How to realize the portable UHF RFID reader

Radio Frequency Identification (RFID) is a non-contact radio frequency automatic identification technology that uses microwaves for two-way data transmission. RFID system has the advantages of long service life, low power consumption, fast data transmission, stability, safety, reliability, adaptability and anti-interference. It has been widely used in industrial control, consumer electronics, medical electronics, modern logistics and campus card etc. RFID technology is the core technology of modern Internet of Things.The research on RFID technology in my country started relatively late. Due to the constraints of software and hardware, RFID technology has not yet achieved large-scale applications. Most of them belong to medium and low frequency data.

Radio Frequency Identification (RFID) is a non-contact radio frequency automatic identification technology that uses microwaves for two-way data transmission. RFID system has the advantages of long service life, low power consumption, fast data transmission, stability, safety, reliability, adaptability and anti-interference. It has been widely used in industrial control, consumer electronics, medical electronics, modern logistics and campus card etc. RFID technology is the core technology of modern Internet of Things. Research on RFID technology in my country started relatively late, and RFID technology has not yet achieved large-scale applications due to software and hardware constraints. Most of it belongs to medium and low frequency data transmission, and there is still a distance from advanced foreign technology. This article focuses on the hardware and software implementation methods of UHF RFID readers based on Altera’s Cyclone series FPGA and ISO1800 0-6C standard.

1 System principle and structure

1.1 Principle of RFID system

A typical RFID system consists of five parts: Reader, RFIDTag, Antenna, Middle Ware and ApplicaTIon Ware. The structure block diagram of RFID system is shown as in Fig. 1. The reader is installed in a fixed location and connected to the PC via USB to realize data communication and reading and writing. The reader can update and store user data for the operation of the radio frequency tag. Since the system is based on the ISO18000-6C standard, the radio frequency tag must comply with this standard to realize user data storage; the reader is composed of a codec circuit, a data storage circuit, a radio frequency front-end transceiver circuit, an antenna, and a power supply circuit; the antenna receives and transmits super high Frequency microwave signals; middleware, intermediate information and data processing software, filtering, aggregation and calculation of RF readers and middleware events, abstracting algorithms with logical meaning to the application software; applications directly facing the user’s human-computer interaction interface . The reader is operated by the application software, and the reader receives and sends microwave signals to modify the user’s radio frequency tag. The application software is an important factor for the user experience and the success of the RFID system.
 

How to realize the portable UHF RFID reader

1.2 Principle of RFID reader

Based on FPGA RFID reader can be divided into 3 Modules: FPGA minimum system, USB serial bus interface circuit, RF signal transceiver circuit. The minimum FPGA system includes FPGA field programmable logic array, JTAG configuration circuit, system clock circuit, data storage circuit, Display circuit, and power supply circuit. The RFID reader control core uses the EP1C3T144 chip of Altera’s Cyclone series. Because FPGA is based on SRAM technology, the download configuration chip uses Altera’s supporting EPCS1, the download mode adopts JTAG mode, 20 MHz active clock crystal oscillator, 0.3-inch common cathode digital tube, external 5 V DC power supply, converted to 3.3 V by two-stage LM1085 And 1.5 V voltage to power FPGA core and pins. The data storage chip selects the CMOS technology chip EEPROMCAT24WC02 from Catalyst Company, which is connected to the FPGA through the I2C bus. The USB serial bus interface chip adopts the USB universal interface chip of Nanjing Qinheng Company. The RF signal transceiver module selects Chipcon’s SmartRF03 technology and 0.18μm CMOS process CC1100 chip as the microwave signal transceiver front end, which is connected to the FPGA through the SPI bus.

In the RFID system, the reader is the core that connects the radio frequency tag and the PC client. By commanding the reader, it can modify and store user data. RFID reader and tag data transmission can be divided into the forward link from the reader to the tag and the backward link from the tag to the reader. The forward link and the backward link use half-duplex communication, and data transmission Adopt ASK modulation, forward link adopts PIE code, backward link adopts Miller code, system adopts CRC-16 check code and special anti-collision algorithm to ensure correct reading and writing. RFID communication can be divided into two steps: the reader first obtains the tag ID number within the radiation range, and then reads and writes the tag whose ID number meets the requirements. The reader and the host computer USB data communication, through the USB device enumeration to complete the HID human-machine interface device identification and data reading and writing.

