Design of wireless data transmission system based on SIM900A and LPC2378

    GSM is a relatively mature and perfect communication system. It can provide short messages, voice, data transmission and other services. Among them, the transmission of short messages is an important service of the GSM network and has been widely used. For example, in the wireless transmission of monitoring data such as substations, electric meters, water towers, and remote wireless control systems, a wireless device with low power consumption, stable performance, and strong anti-interference ability is required. So far, whether it is Bluetooth technology or other radio wave remote control devices, few can meet the above conditions, while the GSM system can meet the above requirements well.

1 System Hardware Design
1.1 System Block Diagram
    Design - A wireless transmission system based on the GSM communication module, using the SIM900A communication module and the LPC2378 controller to make a wireless Modem, and cooperate with the serial port terminal to send and receive SMS data, and use this Modem and the GSM network as a link to achieve long-distance data transmission. The design block diagram is shown in Figure 1.

1.2 Design of control module
    The core chip of the control module is LPC2378, which is a 32-bit ARM7TDMI-S based microcontroller with a reduced instruction set and ultra-low power consumption launched by NXP Semiconductors (NXP). It is suitable for applications that require serial communication for various purposes. Due to its low power consumption, rich on-chip peripherals and convenient and flexible development methods, it has become a research hotspot in many single-chip microcomputer series. Its main features are: low voltage, ultra-low power consumption; operating voltage range of 3.0~3.6 V; 10-bit A/D converter on chip; 4 32-bit timers with flexible clock settings; the operating frequency of the on-chip crystal oscillator is between 1 and 24 MHz. It has 4 UART serial ports, which is convenient for users to communicate with multiple machines. It provides a total of 5 data ports from P0.0 to P4.0, which can provide users with more processing functions. Among the peripheral data ports provided, 4 have interrupt functions. It has a JTAG simulation debugging interface to facilitate software debugging. The chip can provide more memory, with 512 kB of on-chip Flash, and also provides more RAM for calculation processing.
1.2.1 Power supply
    In this system, the LPC2378 microcontroller needs to be powered by 3.3 V and 1.8 V, so the National Company's LM117-33 and LM117-18 chips are used to achieve the voltage reduction from 5 to 3.3 V and 3.3 to 1.8 V. The LED light turns on after the power is turned on, indicating the power on and off; the capacitor is used for filtering to reduce interference to the input end. The power supply schematic is shown in Figure 2.

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1.2.2 Serial communication and buttons
    LPC2378 has two serial ports, USART1 is connected to SIM900A, and USART0 is connected to the host computer. The data to be transmitted by the host computer is temporarily stored in the microcontroller through serial port 0, and then finally transmitted to the SIM900A module through serial port 1, thereby realizing wireless data transmission. Although the working level of the serial port pin of the SIM900A module is CMOS level, and the working voltage of the serial port pin of the microcontroller is TTL level, the high and low level logic judgment level of the microcontroller can be connected to the pin of SIM900A, so the serial port line of the SIM900A module can be directly connected to the serial port 1 of the microcontroller, while the serial port 0 needs a MAX3232 for level conversion to communicate with the host computer, as shown in Figure 3.

1.3 Communication module design
    SIM900A is a new generation of wireless communication GSM/GPRS module launched by SIMCom. It can quickly, safely and reliably realize data, voice transmission, short message service (SMS) and fax in the system solution. The working voltage of the module is 3.2~4.8 V, and the working frequency band is EGSM 900 MHz and DCS 1 800 MHz. SIM900A supports GPRS multi-slot class 10/class 8 (optional) and GPRS coding formats CS-1, CS-2, CS-3 and CS-4. The maximum power consumption of the frequency band is 2 W (900 MHz) and 1 W (1 800 MHz) respectively.
    SIM900 A adopts power saving technology design, so the minimum current consumption in Sleep mode is only 1.0 mA. Common working modes include Sleep, Idle, Talk and other modes. With a size of only 24 mm × 24 mm × 3 mm, SIM900A can meet the space requirements of almost all user applications, such as M2M, data transmission systems, etc. SIM900A is SMT packaged, based on STE single chip solution, using ARM926EJ-S architecture, to achieve two-way transmission of power connection, instructions, data, voice signals, and control signals.
    Keyboard and SPI display interface allow users to flexibly design customized applications. Main serial port and debug serial port can help users easily develop applications. One audio interface, including a microphone input and a speaker output. Programmable general input and output interface (GPIO). SIM900A has built-in TCP/IP protocol, and the extended TCP/IPAT command allows users to easily use TCP/IP protocol, and it is widely used in data transmission.
1.3.1 Power supply part
    SIM900A uses a single power supply of VBAT with a voltage of 3.4 to 4.5 V. In some cases, the burst of signal transmission will cause voltage drop, and the peak current consumption will reach 2 A. Therefore, the power supply must be able to provide enough current of 2 A. To power SIM900A, Micrel's LDO MIC29302BT is used to achieve voltage reduction from 5 to 4 V, as shown in Figure 4.

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1.3.2 SIM card circuit
    The interface level of the SIM card is provided by the voltage regulator inside the module, and the normal voltage value is 2.8 V or 1.8 V. After reset, all pins output low level. The 6-pin SIM card circuit is shown in Figure 5.

1.3.3 LCD and keyboard interaction
    The operation results of the control module can be displayed on the LCD, and the keyboard can be used to debug and set the period.

2 Software Design
2.1 AT Command
    System Program Flow The focus of software design is on the programming of the single-chip microcomputer. By writing different AT commands to SIM900A, various functions can be completed, such as network login, reading the phone number on the SIM card, sending and receiving messages, deleting messages, etc. Here we focus on the sending of Chinese text messages.
2.2 PDU encoding rules
    At present, the Text and PDU (Protocol Data Unit) modes are commonly used to send short messages. The code for sending and receiving text messages using the Text mode is simple and easy to implement, but the biggest disadvantage is that Chinese text messages cannot be sent and received; while the PDU mode not only supports Chinese text messages, but also English text messages. PDU mode can use three encodings for sending and receiving text messages: 7-bit, 8-bit and UCS2 encoding. 7-bit encoding is used to send ordinary ASCII characters, 8-bit encoding is usually used to send data messages, and UCS2 encoding is used to send Unicode characters. The general PDU code consists of 13 items: A, B, C, D, E, F, G, H, I, J, K, L, and M. In the GSM standard, Chinese characters are encoded using PDU, so Chinese characters need to be encoded before they can be sent out.
    A: Short message center address length, 2-digit hexadecimal number (1 Byte); B: Short message center number type, 2-digit hexadecimal number; C: Short message center number, the length of B+C will be determined by the data in A; D: File header byte, 2-digit hexadecimal number; E: Message type, 2-digit hexadecimal number; F: Called number length, 2-digit hexadecimal number; G: Called number type, 2-digit hexadecimal number, the value is the same as B; H: Called number, the length is determined by the data in F; I: Protocol identifier, 2-digit hexadecimal number; J: Data encoding scheme, 2-digit hexadecimal number; K: Validity period, 2-digit hexadecimal number; L: User data length, 2-digit hexadecimal number; M: User data, its length is determined by the data in L. J is set to use UCS2 encoding, which is Unicode characters for Chinese and English.
2.3 Program flow

3 Conclusion
    The system has been debugged and runs stably during the debugging process. The design cost and use cost are low. It can be widely used in remote security monitoring, PLC data acquisition and transmission, smart meters, vehicle communications, wireless POS, wireless medical, home anti-theft and other wireless transmission and automatic alarm systems. In the coming era of the Internet of Things, the application of wireless communication systems will play a key role.