Wireless sensor networks (WSNs) are an emerging research field with enormous potential applications in military, environmental, health, home, and commercial sectors. WSN nodes are the basic units that make up the network, and the rapid development of WSNs has presented many challenges to the design and management of WSN nodes. A WSN is a distributed, self-organizing network that integrates data acquisition, processing, and communication functions. A WSN consists of multiple network nodes within a certain area, each with wireless communication, sensing, and data processing capabilities. Sensor nodes are responsible for acquiring, processing, compressing data, relaying data packets from other nodes, and transmitting the data packets. The structure of sensor network nodes varies depending on the application, but generally consists of a data acquisition unit (sensors, A/D converters), a data processing and control unit (microprocessor, memory), a wireless communication unit (wireless transceiver), and a power supply unit (battery). Compared with traditional wireless network nodes, wireless sensor network nodes have obvious technical characteristics: (1) high network node density and large number; (2) limited computing and storage capabilities of nodes; (3) small node size, usually carrying batteries with very limited energy, and limited node energy; (4) limited communication capabilities, the communication bandwidth of sensor networks is narrow, and the single-hop communication distance between nodes is usually only tens to hundreds of meters. Therefore, how to design a network communication mechanism to meet the communication needs of sensor networks under limited communication capabilities is a problem that must be considered; (5) the locations of each sensor node are randomly distributed and have self-organizing characteristics. Due to the above characteristics of wireless sensor network nodes, the design of nodes requires low hardware cost, low energy consumption, and support for multi-hop routing protocols. The IEEE 802.15.4/ZigBee protocol fully considers the needs of wireless sensor network applications and has the characteristics of low power consumption, reliable communication, network self-organization, strong self-healing ability, low cost, large network capacity, and network security. Based on these basic requirements, the hardware design of wireless sensor network nodes supporting the 802.15.4/ZigBee protocol was carried out. Network Structure Nodes can form three topologies: star, mesh, and cluster tree. Nodes self-organize to form the network; each node can autonomously collect data, which is then relayed to the sink node via single-hop or multi-hop relay. The sink node sends the collected data to a remote control center or transmits it to a PC via an RS232 interface for processing and storage. Node Design The node hardware adopts a modular design, as shown in Figure 1, consisting of a computing and communication subboard, a sensor subboard, and a charging and status display subboard. The computing and communication subboard comprises a microprocessor, data storage circuitry, a wireless communication module, and a power management module. Its main functions are to store and process data, facilitate wireless communication between nodes, and provide power to the system. The sensor subboard consists of several sensors responsible for collecting information within the monitoring area. The charging and status display subboard consists of a charging module and an LCD display module, used to display the node's battery charging status, operating status, and battery level. Microprocessor Circuit The microprocessor circuit uses the Atmel ATmega128L microcontroller, manufactured using low-power CMOS technology and based on a RISC architecture. It features 128KB of on-chip program memory (Flash), 4KB of data memory (SRAM), and 4KB of EEPROM. It has eight 10-bit ADC channels, two 8-bit and two 16-bit hardware timers/counters, eight PWM channels, a programmable watchdog timer, an on-chip oscillator, an on-chip analog comparator, and interfaces including JTAG, UART, SPI, and I2C buses. The ATmega128L can operate in various modes, including a normal operating mode and six different levels of low-power operating modes, making it suitable for low-power applications. Its interface diagram is shown in Figure 2. The ATmega128L's operating clock source can be selected from an external crystal oscillator, an external RC oscillator, an internal RC oscillator, or an external clock source. The operating clock source is selected through the internal fuse bits of the ATmega128L, which can be set via JTAG programming, ISP programming, etc. In this design, the ATmega128L uses two external crystal oscillators: a 7.3728MHz crystal oscillator as the operating clock and a 32.768kHz crystal oscillator as the real-time clock source. Data Storage Circuit Due to the limited transmission capacity of the wireless sensor node's communication module and the very small duty cycle of the node, much data cannot be forwarded in real time. Therefore, a manageable memory is needed to store this data, temporarily storing data collected by the node itself or data collected by other nodes that need to be forwarded. This design uses a 512KB serial FLASH AT45DB041 to store data. Compared with ordinary data memory, this chip has the characteristics of low power consumption, small size, serial interface, and simple external circuitry, making it suitable for sensor nodes. The schematic diagram of the data storage circuit is shown in Figure 3. Wireless Communication Module The wireless communication module uses a wireless RF CC2420 module. The CC2420 is a wireless transceiver module launched by Chipcon in late 2003, compatible with the 2.4GHz IEEE 802.15.4 standard. Based on Chipcon's SmartRF03 technology and manufactured using CMOS technology, it features low operating voltage, low power consumption, and small size. It also boasts programmable output strength and transmit/receive frequencies. This chip requires only a few external components, such as a crystal oscillator, load capacitors, input/output matching components, and power supply decoupling capacitors, to operate normally, ensuring the effectiveness and reliability of short-range communication. Its maximum transmit/receive rate is 250kbps. The CC2420 has 33 16-bit configuration registers, 15 command strobe registers, a 128-byte transmit FIFO buffer, a 128-byte receive FIFO buffer, and a 112-byte security information memory. The connection between the CC2420 and the processor is relatively simple. It uses four pins—SFD, FIFO, FIFOP, and CCA—to indicate the data transmission and reception status. The processor exchanges data and sends commands with the CC2420 through the SPI interface (CSn, SO, SI, SCLK). The RESETn pin resets the chip, and the VREG_EN pin enables the CC2420's voltage regulator, causing it to generate the