Design of campus LED bulletin board system based on ARM

At present, the use of LED bulletin boards on campus is increasing, and they are used for the promotion and dissemination of various notifications. However, the control is still dominated by the control of a single LED bulletin board, and it is very inconvenient to operate and update the information on the display screen. In this context, this design studies and improves the current LED bulletin board system on campus, adds a ZigBee transceiver module to the original LED bulletin board, and designs a campus LED bulletin board system based on ARM devices, using touch screen technology and ZigBee wireless transmission technology.

1 Hardware Design of Control System

1.1 S3C2440 processor main control board module

The Linux real-time operating system is embedded in this hardware platform to manage and control the entire campus LED bulletin board system. The S3C2440 chip supports the touch screen interface, which includes a touch screen controller, four external transistors, and an external voltage source. The touch screen interface control selects the control signals (nYPON, YMON, nXPON, XMON) and analog pins connected to the pins of the touch screen panel and the external transistors.

1.2 PS2 Keyboard Module

The PS2 keyboard transmission protocol uses the following data format for data transmission: 1 start bit (always logic 0), 8 data bits (low bit first), 1 parity bit (odd parity), 1 stop bit (always logic 1), 1 response bit (only used in host-to-device communication). The keyboard transmission timing is shown in Figure 2.

1.3 LCD Module

A 7-inch LCD screen is used, and its video color format is: PAL/NTSC; high definition, wide viewing angle, 16:9 and 4:3 can be switched arbitrarily; optimal resolution: 800×480; contrast: 200:1; the 7-inch LCD screen is mainly used for displaying the main control interface and the input update text.

1.4 Four-wire resistive touch screen module

The four-wire resistive touch screen is the most widely used and popular type of resistive touch screen. Its structure consists of a lower conductive ITO layer and an upper conductive ITO layer, separated by fine insulating dots. When there is no pressure on the touch screen surface, the upper and lower lines are in an open circuit state; once pressure is applied to the touch screen, the upper and lower lines are connected, and the controller applies a driving voltage in the X-coordinate direction through the lower conductive ITO layer, and detects the voltage in the X direction through the probe on the upper conductive ITO layer, thereby calculating the X coordinate of the contact point. By changing the direction of the applied voltage through the controller, the Y coordinate of the contact point can be measured in the same way, thereby clarifying the position of the contact point. Its equivalent circuit is shown in Figure 3.

Figure 3 Equivalent circuit of four-wire resistive touch screen. 1.5 ZigBee wireless transmission module

The ZigBee communication module uses the SZ05 module of Shunzhou Technology. The processor and the communication module are connected through the RT1_TX and RT1_RX pins. The communication module used in the calling terminal uses the terminal node working mode (that is, the DS pin on the communication module is grounded). The connection between the ZigBee wireless transmission module and the main control board is shown in Figure 4.

Figure 4 Connection diagram of ZigBee wireless module and main control circuit.

Here ZigBee forms a star network type, the transmission mode is set to master-slave mode, the baud rate is selected to 9600, and the data bit is set to 8+0+1.

The ZigBee wireless transmission module is used to realize wireless data display information transmission and update, avoiding the additional cost of information update caused by data transmission based on the GSM/GPRS communication network of China Mobile operators.

2 System Software Design

2.1 Design of ZigBee wireless communication protocol

Since the ZigBee wireless transmission module uses serial communication, it is necessary to design a communication protocol for the LED display operating system to ensure accurate operation and update of each LED display. Therefore, its frame format is specified, as shown in Table 1.

Table 1 Frame format of ZigBee communication protocol

① Frame header: indicates the beginning of a frame, the content is FFAA, and the frame header length is 2 bytes.

②Frame length: Indicates the length of the frame data of the data packet excluding the frame header. The frame length is 2 bytes.

③Address identification: The physical address of each ZigBee wireless module, including the destination address and source address, is 1 byte in length.

④Data: The content of the data packet, the length is 0byte~256byte.

⑤ Parity check: In order to reduce the bit error rate in communication, this protocol uses a parity check method. If the number of 1s in the data bit is an even number, the check bit is 1; if the number of 1s in the data bit is an odd number, the check bit is 0.

2.2 Design of the main interface of the QT4 development software based on Linux

QT software is a cross-platform C++ graphical user interface application framework developed by Nokia. It provides application developers with the functions required to build state-of-the-art graphical user interfaces. QT is fully object-oriented, easily extensible, and allows true component programming. Since QT entered the commercial field in early 1996, it has become the basis of thousands of successful applications around the world. QT is also the basis of the popular Linux desktop environment KDE. Basically, QT is the same type of thing as Motif, Openwin, GTK and other graphical interface libraries on Window and MFC, OWL, VCL, ATL on the Windows platform, but QT has the advantages of excellent cross-platform characteristics, object-oriented, rich API, and a large amount of development documentation.

The main interface of the system mainly includes the following aspects:

①The main switch button icon of the entire LED display system;

②Select the interface for operating the LED display screen;

③The window showing the contents on the selected LED display;

④ The switch button icon and send button icon of a single display screen of the selected LED display screen.

3 Conclusion

After adopting this system, the four-wire touch screen technology makes the control main interface more user-friendly. The operator can update the display content of multiple LED bulletin boards and control the switch of each LED bulletin board in the office through the touch screen display main interface. The ZigBee communication module used in smart homes in the Internet of Things is used to realize the remote control and display update of multiple LED display screens on campus, forming a small LED display screen control system.