Design of Transmission Circuit in Wireless Sensor Network

0 Introduction
Wireless sensor network is a kind of RGS system (remote ground sensor system). It is a detection system that uses multiple sensors as comprehensive intelligence collection elements, performs data fusion, encoding and other processing, and sends it to the command center. After processing and restoration, it is displayed on the monitoring platform. It integrates sensor technology, image detection technology, vibration detection technology, sound detection technology, wireless communication technology, digital coding compression technology, information fusion technology and computer technology. It is a comprehensive technology integrated with multiple high-tech technologies. The wireless multi-sensor network system mainly consists of the following parts:
(1) System front-end sensor and GPS module - signal acquisition part: mainly composed of detection units and GPS modules composed of image, sound, vibration and infrared sensors, responsible for completing battlefield information monitoring tasks. (2
) Information transmission part: mainly responsible for compressing and encoding the collected information and performing long-distance wireless transmission.
(3) Command center measurement and control platform part: mainly completes the remote control and signal reception tasks of the monitoring unit, and performs fusion processing and analysis on the various collected information. The processing results are provided to the command center personnel so that they can grasp the battlefield situation in a timely and accurate manner and make corresponding decisions.
This paper mainly discusses the hardware design and software programming ideas of image transmission systems in wireless sensor networks.

1 Design and working principle of transmitter modem hardware circuit
The modem hardware circuit is actually different on the transmitter and receiver sides because of the different tasks to be completed. The schematic diagram of the transmitter modem circuit is shown in Figure 1.

The system uses a +5 V power supply converted from the radio station's battery. MSM7512 B uses a dedicated 3.579 545 MHz crystal. Because it has an internal grounding capacitor, no external compensation components are required. The microcontroller uses a crystal with a frequency of 11.059 2 MHz. This is mainly to obtain an accurate baud rate when setting the baud rate, which can effectively avoid the accumulated error caused by the timer. The external compensation components are two 30 pF capacitors. In order to prevent malfunctions when the microcontroller program is running, the microcontroller EA/VPP terminal (pin 31) should be set to a high level to ensure that the microcontroller accesses the internal program memory. Since both the modem chip MSM7512B and the single-chip computer W77E58 support TTL level, the first serial communication port TXD and RXD of the single-chip computer can be directly connected to the XD and RD of the MSM75125B; the P1.0 and P1.1 of the single-chip computer are respectively connected to: MOD2 and MOD1 of the MSM7512B, according to the communication requirements, switching between sending and receiving to control the working state of the modem chip; P1.4 controls the switching of the receiving/transmitting state of the radio station (PTT). The AO and AI of the MSM75125B are respectively connected to the transmitter/receiver of the radio station through the interface circuit. The display circuit used as the external monitoring of the system is composed entirely of light-emitting diodes and resistors. The red light-emitting diode D1 is a power indicator. If it is on, it means that the system initialization process is correct; the yellow light-emitting diode D2 is a transmission correct indication. It should flash once each time the system completes a data transmission task correctly; D3 is a carrier detection indicator. If it is on, it means that the modem has detected a valid carrier signal in the channel; D4 is a data transmission indicator. It starts flashing when the system sends data until the data is sent. If the front-end sensor has data to be transmitted, a falling edge pulse is generated to trigger the external interrupt INTO (P3.2) of the microcontroller. After the microcontroller responds to the interrupt, the 8-bit parallel data from the front is read in from the P2 port (P2.0~P2.5). Since the P2 port has a pull-up resistor inside, it can be driven by a TTL or MOS circuit when used as an input port, instead of an external pull-up resistor. The serial communication port 2 of the W77E58 can be reserved as a system expansion port.

2 Interface circuit between receiving modem and single-chip microcomputer
The use and control of the receiving modem circuit and battlefield sensor modem circuit in single-chip microcomputer and modulation and demodulation chip are the same. The difference is that the second serial port of the single-chip microcomputer is connected to the RS 232C port of the computer through the level conversion circuit to transmit the received digital signal to the microcomputer. The interface circuit between the receiving modem and the single-chip microcomputer is shown in Figure 2. The function of the light-emitting diode display circuit is not exactly the same. D1~D8 is the display of received data, which can display the correctly received data in the form of binary numbers, D9 is the power supply indicator of the system, D10 is the correct transmission indicator, D11 is the carrier detection indicator, and D12 is the data transmission indicator.