2 System hardware circuit design

2.1 Cyclone EP1C3T144 FPGA minimum system

The FPGA minimum system includes programmable gate array, power supply circuit, clock circuit, reset circuit, configuration circuit, and filter circuit. The field programmable gate array adopts EP1C3T144. It is based on 1.5 V, 0.13μm copper SRAM process, TQFP144 package; 104 I/O ports, supports various I/O standards; supports LVDS, data transfer rate is 311 Mbps; 2910LE logic unit, 288 Kb RAM, 1 PLL phase-locked loop, support 66 MHz 32-bit PCI standard, support external 133 MHz DDR SDROM; separate active clock interface, external active crystal oscillator with 20 MHz frequency; dedicated configuration circuit interface, 3.3 V LVTTL power supply. FPGA is based on SRAM technology, and the data is easy to lose when power off. It adopts the EPCS1JTAG download mode provided by Altera. The system adopts hardware and software reset: the hardware reset is connected to the FPGA nconfig pin, press this button to reconfigure the FPGA code from the EEPROM; the software reset is to set the reset pin when writing the code, and run according to the code.

Due to the limited EEPROM space inside the FPGA chip, a 2KB EEPROM 24C02 is attached to the FPGA chip. The 24C02 is compatible with the 400kHz I2C interface. The power supply voltage is 1.8~6.0V, and the power supply current is 3mA. It has page write buffer and write protection functions, and it is packaged in SOP8. It also has the characteristics of a large number of reads and writes, a long storage time, and low power consumption. The 24C02 hardware interface circuit is shown in Figure 2.
  

How to realize the portable UHF RFID reader

2.2 USB serial bus interface circuit

CH372 simple USB universal serial bus interface integrated circuit, compatible with +5 V and +3.3 V power supply, maximum working current is 30 mA, external 12 MHz frequency crystal oscillator, 20-pin SOP package; compatible with USB2.0 data transmission protocol, support HID The device is hot-swappable and hot-swappable, built-in USB bottom communication protocol, and automatically completes standard USB event enumeration; supports data control transmission, batch transmission, terminal transmission; the default link endpoint 0 all events, FPGA is responsible for data transmission and processing, easy to use, CH372 hardware The interface circuit is shown as in Fig. 3.

2.3 CC1100 RF transceiver circuit

CC1100 is a very low power UHF transceiver integrated circuit. It has good communication effect, strong anti-interference performance, strong penetrating power and high sensitivity; the transceiver frequency can be set freely, the range of which can be set is 300~348 MHz, 400~464MHz, 800~928 MHz, and the programmable control data transmission rate can be up to Up to 500 kbps, 4-wire SPI (SI, SO, SCLK, CSn) interface. The normal working voltage of CC1100 is 3.3 V, and the maximum input voltage is 3.6 V. The input voltage of each pin should be the same. The output power of the 868~915 MHz transmission mode is +10 dBm, the current is 30 mA, the maximum received power is +10 dBm, hardware automatic CRC check, supports ASK modulation and demodulation, and adjustment of peripheral components can facilitate adjustment of the transceiver frequency. CC1100 adopts data buffering technology with 2 64-byte FIFOs to buffer sending and receiving data. CC1100 can be software set RF transmission power, transceiver mode, modulation format, channel selection and bandwidth, data transmission rate, machine working mode, etc.

CC1100 belongs to half-duplex communication, receiving and sending are carried out in time sharing under the control of RF control module RF CONTRON, demodulation is completed by the downward integration of the integrator, and the loop gain is controlled by the AGC circuit; the modulation is based on direct frequency synthesis, and the carrier signal is controlled by the crystal oscillator and internal The PLL circuit is generated, and there is a slight difference between the sending module and the receiving module. The receiving module includes: low noise amplifier LAN, integrator INTEGRATION, analog-to-digital conversion ADC, demodulator DEMODU-LATOR, forward data error processor FEC/INTERLEAVER, packet processing PACKET HANDLER, receiving data buffer RXFI FO, digital interface circuit DIGITAL INTERFACE TO MCU; The different modules of the transmitting end and the receiving end are: voltage-controlled oscillator VCO, mixer MIXER, transmit data buffer TXFIFO, data transmission and reception are completed by different modules in time sharing.

CC1100 has two working states-idle state and working state, each state corresponds to a different current. Idle state can be divided into waiting state, internal current is 1.9 mA; sleep state, internal current is 400~900 nA; the stop state of crystal oscillator only maintains the register value unchanged, internal current is 160 nA; working state can be divided into receiving (RX ) And transmitting (TX) state, the receiving state changes in the range of 14.2 ~ 15.4 mA according to the strength of the received signal. In the sending state, the power of the sending signal varies with the working current. The transmit power at 14 mA is -10 dBm, and the transmit power at 29 mA is +20 dBm. Adjusting the working current according to the working status can achieve the lowest power consumption of the whole machine.

