Author: Fan Yu Cheng Xu Zhaohui whole
With the development of communication technologies, service functions based on mobile communication networks are expanding in the field of industrial control and remote control. In order to meet this need, Siemens introduced a new generation of TC35i wireless communication GSM module. Based on the short message function of the module, it can realize user information processing functions quickly, safely and reliably, especially for the distance between the control center and the terminal equipment. It is far from or inconvenient to use wired communication media. The system consists of a control center and a number of display screens. The control center sends the latest information to each display in real time to form a communication link between the control center and the display terminal.
2 TC35i and microcontroller hardware interface circuit
The TC35i provides fast, secure and reliable data, voice transmission, short message service and fax in system solutions. The module operates from 3.3 V to 5.5 V and can operate from 900 MHz to 1800 MHz. The module has an AT command set interface that supports short messages in text and PDU mode. In addition, the module also has functions such as phone book, multi-party call, roaming detection, etc. The common working modes are power saving mode, I-DLE and TALK modes. Bidirectional transmission of power connections, commands, data, voice signals and control signals via a 40-pin ZIF connector. The SIM card holder and antenna can be connected separately via a ZIF connector and a 50 Ω antenna connector.
In the control system, the AT89C51 type MCU is used to simultaneously expand the RAM6264 for
Stores the 128 B data to be displayed corresponding to the display and the information read from the GSM module. In order to increase the capacity of data storage, this design uses AM29F040 FlashMemory (512 KB), which has large storage capacity, high integration, low cost, flexible read and writeability and good data non-volatility. It has 15 address lines, of which the lower 8 bits are provided by the P0 port after being latched by the 74HC373, and the upper 11 bits are provided by the P2 port together with P1.4, P1.5, and P1.6. The 8-bit data is provided directly by the PO port.
Using dynamic scanning mode, 16 lines of progressive scanning with 74HCl54 and 74HC595, using the visual persistence effect of the human eye, can save I / O resources and reduce heat.
3 Working principle
As shown in Figure 1, at the receiving terminal, the MCU reads the data carried by the TC35i module in the form of a short message, and displays it on the LED display after processing. Because the Chinese characters in the short message are only one code, the MCU application system converts the code into Chinese character dot matrix data, and the Chinese character font must be configured. Therefore, when designing a single-chip application system, the control center edits the PDU data packet data and adopts a custom data method. Among them, the data representing Chinese characters directly uses the internal code of Chinese characters, and the font configured by the system is the Chinese character library encoded by GB-2312, that is, the location code Chinese character library. The data format of the SMSPDU is introduced by analyzing the information to be sent stored in the mobile phone. First, write a message on your mobile phone, send the mobile number to 13560960631, and the content of the message is "HelloWorld!". This piece of information can be read by executing AT+CMGL=2.
The operation is as follows (the italic characters are response information, and the comments in {} are:)
The software interface between the MCU and the mobile phone is actually the technology that the MCU controls the mobile phone through the AT command related to the GSM short message, such as reading the short message content of the mobile phone, deleting the short message content, and listing the short messages that have not been read in the mobile phone. Executing one instruction is not as simple as some information. In fact, the execution of the instruction requires the interaction between the microcontroller and the mobile phone. The number of bytes sent or received is strictly regulated. The two must be implemented according to these regulations. Data exchange, otherwise, communication is a failure. Table 1 lists the AT instruction execution process.
The command symbols, constants, PDU packets, etc. of all AT commands are transmitted in ASCII code format. For example, the ASCII code of "A" is 41H, the ASCII code of "T" is 54H, and the ASCII code of the number "0" is 30H.
When the MCU controls the operation of the mobile phone, the short message working mode of the mobile phone must be set to the PDU format, that is, the command AT+CMGF=0 is completed.
After the MCU sends every l instruction to the mobile phone, it must use the carriage return as the end of the instruction. The ASCII code of the carriage return is ODH. For example, the MCU sends the “AT+CMGF=0†command to the mobile phone, and its ASCII code is listed as “ 41H, 54H, 2BH, 42H, 4DH, 47H, 46H, 3DH, 30H, 0DH", the last byte 0DH is a carriage return, indicating that the instruction ends. If there is no such carriage return, the phone will not recognize this. Instructions.
When the mobile phone receives a complete AT command, the mobile phone does not immediately execute this command, but first sends all the ASCII code sequences of the AT command just received (including ODH), and then sends a carriage return. The ASCII codes of the characters and newline characters are 0DH and OAH, and the instruction is executed last.
When the mobile phone sends the short message content to the MCU, the content of the PDU data packet is the data expressed in hexadecimal notation, but it does not directly transfer the hexadecimal data to the MCU, but still encodes each hexadecimal number in ASCII. Send, so that the 2-byte hexadecimal number becomes 4 bytes of ASCII code. However, the length of the data byte in the PDU packet is still the actual byte length, not the length of the ASCII code. This should be paid attention to during programming. Otherwise, the received data is incomplete. After receiving the PDU packet data, the MCU must restore it to hexadecimal data. The algorithm is as follows:
Let a be the received ASCII code. b is the converted hexadecimal number, then b=a-30H if a<39H, b=a-30H-07H if a>39H. Finally, the two numbers before and after are combined into one byte.
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