introduction
RF Identification (RFID) is an emerging identification and tracking technique for discovering targets and wireless data exchange through radio frequency carrier. Compared to other technologies, RFID has the advantages of fast recognition, strong anti-interference ability, high security, non-contact. Therefore, the technology has been widely used in access control systems, logistics distribution, campus cards and other daily lives, key technologies for work networks, and huge development potential for future development. However, existing card reader has generally mainfree, slow processing, and the shortcomings of portability and poor portability are difficult to meet the growing needs. In response to these shortcomings, this paper is based on the high frequency and powerful S3C6410 embedded microprocessor. The new CR95HF RF chip is used to develop a hand-held RFID card reader that works in high frequency 13.56 MHz, and is innovatively equipped. Emerging Android embedded systems, support ISO14443, ISO15693 and other protocols, with high processing speed and high accuracy.
1 System structure and work principle
The embedded RFID card system designed herein is composed of microprocessor, touch screen, power, radio frequency, storage, antenna, and the like. The overall structure is shown in Figure 1.
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The card reader requires fast processing speed, can display information in real time, so the performance powerful, high power consumption S3C6410 embedded microprocessor is used as the core, which is based on the advanced ARM11 kernel, bonding the Android embedded system. The RF chip uses ST company's new non-contact chip CR95HF, and communicates with the microprocessor. When reading the label data, the label is close to the card reader, and the touch screen is transmitted to the S3C6410. The control RF module sends the command to the label. After the label is received, the required data is returned to the card reader. Display; When the write operation is performed, the S3C6410 receives the touch screen writing command to the control RF module writes data to the label.
2 system hardware design
2.1 Microprocessor Peripheral and Power Circuit Design
This design uses Samsung's S3C6410 embedded microprocessor as a master chip, its main frequency up to 667 MHz is a high-performance RISC processor based on ARM11 kernel. S3C 6410 includes a variety of hardware interfaces such as power management, serial port, SPI, I2C bus, USB, and I / O, with advantages of powerful, fast processing speed and low power consumption, and can meet the design needs of the system. 2 128M DDR chip K4X1G163PC is used in parallel to implement 256m RAM circuitry. The master chip communicates with the serial port and RF modules and communicates with the USB interface and the host computer.
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The system is designed with 5 V DC and 3.7 V lithium batteries to supply power to satisfy the card holder, and select it with jumper cap. The card reader can be used when the lithium battery is powered. System power requires two types of 5 V and 3.3 V. 3.7 V Lithium battery is converted to 5V by a boost chip, and then converts the microprocessor, the radio frequency chip is powered by the regulator chip LM1117. Power supply can be accomplished by the LM1117 when the DC power supply is powered. The LM 1117 supports a large current output close to 1 A, which is shown in Figure 2. The input is VCC5, the output is Vcc3.3.c1 ~ C4 is a decoupling capacitor, which is used to eliminate the power supply pin self-motivation, and hold the power supply stability.
2.2 RF circuit and matching network design
This article is designed by CR95HF, which is ST company with SPI and serial port high-frequency 13.56 MHz new transceiver chip, support ISO14443, ISO15693, ISO 18092 and other protocols, mainly for RFID and NEFC near field communication. . The RF circuit is shown in Figure 3, and the CR95HF uses the serial port to communicate with the microprocessor, and its serial pin is UART_TX and UART_RX. The SSI_0, SSI_1 is grounded 0 to implement serial mode. The pull-up resistor R5, R6 clamps the level at a high level and acts as a current limiting protection. C2, C5 is a decoupling capacitor. Two TX and RX pin connections match the network and antenna.
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When the matching network is designed, the low-pass filter is first formed by 0 Ω resistors and non-welded grounding capacitors to suppress the high harmonic, and then the matching capacitance is designed. With the PCB rectangular antenna, the antenna design can be obtained by the LC resonant circuit of 13.56 MHz as the matching network is equivalent to the matching network. When designing the PCB, pay attention to widen the power cord and isolate the radio frequency part, minimize the connection length between the radio frequency circuit, and reduce the circuit area of the loop area from signal crosstalk and electromagnetic interference (EMI) of each line of PCB, and improve the stability of the board. sex.
2.3 Touch Screen and Storage Circuit
The card reader is 4.3 inches, a LCD screen with a resolution of 272 × 480, and a good interface display can be realized. The screen is a resistive touch screen, using 24 I / O ports to communicate with the microprocessor to display information. The microprocessor connects the control function through 10 control I / O ports and screens.