required 1.8V voltage, thus putting the CC2420 into normal operation. The CC2420 communicates via a monopole antenna or a PCB antenna. Its module diagram is shown in Figure 4. The CC2420 requires a 16MHz reference clock for data transmission and reception. The reference clock can come from an external clock source or be generated by an internal crystal oscillator. If an external clock is used, it is directly input from the XOSC16_Q1 pin, with the XOSC16_Q2 pin left floating. If an internal crystal oscillator is used, the crystal is connected between the XOSC16_Q1 and XOSC16_Q2 pins. Crystal oscillator startup requires enabling the CC2420 strobe command register SXOSCON. Power Management Module Power is a very valuable resource in sensor networks. To ensure low-power design of the hardware circuitry, low-power, low-voltage chips are selected for node chips. The system operates using ordinary batteries or rechargeable lithium-ion batteries. The power management chip is the ADP3338-3.3 from Analog Devices, in an SOT-223 package. Charging and Status Display Module When nodes can be charged, they operate using lithium-ion batteries. The charging module replenishes the nodes' power, ensuring continuous operation and preventing interruptions caused by battery replacement. The charging module uses the Dallas Semiconductor DS2770 and the DS2720 battery protection chip, providing functions such as charging control, power control, battery level counting, and battery protection. The processor and DS2770 use a one-wire interface to transmit information, requiring an external pull-up resistor of approximately 4.7kΩ. A schematic diagram of the charging module is shown in Figure 5. The LCD display module uses an LCM6432ZK LCD display, connected to the main MCU via a serial interface, for displaying system operating status information, charging progress, battery level, etc. The node hardware has an LCD interface for easy connection of the LCD display module when needed. Sensor Module The node sensor module is separated from the computing and communication daughterboards. This modular design improves the node's flexibility in different applications. The sensor module can be configured with appropriate sensors based on actual needs, such as temperature, humidity, vibration, light intensity, gas alarm, magnetoresistive, and infrared sensors, to meet diverse requirements. Since the nodes are mostly battery-powered, sensors must be small, low-power, and have simple peripheral circuitry; ideally, digital sensors that do not require complex signal conditioning circuitry should be used. Some of the sensors selected in this design are: The DS18B20 temperature sensor is a new type of digital temperature sensor with a very simple external circuit, using a one-wire bus interface. Its measurement range is -55℃ to 125℃, with a measurement accuracy of ±0.5℃ between -10℃ and 85℃. The maximum resolution can be designed to be 12 bits, providing accurate and reliable measurement data. The PD632 infrared sensor is a digital pyroelectric sensor with an operating wavelength of 7.5 nm to 14 nm. Its detection range is 6 m to 15 m in an operating environment of -20℃ to 60℃. The ADXL202 accelerometer is a two-dimensional digital accelerometer from Analog Devices, with an operating temperature of -40℃ to 85℃. Utilizing advanced MEMS technology, it can measure both vibration and static acceleration. External Interfaces The node's external interfaces include a JTAGE interface, an ISP programming interface, an RS232 interface, a charging interface, a sensor interface, and an SMA antenna mount interface. The node uses JTAGE and ISP to download programs; it connects directly to a PC's serial port using the RS232 interface; different sensor modules can be connected via the sensor interface according to different needs; and the node can be quickly recharged via the charging interface when charging is available. Figure 6 shows a schematic diagram of the RS232 interface. Key Design Considerations and Points to Note for Nodes The radio frequency (RF) section is the focus and challenge of this design, and is crucial to the success of the system design. The main problems encountered during the module design process and their solutions are as follows: The CC2420's carrier frequency is 2.4GHz, with a new channel added every 5MHz. The accuracy of the crystal oscillator will affect the carrier frequency, thus affecting communication establishment and stability. The CC2420 requires a clock source accuracy within ±40ppm. If an external crystal oscillator is used, a high-precision, stable four-pin surface-mount crystal oscillator should be used whenever possible. The CC2420 RF circuit operates in the high-frequency band of 2.400GHz to 2.4835GHz, and anti-interference design is directly related to RF performance and the operation of the entire sensor node. During RF wiring, reasonable layout and routing, as well as the use of multilayer boards, are not only necessary for wiring but also effective means to reduce electromagnetic interference and improve anti-interference capabilities. When routing, pay special attention to the following points: First, all areas of the RF circuit not used for routing should be filled with copper and connected to ground to provide RF shielding for effective anti-interference. Second, the bottom of the CC2420 chip should be grounded. To reduce latency, minimize crosstalk, and ensure high-frequency signal transmission, multiple grounding vias should be used to connect the bottom of the CC2420 chip to the ground plane. Third, minimize crosstalk and the influence of distributed parameters as much as possible; components should be densely distributed around the CC2420 and a smaller package should be used. For wireless communication networks, antennas play a crucial role. The selection and placement of antennas directly affect the overall operating quality of the wireless communication network. The CC2420 RF chip in this node can use two design schemes: a metal inverted F-type PCB leaded antenna and a monopole antenna. PCB leaded antennas are wires printed on a circuit board that sense radio waves in the air and receive information. The shape and size of the PCB antenna should strictly follow the datasheet design. In recent years, with the decrease in computer costs and the miniaturization of microprocessors, wireless sensor networks have received increasing attention. This design, based on a summary of domestic and international research findings on wireless sensor networks, presents a low-power, low-cost, and practical wireless sensor network node. The node employs an independently selectable charging module, an LCD status display module, and a rich set of external interfaces, making it highly practical. It can operate in various environments, be configured to fulfill diverse system functions, and offers significant advantages in terms of cost, power consumption, and flexibility.