Design of RF wireless sensor network node based on ZigBee and 51 core

0 Introduction

　　In recent years, wireless sensor network technology has developed rapidly. Due to the free and open characteristics of the 2.4 GHz communication frequency band, various communication protocols based on this frequency band, such as Wi-Fi, Bluetooth and other technologies, have become quite mature and widely used. ZigBee is a low-power personal area network protocol based on the IEEE802.15.4 standard. The protocol is based on the 2.4 GHz frequency band and is a low-cost, low-power short-range wireless networking communication technology. In recent years, it has been widely used in various radio frequency communication fields, such as regional positioning, line-of-sight data transmission, Internet of Things tags, and automotive wireless electronic devices.

　　SOC (system on chip), represented by Chipcon's series of products based on ZigBee protocol, is also becoming more mature. Therefore, designing a low-cost, stable and fully functional development system has always been an important part of related research. This paper will propose a RF wireless sensor network node hardware design based on ZigBee and 51 cores. The design is based on Chipcon's CC2430 chip, which meets the physical layer requirements of the ZigBee protocol and integrates a 51 core MCU. It is low-priced and has great development potential. The design adopts a modular design method and can be applied to various software and hardware development based on the ZigBee protocol. This solution will introduce the principles and design methods of each module in detail.

　　1 System overall framework

　　The system is generally divided into two parts: the first part is the controller and RF module part; the second part is the peripheral expansion circuit part. The specific system framework diagram is shown in Figure 1.

　　2 Controller and RF Module Design

　　The main control circuit is the core of the whole system, which is responsible for the overall scheduling and control of the whole node. Considering the convenience of equipment operation and maintenance, the integration of the system and other characteristics, the main control circuit has the ability to process data and can also store a certain amount of data. This design uses the RF chip CC2430 based on ZigBee technology as the core. The device integrates a 51-core MCU controller and an RF transceiver, so the controller module and the RF module adopt an overall design mode. At the same time, the chip also has FLASH memory, which can store data conveniently. The device is small in size, stable in performance, fast in computing speed, and has good scalability, which can better meet the various needs of this design.

　　2.1 CC2430 controller circuit configuration

　　In this design, the main control unit is responsible for peripheral device expansion and control, A/D conversion, data transmission and other functions. CC2430 is a highly integrated SOC system with compact I/O port design and multiplexing function. Therefore, the use of I/O ports should be saved as much as possible in the design, and they can be expanded when necessary. At the same time, the design should also have online download and debugging functions to facilitate the needs of engineering applications.

　　2.1.1 I/O port configuration

　　CC2430 has 21 digital I/O port pins, namely P0, P1, and P2. They are all 8-bit I/O ports. Each port can be set as a general I/O or external device I/O. Except for the two high-output ports P1_0 and P1_1, the rest are used for output. The relevant I/O ports of this design are reserved in the form of connectors to facilitate use and expansion in different occasions, as shown in Figure 2.

　　2.1.2 Debug interface

　　The CC2430 in this design has online debugging and downloading functions and can be freely configured as needed. Figure 3 shows the CC2430 debug interface diagram, which is composed of debug interface pins P2.2 and P2.1, which are used as debug clock and debug data signal pins respectively.

　　2.2 Clock and Reset

　　The crystal oscillator of CC2430 adopts a two-stage design, one is 32 MHz and the other is 32.768 kHz. In the CC2430 whole machine working mode (PM0), these two crystal oscillators need to work together; in PM1 and PM2 power modes (power saving mode), only the 32.768 kHz crystal oscillator works; in PM3 mode, both are turned off. At the same time, a 1% precision resistor must be connected to the RBIAS1 and RBIAS2 (22, 26 pins) pins to provide accurate bias current for the 32 MHz crystal oscillator. The specific circuit is shown in Figure 4.

　　CC2430 has a power-on reset function, and can also be reset manually. Simply pull the 10th pin RESETn to a low level to complete the reset.

　　2.3 CC2430 RF Module

　　The design of the CC2430 RF module is shown in Figure 5. In this design, except for the P2_3 and P2_4 pins reserved for external crystal oscillators, all the P0_0 to P2_2 pins of CC2430 are brought out as interfaces.

　　The RF input and output adopt high impedance differential type, and the pins are RF_n and RF_p.

　　This design uses a monopole antenna. In order to obtain the best communication performance, an unbalanced transformer should be used to achieve impedance matching.

　　As shown in Figure 5, discrete components L321, L331, L341 and C341 form an unbalanced transformer to connect the differential output terminal and the monopole antenna. Since the antenna is some distance away from the RF pin, it is necessary to design impedance matching for the feedback transmission line from the antenna to the RF pin. Since it is a monopole antenna, the matching impedance is 50 Ω, which is composed of the unbalanced transformer and the PCB microstrip transmission line. λ is the microwave wavelength on the PCB transmission line, and the microstrip transmission line is actually λ/2 impedance matching.

