Design of LED Display Screen Based on a Modularization

LED large screen display has been more and more used in advertising, information release, traffic instructions and other public places because of its eye-catching and flexible content. LED display is mainly divided into digital display and dot matrix display. This paper only discusses dot matrix display. At present, the requirements of the LED display screen are basically put forward by the user first, which is customized by the manufacturer according to the needs. Each time, the circuit and mechanical structure must be designed repeatedly, resulting in a waste of resources. Moreover, if the needs of the user change, it will be very difficult to change.

In fact, regardless of the size of the display screen, its principle is the same, so it is completely possible to design a standardized and modular LED display screen, which only needs to combine the corresponding modules for different needs. This paper introduces a modular LED display, which can flexibly change the size according to needs, run independently of the computer, and realize special effects such as flashing, rolling display and so on. The common problem of flashing due to insufficient refresh rate of the integral display screen is no longer a problem because it is divided into small modules in this design.

1 basic principleThe basic design idea is to divide the whole display screen into several display modules with the same size and function, and use a central control module to control and synchronize the operation of all display modules to realize the overall coordinated display.Therefore, the system is mainly composed of two parts: display module and control module.

The display module is actually a small LED driver board of the same size, which can drive the LED array, has basic display functions, and can receive data and commands from the control module. In addition, some common display effects such as flashing and scrolling are also included in the display module. The existing national standard dot matrix Chinese character library has 16 16 and 24 24. Considering the convenience of software writing and the ability of hardware, we regard the display module as 16 16 dot matrix size.The control module is responsible for controlling each display module and sending data and commands to each display module. The control module stores the lattice data information to be displayed and transmits it to each display module regularly. The control module is also equipped with a keypad and a universal asynchronous serial port, which can be operated by the keyboard or controlled by the computer, and the computer can also be used to update the data in the control module.The display module and control module are connected by bus, and we choose I2C bus.

The design and use of the whole system has a great degree of freedom: the number of display modules controlled by the control module can be changed, and the arrangement of display modules can also be changed (for example, 16 display modules can be 2) 8 can also be 4 4) , for the same display screen, several conditions can be saved and displayed separately with information. The lattice size of each information can be different, and the information exceeding the screen size can be scrolled. This can basically meet a variety of needs.2 circuit structurePIC16C73 is used as the central controller. The MCU has built-in I2C interface and serial port, and has strong port line driving ability (25mA per port line). It can simplify many peripheral circuits.

Two EEPROM are used for data storage: x24256 provides a total of 64K bytes of data space, which can be erased and reset repeatedly, and the data will not be lost after power failure. If all are used to display 16 16 dot matrix Chinese characters can display about 2000. The data memory shares the I2C bus with each display module, and depends on their respective addresses to distinguish whether it is a memory or a display module.The keypad is externally connected with 4 current limiting resistors, which are directly connected to the port line of the CPU, and 4 current limiting resistors are used for the keyboard 4. Scanning mode.The connection between the control module and the computer uses RS232 serial port and a max202 for level conversion.

In addition, the control module also has a light intensity sensor, which is used to adjust the brightness of the display screen. Due to the great difference in ambient light intensity between day and night, LiangFang who feels appropriate during the day may sleep at night. Therefore, use this sensor to automatically adjust the brightness of the display screen according to the external light intensity. Tls235 optical frequency conversion element of TI company is used here.Each display module shall drive one 16 16 LED array, due to the small number of port lines of single chip microcomputer, it is impossible to use static display. Here, dynamic scanning display is used. Considering the refresh speed, there may be flicker when refreshing with 16 lines of scanning, so the method of 8 lines of scanning and 32-bit data is used.Each data line only corresponds to one light-emitting unit at each time, and the current is required to be small; When the display screen is fully open, 8 common anode scanning lines will correspond to 32 light-emitting units, and the current requirements are large, so the driving circuits at both ends are designed separately. Four tpic6b595 chips of TI company are used at the data line end. The scanning end uses two pieces of 2944 from allegro.

