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Design a module that can realize SMS sending and receiving and data wireless transmission. This paper adopts the mainstream product STM32 of ARM Cortex-M3 core as the main control chip, and uses SIMCom's SIM900A as the communication chip. After consulting a large number of related documents and data sheets of related chips, this paper designed a remote wireless communication module. After one week of trial operation of the module, there was no disconnection, and the speed of data transmission and reception was also fast. The module has the advantages of stable performance, small size and high cost performance. After the factory was put into use, the reaction was good.
With the continuous development of networks and modern communication technologies, remote wireless communication technology has played a very important role in the field of industrial control after years of research and practical application. According to the requirements of the manufacturer, the wireless communication module designed mainly realizes the function of sending and receiving short messages and data, and achieves the stability of the module. It can be automatically reconnected after the line is dropped.
1 wireless communication module overall design
The module mainly consists of a power supply part, a main control part, a communication part, and a data transmission part. The communication module uses SIM900A for wireless communication. The main control module uses STM32 as the main control chip to control the smooth transmission and reception of SMS and data transmission. The remote terminal can be the data center software of the mobile phone or the upper computer, and after being processed, it is stored for future inquiry. This module has low power consumption, is convenient and flexible, simple and stable in operation, can be automatically connected after disconnection, and the operation process is very stable and the cost is low.
2 wireless communication module hardware design
2.1 Functional requirements
The wireless communication module designed in this paper requires a single 12V power input. After the module is powered on, there is a corresponding indicator to indicate the operating status of the module. In addition, the module is required to implement two main functions of short message and data transparent transmission. After the mobile phone sends a short message to the module, the module can act and give a response. Finally, it is required to be able to send and receive text messages while establishing a TCP link.
Figure 1 Wireless communication module structure
2.2 Power section
In the main circuit, the working voltage of the main control chip STM32 is 2.0~3.6 V, and the working voltage of the communication chip SIM900A is 3.1~4.6 V. In order to make the various parts of the module work normally, the two must be separately powered. The power supply circuit is as follows :
In the figure, the external 12 V power supply of V12 is capacitively filtered and input to the LM2576 to realize the conversion from 12 V to 4 V. R1 and R2 play a voltage dividing role in the line. When the D2 light is on, it indicates that the module has been powered normally. LM2576 is a 3 A current output step-down switching type integrated voltage regulator circuit produced by National Semiconductor Corporation. It has a complete protection circuit and is relatively stable.
Figure 2 Power Module
2.3 main control chip
The main control module uses STM32 MCU as the microcontroller. The chip can work in the temperature range of -40~105°C. The MAX3232 chip is used for level conversion of the serial port to realize communication between the controller and the communication interface. Serial port 1 is connected to the power level conversion chip Max3223, USART1_TX (output, so when configuring GPIO, the mode of the port is defined as push-pull output, USART1_RX is input, defined as floating input mode. Serial port 2 communicates with SN65LBC184D to realize data Transceiver, USART2_RTS, USART2_RX are input ports, mode is defined as floating input mode, USART2_CTS, USART2_TX are output ports, mode is defined as push-pull output. Serial port 3 is used to control SIM900A chip, USART3_RTS, USART3_RX is input port, mode is defined as floating input, USART3_CTS and USART3_TX are output ports, and the mode is defined as push-pull output.
Figure 3 main control chip STM32
2.4 Communication chip
The communication chip adopts SIMCom's new compact product SIM900A, which belongs to the dual-band GSM/GPRS module. It is completely in SMT package form, with stable performance, exquisite appearance, high cost performance, and can meet various needs of users. In the implementation of the automatic disconnection function of the disconnection, the use of the two pins of DCD and RI is involved. The DCD pin is used to implement analog-to-digital conversion. When the module is disconnected, it will give the DCD pin a high level. When this level is detected by the DCD pin, the module takes the corresponding action to reconnect the line. The RI pin stays high after the module is online. When there is a call and a text message, the RI pin will have a low level. When the RI pin detects this low level, the module Will take the appropriate action, enter the SMS or phone mode.
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