Abstract: This paper introduces the design and implementation of a wireless sensor network node based on the ATmega128L and CC2420 operating in the 2.4GHz frequency band, and discusses the hardware composition of each part in detail. Experiments show that the nodes can flexibly form a wireless sensor network. The network system composed of the nodes has stable performance, high communication efficiency, and low power consumption, and can be widely used in control, signal acquisition and transmission fields. Keywords: ATmega128L microcontroller; CC2420 chip; wireless sensor network node. Wireless sensor networks are an emerging research field with huge potential applications in many fields such as military, environment, health, home, and commerce. Wireless sensor network nodes are the basic units of the network. The rapid development of wireless sensor networks has brought many challenges to the design and management of wireless sensor network nodes. Wireless Sensor Network Node A wireless sensor network is a distributed self-organizing network that integrates data acquisition, processing, and communication functions. A wireless sensor network consists of multiple network nodes with wireless communication, sensing, and data processing functions within a certain area. Sensor nodes are responsible for collecting, processing, compressing data, relaying data packets from other nodes, and sending the data packets out. The structure of sensor network nodes varies in different applications, but they are generally composed of data acquisition units (sensors, A/D converters), data processing and control units (microprocessors, memory), wireless communication units (wireless transceivers), and power supply units (batteries). 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, resulting in limited node energy; (4) limited communication capabilities, with narrow communication bandwidth in sensor networks, 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, featuring low power consumption, reliable communication, network self-organization, strong self-healing capabilities, low cost, large network capacity, and network security. Based on these basic requirements, a hardware design for a wireless sensor network node supporting the 802.15.4/ZigBee protocol was developed. Network Structure Nodes can form three topologies: star, mesh, and cluster tree. Nodes form a self-organizing 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 sub-board, a sensor sub-board, and a charging and status display sub-board. The computing and communication daughterboard consists of a microprocessor, data storage circuit, wireless communication module, and power management module. Its main functions are to store and process data, complete wireless communication between nodes, and provide power to the system. The sensor daughterboard consists of several sensors responsible for collecting information within the monitoring area. The charging and status display daughterboard consists of a charging module and an LCD display module, used to display the node's battery charging status, node operating status, and battery level. Microprocessor Circuit The microprocessor circuit uses the Atmel ATmega128L microcontroller, manufactured using low-power CMOS technology, based on a RISC architecture. It has 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 such as JTAG, UART, SPI, and I2C buses. The ATmega128L can operate in multiple 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 an external crystal oscillator, an external RC oscillator, an internal RC oscillator, or an external clock source. The selection of the operating clock source is designed through the ATmega128L's internal fuse bits, 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 the data. Compared to ordinary data storage devices, this chip features low power consumption, small size, serial interface, and simple external circuitry, making it suitable for sensor nodes. A schematic diagram of the data storage circuit is shown in Figure 3. [align=center][align] Wireless Communication Module The wireless communication module uses the CC2420 wireless RF module. Launched by Chipcon in late 2003, it is a wireless transceiver module 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, small size, and programmable output strength and transmission/reception frequency. This chip requires only a few external components, such as a crystal oscillator, load capacitor, input/output matching components, and power supply decoupling capacitors, to operate normally, ensuring the effectiveness and reliability of short-range communication. Its maximum transmission/reception 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 the 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 avoiding 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 communicate via a single-wire interface, 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. 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, the 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, 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 JTAG interface, an ISP programming interface, an RS232 interface, a charging interface, a sensor interface, and an SMA antenna mount interface. The node uses JTAG 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, and Figure 7 shows a schematic diagram of the JTAG/ISP interface. Key Points and Considerations for Node Design 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 affects the carrier frequency, thus impacting 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 directly affects 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 conductors 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.