LED dot matrix display system design

In places such as stations, shopping malls, and schools where changing real-time information or advertising is required, traditional media such as blackboards, paper, or neon billboards can no longer meet current needs in terms of display effects or modifiability. LED dot matrix displays have the characteristics of low power consumption, low cost, long life, small space occupation, and real-time display. In addition, the amount of information displayed is large, and users can edit and modify the displayed content at any time. Therefore, it has been widely used in recent years.

1. Overall design

The system adopts the structure of upper computer and lower computer. The upper computer is a PC, which communicates with the lower computer display system through a serial communication interface [1] to realize real-time erasing and updating of the displayed content. The lower computer system mainly includes two parts: the single-chip microcomputer control circuit and the display circuit. The Chinese character display adopts the 16x16 dot matrix mode. Through the control of the single-chip microcomputer, the dynamic display effect of characters scrolling from right to left is realized. The basic block diagram is shown in Figure 1.

2. Hardware structure of the system

2.1 MCU selection

MCU is the core component of the whole lower computer system. Its performance and on-chip resources largely determine the flexibility, advancement and stability of the system. Based on this, this system uses the STC89C55RD+ enhanced 51 single-chip microcomputer produced by STC .

The instruction system, hardware structure and on-chip resources of the STC89C55RD+ microcontroller are fully compatible with the standard 8052 microcontroller and are packaged in DIP40. The maximum supported clock frequency is 80M, which can maximize the running speed of the MCU. The chip contains a large-capacity 20KB FLASH program memory and 1KB data memory, and the internal available Data FLASH is 58 sectors totaling 29KB. It has in-system programmable (ISP) and in-application programmable (IAP) functions, which can realize remote software upgrades without a programmer, thereby greatly reducing the complexity of development and saving the additional investment in purchasing a programmer.

2.2 Serial Communication

The system host and slave communicate through a serial communication interface. The STC89C55RD+ microcontroller contains a programmable full-duplex serial communication interface, namely RXD (P3.0) and TXD (P3.1), which has all the functions of UART. The interface circuit can send and receive data at the same time. In general, serial communication with the host PC can be achieved through the three wires of RXD, TXD and GND. According to the distance of communication, RS232 or RS485 communication mode can be selected. Since the display control circuit in this system is close to the host PC, the RS-232 standard bus interface is selected. The specific circuit diagram is shown in Figure 2.

2.3 Driving and scanning circuit

The LED dot matrix uses 8x8 modules, and every 4 modules are arranged into a 16x16 dot matrix to display a Chinese character. All LEDs in each row of the dot matrix share a cathode, and all LEDs in each column share an anode.

Because the driving capability of the I/O port of the single-chip microcomputer is limited, the cathode of each row of LEDs is connected to the power supply through a transistor 8550, and the I/O port only needs to provide a few milliamperes of input current to control its on and off. Considering that this system can display six Chinese characters at the same time, the anode of each column of LEDs is connected to the power supply through a current limiting resistor and a transistor 8550. When the Chinese character fonts of the same row of six Chinese characters (i.e. 12 bytes) are sent out in parallel through the latch 74HC377, the shift register 74HC595 outputs a row selection signal to light up the LED of the row, and then sends the next row of data, and then selects the next row to be valid, until all 16 rows are scanned once. At this point, a complete text message is displayed, and then it is scanned repeatedly in this way. With the help of program control, the information can be dynamically displayed from right to left. The specific circuit diagram is shown in Figure 3.

The main functions implemented by the software of this system include: sending information (or commands) from the upper computer and controlling the content and mode of the dot matrix display of the lower computer.