Design of LED display screen control circuit based on CPLD

In recent years, with the rapid development of computer technology and integrated circuit technology, the most widely used large-screen display system is the video LED display system. In LED display technology, the brightness, light effect and color difference of red and green light-emitting diodes have been greatly improved. In addition, with the development of computer multimedia production software, the manufacturing cost of pseudo-color video LED display systems has been greatly reduced, and the application field has been continuously increased. This pseudo-color video LED display system uses computer multimedia technology to display video images in full synchronization and dynamic mode. The image is clear, the brightness is high, and there is no seam. The grayscale level of each color has increased from the early 16 grayscales to the current 256 grayscales. With the development of large-scale integrated circuits and special components, a full-color video LED display system with 256 grayscales may be realized at any time.

　　LED electronic display technology has developed rapidly and has become one of the leading technologies in the field of flat panel display. This article focuses on the design of the control circuit of LED display screen using M4A5-128P64-10VC.

　　1 Composition of LED display

　　In the LED display system, the dot matrix structure unit is its basic structure. Each display driver unit is composed of several 8×8 dot matrix LED display modules. Multiple display driver boards are assembled together to form a display screen of several square meters, which can be used to display various texts and images. The LED display screen includes computer video acquisition circuit, control circuit, drive circuit and power supply, as shown in Figure 1.

　　The LED display has two primary colors, red and green, with 256 gray levels for each primary color. The pixel pitch is 7.62mm, and the pixels can reach 1024 dots in the horizontal direction and 768 dots in the vertical direction.

　　2 LED electronic display features

　　The LED display screen is composed of several display units. Its display method adopts the principle of mapping LED dot matrix with computer display screen, that is, one pixel of LED dot matrix corresponds to one pixel of computer display screen. For example, the picture on the computer screen is divided into 640 columns and 480 rows according to the resolution, that is, 640×480 dot matrix units on the LED display screen. Each dot matrix unit includes three kinds of light-emitting diodes: red, green and blue. These three kinds of light-emitting diodes emit three kinds of colors of light, and after mixing, they get the color perceived by human eyes. According to the principle of optical three primary colors, we only collect the image of each point on the computer screen, digitize it and decompose it into three kinds of signals: red, green and blue. After system processing, it is transmitted to the dot matrix unit on the LED dot matrix screen, and the light-emitting diodes of corresponding colors are driven respectively, thus realizing the mapping of computer screen on LED dot matrix screen.

　　3 LED electronic display driving principle

　　In most LED display systems, a refresh drive method is used, that is, data is latched in the column direction for each LED display driver unit and scanned in the row direction. According to the LED display driver board structure, a 1P16 scan duty cycle is used.

　　The LED display driver board driver circuit we designed uses two 74HC595 chips to form a 4:16 line decoder, which provides the line signal required by the entire scanning circuit. At the same time, the 74HC595 chip is also used as a serial shift register, which shifts the serial data from the system into parallel signal output. In this way, the driver column needs to provide a serial shift clock, a parallel lock-in signal and an output enable signal. The line scan requires serial data input and a serial shift clock signal, as shown in Figure 2. Therefore, we need to design a timing control circuit.

　　4 Conclusion

　　M4A5-128P64-10VC is a CPLD device produced by Lattice, with 128 macro units and 64 IPO pins. Lattice's development software ispDesignEXPERT integrates all the work of design input, compilation, verification and programming. First, design input is performed, and the schematic diagram can be directly drawn or programmed in VHDL language. We need to get a 100KHz signal from a 10MHz clock source. Use VHDL language to write a file named F100K.HDL in the VHDLMODULE text editor as follows:

　　After the design file is entered, it is compiled, and the user can adjust the pin assignment. After the compilation is passed, the chip can be programmed. Use the Byte-Blaster download cable to connect the computer parallel port to the JTAG socket on the PCB, and program the installed chip after power is turned on.