Design and implementation of wireless sensor network nodes

Wireless sensor network is a kind of comprehensive technology that appears after the development of information technology to a certain stage, which integrates sensors, embedded systems, modern networks, wireless communications, distributed information processing, etc. Sensor networks can be widely used in military, environmental monitoring and forecasting, health care, smart homes, building status monitoring, complex mechanical control, urban transportation, space exploration, large workshop and warehouse management, as well as security monitoring of airports and large industrial parks. A wireless sensor node is designed in this paper. The hardware design is based on the Moteiv solution, using the ultra-low power microcontroller MSP430F1611 as the data processing chip, the CC2420 wireless RF chip as the transceiver chip, and has JTAG and other expansion interfaces. Through hardware testing and software debugging, the node meets the design indicators.

1 System Overview

The wireless sensor network is composed of a large number of wireless sensor nodes. Each node collects data through sensors. The data processing chip is responsible for receiving and processing the data collected by the sensor. The wireless transmission and reception of data and the wireless networking function are performed through the wireless radio frequency chip. The USB interface can be used as a power supply and programming interface, the first-line silicon serial number can be used as the unique identification of the node, the Flash chip is used to store data, and the JTAG port is used for debugging and programming.

2 Hardware Design

The node mainly consists of six parts: power supply unit, wireless RF module, sensor module, USB communication module and microprocessor.

2.1 Power supply unit

Sensor nodes are small and usually carry batteries with limited energy. The energy supply module is crucial in wireless sensor nodes, providing energy for various parts of the sensor nodes. Some sensors that require long-term data collection require solar energy and other methods to maintain the normal operation of the node. Each part of the node also needs to be carefully designed. The operating voltage of the node microprocessor is 1.8~3.6V, the operating voltage of the wireless RF chip is 2.1~3.6V, the Flash power supply voltage is 2.7~3.6V, and the USB conversion chip is powered by USB. Therefore, the power supply can be powered by a 3V button battery, which effectively reduces the size of the node. In addition, USB can also be used as a power supply and a programming power supply. The power supply unit is shown in Figure 2. The LC filter unit has a good filtering effect on AC power without reducing the DC output voltage. The low-dropout regulator ADP3339 ensures good voltage stability of the power supply.

2.2 Wireless RF Module

The wireless transceiver in the node uses a wireless transceiver chip CC2420 that is compatible with 2.4GHz IEEE802.15.4 launched by Chipcon. It is based on Chipeon's Smart RF03 technology, produced using 0.18μm CMOS technology, and has a high degree of integration. The chip is small in size, low in power consumption, and has a fully integrated voltage-controlled oscillator. It only needs a small number of peripheral circuits such as an antenna and a 16MHz crystal to work in the 2.4GHz frequency band. CC2420 uses O-QPSK modulation; ultra-low current consumption, a receiving sensitivity of -94dBm, strong resistance to adjacent channel interference, and its selectivity and sensitivity index exceed the requirements of the IEEE802.15.4 standard, which can ensure the effectiveness and reliability of short-distance communication. The wireless communication equipment developed using this chip supports a data transmission rate of up to 250kbit·s-1, and can achieve multi-point to multi-point rapid networking. [page]

The peripheral circuit of the wireless RF module in Figure 3 uses the device values ​​of the typical application circuit provided in the CC2420 manual, which ensures that the chip can work in a normal state. In addition, in order to achieve optimal performance, power decoupling must be used. The setting and size of decoupling capacitors and power filtering play a key role in obtaining optimal performance in the application. TI provides a compact reference design that must be strictly followed. At the same time, digital and analog power supplies are added with capacitor filtering. CC2420 requires a 16MHz reference clock for data transmission and reception with a transmission rate of 250kbit·s-1. The reference clock can come from an external clock source or be generated by an internal crystal oscillator. Here, the internal crystal oscillator is used. Antenna impedance matching is crucial. In order to obtain better transmission power, some capacitors and inductors are appropriately adjusted on the original basis. The communication between CC2420 and the microcontroller is realized through a 4-wire SPI bus (SI, SO, SCLK, CSn). By controlling the interface status of the FIFO and FIFOP pins, the chip can work in the transmission or reception mode. In addition, CCA is used for idle channel assessment, and SFD is used to control the input of clock or timing information.

2.3 Sensor Module

The sensor is the data acquisition unit of the wireless sensor node. Different sensors can be selected according to actual needs. The node uses the temperature and humidity sensor SHT11. SHT11 is a single-chip fully calibrated data output sensor that integrates temperature and humidity sensors, signal amplifiers and conditioners, A/D converters, and bus interfaces on one chip. It can directly provide digital outputs with a temperature range of -40 to 120°C and a resolution of 14 bits and a humidity range of 0 to 100%RH and a resolution of 12 bits. The sensor and peripheral modules are shown in Figure 4.

