A DESIGN OF ACTIVE RFID TAGS BASED ON NRF24L01

978-1-4799-2446-2/13/$31.00 ©2013 IEEE 210 A DESIGN OF ACTIVE RFID TAGS BASED ON NRF24L01 SHUNYU-SHI1, TINGTING-LU1, HAO-ZHANG1,2, LINGWEI-XU1 AND T. A. GULLIVER2 1College of Information Science and Engineering, Ocean University of China, Qingdao, China

2Department of Electrical Computer Engineering, University of Victoria, Victoria, Canada

E-MAIL: zhaohao@ouc.edu.cn, {shunyu.2006, lvtingting33, gaomilaojia2009}@163.com, agullive@ece.uvic.ca

Abstract: An active RFID tag design based on nRF24L01 is proposed in this paper. This active RFID tag works in 2.4GHz. The requirements for low power consumption and low system complexity of RFID tags have been well considered. The circuit diagram and the communication protocol are fully described in the paper. Particularly, an anti-collision algorithm has been proposed to improve the effectiveness of thecommunication protocol.

Key words: Active RFID tags; communication protocols; anti-collision

1. Introduction Radio frequency identification (RFID) has been a hot automatic identification technology since 1980s. As the core technology of the “world of things”, it has many advantages, such as small sizes, low price and power consumption, etc. One of these advantages is that RFID is a non-contact identification technology[1]. It can read and write target tags through fog, dust and other obstacles. With the development of RFID, the effective communication distance of it has beenfrom several centimeters to several meters, and even hundreds meters. And the data transmission rate has been to Mbps. Recently, active RFID has drawn wide attentions from both academic and industrial fields. It has a great advantage over passive RFID incommunication distance, read-and-writespeed and data storage, and has huge applications in smart hospitals, parking lots and intelligent transportations, etc. Currently, most studies on active RFID focus on anti-collision algorithm[2]-[5] and its applications, such as tracking and positioning systems [6]-[7]. In this paper, we

optimize the whole active RFID system by improving circuit design, communication protocol and anti-collision algorithm. The integrated RF transceiver chip nRF24L01 is employed due to its low power consumption. The whole circuit diagram and the reference values of components in the circuit are given in Section 2.Considering the characteristics of nRF24L01, a new anti-collision algorithm is proposed based on the current mainstream anti-collision algorithmin section 3.The proper communication protocol is also given in this section. Finally, conclusions are given in section 4.

2. Hardware Design for Active RFID Tags As the information carrier in the active RFID systemˈRFID tags are used to store , send and receive data. A typical active RFID tag consists of antenna, RF front-end circuit [8], logical control unit, storage devices, power control module and some other components. Fig. 1 shows the structure diagram of the active RFID tag.

LogicalControlUnitDataStorageUnitsPowerControlModuleModemBand-passFilterPowerAmplifierFrequencyGenerator

RFfront-endAntenna

Fig.1 Structure diagram of the active RFID tag The nRF24L01 chip of NORDIC Corporation is employed as the RF transceiver front-end of the tag in this design. It is an integrated RF transceiver chip with 211

ultra-low power consumption, and works in 2.4-2.5GHz worldwideISM band. The chip has integrated all of the physical layer and the data link layer operations, such as wireless signal transmitting and receiving, modulating and demodulating, encoding and decoding. The logic control unit can be ARM series microprocessors or 8 bit single chip microprocessors which are compatible with industry standard MCS-51.The nRF24L01 chip is used to send and receive data, whilethe MCU is used to store and process data. If the internal storage memory is not big enough, external storage can also be used. NRF24L01 is anultra-integrated RF front-end chip, and only need very small amount of peripheral devices, such as crystal oscillation circuit, rectifier, filter circuit, reset circuit.The 16MHz crystal oscillator is employed. Considering the requirements of low power consumption and quick startup, the circuit tends to use small capacitors. Here we use 3.3pF capacitors. Fig. 2 shows the whole circuit diagram of MCU and RF front-end.

Fig.2 The circuit Diagram of MCU and RF front-end The P0.0~P0.5 pins of MCU are connected with CE pin, CSN pin, SCK pin, MOSI pin, MISO pin and IRQ pin of nRF4L01 respectively via a pull-up resistor. The microcontroller controls the chip working state by the CE pin, namely, RX or TX mode (CE=1) and standby mode(CE=0). CSN, SCK, MOSI, MISO are four pins of nRF21L01 SPI bus. The four pins simulate SPI bus timing sequence to enable the communication between the NRF24L01 chip and the microcontroller. IRQ is an interrupt pin. If IRQ=0,the microcontroller will determine the interrupt source by scanning the RX_DR bit, TX_DS bit or MAX_RT bit in the status register, and make the corresponding responses. Reader command parsing, data storing and retrieving, data sending and receiving are the main work of RFID