Design of rotating LED screen control system based on SOPC

1. Introduction

LED (light emitting diode) display screen is composed of light emitting diode array. Light emitting diode (LED) is a current control device with the advantages of high brightness, small size, good monochromaticity, fast response speed, simple driving and long life. It can be competent for real-time, diverse and dynamic information release tasks in various occasions, so it has been widely used. LED large screen is a display screen composed of LED device array, which is used to display various information such as text, image market information and TV and video signals through a certain control method. 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, and is widely used in industry, transportation, commerce, advertising, finance, sports competitions, electronic landscapes, etc.

At present, most of the LED screens on the market are flat-panel LED screens, which have the advantages of stable display and easy modification of display content. However, the principle of flat-panel display screens determines that each pixel on the display screen corresponds to a single LED, which inevitably leads to the disadvantages of high cost of flat-panel display systems and large display control systems. In addition, due to the limitation of geometric structures, the field of view of flat-panel display screens is small. The new rotating column LED screen can overcome the above shortcomings of flat-panel LED screens - it uses mechanical rotation scanning instead of line-by-line scanning, greatly reducing the number of LEDs used, 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. The organic combination of SOPC technology and rotating LED control can realize a rotating LED control system with compact structure and stable functions, which is also conducive to the functional upgrade and maintenance of the control system.

2. Basic principles and key technologies of rotating LED screen

2.1 LED array rotation display

The 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 360° visual range. Its core technology lies in the precise control of the synchronization of LED position and lighting state. The rotating LED screen has only one column of display devices, and this column of LEDs is driven by LEDs with a constant speed to rotate. At the same time, the control circuit synchronously controls the lighting state of the LEDs, so that the display content of this column of LEDs changes once every time the motor rotates a certain angle. There is a unique display content when it rotates to any position. In other words, the rotating LED screen uses column-by-column display and uses mechanical rotation to replace scanning display. In the figure below, a, b, c, and d are the display states of the rotating LED screen at different times, and d is the complete picture "3" seen by human eyes.

2.2 Color grayscale control of three-primary color LED

This design uses three-primary color LED to realize the display of color pixels. In order to realize rich and colorful color display, it is necessary to control the color grayscale of LED. There are two main methods, namely driving current control method and driving pulse duty cycle control method. However, the former method is difficult to implement and has high cost, while the latter is simple in principle and easy to implement using digital design methods. Therefore, the duty cycle control method is used in this article to control the LED grayscale.

The duty cycle control method controls the brightness of the LED, which actually controls the lighting time of the LED. The LED brightness is changed by periodically changing the lighting time of the LED in a cycle. The continuous change of the LED brightness grayscale level can be achieved by continuously changing the lighting time of the LED in the cycle under working conditions. For 256-color display, only 8-bit color data is needed to control the grayscale of the LED. The relationship between the column data bit and the duty cycle is shown in Figure 2 below:

2.3 Key points of rotating LED screen design

The principle of the rotating LED screen is not complicated, and its hardware structure is relatively simple. However, in order to make the rotating LED screen able to stably display the set image or text, three technical difficulties need to be overcome:

1. Power supply of the rotating screen control circuit. 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 LED.

2. The rotating screen has the problem of horizontal display ghosting. Since the rotating screen uses a single-row LED rotating scan to realize cylindrical image display, the pixel display becomes sticky in the horizontal direction, forming a ghosting phenomenon.

3. Solve the problem of insufficient display brightness. Due to the working principle of the rotating LED screen, compared with the ordinary flat LED screen, the display time of each column is much shorter, 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 a relatively simple and reliable method to use the brush piece fixed on the base and the rotating metal guide rail or metal wheel axle to power the system. The design of this article is to provide a constant 12V DC voltage through the brush, and then the DC-DC chip converts it into 2.5V, 3.3V, 5V and other voltages to maintain the control system. To solve the horizontal smear of the rotating screen, it is mainly based on the characteristics of rotating scanning, inserting a full black time slot between two columns of pixel points, so that the stickiness of the two columns of image display points can be eliminated. As for solving the problem of insufficient brightness display, the design of this article uses four columns of LED lights to display each pixel on each rotating screen in turn without continuing to increase the brightness of the LED lights, so that the brightness becomes four times the original when the rotation rate remains unchanged.

3. Overall solution system design

The design of this paper is based on FPGA, using Hall sensor, infrared remote control and three-primary color LED display array to build a columnar rotating LED screen system. The system mainly consists of three parts: motor, control circuit and LED display array.