The power supply requirement of SHT11 is 2.4～5.5V. The power supply and clock signal are provided by the microprocessor. The data line pin is tri-state output, so an external pull-up resistor is required to pull the signal high. [page]

2.4 USB Communication Module

The wireless sensor network is composed of a large number of nodes. These nodes fuse and transmit the collected data according to a certain protocol, and finally transmit the data to a personal computer for processing and observation. At the same time, the node operation requires specific programs to be written into the Flash. At present, the most commonly used interface for personal computers is USB, while the microprocessor uses the USART interface. In order to achieve conversion between the two interfaces, the node uses the FT232BM chip as a conversion chip. FT232BM is a single-chip USB to asynchronous serial port conversion chip that supports full handshake and modulation and demodulation interface signals. The data transmission speed range at the TTL level is 300bit·s-1~3Mbit·s-1. The chip is powered by the USB bus, the operating voltage is 4.35~5.25V, and an external 6MHz clock is used. USB PID, serial number and product information can be saved in an external EEPROM. The USB communication module interface is shown in Figure 5.

2.5 Microprocessor Module

The microprocessor is the core of the wireless sensor node. The data processing of sensor data, the transmission and control of the serial port and the wireless module all require the participation of the microprocessor. The node microprocessor TI's 16 is the ultra-low power MSP430F1611. The operating voltage of this microprocessor is 1.8~3.6V, and the power consumption is only 0.2μA in RAM data retention mode, and 330μA when activated at 1MHz. It can work in 5 low-power modes, and the wake-up time is <6μm. The Flash size is 48kB and the RAM size is 10kB. The chip has 16-bit timers Timer_A and Timer_B with capture/compare functions; a large number of capture/compare registers can be used for event counting, timing generation, etc.; the multi-function serial port (USART) can realize asynchronous, synchronous and I2C serial communication, and can easily realize multi-level communication applications; it has more I/O ports, up to 6×8 I/O lines, and P1 and P2 ports can also receive external rising or falling edge interrupt inputs; the 12-bit A/D converter has a high conversion rate, up to 200kbit·s-1, which can meet most data acquisition applications. The microprocessor module is shown in Figure 6. The temperature and humidity sensor is connected to the 5, 6, and 7 pins of the P1 port, the wireless module SPI is connected to the 1, 2, 3 and P4.4 pins of the P3 port, and the USB communication module data line is connected to the 6 and 7 pins of the P3 port. The other pins are used for control and expansion interfaces respectively. The external crystal size is 32MHz.

2.6 Circuit Board Design

Circuit boards are classified according to different standards. In design, most of them are classified according to the number of boards. In the design of circuit boards with complex electrical connections, double-sided boards are difficult to meet the requirements of circuit wiring. In this case, multi-layer boards must be considered. This node uses a 4-layer board. The top layer is mainly USB module and wireless module, the bottom layer is microprocessor module, and the inner layer is power layer and ground layer. The design result is shown in Figure 7. [page]

The actual node is shown in Figure 8.

3 Design Verification

3.1 Wireless Module Verification

To verify the node function of the design, the test program of CC2420 is first written using the IAR integrated development environment, and the program is burned into Flash through JTAG. After testing, the RF part meets the expectations well. The chip operating frequency band range is 2.4~2.48GHz, and the transmission power is 0dBm. The design meets the requirements through the spectrum analyzer and frequency meter. The verification results are shown in Figure 9.

3.2 Sensor and USB conversion module verification

TinyOS is an open source operating system developed by UC Berkeley, designed for embedded wireless sensors. The component-based architecture of the operating system makes fast updates possible, which in turn reduces the code length limited by the memory of the sensor network. The components of TinyOS include network protocols, distributed servers, sensor drivers, and data identification tools. Its good power management comes from the event-driven execution model, which also allows flexibility in timing scheduling. Therefore, the TinyOS operating system is used for the verification of the entire wireless sensor network. It is designed as two nodes. Node A is responsible for collecting temperature and humidity data, and then sends the collected data to another node B. After receiving the data, node B transfers the data to a personal computer via USB and displays the data in a chart, as shown in Figure 10.

4 Conclusion

A wireless sensor node designed in this paper has a hardware design based on the Moteiv solution. It uses the ultra-low power microcontroller MSP430F1611 as the data processing chip, the CC2420 wireless RF chip as the transceiver chip, and has JTAG and other expansion connections. Through hardware testing and software adjustment, the node meets the design indicators.