The principle of high performance LED display design

The principle of high performance LED display design

The LED display is a flat display screen composed of a light emitting diode dot matrix module or a pixel unit. It has the advantages of high luminous efficiency, long service life, flexible configuration, rich colors and strong adaptability to indoor and outdoor environments. Therefore, with the continuous improvement of LED manufacturing technology, LED displays have been widely used at home and abroad.

The LED display is a display mode that controls the semiconductor light-emitting diodes. Its approximate appearance is composed of a plurality of light-emitting diodes that are usually red, and the characters are displayed by the blinking of the light. Used to display text, graphics, images, animation, quotes, video, video signals and other information display screen.

This article mainly explained the principle of high-performance LED display design:

The principle of LED display design mainly from the LED display system composition, LED display hardware design and LED display software design to analyze the three aspects.

1. LED display system configuration LED display system screen by the microcontroller control unit and display unit consists of two parts, a single display unit can display 16 × 16 Chinese characters 4, if the production of a certain size of the LED graphic display system, as long as With several intelligent display units, a “building block” method can be used to achieve serial communication between the display units in the system. In addition to the control unit is responsible for controlling the display unit and the transmission of the host computer instructions and signals, but also embedded a single bus digital temperature sensor 18B20. Thanks to the control circuit module design, if the humidity measurement is also required, 18b20 can be upgraded The module circuit consists of the DS2438 from Dallas and the HIH23610 from HoneywELl. In order to meet the communication needs of the vehicle, the CAN bus communication is used between the host computer and each control unit in the vehicle.

2. LED display hardware design The display unit is composed of LED display panel and display circuit. The LED display panel is a 16×64 dot matrix universal smart display unit composed of 4 dot matrix modules. The single display unit can display 4 full screens. A 16 × 16 dot matrix character or symbol, the serial communication between the display units in the system, so that the entire system work coordination and unity. The display circuit consists of two 16-pin cable ports, two 74H245 tri-state bus drivers, a 74HC04D six-inverter, two 74H138 eight decoders, and eight 74HC595 shift latches. The core of the control circuit is WINBOND's high-speed single-chip microcomputer 77E58, with a crystal frequency of 24MHz. AT29C020A is a 256K ROM for storing 16×16 dot matrix Chinese character library and 16×8 dot matrix ASCII code table. The AT24C020 is an EP2ROM based on the I2C serial bus, which stores preset statements such as subway station names, greetings, and the like. The interior temperature is measured by a single-bus digital temperature sensor 18b20. The SJA1000 and TJA1040 are CAN bus controllers and transceivers, respectively.

(1) Control circuit unit design The whole system is based on winbond's dynamic single chip microcomputer 77E58. The 77E58 uses a redesigned microprocessor core. Its instructions are compatible with the 51 series, but the clock cycle is only 4 cycles at the same clock frequency. Below, its operating speed is generally 2 to 3 times higher than the traditional 8051, so a good solution to the large-capacity Chinese character dynamic display when the frequency requirements of the microcontroller, and also comes with a watchdog. 77E58 controls the flash memory AT29C020 through the latch 74LS373, the size is 256K. Because the memory capacity is bigger than 64K, therefore has used the page addressing method in the design, namely uses P1.1 and P1.2 to carry on the page selection to the flash memory, Divided into 4 pages, the addressing size of each page is 64K. P1.5 is not only used for chip selection of AT29C020, but also ensures that P1.1 and P1.2 are not caused to AT29C020 when multiplexed on the 16-pin cable interface. Misoperation. The CAN controller is a key part of communication. In order to improve the anti-jamming ability, 6N137 high-speed optocoupler is added between the CAN controller SJA1000 and the CAN transceiver TJA1040. The microcontroller selects the CAN controller SJA1000 through P3.0. The 18B20 is a single-bus device. The device interfaces with a single-chip microcomputer and only needs one I/O port. It can directly convert the temperature into a digital signal and output it serially in a 9-bit digital code. Select P1.4 in the control circuit. 18B20 Chip Select and Data Transfer Function. The clock line SCL of the AT24C020 and the bidirectional data line SDA are connected to the P1.6 and P1.7.16 pins of the single-chip microcomputer. The interface of the cable is the interface between the control circuit and the display circuit.

