM. The entire process of downloading program code only takes a few seconds, which is very convenient to use. The internal program memory of 8051 is a 4KB PROM, which can only write program code once and cannot be modified later. In addition, AT89S51 provides an ISP download interface. It is very suitable for the design or development of single-chip microcomputer application systems.

3 Introduction to system hardware circuit

The hardware circuit of the display controller is mainly composed of three parts: external memory expansion, serial communication interface and LED dot matrix display driver and interface circuit.

Generally speaking, there are many contents to be displayed, so it is necessary to expand the external data memory. Static data memory has low cost, low power consumption, and fast read and write speed. We use two pieces of static RAM 62256 data memory (total 64KB). All the content codes to be displayed are stored in 62256.

The interface circuit between 62256 and single-chip microcomputer is shown in Figure 2. The displayed content code is downloaded to 62256 through the serial interface. The serial communication interface circuit is shown in Figure 3, which is connected to the serial port of PC through MAX232E. The LED display driver and interface circuit is shown in Figure 4. The control signal is divided into two paths after passing through the buffer 74LS245. One path is decoded by CD4515 and then driven by TIP127 to drive the row of the dot matrix; the other path is driven by the data shift register 74HC595 to drive the column of the dot matrix. When multiple display units are cascaded, just connect the input of the next level to the output of the previous level accordingly.

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Figure 3 Communication circuit between AT89S51 and PC

4 Software Programs

The software program is the core part of the entire control system. The entire software design is mainly divided into two parts: the display part and the communication part. The display part adopts a dynamic scanning method to realize the transmission control and display functions of the Chinese characters, images, characters and other data information to be displayed on the display screen. The data storage is divided into three areas in the program: display buffer, data storage area and receiving buffer. The single-chip microcomputer receives the data from the PC through the serial port interrupt, temporarily stores it in the receiving buffer, and after analysis and processing, it is stored in the data storage area according to a certain rule, and then the data is taken out from the data storage in turn according to the display mode and put into the display buffer for display. The display adopts a line-by-line scanning method. Figure 5 is a program flow chart for displaying a screen of characters. The real-time communication part with the PC mainly uses the single-chip microcomputer serial port interrupt to receive data information to realize data information transmission with the computer. Its program flow chart is shown in Figure 6.

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The upper computer software of the communication part is programmed in Visual Basic (VB for short). VB provides a powerful communication control MSCOMM, which can set the serial port status and the information format and protocol of serial communication, and directly use the serial port of the PC to send data. In order to achieve reliable communication between the microcontroller and the PC, it is necessary to ensure that both parties have the same data format and baud rate. The use of this software is relatively simple, as shown in Figure 7. When using it, you can first use Microsoft Word to process the content to be displayed (note that the width of the page setting should be the same as the width of the LED display), then select the Select All command and Copy command in the Edit menu, and then run the LED display download software V1.1. Select the "From Clipboard" button in the interface, and the content to be downloaded and displayed can be displayed in the preview box on the right. After checking that it is correct, select the appropriate baud rate and the connected serial communication port, and press the "Download" button to download it to the display screen for display. Repeat the above operations when you need to modify or display new content. It is very convenient to use.

5 Conclusion

The dot matrix LED display control system designed in this paper is based on the AT89S51 single-chip microcomputer, uses static RAM as data storage, and uses a serial interface to achieve data transmission with a PC. The anti-interference ability of the single-chip microcomputer is also taken into account in the system design, which effectively improves the reliability of the system operation. It can be directly applied to actual occasions. It can realize the display of Chinese characters, characters, numbers and other contents. The displayed content can be updated at any time. It is very convenient to use.