Design of UHF RFID reader/writer

Abstract: The ultra-high frequency radio frequency identification system has the characteristics of fast reading and writing speed, large storage capacity, long recognition distance and simultaneous reading and writing of multiple tags, and has been widely used in logistics and other fields. The main characteristics, structure, working principle and reading and writing method of ultra-high frequency RFID electronic tags that meet the ISO 18000-6 standard are introduced, and the solution of the corresponding reader is proposed, focusing on the hardware design and software program flow of the reader. The actual application results show that the reader has a fast reading and writing speed (single tag 64bit/6ms), a high recognition rate and a long recognition distance (≥4m).

0. Introduction

Radio Frequency Identification (RFID) technology is an emerging automatic identification technology. It uses wireless radio frequency to carry out non-contact two-way data communication to achieve the purpose of target identification and data exchange. It can be used to track and manage almost all physical objects, and has broad application prospects in many fields such as industrial automation, commercial automation, transportation control management, anti-counterfeiting and military. According to the different working frequency bands, RFID systems can also be divided into low frequency (below 135kHz), high frequency (13.56MHz), ultra-high frequency (860~960MHz) and microwave (above 2.4GHz). At present, most RFID systems are low-frequency and high-frequency systems, but the RFID system in the ultra-high frequency (UHF) band has the advantages of long operating distance, fast communication speed, low cost and small size. It is more suitable for applications in the future logistics and supply chain fields, and also provides the possibility of realizing the "Internet of Things". Therefore, the development of ultra-high frequency RFID system is the focus of the current development of RFID system. This paper introduces the main features, structure, working principle and reading and writing methods of ultra-high frequency RFID electronic tags that meet the ISO1800026 standard, proposes a solution for the corresponding reader/writer, and focuses on the hardware design and software program flow of the reader/writer. The actual application results show that the reader/writer has the following characteristics: fast reading and writing speed (single tag 64bit/6ms), high recognition rate, and long recognition distance (≥4m).

1. Tag working principle and characteristics

1.1 Working Principle

RFID systems are generally composed of readers and tags (also known as transponders, electronic tags, smart tags) and antennas. This article uses a certain company's UCODEHSL tag, which complies with ISO18000-4 and ISO18000-6 standards. It has no power supply itself and obtains energy from the radio frequency field of the reader. It uses load modulation and the operating frequency band is UHF or 2.45GHz. The working principle is shown in Figure 1.

Figure 1: Working principle

The PC remotely controls the reader/writer through the RS232 interface. After receiving the command, the reader/writer sends the RF command through the antenna to operate the tag and receive the data returned by the tag. The tag obtains energy through its dipole antenna and the chip (IC) controls the reception and transmission of data.

1.2 IC structure

The tag IC is mainly composed of three modules: analog, data processing and EEPROM, as shown in Figure 2.

Figure 2: Tag IC structure

The analog RF interface module provides a stable voltage for the IC, demodulates the acquired data for processing by the data module, and modulates the data and returns it to the reader. The digital processing module includes functions such as state conversion machine, read/write protocol execution, and data exchange processing with EEPROM.

1.3 Storage Characteristics

The tag has a built-in 2048-bit EEPROM, which is divided into 64 blocks, each with 32 bits. 8 bytes are ID storage space and 216 bytes are user storage space. Each byte has a corresponding lock bit, which cannot be changed when it is set to "1". It can be locked through the LOCK command, and the status of the lock bit can be read through the Query locK command. The lock bit is not allowed to be reset. Byte 0~7 is locked and is the tag's identification code (Unique ID). The 64-bit UID contains a 50-bit independent serial number, a 12-bit boundary code and a two-bit check code. Byte 8~219 is an unlocked space for user use. Byte 220~223 is also unlocked, serving as a flag bit or user space to indicate that the write operation is complete.
2 Reading and writing tags

2.1 Command Format

2.1.1 Command format of the reader

The command format of the reader is as follows:

The frame header detection segment is a stable unmodulated carrier that lasts for at least 400Ls (equivalent to the transmission of 16-bit data); the frame header is a 9-bit NRZ format Manchester "O", that is, 010101010101010101; the start character is used to mark valid data, the original return rate uses a 5-bit start character (1100111010), and the 4-fold return rate uses a start character (11011100101); the CRC uses a 16-bit CRC encoding.