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Nuclear code, display data, application, and read tag information need to be stored, so the card reader is designed with Flash and SD cards. Flash uses the K9G8G08U0A chip, 1GB capacity, and uses the chip select signal CSN2 to store kernel code and application software. The SD card capacity is 8GB, and the microprocessor is connected to the high speed MMC interface, which is used to store display data and tag information, which is shown in Figure 4. The clock pin is MMC0_CLK, MMC0_CDN, MMC0_WPN, MMC0_CMD as the control pin, used to control the SD card read and write. R17 ~ R24 is a pull-up resistor, MM C0_DATA0 ~ MMC0_DATA3 is a data communication pin for transmitting read and write data.
3 system software design
3.1 Embedded System Transplantation
This card reader is transplanted embedded Android 2.3 operating system and develops RFID applications. Android is a Linux kernel-based mobile phone operating system launched by Google. It is a true open source and powerful embedded mobile system with a software stack layer architecture. The embedded system transplant is shown in Figure 5. The PC constructs a development environment via an ARM-Linux-GCC cross-co-serving tool, compiles UboT generation to burn the flash boot process, compile the underlying image of the Linux kernel to generate the underlying image Zimage required for Android, and cut, compile the Android source code generating root system rootfs. YAFFS2, Import the above file into the SD card to achieve the system's transplantation and sink.
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3.2 Card reader driver design
The card reader software is mainly composed of underlying drive, data processing, and interactive interface. Microprocessors and RF chips via serial communication, serial port-driven development uses Android NDK to generate the Linux's C function through the JNI interface to Android software developed for Java language development. Since the data is transmitted with hexadecimal transmission, serial communication needs to be configured as original data input / output. In order to improve communication rate, this paper uses high-speed baud rate 921 600 b / s, and the error rate is effectively reduced by parity, and the modification register implements the transmission of 8 data bits, which is 1/8 compared to the traditional 7 data bit rate, The main C code is as follows:
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CR95HF supports wireless protocols such as ISO14443, 693, call serial port functions and delay functions and load data according to the protocol standard and load data. The RF instruction format is shown in Figure 6.
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From different CMD commands to select different protocols, the protocol command will be seen in Table 1.
3.3 Data processing
The card reader needs to have functions such as search, anti-conflict, and read and write cards. Set the serial baud rate and selection protocol, and conflict to determine that there is a multi-tag card in the vicinity. Data processing is implemented based on the selected CMD and the return result to implement wireless communication of the card reader and the label, and the processing flow is shown in Figure 7.
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Pick up the card, select the address after the conflict is selected, and 32-bit data can be stored per address, and the ISO15693 protocol can be stored. Due to the original data input / output, the information is converted to the HEX string by the function byterrtohex (Byte []) after receiving the information.
After searching the label card, the unique identifier UID data must be globally universally in the Android card reader software to let the read card interface and the write card interface together to manipulate the data.
Since each Activity interface data is independent, and the traditional INTENT class transmits the same set of data operations between the multiple interfaces and is prone to data conflicts. This article is deposited by the Applicat ION class to the received tag card information in the array of RCVBuffer. shared. Implementation Global Sharing Java programs are as follows:
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3.4 human machine interaction interface software design
Android interface development is made through the XML layout file and the Java program. When designing, register the desired interface in the AndroidManifest.xml file and implement the image button of the main interface through the grid view grid view. The main interface includes setting, search, card read, write card, etc., which will then call the corresponding sub-interface component Activ ITY through the INTENT class, which starts the corresponding XML through the setContentView () function.
4 system test
The active card developed is shown in Figure 8, the left side is the main body, and the right side is an antenna. The test results after downloading the RFID software to the card reader are shown in Figure 9, shown in Figure 10. Figure 9 is a selection of 4 sheets that support the ISO15693 protocol to pick up the logo results near the card reader, each tag corresponding to the unique UID identifier, and the reader finds all the labels nearby. Figure 10 is a label data "AA1107FF" that can be successfully read when the card is read, select the UID and the input address. The visible card reader can be successfully communicated with label wireless communication.
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Tested 400 reader and read and write cards, only one data loss occurred, indicating that the card reader was very high. After testing, the read-write distance of the card reader is at least 6.4 cm, and the read and write distance is at least 5 cm when there is an obstacle such as a wooden board, a book, and leather. It can be seen that the identification distance can meet the needs. At the same time, the label test of the ISO14443 protocol also indicates that the card reader can stably read and write.
Conclusion
This article details the new embedded RFID card system solution based on S3C6410 and CR95HF. The card reader operates at a high frequency band of 13.56MHz and supports a variety of protocols. After testing, the card reader can perform stable reading and writing, with portable, powerful power, and low power. This design is widely used, which can be used in an occasion of the aviation logistics industry, which requires strong portability and quick speed.
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Reprinted from-Electronics World
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