Design of single chip microcomputer for sending and receiving short messages

0 Introduction

The GSM (Global System for Mobile communication) system is a relatively mature, complete and widely used system in the current mobile communication system based on time division multiple access technology. The GSM digital cellular mobile communication network that has been built to cover the whole country is the main mode of public mobile communication network in my country. The short message service based on GSM is a wireless application for transmitting short messages on the mobile network, and is a process of storing and forwarding information on the mobile network. The GSM module is another very important GSM mobile communication system terminal device after the GSM mobile phone. It is a data terminal device that combines the traditional modem with the GSM wireless mobile communication system, also known as the GSM Modem.

1 System Principle and System Structure

This system mainly studies how to use the GSM mobile phone module to realize the sending and receiving of short messages on the computer terminal (i.e. the dotted part in Figure 1). The design uses the STC89C52 microcontroller and Siemens' GSM module TC35 to realize the GSM Modem prototype with the function of sending and receiving text messages. The modem is connected to the PC through the RS232 serial port, and the front-end software is developed on the PC to realize the point-to-point two-way sending and receiving of Chinese and English text messages, thereby realizing the remote communication function with the user's mobile phone or GSM module. The system design consists of two parts: hardware design and software design. The hardware design mainly studies the control of the GSM module by the microcontroller and the communication between the microcontroller and the computer.

Figure 1 System working principle block diagram

2 System Hardware[1]

The hardware part of the system includes the TC35 communication module circuit, power supply circuit and single-chip microcomputer system. The TC35 communication module circuit consists of the ICT (Ignition) startup circuit, the SYNC (Synchronization) indicator circuit, and the SIM (Subscriber Identification Module) card circuit; the single-chip microcomputer system includes the single-chip microcomputer minimum system, the single-chip microcomputer and TC35 communication circuit, and the single-chip microcomputer and PC communication circuit. The system hardware principle is shown in Figure 2. The single-chip microcomputer communicates with the TC35 module through AT instructions. AT instructions can be used to control calls, text messages, phone books, data services, supplementary services, faxes, etc. The single-chip microcomputer sends AT commands directly to the TC35 module through the serial interface, which can complete a variety of functions. Such as network login, reading SIM card numbers, sending SMS messages, receiving SMS messages, etc. In this way, the sending, receiving, querying and management of short messages can be realized conveniently and concisely.

Figure 2 System hardware structure diagram [page]

Figure 3 Main program design flow chart

3 Software design[2]

The system programming includes PC interface design and MCU programming. Instrument users only introduce MCU programming here. MCU programming mainly includes MCU and TC35 module initialization program; MCU control program for TC35 module; TC35 module SMS sending and receiving program.

The main program is a description of the entire system framework [3]. The function of the main program of this system is to complete the system initialization after power-on, then wait for the command from the PC or TC35 module to arrive, and complete the corresponding action according to the content of the command, such as sending or receiving text messages. The flow chart is shown in Figure 3.

System initialization includes the initialization of the MCU itself and the TC35 module. The MCU initialization mainly sets the serial port and timer registers. In this system, the serial port is set to a baud rate of 9600Bps, and timer 0 is set for delay to generate the baud rate required for the simulated serial port. The initialization of the TC35 module is done by sending AT commands to the MCU to set the TC35 module.

The microcontroller controls TC35 through AT commands, and TC35 responds to the commands to make corresponding actions. For receiving SMS, the microcontroller uses serial port interruption. When the serial port is interrupted, it determines whether it is a new message prompt, and then takes the next step. Sending SMS uses query mode, and the main program performs cyclic query on the PC side and performs corresponding actions on the query results.

4 Conclusion

This system is not limited to application in specific fields. It mainly analyzes and studies the common key technologies in short message systems through this design. After mastering this technology, it can be applied to specific fields such as remote data acquisition, monitoring, and alarm by simply adding microcontroller peripheral circuits.

References

[1] Cheng Xiang. Short Message Service in GSM System[J]. Mobile Communications, 2004, (6): 7-10

[2] Li Hui. Wireless communication system based on GSM short message[D]. Nanjing: Nanjing University of Science and Technology, 2004

[3] Huang Weigong, Principles and Application Technology of Single Chip Microcomputers[M]. Xi'an: Xi'an University of Electronic Science and Technology Press, 2007