The function of the motor is to drive the circuit part to rotate. The key to this part is that the motor speed should be uniform, so as to ensure that the image will not be partially stretched or compressed in the horizontal direction. In order to keep the LED speed stable, a uniform DC motor or a stepper motor is generally used in the design.

The entire LED array is the main display body of the rotating screen. In order to achieve a better display effect, the design of this article uses three-primary color LED lights. By controlling the color grayscale level of the LED through the circuit, the LED screen can display complex and changeable 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 by scanning the column according to the rotation position under the synchronization of the rotation position sensing signal. This part is the core of the whole system.

4. SOPC Design of Rotating LED Control Circuit

The hardware platform of this design adopts EP2C20Q204C8 in Altera's CycloneⅡ series as the FPGA platform. The control system of the rotating LED screen is built by using Quartus II software, Nios soft core processing, DMA, SDRAM controller and other peripheral modules, as well as a customized LED drive control module.

4.1 Structure of 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 power supply module mainly provides continuous and stable power supply for the rotating LED screen control system and LED display array.

FPGA and its configuration circuit, FPGA circuit is the core of the entire control system and is the carrier for realizing Nios II processor and related functional modules.

The color LED array drive control circuit is mainly composed of LED driver chips and related devices. Under the control of the controller, the LED driver chip displays graphic and text content in the corresponding order, color and brightness.

Peripheral storage circuits include SDRAM and CF cards. SDRAM is used as the program running space of the Nios II processor, while the large-capacity CF card is used to store the images or text content to be displayed on the LED screen.

The rotation position sensing circuit, which is composed of an integrated Hall sensor device and related devices, is used to sense the rotation position of the LED screen so that the output content corresponds to the rotation position.

The wireless control circuit is composed of an infrared (irDA) receiving chip and a handheld infrared remote controller, and is mainly used to realize infrared remote control of switching the display content of the rotating LED.

The real-time clock circuit (RTC) is composed of an external real-time clock chip to realize the function of displaying the clock.

4.2 LED drive control module design

The LED drive control module is the most important part of the rotating LED screen control system. It is mainly divided into Avalon interface submodule, dual-port RAM submodule, serial shift output submodule, output synchronization submodule, grayscale control submodule, etc. Its structure diagram is shown in Figure 4 below.

In the design, the dual-port RAM submodule enables the system to update the display content data synchronously while outputting the display image. The rotating LED screen has four display LED arrays, and the entire display cylinder is divided into four areas, each area occupies 1/4 of the cylinder. In order to achieve synchronous scanning of the four areas, four independent dual-port RAMs are required, with a bit width of 24 bits, which can just store one pixel data, so that R, G, and B each occupy 8 bits to meet the needs of 256 display. The dual-port RAM is generated using the MegaCore embedded in the QuartusII software, and the size of each RAM is 768x24bit.

The serial shift output submodule and the output synchronization control submodule are used to take out the parallel image data from the RAM and serialize and output the data under the synchronization control of the position signal input by the Hall sensor. The image grayscale control method is implemented by the duty cycle control method mentioned above. The grayscale control submodule is mainly composed of a counter and a comparator. The counter counts at the rising edge of the grayscale clock GRY_CLK. When the count value is greater than or equal to 0 and is less than the comparator value, the grayscale control output signal GRY is high, otherwise it is low. This signal is sent to the enable terminal EN of the driver chip 74HC595 to control the lighting time of the LED lamp, which can control the color grayscale and increase the black time slot in the two-column image display column.

5. System software design

The control core of the entire rotating LED screen control circuit is the Nios processor. The reading of image data, the sensing of the rotation position, and the display drive of the LED are all controlled and scheduled by the Nios processor. For the Nios soft-core processor, its software development is completed in the Nios IDE integrated development environment under SOPC Builder. The flow structure of the Nios processor software is shown in Figure 5 below:

VI. Conclusion

As a new type of LED display screen, the rotating LED screen is gaining more and more attention due to its unique advantages such as low cost and wide viewing range. According to the working principle and design requirements of the rotating LED screen, this paper introduces the SOPC technology based on Nios soft core into the design, and realizes the design of a colorful rotating LED screen based on SOPC. This design integrates the microprocessor and the user logic interface on an FPGA chip, and its interface can be flexibly defined by programmers. Users can flexibly adjust the hardware logic design according to the size of the display screen to realize the control of the screen without changing its original hardware structure. Therefore, it has great flexibility and is a new direction for the design of rotating LED screen control circuits.