The prosperity of the social economy has led to the rapid growth of various industries, and the way of messaging is also diverse. The taxi industry is obviously a very large and effective advertising media. In the non-stop motion of the taxi, the information can be infiltrated into every corner of the city through the LED advertising screen at the rear to achieve the purpose of information transmission. LED displays have made great progress since the rapid rise in the world, and have developed into an important means of information dissemination media and have been widely used. The advertising screen composed of LED has better shock resistance, cold and heat resistance, corrosion resistance, and is more suitable for taxi tail installation than other types of information dissemination systems such as liquid crystal display, and the low cost is beneficial to save. cost. In the use of outdoor advertising screens, LED display is a good choice. Nowadays, China has entered a fast-developing information age. Although the LED industry has great potential and good development prospects, it also recognizes that there is still a gap in related technologies in China compared with the world advanced level. Therefore, the implementation of this system has certain practical value and significance for the further application of LED technology. 
1. System overall framework and hardware system design
The LED dot matrix display system at the tail of the taxi belongs to the outdoor work and needs good earthquake resistance, so it has high requirements on the quality of the display. Usually, in order to facilitate the maintenance of the hardware system circuit, the overall system is divided into several modules to design. As shown in Figure 1, the system is divided into two parts according to the display function of the system, which are the core control system and the display circuit.
The control system is a very important component of the overall system. As a core control part, the program written by the developer is processed by it and then controls the display. This design has selected the single-chip microcomputer as the central controller. After writing the program, it sends instructions to the peripheral devices through each pin to control it to achieve the purpose of display.
In Figure 1, the communication system is the channel through which the computer sends data to the display control system.
1) Parallel communication.
Each of the data in parallel communication is a communication method in which transmission is simultaneously performed. In this way, the data transmission rate is rapid, but the shortcoming is that there are many communication lines required, the cost is not low, and it is not suitable for long-distance transmission.
2) Serial communication.
Data in serial communication is a one-bit communication method that transmits in sequence. It only needs a few communication lines, but the transmission speed is very slow, but it can transmit a long distance. Hardware is usually used to switch between data modes. The hardware circuit generally uses a Universal Asynchronous Transceiver (Universal Asynchrono nous Receiver/Transmitter), which is commonly referred to as a UART. This design selects the serial communication method.
As shown in Figure 1, the display portion of the system includes a row and column driver circuit that can display eight words of LED screens and screens. Because the I/O pins of the microcontroller are limited. The MCU and the screen cannot be connected without any chip in between. Need to choose the method of adding I / O port to increase the pulse of the microcontroller. LEDs are also known as light-emitting diodes. This design requires a dot matrix screen with a size of 16×128, which is spliced ​​by a dot matrix module composed of light-emitting diodes, usually 8×8, 16×16 and so on. At the same time, in order to achieve cost savings, the module area is relatively small to reduce the cost of maintenance. Therefore, the LED screen of this design is selected from a total of 32 combinations of 8×8 type modules. At the same time, the number of expansions of the I/O port of the microcontroller must be implemented in the design. Because the low-level port of the microcontroller is equivalent to having no driving capability. Therefore, this type of circuit also requires functional (current) amplification.
In the overall framework, the detailed system hardware design is shown in Figure 2. The whole system includes clock, reset, keyboard scan control and the microcontroller is part of the core control part; the LED driving circuit and the dot matrix screen are part of the display.
2, system software design and program flow chart
After the hardware circuit design of the MCU system is determined, the software design is based on the hardware circuit, which is divided into the main program, the keyboard scan control program, the display mode program, and the external interrupt.
1) In this design, the tact switch is used to control the final interruption. Since there is more than one mode, it needs to be switched with the refresh button.
2) The function of the main program is to initialize the system environment, set the timer interrupt, the total interrupt, enable the interrupt work, etc., and call the subroutine at the same time.
3) The control program mainly sends the control data to the column drive circuit, because the row driver scans the screen line by line, and the column driver controls the high and low level output by the quality command sent by the single chip. First enter a font of 8 words into the 595 module, and then perform a line scan.
4) The display program mainly describes the principle of these several display modes, because one screen can display eight words, so the display program is written to implement the 8-word font first, then the last 8 words, and so on.
The flow chart of the system general program is shown in Figure 3. The first step is to initialize the system running environment; then the entire system has entered the display state. The default initial display mode in this design is the display mode. You can press other buttons to switch modes as needed.
3, debugging and simulation results
In a row driver circuit, it may happen that current flowing through the decoder is insufficient to drive so many LED modules, leaving the circuit in a state of no level. Therefore, a 74HC04 high-power amplifier or a triode is added between the row driver circuit and the LED display to amplify the current so that the LED screen can be stably displayed. After the system is debugged by hardware and software, a good dynamic demonstration effect is obtained. The interception effect diagram is shown in Figure 4.
4 Conclusion
This design implements a system of taxi advertising screen based on single-chip LED. The content displayed on the advertisement screen can be controlled by a program written in the single chip microcomputer, and can realize a plurality of display modes, and a better dynamic display effect is obtained.
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