As the ISO18000-6C standard radio frequency air interface protocol is adopted, the system adopts 889 MHz transmitting and receiving frequencies. The parameters of the 12th and 13th pins of CC1100 are changed to connect capacitors C1 and C2, and the parameters of inductances L1 and L2 can be adjusted to transmit and receive radio frequency. The transmitting antenna adopts 50 Ω dipole antenna. The parameters of the 12 and 13-pin LC network devices can be simulated by the software SmartRF studio, refer to the official DATASHEET of CC1100, and fine-tune according to the actual situation. The CC1100 data transceiver principle diagram is shown as in Fig. 4.
  

How to realize the portable UHF RFID reader

3 System software design

3.1 Software design of RF transceiver chip CC1100

CC1100 operation includes chip data reading and writing, register and command filter setting access, data packet and data transmission rate setting, FIFO access, low power consumption strategy, etc. The CC1100 data transmission in the RFID reader adopts a synchronous continuous mode, which supports hardware data packet processing, FIFO buffering, data whitening, interleaving and forward processing. The CC1100 data frame includes preamble, synchronization data, label address, data length, read and write commands, data address, data, CRC check code, and end bit. The leading data and synchronization data are automatically completed by the CC1100 register setting hardware, and the CRC check code is automatically completed by the hardware. Other data requires software setting or configuration registers. The data frame structure is listed in Table 1.
 

How to realize the portable UHF RFID reader

CC1100 is connected to FPGA through a 4-wire SPI interface, and completes data transmission and reception for different internal registers and different command filter operations. When the SO pin reads data, SCLK must be pulled low before the first rising edge. Within one cycle, the falling edge of SCLK completes 1-bit data establishment, and the rising edge data is maintained; CSn=0 chip select signal is valid and data is transmitted normally; Under SCLK control, SI is synchronous data input, and SO is synchronous data output. After the data communication is completed, SCLK remains low and waits for the next data communication. When CSn=1, CC1100 is in sleep or crystal stop state to reduce power consumption. CC 1100 uses electromagnetic waves to activate the WOR low-power technology. The CPU is in a deep sleep waiting for reception. After the data arrives, it is directly stored in the RX FIFO without waking up the CPU.

CC1100 can be divided into configuration register address (0x00~0x2F) and command filter address (0x30~0x3F). The configuration register sets the SPI pin status, data modulation method, encoding method, check method, data transmission rate, channel bandwidth, etc.; the 6th to 4th bits of the register status word STATE query the current working state, 000 is the idle state, 001 is Receiving status, 010 is the sending status; command the filter to set the current working status, the receiving mode is enabled when SRX 0x34=0xFF, and the sending mode is enabled when STX 0x35=0xFF. CC1100 has a 64-bit RX FIFO and TX FIFO respectively, and the STATE status word monitors whether the FIFO overflows. Make sure that the FIFO is not empty before reading the RX FIFO, and make sure that the FIFO is empty before writing to the TX FIFO, otherwise the read and write data will make an error. The CC1100 register and filter settings can be optimally configured through the software SmartRF Studio.

The CC1100 data transmission rate is set through the MDMCFG3.DRATE_M and MDMCFG4.DRATE_E configuration registers and the crystal frequency. First set the data transmission rate and crystal frequency, and calculate the MDMCFG3.DRATE_M and MDMCFG4.DRATE_E configuration register values.

3.2 ISO18000-6C air interface protocol

Transmission frequency range: 860~960MHz.

Modulation method: ASK, each channel is 500kHz, 52 channels can be set, the label adopts backscatter modulation.

Maximum transmission power: 4W.

Coding method: PIE, Miller, FM0.

Check method: CRC-16.

Data transmission rate: 40~640kbps.

4 System experiment results and conclusions

RFID system testing includes software testing, hardware testing, stability and EMI testing. Software testing includes user software, middleware, label software, and codec signal testing; hardware testing includes transmit power, transceiver distance, transceiver angle, and number of tags; EMI and stability testing includes complex electromagnetic interference testing, etc. The instruments used are digital storage oscilloscope, vector analyzer, logic tester, spectrum analyzer, NRT power meter tester. They perform related tests and data analysis on the number of reads and writes and data accuracy, reader-to-tag distance and data accuracy, antenna transmission power and data transmission distance, and the relationship between antenna amplification and signal frequency.

After testing, the RF amplifier has the largest amplification factor for the 889 MHz signal, the maximum data transmission rate of the reader and the tag is 240 kbps; the transmission distance is within 1 m, and the antenna transmission power is 1.1 dBm. Adjust the transmission power, and the maximum transmission distance can reach 3 m. . The RFID portable system based on IS018000-6C standard radio frequency tag protocol works stably and reliably. It only needs to connect USB and PC to realize data communication between reader and PC. It is simple, convenient and reliable. It can be used in industrial control, consumer electronics, food Security tracking, modern logistics, campus all-in-one card and other places.

The Links:   LM215WF3-SDA1 M170EG01-V3

Author: Fymicohuang

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