(2) Display unit connection and control The display circuit section is connected to the 16-pin cable port of the control circuit section through the 16-pin cable (1) port. The 16-pin cable port (1) transfers the instructions and data from the microcontroller to the LED display. The circuit, 16-pin cable (2) is used for cascading multiple display screens, and its connection is basically the same as the 16-pin cable outlet (1), but note that the R terminal is connected from left to right of Figure 2. The DS terminal of the 8th 74H595, when cascaded, will be connected in series with the 16-pin cable (1) of the next display (Figure 1). CLK is the clock signal terminal, STR is the line latch terminal, R is the data terminal, G (GND), LOE is the line illumination enable terminal, and A, B, C, and D are the line selection terminals. The specific role of each port is as follows: A, B, C, and D are row selection terminals, which are used to control sending specific data from the upper computer to a specified row on the display panel. R is a data terminal and receives data transmitted by the single chip microcomputer. The working sequence of the LED display unit is as follows: After the CLK clock signal terminal receives a data at the R terminal, the control circuit artificially gives a rising edge of the pulse, and the STR gives after the transmission of 64 data of one line of data (16×4). A rising edge of the pulse latches the data; the LOE is set by the microcontroller to light the line. Display circuit schematic shown in Figure 3.

(3) Modular design Metro vehicles have different requirements for LED display screens according to actual conditions, so we have taken full consideration of this when designing circuits, that is, to ensure that the main functions and structures are not changed, specific modules can be implemented. Make swaps. This structure makes the LED control circuit has good expandability and ease of use.

(4) Temperature and Humidity Modules In the hot and rainy southern regions, despite the constant temperature and air conditioning in the car, humidity is an important indicator that visitors are concerned about. The temperature and humidity module we designed has the function of measuring temperature and humidity. The temperature module and the temperature and humidity module have the same socket interface, and all of them are single-bus structures, all controlled by the P1.4 port, so the exchange is very convenient. The HIH3610 is a three-terminal integrated humidity sensor manufactured by Honeywell, Inc. The DS2438 is a 10-bit A/D converter with a single-bus communication interface. It contains a high-resolution digital temperature sensor, which can be used for the temperature of a humidity sensor. make up.

(5) 485 bus expansion module The 485 bus has a irreplaceable position as a mature and inexpensive bus in the industrial field and traffic field. For this reason, we designed the 485 bus expansion module, which can replace the existing CAN module for external communication. . This module uses MAXIM's opto-isolated MXL1535E as the 485 transceiver. To ensure compatibility in control, both the MXL1535E and SJA1000 are chip selectable via P3.0. In addition, a 2500V RMS galvanic isolation is provided between the RS2485 side and the controller or control logic side through a transformer. TVS diode circuits are added to the output section of the module to reduce line surge interference. Jumpers can also be used to determine whether to load the terminating resistor of the bus.

3. The LED display software design system software consists of upper computer management software and unit controller control software. PC management software is developed on the Windows 22000 operating platform using C++BUILD6.0, including display mode selection (including static, flashing, scrolling, typing, etc.), scrolling direction selection (including scrolling up and down and scrolling left and right), Dynamic display speed adjustment (ie text flash frequency, scroll speed, typing display speed, etc.), display content input and display preview, etc.

When the system is running, the system can not only display characters such as newsstands, advertisements, etc., but also manually input required display characters. Unit controller control software uses 8051's KEILC programming, solidified in the EEPROM 77E58 MCU, mainly to complete the upper and lower computer communication, temperature and humidity data acquisition, I / O interface control and other functions. In actual operation, the temperature measurement accuracy reaches ±0.5°C and the humidity measurement accuracy reaches ±2%RH.

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