Design of LED Display System Based on AT91 M42800A

Recently, the author found in a logistics call system project based on fieldbus on a large production line in a factory that due to the large amount of information flow to be displayed, the existing LED display screen control system based on AT89C51 chip is limited by the processing speed, system architecture, addressing range, peripheral interface resources, etc. of the microprocessor. It is difficult to obtain good dynamic visual effects when more pixels are required to be displayed, the display content has a high frame rate, and the dynamic display effect is complex. In view of the above situation, based on the use of existing resources, a new LED display screen control system composed of a 32-bit high-performance ARM microprocessor was redesigned and developed, and real-time data communication with the host computer in the field bus was carried out through the RS485 interface to realize the information display of the entire system.

1 System Hardware Structure
The hardware block diagram of the system is shown in Figure 1. In Figure 1, the microprocessor is AT91M42800A produced by AtmEL. It uses a high-performance 32-bit RISC architecture processor based on the ARM7TDMI core and has rich peripheral interface resources. AT91M42800A has two USART peripheral interfaces. The system uses USART0 port and MAX485 to form a 485 interface circuit. The specific interface circuit is shown in Figure 2. AT91M42800A also has two SPI ports. Each SPI port has 4 chip select signals, which can support 15 external devices through chip select. The system connects the two SPI ports to the column drive circuit and the row drive circuit respectively, and uses their respective two chip select signals CS0 and CS1 to complete the signal latching and output control of the drive circuit. The CLK output of SPI is used as the clock signal input of the drive circuit, and the operating frequency is 4 MHz.

The SRAM interface circuit is composed of two HY57V641620 chips in parallel. HY57V641620 is a 4 BanksX1M×16-bit SDRAM chip produced by Hynix. The storage capacity of a single HY57V641620 is 4 groups×16 M bits (8 MB), supporting automatic refresh and 16-bit data width. In order to give full play to the data processing capability of the 32-bit CPU, the system uses two 8 ns HY57V641620 chips to form a 32-bit SDRAM memory system. The Flash memory interface circuit is composed of a HY29LV160 chip. HY57V641620 is a Flash memory chip with a single chip storage capacity of 16 M bits (2 MB) and 8/16-bit data width. This system uses a 16-bit data width working mode. For specific circuit connections, please refer to the references.

The row drive circuit is composed of 36 cascaded A68595 chips from Allegro. The data line of each row on the back of the display is composed of a cascaded A68595 serial-in-parallel-out shift register. The A68595 chip has a driver composed of MOS tubes integrated in it, which is enough to drive the light-emitting diode to emit light. The column drive circuit is composed of 24 cascaded A6276 chips from Allegro. A6276 is a 16-bit serial-in-parallel-out shift LED driver chip with latch. The pins and connection methods of A68595 and A6276 cascade are shown in Figure 2. The circuits are relatively simple (the pins with port box marks are the corresponding pins of AT91M42800A). For other detailed performance information, please refer to the relevant product documents of Atmel and Allegro. The SPI port of AT91M42800A adopts 16-bit serial output working mode, which can fully improve the data transmission speed by utilizing the high-speed performance of the 32-bit ARM processor.

2 Working Principle
The communication between the system and the host computer is completed by the USARTO port of AT91M42800A and the 485 interface circuit. The host computer only needs to transmit the data to be displayed to AT91M42800A. After booting up, Ar91M42800A is initialized. After reading the startup code, the program code and the font data to be displayed stored in the Flash memory are remapped to SDRAM, so that the system data access is all completed in the high-speed SDRAM. After receiving the data from the host computer, AT91M42800A converts the data to be displayed into the corresponding LED screen display drive signal, and then adds the corresponding dynamic display effect control program (screen left shift, up shift, opening, covering, flashing and direct display, etc.), and outputs it to the row and column drive circuits respectively through the SPI port. At the same time, if necessary, the data or image screen sent by the host computer can also be saved in the Flash memory.

The display screen adopts 1/16 dynamic line scanning mode. First, the 24 bytes of data in the SPIA port are serially shifted into the corresponding 24 A6276 column drive circuits and latched. Then, the SPIB port serially shifts the row selection signal into the row drive circuit to complete the LED display of one row. Then, the rows of the LED screen are displayed one by one.

The duty cycle of the diode on and off time can be set by software to select the appropriate brightness and increase the service life of the light-emitting diode. The LED display screen actually installed on site has an effective display area of ​​about 4.6 m2, a total of 288×384=110,592 pixels, a full frame refresh time of less than 8 ms, and a frame change frequency of more than 125 Hz, which is more than 10 times higher than the traditional display system composed of single-chip microcomputers, ensuring the visual effect of dynamic display. At the same time, under the same conditions, the actual visible pixels can also be increased.

3 Software Description
The software of this system adopts μC/0S-II operating system, which enables the system to have powerful multi-task management, timer management, interrupt management, storage management and other functions. Through real-time monitoring of related registers, the stability of the system can be greatly improved, which is impossible to achieve with single-chip microcomputers and some DSP processors in the past.

The display application uses the timer interrupt method. By setting a suitable interrupt entry time constant, an LED refresh frame rate higher than 40 Hz can be obtained, giving the human eye a stable dynamic visual effect.

Real-time dynamic processing of the screen, that is, various dynamic display modes are written in the form of subroutines, and each display mode is an independent subroutine. Specific dynamic display modes include: left and right movement of the screen, up and down movement, curtain pulling, covering, flashing, direct display and other modes.

4 Advantages of this system①
The use of high-performance 32-bit RIS-based ARM microprocessor overcomes the shortcomings of traditional 8/16-bit microcontrollers in terms of processing power, system architecture, addressing range and peripheral interface capabilities in hardware; the use of real-time multi-tasking operating system in software makes the system management function powerful, can perform real-time monitoring, realize complex program control, and is also convenient for program development and expansion. Compared with similar systems composed of previous microcontrollers, the software stability and reliability of this system have been greatly improved.②
The system omits the bus drive and decoding circuit of the LED display part in the traditional practice, unlike some other microcontroller systems that use multiple processors to increase the display speed, use dual-port RAM, or divide the LED screen into multiple blocks. The system uses the SPI interface of AT9lM42800A to directly realize the LED display logic drive, which not only simplifies the circuit, but also simplifies the relevant software programming and saves the MCU's GPIO hardware resources.
③The SPI interface of AT91M42800A can adopt 16-bit transmission mode, and when combined with A6276 high-speed 16-bit dedicated LED driver chip, the LED display refresh speed is greatly improved compared with traditional single-chip microcomputers.

Conclusion
Compared with the traditional LED display system based on 8/16-bit single-chip microcomputer, the large-screen LED display system composed of 32-bit embedded RISC microprocessor has greatly improved performance without significantly increasing the system cost. Compared with the display system using DVI interface, it saves the relevant circuits of video processing, and has the advantages of simple hardware structure and low cost. The adoption of this design scheme can save the port resources of the single-chip microcomputer, effectively simplify the circuit structure of the display screen, and improve the reliability of the entire display system. In terms of LED information display such as monochrome video, animation, and text, this system has certain application value. After long-term actual operation on a large logistics production line, it has been proved that its design scheme is successful.