Design of distributed multi-screen electronic bulletin system based on USB and OLED

Design of distributed multi-screen electronic bulletin system based on USB and OLED

introduction

The new generation of display devices, organic electroluminescent display devices OLED (Organic Light Emitting Diode), are 1/3 of the thickness of LCD, and the response speed is 1,000 times that of LCD components. They have good low-temperature characteristics and can display normally at -40°C. The active luminescence characteristics of OLED make it almost free of viewing angle problems. When viewed at a large angle, the display surface is still not distorted; OLED can be manufactured on substrates of different materials and can be made into flexible displays that can be bent; the appearance is not restricted and can be any shape; it can be placed on the surface of any object, so it is particularly suitable for outdoor large-screen electronic screen advertising display. Based on the above advantages, a set of multi-electronic graphic advertising screens that can simultaneously publish electronic bulletins in multiple public places (such as factory areas and campuses) is designed and developed. The system is based on universal serial bus (USB) transmission, with fast speed and high reliability, effectively overcoming the defects of traditional advertising graphic transmission.

2 System Design

The electronic advertising screen system for multiple occasions is composed of a PC (host computer) located on the Internet network and a single-chip computer (slave computer) located on each LED controller, and uses the USB bus for master-slave communication. Figure 1 is a logical structure block diagram of the control system. The host computer mainly completes tasks such as image and text editing, font extraction and storage, image preview and file transfer; the slave computer controls the OLED dot matrix row and column driver through a certain scanning drive method to realize the device drive and finally receive and display the image and text. Due to the use of the USB bus, multi-screen connection can be smoothly realized.

3 OLED drive circuit design

The driving mode of OLED is current driving. Figure 2 is a driving principle diagram of an active matrix OLED pixel.

Compared with liquid crystal pixels, OLED pixel drivers have one more transistor VT2, which can provide OLED pixels with luminous current when VT1 is turned off. When a high voltage selection signal is input to the row signal end and VT1 is in the on state, the column synchronously inputs the corresponding voltage to charge capacitor C while controlling the current flowing through VT2. When a low voltage signal is input to the row signal end, VT1 is turned off, and capacitor C provides gate voltage for VT2, continuing to control the drain current of VT2 and maintaining the pixel to continue emitting light.

The OLED in the system adopts a passive drive mode. Assuming that the OLED display screen has n rows and m columns of pixels, if the light-emitting element crossing the xth row and the yth column is to emit light (turn on), it is only necessary to close the yth column selection switch of the column driving circuit, output a high level, and make the xth row of the row scanning circuit output a low level. Its scanning timing diagram is shown in Figure 3. Among them, TFRM is the frame period, TLINE is the row scanning period, and after determining the frame frequency fFRM, the time of one frame is also determined, TFRM=1/fFRM; the scanning time TLINE of each row is determined when the frame frequency is determined, that is, TLINE=TFRM/N.

The designed driving circuit uses PT6807 and PT6808. PT6807 is a 64-way row driver for the dot matrix 0LED graphic display system. It uses CMOS technology to provide 64 shift registers and 64 output drivers. PT6807 generates its own clock signal to control the PT6808 column driver. Take 128x128 dot matrix as an example to illustrate the specific operation. The interface of the dot matrix driver module is shown in Figure 4. To read the content in the display data RAM, it is necessary to access the read instruction twice. In the first access, the data in the display data RAM is latched into the output register. In the second access, the MPU reads the latched data. That is, a dummy read is required when reading the display data RAM, but no dummy read is required when reading the status data.

4 MCU Memory Design

OLED electronic screen mainly displays Chinese characters, and storing one Chinese character requires 32x32=1 024 b=128 B of space. However, the internal storage space of AT89C51 is limited, so the data memory is expanded, using 62256 SRAM, each chip can store 256 Chinese characters, and 8 chips can display more than 2 000 common Chinese characters. In addition, since the system needs to call a lot of programs and data, an external program memory is expanded, so that except for frequently used structures such as stacks and queues, other structures such as initialization status words do not occupy the on-chip memory to avoid on-chip data blocking.

5 USB interface hardware design

The data transmission from the upper computer to the lower computer is realized through the dedicated USB communication control device PDIUSBD12. This device supports 3 USB endpoints and uses multiple interrupt modes to facilitate bulk and isochronous transmissions. The data transmission rate of both bulk and isochronous transmissions can reach 1 Mb/s. The interface circuit between it and the AT89C51 microcontroller is shown in Figure 5.

6 Software Design

The software system includes two parts: USB driver firmware and graphic data processing. The device firmware is the core of the device operation and is written in C language. Its main function is to download data to the microcontroller in real time through PDIUSBD12. The program is mainly divided into four modules: initialization, data processing, control and USB communication. After the device is connected to the microcomputer, the initialization module initializes the device configuration; the data processing module preprocesses the front-end data; the control module is used to control the front-end hardware; and the USB communication module is responsible for the communication between the host and the front-end hardware, which is the key to the same-piece design. The goal of the USB communication same-piece design is to enable PDIUSBD12 to achieve the maximum transmission rate on USB. The GUID of the USB device needs to be defined in the program, and the USB device needs to be connected through the USB driver, and the Handle of the USB device needs to be obtained using the Windows API function. According to the USB protocol, any transmission is started by the host. The microcontroller performs its foreground work and waits for interrupts. The host first sends a token packet to PDIUSBD12, and PDIUSBD12 sends an interrupt to the microcontroller after receiving the token packet. The MCU enters the interrupt service routine, first reads the interrupt register of PDIUSBD12, determines the type of USB token packet, and then performs the corresponding operation. PDIUSBD12 uses SIE to manage USB communication. When the host and the device communicate via USB, external interrupt 0 will be generated, which is determined by the interrupt vector register. The three interrupts Setup_packed_Int, Input_endpoint0_Int, and Output_endpoint0_Int are mainly used to establish a connection with the host, perform control transmission or interrupt transmission; the two interrupts Input_endpoint1_Int and Output_endpoint1_Int are mainly used for batch transmission. Different interrupt programs are executed in the same device to realize USB data transmission. The external interrupt 0 program is as follows:

7 Conclusion

OLED display technology has broad application prospects. It can display normally at -40℃ and can be viewed within a range of 180°. It can be made into any shape and placed on the surface of any object. It is particularly suitable for outdoor large-screen electronic screen advertising displays. The USB interface is suitable for high-speed data transmission and can be used as an interface for portable systems. The perfect combination of the two makes the performance of the large-screen electronic bulletin synchronization release system more superior.