I. Introduction
The LED (1ight emitting diode) display is composed of an array of light emitting diodes. Light-emitting diode (LED) is a current control device with high brightness, small volume, good monochromaticity, fast response, simple driving, long life, etc. It can be used for real-time, diversity and dynamic in various occasions. The information release task has been widely used. The LED large screen is a display screen composed of an array of LED devices through a certain control method for displaying various information such as text and image quotes, as well as television and video signals. As an important media for modern information release, LED large screen is receiving great attention from all walks of life, especially the business and advertising circles. It is widely used in employment, transportation, commerce, advertising, finance, sports competitions, electronic landscapes, etc.
At present, the LED screens on the market are basically flat panel LED screens, and the screen has the advantages of stable display and easy modification in the display. However, the principle of the flat panel display determines that each pixel on the display screen is in one-to-one correspondence with a single LED, which inevitably causes disadvantages such as the high cost of the flat panel display system and the huge display control system, and is determined by the limitation of the geometric mechanism. The flat panel display has a smaller field of view. The new type of rotating column LED screen can overcome the above-mentioned shortcomings of the flat panel LED screen - it replaces the progressive scan with a mechanical rotary scanning method, and the number of LEDs used is greatly reduced, which greatly reduces the cost, and the screen range reaches 360°.
SOPC is a new direction of embedded design. SOPC technology can be used to integrate multiple functional modules into a programmable chip to form a programmable chip. The combination of SOPC technology and rotating LED control can realize a compact and stable rotating LED control system, and also facilitate the function upgrade and maintenance of the control system.
Second, the basic principle and key technology of rotating LED screen
2.1 LED array rotation display
Rotating LED display is a new type of display that realizes graphic display by synchronously controlling the position and lighting state of light-emitting diodes (LEDs). It has been rapidly developed due to its novel structure, low cost and visible range of 360°. The core technology is to precisely control the synchronization of the LED position and the illumination state. The display device of the rotating LED screen has only one column, and the LED of this column is rotated by the LED with constant rotation speed, and the lighting state of the LED is synchronously controlled by the control circuit, so that the motor rotates every certain angle, the display content of the LED of this column It changes once and has a unique display content when rotated to any position. That is to say, the rotating LED screen is displayed column by column, and mechanical rotation is used instead of the alternative scanning display. In the following figure, a, b, c , d is the display state of the rotating LED screen at different times, and d is the complete picture "3" seen by the human eye.
2.2 Color gradation control of three primary color LEDs
This design uses three primary color LEDs to realize the display of color pixels. In order to realize colorful color display, it is necessary to control the color gradation of the LED. There are two main types of driving current control method and driving pulse duty control method, but the former method is more difficult to implement and the latter method is simple, and the digital design method is easy to implement. Therefore, the LED gray scale of this paper The control uses a duty cycle control method.
The duty cycle control method controls the brightness of the LED to actually control the time the LED is lit. The length of the LED's lighting time in one cycle is periodically changed to achieve a change in the brightness of the LED. Continuous changes in the LED gradation level can be achieved by continuously changing the lighting time of the LED during the cycle in the operating state. For the 256-color display, only the 8-bit chrominance data is needed to control the gradation of the LED. The relationship between the column data bits and the duty cycle is as shown in Figure 2:
2.3 The key to rotating LED screen design
The principle of rotating the LED screen is not complicated, and its hardware structure is relatively simple. However, in order to enable the rotating LED screen to stably display the set image or text, it is necessary to overcome three technical difficulties:
1. Rotating screen control control circuit power supply. When the system is working, the control circuit rotates with the rotor of the motor, so it is difficult to provide a stable power supply to the rotating control circuit and the LED.
2. The rotating screen displays the smear problem horizontally. Since the rotating screen adopts a single-row LED rotation scan to realize the cylindrical image display, the image-like display appears viscous in the lateral direction, forming a smear phenomenon.
3. Solve the lack of display brightness, because the working principle of rotating LED screen determines that the time of displaying each column is much less than that of ordinary flat LED screen, so the brightness will be greatly reduced.
In order to make the designed rotating LED screen have a better display effect, it is necessary to solve these three problems in the design. For the power supply of the rotating LED screen, it is relatively simple and reliable to supply the system with the brush piece fixed on the base and the rotating metal rail or the metal wheel shaft. The design of this paper is to provide a constant 12V DC voltage through the brush, and then convert the DC-DC chip into 2.5V, 3.3V, 5V and other voltages to maintain the control system. To solve the horizontal smear of the rotating screen, the main purpose is to insert a full black time slot between the two columns of pixels according to the characteristics of the rotating scanning, so that the stickiness of the display points of the two columns of images can be eliminated. In order to solve the problem of insufficient brightness display, the design of this paper does not continue to increase the brightness of the LED light, using four columns of LED lights to display each pixel on each rotating screen in turn, so that the brightness is When the rotation rate is constant, it becomes four times the original.
Third, the overall program system design
The design of this paper is based on FPGA as the core, using Hall sensor, infrared remote control and three-color LED display array to build a column-type rotating LED screen system. The system is mainly composed of three parts: motor, control circuit and LED display array.
The function of the motor is to drive the part of the circuit to rotate. The key to this part is that the speed of the motor should be uniform, so that the image will not be partially stretched or compressed in the horizontal direction. In order to keep the LED speed stable, it is generally designed to use a uniform DC motor or a stepper motor.
The LED column is the display body of the rotating screen. In order to achieve better display effect, the design of this paper adopts three primary color LED lights. Through the circuit control the color gray level of the LED, the LED screen can display complex and variable colors. Although this is more complicated in the design of the control circuit, it has a better display effect.
The function of the LED display control circuit is to display the image or symbol stored in the RAM on the LED array in a column scan according to the position of the rotation under the synchronization of the rotational position sensing signal. This part is the core of the entire system.
Fourth, SOPC design of rotating LED control circuit
This design hardware platform uses EP2C20Q204C8 in Altera's CycloneII series as the FPGA platform. It uses the Quartus II software to build a rotating module using Nios soft core processing, DMA, SDRAM controller and other peripheral modules and custom LED driver control modules. LED screen control system.
4.1 Structure of the control circuit
As the core part of the rotating LED screen, the rotating LED screen control system circuit designed in this paper is mainly composed of the following parts, as shown in Figure 3.
The main function of the power supply module is to provide a continuous and stable power supply for the rotating LED screen control system and the LED display array.
FPGA and its configuration circuit, the core of the entire control system of the FPGA circuit, is the carrier that implements the Nios II processor and related functional modules.
The color LED array drive control circuit is mainly composed of a driving chip of LED and related devices. The LED driver chip displays the graphic text content in the corresponding order, color and brightness under the control of the controller.
Peripheral memory circuits, including SDRAM and CF cards. SDRAM is used as the program running space of the Nios II processor, while a large-capacity CF card is used to store images or text content to be displayed on the LED screen.
The rotary position sensing circuit is composed of an integrated Hall sensing device and related devices for sensing the position of the LED screen to rotate, so that the output content corresponds to the rotating position.
The wireless control circuit is composed of an infrared (irDA) receiving chip and a hand-held infrared remote controller, and mainly realizes switching infrared remote control for rotating the display content of the LED.
The real-time clock circuit (RTC) is composed of an external real-time clock chip to realize the function of displaying the clock.