LED display system design based on AT91M42800A

Introduction :

　　Recently, the author found in a fieldbus-based logistics call system project on a large production line in a factory that due to the relatively large information flow that needs to be displayed, using the existing LED display control system based on the AT89C51 chip is problematic due to micro-controllers. There are many limitations in the processor's processing speed, architecture, addressing range, peripheral interface resources, etc., which have made it difficult to obtain good dynamic visual effects when more pixels are displayed, the frame rate of the display content is higher, and the dynamic display effect is complex. . In response to the above situation, on the basis of utilizing existing resources, a brand new LED display control system composed of a 32-bit high-performance ARM microprocessor is shown in Figure 1. Hardware structure block diagram of the system, and through RS485 The interface conducts real-time data communication with the host computer in the field bus to realize 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 Company. It uses a high-performance 32-bit RISC architecture processor based on the ARM7TDMI core and has rich peripheral interface resources. AT91M42800A has 2 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 2 SPI ports, each SPI port has 4 chip select signals, and can support 15 external devices through chip select. The method of this system is to connect the two SPI ports to the column driver circuit and the row driver circuit respectively, and use the two chip select signals CS0 and CS1 to complete the signal latch and allow output control of the driver circuit. The CLK output of the SPI As the clock signal input of the driver circuit, the operating frequency is 4 MHz.

　　The SRAM interface circuit is composed of two HY57V641620 chips connected in parallel. HY57V641620 is a 4 Banks × 1M × 16-bit SDRAM chip produced by Hynix. The storage capacity of a single HY57V641620 is 4 groups × 16 M bits (8 MB). It supports automatic refresh, 16 Bit data width. In order to give full play to the data processing capabilities of the 32-bit CPU, the system uses two 8 ns HY57V641620 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), 8/16-bit data width, and this system uses a 16-bit data width working mode. For specific circuit connections, please refer to reference [1].

　　The row drive circuit is composed of 36 A6B595 cascades from Allegro Company. Each row of data lines on the back of the display screen is formed by a cascade of serial input and parallel output shift registers A6B595. The A6B595 chip integrates a driver composed of MOS tubes, which is enough to drive light-emitting diodes. glow. The column driver circuit is composed of 24 cascaded A6276 chips from Allegro. A6276 is a 16-bit latched series-in and parallel-out shift LED driver chip. When A6B595 and A6276 are cascaded, the pins and connection methods are shown in Figure 2. The circuits are relatively simple (the port boxes marked are the corresponding pins of AT91M42800A). For other detailed performance information, please consult the relevant product documents of Atmel and Allegro [2, 3]. The SPI ports of AT91M42800A all adopt 16-bit serial output working mode, taking advantage of the high-speed performance of the 32-bit ARM processor to fully improve the data transmission speed.
 　　　　　　　　　　

　Figure 1 Hardware structure block diagram of the system
　　

　　Figure 2 485 interface circuit, A6B595 and A6276 cascade circuit principle Figure

2 Working principle

　　The communication between the system and the host computer is completed by the USART0 port of AT91M42800A and the 485 interface circuit. The host computer only needs to transmit the data to be displayed to AT91M42800A. After powering on, the AT91M42800A initializes. After reading the startup code, the program code and the font data to be displayed are remapped into the SDRAM so that all system data access is 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 (picture left move, up move, opening, overlay, flashing and direct After displaying etc.), use the SPI port to output to the row and column driver circuits respectively. At the same time, if necessary, the data or images transmitted from the host computer can also be saved in the Flash memory.

　　The display screen adopts 1/16 dynamic sequential line scanning method. First, the 24 bytes of data in the SPIA port are serially moved into the corresponding 24 A6276 column driver circuits and latched. Then, the SPIB port serially moves a row of strobe signals into the row driver circuit to complete a row of LED display. Then follow the logic and display each row of the LED screen one by one.

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

3 Brief description of the software part

　　The software of this system adopts the μC/OSII 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 relevant registers, the system performance can be greatly improved. Stability, these cannot be achieved with microcontrollers and some DSP processors in the past.

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

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

4 Advantages of this system　

　　① Using a high-performance 32-bit RISC architecture ARM microprocessor, the hardware overcomes the shortcomings of traditional 8/16-bit microcontrollers in processing power, system architecture, addressing range and peripheral interface capabilities; the software uses The real-time multi-tasking operating system makes the system's management functions powerful, enables real-time monitoring, and realizes complex program control. Program development and expansion are also very convenient. Compared with similar systems composed of previous microcontrollers, the software stability and reliability of this system have been greatly improved.

　　② This system omits the bus driver and decoding circuit of the LED display part in the traditional practice. Unlike some other single-chip computer systems, in order to increase the display speed, multi-processors are used, dual-port RAM is used, or the LED screen is divided into Multi-block scheme. The system uses the SPI interface of AT91M42800A to directly implement the LED display logic driver, which not only simplifies the circuit, but also simplifies the related programming of the software and saves the GPIO hardware resources of the MCU.

　　③ The SPI interface of AT91M42800A can adopt 16-bit transmission mode, and is equipped with the A6276 high-speed 16-bit dedicated LED driver chip, which greatly improves the LED display refresh speed compared with traditional microcontrollers.

Conclusion

　　: Compared with the traditional LED display system based on 8/16-bit microcontrollers, the performance of the large-screen LED display system composed of 32-bit embedded RISC microprocessors has been greatly improved without significantly increasing the system cost. . Compared with the display system using the DVI interface, it eliminates the need for video processing circuits and has the advantages of simple hardware structure and low cost. Adopting this design scheme can save the port resources of the microcontroller, effectively simplify the circuit structure of the display screen, and improve the reliability of the entire display system. This system has certain application value in terms of LED information display such as monochrome video, animation, text, etc. After long-term actual operation on a large logistics production line, the design scheme has been proved to be successful.

References

　　1 Li Juguang. Detailed explanation of ARM application system development - system design based on S3C4510B. Beijing: Tsinghua University Press, 2003.

　　2 Allegro Corp. A6B595 & A6276 Products Datasheet. 2004

　　3 Atmel Corp. AT91M42800A Series ARM Thumb Microcontrollers Datasheet. 2002

　　4 Zhang Hua, Fan Qingwen, Hou Li. Large-screen LED display system based on industrial Ethernet. Mechanical and Electrical Product Development and Innovation, 2004.17(2):28~30

　　5 Li Juguang, Zhang Hua, Li Zhongli. AVR high-speed microcontroller LED display system. Electronic Technology, 2002(5)

　　6 Du Chunlei. ARM Architecture and Programming. Beijing: Tsinghua University Press, 2003