2.1.2 Tag response format

The tag's response format is as follows:

Silence means that the tag continues for 2 bytes without backscattering (equivalent to a duration of 400Ls at a rate of 40kb/s); the returned frame header is: "00000101010101010101000110110001"; CRC uses 16-bit CRC encoding.

2.2 Anti-collision mechanism

After charging, the IC has three main digital states: READY (initial state); ID (the state in which the tag expects the reader to identify it); and DATE EXCHANGE (the state in which the tag has been identified).

Figure 3: State transition diagram

First, the tag enters the RF field of the reader and enters the ready state from the power-off state. The reader selects all or part of the tags in the ready state within the working range through the "group selection" and "cancel selection" commands to participate in the conflict judgment process. To solve the conflict judgment problem, there are two devices inside the tag: an 8-bit counter; a 0 or 1 random number generator. When the tag enters the ID state, its internal counter is cleared to "0". Some of them can return to the ready state through the UHF RFID system reader design to receive the "cancel" command, and other tags in the identification state enter the conflict judgment process. The selected tag starts the following cycle:

① All tags in ID state and with internal counter at 0 will send their UID.

②If more than one tag sends, the reader will send a failure command.

③ All tags that receive the failed command and whose internal counter is not equal to 0 will increase their counter by 1. Tags that receive the failed command and whose internal counter is equal to 0 (tags that have just sent a response) will generate a random number of "1" or "0". If it is "1", it will increase its own counter by 1; if it is "0", it will keep the counter at 0 and send their UID again.

④If there is more than one tag to send, repeat step 2;

⑤ If all tags randomly select "1", the reader will not receive any response, it will send a success command, the counters of all transponders will be reduced by 1, and then the transponder with a counter equal to 0 will start to send, and then repeat step 2;

⑥ If only one tag sends and its UID is correctly received, the reader will send a data read command containing the UID. After the tag receives the command correctly, it will enter the data exchange state and then send its data. The reader will send a success command to reduce the counter of the tag in the ID state by 1;

⑦ If only one tag has a counter equal to 1 and returns a response, repeat steps 5 and 6; if more than one tag returns a response, repeat step 2;

⑧ If only one tag returns a response and its UID is not received correctly, the reader will send a resend command. If the UID is received correctly, repeat step 5. If the UID is received several times (this number can be set based on the error handling criteria desired by the system), it is assumed that more than one tag is responding and repeat step 2.

3. System hardware composition

This system uses W77E58 single-chip microcomputer as the main control module, which together with the transmitting module, receiving module and serial communication module constitutes the reading and writing system of the radio frequency tag. The hardware principle of the system is shown in the reader-writer part in Figure 1.

3.1 Main control module

The main control module is W77E58 from WINBOND, which is a high-speed, highly integrated, high-performance 8051-core microcontroller. It has a built-in 32kbit reprogrammable Flash EPROM, 1kbit internal SRAM accessed by MOV instructions (saving 16 data/address I/O lines), and 2 enhanced full-duplex serial ports. The system speed using W77E58 is about 2.5 times faster than the traditional 51 series microcontroller. The operating frequency of W77E58 is 40MHz, which is equivalent to 8051 of about 100MHz.

3.2 Transmitter Module

The transmitter module consists of an RF modulation/transmission chip and a power amplifier chip. The principle is shown in Figure 4. The modulation/transmission chip is MC33493 from Motorola, which is a UHF band modulation/transmission chip tuned by a phase-locked loop. It uses OOK or FSK modulation, has an integrated VCO, loop filter, and adjustable output power. The operating frequency band can be selected from 315~434 or 868~928MHz. The operating frequency band is controlled by the BAND (3) pin, and the modulation mode is set by the MODE (14) pin. The output frequency F (oUt) of the RFOUT (10) pin = F (Y1) × [Ratio] (PLL).