The display module and the control module are connected by I2C bus. Since the task of the display module is only display, no additional memory is added, and only the internal RAM unit saves the lattice data to be displayed and scans and displays it on the LED array continuously. Display effects such as scrolling and blinking only perform additional processing on the display data, such as the shift operation of scrolling the actual corresponding data. These functions are also written in the display module, and the control module sends commands to activate these functions.In addition, it should be noted that the driving circuit of the display module and the LED array can be regarded as two separate parts. As long as within the driving capacity of the driving circuit, LED arrays with different light-emitting units can be used, such as a small display board with only one LED or a large display board with 5 7 LEDs.3 software design

The software of the system includes a part: display module, control module and PC.

The software of the display module mainly includes three parts: scanning display, special effect processing and I2C communication. 32 bytes are opened in the ram of the three display module CPU as the display buffer. The scanning display program only takes data from the display buffer regularly and sends it to the mobile register, and sets the corresponding scanning line to realize one-time scanning. The scan display does not recognize what the data in the buffer is. Scanning takes advantage of the image retention effect of human eyes. If the scanning frequency reaches more than 60Hz, it is not sensitive to the fluctuation of scanning frequency. Therefore, the scanning display program here does not use timer interrupt, but is only completed by cycle.

Special effect processing refers to the operation of shifting the data in the scanning buffer, which mainly corresponds to the scrolling display effect. When scrolling, scroll one pixel row or column at a time. Most of the data already exists in the display module. When the control module sends the scrolling command, the United Nations adds two bytes (1 row or 1 column) of new data. Because the scanning method used is 32 8. Therefore, the data storage is not completely linear, but there is a jump storage part. In addition, the character rolling display has four directions up, down, left, right, so it is necessary to move the new data into the buffer and remove the two byte old data, so it is necessary to calculate the RAM address more complicated.

I2C communication is a function realized by PIC16C73 internal hardware. It is relatively simple and can be realized only by interrupt.

The adjustment function of display brightness is realized by adjusting the lighting time of the light-emitting unit. Specifically, due to the scanning display, each light-emitting unit can only be lit for 1 / 8 of the time at most. If all of this time is lit, the effect is the maximum brightness. If it is still not lit for some time within this 1 / 8 of the time, the overall display brightness will decrease. The display module receives the ambient brightness data from the control module and adjusts the proportion of the lighting time to realize the function of adjusting the brightness.

The control module program includes I2C communication with display module, I2C communication with EEPROM, serial communication with computer, data address calculation and external light intensity detection.Two I2C communication parts and serial communication are supported by hardware, which is relatively easy. The external light intensity sensor directly converts the light intensity into frequency and sends it to the CPU, so it only needs to count regularly. The more complex part is the address calculation of data. Because the degree of freedom and data of the system are stored in EEPROM by index, the address calculation logic is simple, but the implementation is complex. Different contents should be sent to each display module. The case of scrolling display is more complex.If it is a static display, the control module will only transmit data once after receiving the command and enter idle waiting: if it is a flashing, scrolling and other display, it is necessary to take data from EEPROM regularly to generate each display module.

The program of PC is mainly dot matrix editing function and RS232 communication function. Dot matrix editing can not only write characters of various fonts, but also draw simple graphics; RS232 communication not only includes all functions that can be realized by keyboard, but also can directly display graphics on the display screen without saving it to the control module to realize real-time information release.

The system has made a beneficial attempt to standardize and modularize the commonly used LED large screen display device, and has been applied to the occasion of traffic condition display, and achieved good results, which proves that the design is reasonable and feasible. Because the design retains a lot of freedom for use, it can be flexibly combined and applied to various occasions with different needs without repeated design, which effectively saves resources. At first, the system corresponds to a relatively small display screen, so I'm afraid I2C bus is selected. If you want to do a considerable display of zero, you can also replace it with RS485 bus to provide more room for expansion. In addition, based on this design idea, products such as color LED large screen display can be further developed.

Source; International LED network