　　TXRX_SWITCH is an analog power output pin that can provide calibration voltage for the low noise amplifier (LNA) and power amplifier (PA) inside CC2430. This pin must be connected to the RF_n and RF_p pins through an external DC circuit. When CC2430 is in the receiving state, TXRX_SWITCH is internally grounded to provide bias voltage for LNA, and a low level can be obtained on the pin; when the chip is in the transmitting state, TXRX_SWITCH is internally connected to the power supply voltage to provide bias voltage for PA, and a high level can be measured on the pin. In addition, the external antenna of this circuit uses an SMA interface.

　　3 Peripheral expansion circuit

　　In actual use, wireless sensor network nodes with CC2430 as the core can be equipped with corresponding peripheral circuits, mainly including external power supply circuit, display and key circuit, serial port and USB communication circuit, etc. Through these circuits, the RF and main control modules can be developed and debugged accordingly.

　　3.1 External power supply circuit

　　The power supply circuit of this design is mainly composed of TPS79533 low voltage regulator and its peripheral devices. TPS79533 outputs 3.3 V voltage, its input voltage range is 2.7 ~ 5.5 V, and has high power supply rejection ratio, ultra-low noise, good voltage linearity and load transient effect, and small voltage drift. Its specific circuit is shown in Figure 6.

　　3.2 LCD display and keyboard circuit

　　3.2.1 LCD display circuit

　　The LCD display circuit can use a 128×64 dot matrix LCD. At the same time, in order to save the I/O port resources of the main control chip, the serial/parallel port conversion chip 74HC595d is used. The specific circuit is shown in Figure 7.

　　In order to make the LCD display have appropriate backlight brightness, corresponding amplifier tubes, such as 9015, can also be used in the design to drive the LCD display backlight display.

　　3.2.2 Keyboard Circuit

　　This design can adjust various parameters through the key circuit and display them through the LCD circuit. As shown in Figure 8, the keyboard has 6 keys: up, down, left, right, confirm, and exit. Among them, the circuit of the direction key is a voltage divider circuit, and its voltage divider value is input into the P0.6 terminal of CC2430. This I/O port has the function of A/D conversion, and the keyboard function can be realized through software, thus saving I/O port resources.

　　3.3 Communication Circuit

　　The communication circuit is responsible for data transmission and reception between the node and the PC to realize data download, debugging and other functions. CC2430 adopts RS232 communication mode, and the specific circuit is shown in Figure 9. This design adopts the classic RS232 circuit, and the control chip adopts the widely used SP3223E, whose RXD1 and TXD1 pins can be directly connected to the P0.2 and P0.3 pins of CC2430.

　　It should be noted that in actual use, people often use laptops to perform online debugging and program downloading on nodes, but laptops generally do not have serial ports and require external USB-RS232 conversion circuits. The author found that in the selection of conversion circuits, there are conversion circuits based on devices such as PL2602 and SP3223E on the market. Although PL2602 is cheap, it is not suitable for the high bit rate transmission of CC2430, and although SP3223E is more expensive, it has better support for CC2430, which is also something that needs to be paid attention to in actual use.

　　4 Hardware Process Characteristics

　　Since the wireless sensor network node with CC2430 as the core works in a high-frequency environment of 2.4 GHz, the EMI requirements are relatively high. The PCB of the wireless sensor network node also has corresponding specific design requirements.

　　Since the RF module operates at a high frequency, in the specific PCB design, according to the relevant documents of TI, a double-layer PCB can be used. If you want to reduce the PCB size, a 4-layer PCB design can also be adopted. The specific requirements are as follows:

　　(1) If a double-layer PCB design is used, the top layer is used for component placement and signal connection, and a large area of ​​copper is used to reduce interference.

　　(2) The power supply filtering requirements are high, and the decoupling capacitor should be as close to the power supply pin as possible and connected to the ground plane of the printed circuit board through a separate via.

　　(3) The ground pin of the chip should be as close to the package pin using a separate via as possible to reduce interference.

　　(4) The smaller the external components are, the better. Surface mount devices must be used. For specific designs, 0603 or 0402 packaged chip components can be used.

　　(5) If high-speed external digital devices are to be used on the PCB, RF circuits must be avoided.

　　(6) The system should adopt large-scale grounding to eliminate interference. The bottom layer of the PCB can be designed as a ground layer.

　　5 Conclusion

　　This paper first introduces the components of wireless sensor networks, and proposes a hardware design scheme for wireless sensor network nodes and their peripheral expansion circuits based on CC2430. The scheme introduces the working principles and design methods of each hardware module. Among them, the scheme introduces in detail the controller and RF module circuits and peripheral expansion circuits, including external power supply circuits, LCD display and keyboard circuits, and communication circuits, and introduces the relevant process points that should be paid attention to in PCB design of this scheme. The scheme has stable performance and works well in actual use, and it also has certain guiding significance for similar design schemes.