Figure 4: Transmitter module

In this design, the BAND (3) pin is set to a low level, and the frequency band of 868~928MHz is selected; the operating frequency is set at 915MHz, f (Y1) = 915MHz/64 = 14.297MHz; the MODE (14) pin is set to a low level, and the OOK modulation method is adopted; the DATACLK (1), DATA (2), and ENABL E (13) pins are respectively the clock, data input, and chip working switch, which are controlled by the microcontroller.

In order to improve the transmission power of the system, this design uses RFMicroDevice's RF2132 power amplifier chip to amplify the RF signal output by MC33493; RF2132 is a high-power, high-efficiency linear amplifier with a linear output power of 29dBm.

3.3 Receiving Module

The receiving module consists of a RF receiving/demodulating chip and a signal amplifying chip. The principle is shown in Figure 5. The RF receiving/demodulating chip uses the MC33593 from Motorola, which is a UHF band low-power RF receiving/demodulating chip tuned by a phase-locked loop. The operating frequency band is 868~928MHz, the intermediate frequency bandwidth is 500kHz, and OOK or FSK modulation is used, which is set by the DMDAT (13) pin. It has an integrated VCO and loop filter.

In this design, the DMDAT (13) pin is set to low level and OOK modulation is used. The frequency selection of the crystal oscillator is the same as that of MC33493. The system clock (11), data interface (15, 16) and input control switch (14) are controlled by the microcontroller.

In order to improve the receiving sensitivity of the system, this design adds a set of RF signal amplification circuit between the antenna and the RF receiver/demodulator, which is mainly composed of RF2173. Its function is to amplify the RF signal received by the antenna to improve the signal strength of the MC33593 input RF signal; RF2173 has a maximum gain of 32dB.

Figure 5: Receiver module

3.4 Serial communication module

The reader uses RS232 interface to communicate with the computer, and the level conversion chip uses ICL232. Through this interface, the computer sends commands such as reading and writing tags to the reader, and the reader can send the results back to the computer.

4. Software Design

4.1 Main Program

Since the system works under the supervision of the PC, the communication between the two is master-slave. After the main control module is powered on and completes the normal initialization process, it enters the waiting state and waits for instructions from the PC. After receiving the instructions from the PC, it turns to process the corresponding program. After processing, the execution result information is returned to the PC. The main program block diagram is shown in Figure 6.

Figure 6: Receiver module

4.2 Anti-collision procedures

When there are multiple tags within the coverage of the reader antenna, after the reader sends a command, it will cause a response conflict, resulting in communication failure. When the reader detects a conflict, it can use commands to handle the conflict. By sending commands, the UID of the tags within the coverage of the reader antenna can be recorded, and then using the uniqueness of the UID, the reader and each tag establish independent channels for communication, thereby eliminating conflicts. The reader first sends a command to the tag, and the UID mask and mask length are included in the data field and parameter field of the command respectively. The mask transmitted to the tag is required to be an integer byte. If the mask is not an integer byte, it will automatically fill the high bit with zeros. By setting the corresponding flag bit in the flag field, the reader can set the time slot for receiving the tag response to 3 or 6. In each time slot, the reader can receive the UID returned by the tag. The reader compares the UID of the end signal sent and the lowest 4 bits of the current time slot number plus the mask in the command data field. If there is no match, there will be no response. If there is a match, it will send back its own UID. In a certain time slot, multiple tags may respond at the same time. At this time, the reader must record the conflicting tag mask and the value of the time slot counter for further conflict processing. The flow chart is shown in Figure 7.

Figure 7: Receiver module

5. Conclusion

The UHF RFID reader/writer designed in this paper can read and write a variety of UCODE HSL series tags, with the fastest reading and writing speed (reading 64 bits from a single tag on average takes no more than 6ms, and each additional 32 bits takes an accumulated 1ms; writing 32 bits on each single tag on average takes no more than 25ms, and each additional 32 bits takes an accumulated 25ms), reading and writing distance (≥4m), and effectively solves the problem of multi-tag anti-collision. This UHF RFID system is especially suitable for logistics and supply chain fields.