UHF RFID reader design

　　Radio frequency identification (RFID) is a non-contact automatic identification technology. It mainly consists of readers and tags, which has the advantages of long reading and writing distance, high service life, convenient reading and writing, and quick response. With the in-depth study of RFID technology, RFID series of electronic products are gradually applied to millions of households, UHF is the ultra-high frequency band, this band reader read and write distance, read and write speed, has been used in supply chain management, aviation management and Logistics management and other industries.

　　This article describes a RFID reader UHF band design, single-chip Philips LPC2103, a simple design, based on the ISO / IEC 18000-6C agreement, you can do multi-label environment successfully label and interact with their data. This paper gives a general introduction of the protocol, elaborates the hardware architecture of the whole system design, focuses on the data baseband processing part of the software design, the anti-collision part, pulse interval coding (PIE coding) and bidirectional interval decoding (FMO decoding) A detailed introduction.

1 ISO / IEC 18000-6C agreement

1.1 ISO / IEC 18000-6C protocol profile

　　ISO / IEC 18000 is an international standard for radio frequency identification (RFID) based on item management. It is divided into 7 parts according to different working frequencies. The 6th part has a frequency of 860 to 930 MHz, that is, UHF band. ISO / IEC 18000-6 series of standards include the three types of ISO / IEC 18000-6A, ISO / IEC 18000-6B and ISC / IEC 18000-6C, 6B is mainly used in the field of transportation, 6C is mainly used in logistics, production management and Supply chain management. As the number of production and sales far exceeds the B class, C class has gradually replaced the trend of Class B, becoming the current focus of RFID research.

　　The ISO / IEC 18000-6C protocol defines the physical and logical requirements for readers and tags, using a reader-first format. Table 1 for this set of reader and tag communications part of the parameters.

　　Reader and tag communication process state of the writer is divided into Select, Inventory and Access 3 states, and the tag state is divided into Ready, Arbitrate, Reply and other seven states.

1.2Anti-collision algorithm

　　The ISO / IEC 18000-6C protocol adopts a random slot slot anti-collision algorithm. The tag uses the slot counter value to determine whether to communicate with the reader. The reader uses QueryRep to set the value of the slot slot counter. QueryAdjust sets the Q value , Thus decides the range of the label slot time slot counter, Fig. 1 is the algorithm that chooses Q value.

　　The parameters Q, Qfp and C are positive integers. The information frame length is F = 2 ^ Q-1. Q is dynamically changing. The initial value is round (Qfp). After a slot, if the slot is a collision slot, Qfp is added to parameter C; Qfp is subtracted from parameter C if it is an idle slot; Qfp remains unchanged if a slot is successful. Based on the new Q = round (Qfp), the reader decides whether to continue sending the next slot or restarting a new frame.

2 Reader hardware design

　　RF transmitter front-end design of the transmitter channel using radio frequency, receive channels using I, Q coherent demodulation, the use of two amplifiers and comparators.

　　According to the agreement, the main parameters of reader / tag communication in this design were drawn up.

　　In order to ensure the expected communication parameters, the Philips LPC2103 is designed with low cost, with a serial port and rich resources. It has 8 KRAM, 32 K Flash, 32 bit and 16 bit timers, 30 available I / O ports , Clock frequency up to 70 MHz.

　　The chip is a programmable phase-locked loop chip, low power consumption, support for FSK, GFSK and OOK modulation, the data rate of 1 ~ 128 Kb / s, you can control the output frequency through digital programming, Frequency hopping in the frequency range of 240 ~ 930 MHz. The receiving circuit adopts the relevant demodulation, and the demodulation circuit is set up by the separate components. The return information of I and Q are sequentially passed through an inert circuit, a filter circuit, and then to two differential amplifying circuits with constant current source compensation. Finally, the comparator is completed demodulation.

　　3 Reader software design

3.1 Digital baseband processing module

Digital baseband processing module is the core of the entire software design, which includes codecs, data analysis, anti-collision and other parts of the digital baseband processing part of the basic block diagram.

3.2 Reader anti-collision process

Part of the digital baseband processing, anti-collision part is the top priority, but also a key part of the entire reader design. This design uses a random slot slot arbitration anti-collision algorithm to solve the reader in a multi-label environment can not identify the label, the reader anti-collision general process.

 

3.3 Reader PIE coding design

　　PIE (Pulse interval encoding) encoding, that is, the pulse interval encoding, data is defined by defining the time width between different falling edges of a pulse. Defining a time interval named "Tari" in the standard also becomes the reference time interval, which is the time width of the falling edge of two adjacent pulses, with a duration of 25 μs. The PIE encoding of data 0 and 1 in this design is shown in Table 3 in relation to the "Tari" time segments. The timer is used in the design to make more accurate PIE code.

PIE encoding process: first set the encoding logic, that is, the value of the timer is greater than or equal to T output is 1, less than T, output 0, and then wait for the encoded signal to come. After the code signal arrives, fetches the data from the FIFO, if it is 0, then set the timer value as 2T, the clock will do a self-decrement operation every time; When the data is 1, set the timer value as 4T, Once done self-decrement operation.

 

3.4 Reader decoding design

　　FMO (Bi-Phase Space) decoding, that is, two-phase interval decoding, the principle of work is to use a level change in a bit window to represent logic. A logic "1" if the level is only toggled from the beginning of the bit window, a logic "0" if the level is toggled in addition to the beginning of the bit window, as shown in Figure 7 Show. The duration of a bit window is 25μs.

　FMO decoding process: First reader at the same time I and Q two signal sampling, the use of the state machine to detect the correctness of the return frame header, the reader according to the correct frame header to decide on the I or Q signal decoding . According to the characteristic of FMO coding, we can see that the beginning of each data unit of FMO is flipped, so that the data represented by this bit window can be judged according to the rising or falling edge at the beginning and the sampling points in the bit window. Set a bit window time length T, one case is the beginning of the bit window for the falling edge of the bit window in the 3 / 4T sampling, sampling for a bit window that the data "0", the sample is 0 bit The window indicates the data "1"; the other is the rising edge of the bit window at the beginning of the bit window 3 / 4T sampling, the sample is 1 bit window that data "1", the sample is 0 bit window that Data "0".

4 Conclusion

      In this paper, the design of the main chip using LPC2103 and Si4031, the hardware circuit is simple and easy to implement; based on ISO / IEC 18000-6C, the use of anti-collision algorithm designed to achieve the UHF band reader in a multi-tag environment to communicate smoothly with the label, enhanced the encoding performance of reader; Finally, through a simple analysis, introduced PIE encoding and FMO decoding process.