3 Design of the interface circuit between modem and PC
The interface between modem and PC is actually the interface circuit between the single-chip microcomputer W77E58 in the modem and the PC. W77E58 supports TTL level, while the serial communication port RS 232C of the microcomputer supports EIA level. Therefore, when realizing serial communication between them, a level conversion circuit must be designed to meet their respective needs.
The level conversion circuit is the interface circuit between the modem and the microcomputer on the command center side, and it is also a component of the hardware circuit of the data wireless transmission system (command center side). Its working process is as follows: the digital signal demodulated by the modem is processed by the single-chip microcomputer, and then transmitted from the serial communication port 2 of W77E58, through the level conversion chip MAX232, the RS 232C port (DB9) of the PC and the UART inside the microcomputer, and finally passed to the CPU and displayed on the monitoring platform. The circuit schematic is shown in Figure 3.

4 Software Design for Wireless Image Transmission
The program is divided into five parts, three main programs: the sender's single-chip program, the receiver's single-chip program and the microcomputer receiving program; two subprograms: the error handling subprogram and the sending delay subprogram.
The communication between the sender and the receiver and between the single-chip computer and the PC uses the software "handshake" signal. All communication "handshake" signals are #0AAH, the response signal is #00H after correct reception, and the response is #0FFH if the reception is wrong.
When there is no data to be transmitted on the sensor side, the MSM7512B is controlled by the programming of the single-chip computer to work in the power saving mode. At this time, the power consumption of the modem chip (excluding W77E58) is only 0.1 mW, which can maximize the battery life.

The logic control relationship between the single-chip microcomputer and MSM7512B is: P1.0→MOD2, P1.1→MOD1, P1.5→AOG, and P1.4→radio station PTT. The single-chip microcomputer controls MSM7512B and the radio station to perform receiving and transmitting conversion. When the front-end sensor has data transmission, a falling edge pulse signal is generated to start the program of the entire system. After the data transmission is completed, the system returns to the initial state. The P1.5 port of the single-chip microcomputer controls the output level of MSM7512B.
Set the two serial ports of the single-chip microcomputer to work in serial port working mode 1; timer T1 works in mode 2 (automatically reloads the initial value) and acts as a baud rate generator. By adjusting the initial value of T1, it is used to select three rates of 1200 b/s, 600 b/s and 300 b/s; timer T2 works in mode 1 and acts as a timer to design and arrange delays.
In the process of system design, in order to reduce the influence of low sensitivity of radio station and poor channel quality and error, the sender needs to send "handshake" contact signals 5 times in a row. The receiver will confirm that it is a valid contact and respond after receiving the correct contact signal twice in a row. Otherwise, it will be considered as an interference signal and will not respond. This can not only reduce the possibility of missed reports caused by various reasons that the receiver program does not start running, but also ensure that the receiver does not operate incorrectly due to interference signals, reducing the probability of false reports. In addition, considering the various delay times required for the radio station's transceiver conversion and the transceiver conversion of the modulation and demodulation chip, a delay time is specially arranged when designing the program. After a lot of experiments, a more appropriate delay time is obtained, that is, no matter which party is communicating, after switching from the receiving state to the transmitting state, a delay of 70 ms is first made, because if the time is too short, the system cannot work normally, and if it is too long, it may affect the data transmission rate and reduce the timeliness of data transmission. The program flow chart of the system data transmitter and receiver microcontroller is shown in Figure 4.

5 Conclusion
Through understanding the performance characteristics of the MSM7512B modem chip, the actual circuit of the transmitter and receiver modems is designed. Then, the performance characteristics of the single-chip microcomputer W77E58 with dual serial port function are briefly introduced. The hardware circuit diagram of the serial communication between the receiving single-chip microcomputer and the PC of the data wireless transmission system is given, and the design process of the Mod-dem and radio interface circuit is described. Finally, the characteristics of the single-chip microcomputer software of the whole system are described. The design schematic diagram of the wireless sensor network data wireless transmission system is given as a whole.
Wireless sensor networks involve sensor technology, network communication technology, wireless transmission technology, embedded computing technology, microelectronics manufacturing technology, software programming technology and other fields. They have interdisciplinary characteristics and have broad application prospects in military, civil defense, environment, ecology, agriculture, health, family and other fields. In special fields such as space exploration and disaster relief, sensor networks have their unique technical advantages.