Design of LED display screen based on GSM moduleAbstract: This paper introduces the hardware interface circuit between TC35i module and AT89C51 single chip microcomputer, analyzes in detail the process of information transmission between AT89C51 and TC35i and how to read Chinese character code from memory and provide it to LED display screen, and explains the problems needing attention in information transmission.1 Introduction With the development of communication technology, the service function based on mobile communication network is expanding to the field of industrial control and remote control. In order to meet this need, Siemens has launched a new generation of TC35i wireless communication GSM module. Based on the short message function of the module, user information processing and other functions can be realized quickly, safely and reliably. It is especially suitable for occasions where the control center is far away from the terminal equipment or it is not convenient to use wired communication media. The system is composed of control center and several display screens. The control center sends the latest information to each display screen in real time to form the communication link between the control center and the display screen.2 hardware interface circuit between TC35i and MCU TC35i can realize data, voice transmission, short message service and fax in the system scheme quickly, safely and reliably. The working voltage of the module is 3.3 V 5.5 V, which can work at 900 MHz 1800MHz. The module has at command set interface and supports text and PDU mode short messages. In addition, the module also has the functions of telephone book, multi-party call and roaming detection. The common working modes include power saving mode, i-dle and talk. The bidirectional transmission of power connection, command, data, voice signal and control signal is realized through 40 pin ZIF connector. The SIM card holder and antenna can be connected respectively through ZIF connector and 50 antenna connector. In the control system, AT89C51 single chip microcomputer is used to expand ram6264 at the same time to store 128 B data to be displayed corresponding to the display screen and information read from GSM module. In order to increase the capacity of storing data, am29f040 flashmemory (512 KB) is adopted in this design. It has large storage capacity, high integration, low cost, flexible reading and writing and good data nonvolatile. It has 15 address lines, of which the low 8-bit address is provided by port P0 after being latched by 74HC373, and the high 11 bit address is provided by port P2 together with P1.4, P1.5 and P1.6. 8-bit data is directly provided by Po port. Using dynamic scanning mode, 16 lines of progressive scanning are carried out with 74hcl54 and 74HC595. Using the visual persistence effect of human eyes, I / O resources can be saved and calorific value can be reduced.3 working principle As shown in Figure L, at the receiving terminal, the single chip microcomputer reads the data carried by TC35i module in the form of short message, and displays it on the LED display screen after processing. Because the Chinese character in the short message is only one code, the MCU application system changes the code into Chinese character dot matrix data, and the Chinese character library must be configured. Therefore, when designing the MCU application system, the control center edits the PDU packet data and adopts the user-defined data mode. Among them, the data representing Chinese characters directly uses the internal code of Chinese characters, and the word library configured by the system is the Chinese character library encoded by gb-2312, that is, the location code Chinese character library. Next, the data format of smspdu is introduced by analyzing the outgoing information stored in the mobile phone. First, write a message with your mobile phone. The sending mobile phone number is 13605696031 and the message content is "HelloWorld!". This message can be read out by executing at cmgl = 2.The operation process is as follows (italicized characters are the response information and is the comment):The software interface between MCU and mobile phone is actually the technology that MCU controls mobile phone through at command related to GSM short message, such as reading the short message content of mobile phone, deleting the short message content, listing the unread short messages in mobile phone, etc. Executing l instructions is not as simple as some materials. In fact, the execution process of instructions requires the interactive response between MCU and mobile phone. There are strict regulations on the number of bytes sent or received each time. The two must realize data exchange according to these regulations, otherwise, the communication will fail. Table L lists the at instruction execution process.The instruction symbols, constants, PDU data packets, etc. of all at instructions are transmitted in the form of ASC coding, such as ASC coding of "a" is 41h, "ASC coding of" t "is 54H, ASC coding of number" 0 "is 30h, etc.When the single chip microcomputer controls the operation of the mobile phone, the SMS working mode of the mobile phone must be set to PDU format, that is, it is completed through the instruction at cmgf = 0.After the single chip microcomputer sends each l instruction to the mobile phone, the carriage return character must be used as the end of the instruction. The ASC code of carriage return is ODH. For example, the single chip microcomputer sends the "at cmgf = 0" instruction to the mobile phone, and its ASC code is listed as "41h, 54H, 2bh, 42h, 4dh, 47h, 46h, 3dH, 30h and 0dh" , the last byte 0dh is the carriage return, indicating the end of the instruction. If there is no carriage return, the mobile phone will not recognize the instruction.When the mobile phone receives a complete at instruction, the mobile phone does not execute the instruction immediately, but first reverses all ASC II coding sequences of the at instruction just received (including ODH), then sends one carriage return and line feed ASC II coding, i.e. 0dh and OAH, and finally executes the instruction.When the mobile phone sends short message content to the single chip microcomputer, the content of its PDU data packet is hexadecimal data, but it does not directly transfer hexadecimal data to the single chip microcomputer, but still encodes each hexadecimal number ASC to send, so that the hexadecimal number of 2 bytes becomes ASC code of 4 bytes. However, the length of data bytes in the PDU data packet is still the actual word The section length, rather than the byte length of ASC code, should be paid special attention during programming, otherwise the received data will be incomplete. After receiving the PDU packet data, the single chip microcomputer must restore it to hexadecimal data, and its algorithm is as follows:Let a be the received ASC code. B be the converted hexadecimal number. Then, if a 39H, B = a-30h-07h. Finally, combine the first and second numbers into one byte.