"Abstract: Aiming at the shortcomings of low main frequency, slow speed and poor portability of current card readers, a design scheme of RFID card reading system based on S3C6410 is proposed. This paper takes the high-performance S3C6410 embedded microprocessor as the core, selects the new cr95hf RF chip, and develops and designs a new handheld RFID card reader with high main frequency and Android embedded system. The card reader works at high frequency 13.56 MHz and supports iso14443, 693 and other protocols. Experiments show that the card reader can read and write labels that meet the protocol, the reading and writing distance can meet the needs, and has the characteristics of portability, high stability and fast processing speed.
introduction
Radio frequency identification (RFID) is a new identification and tracking technology to find targets and exchange wireless data through radio frequency carrier. Compared with other technologies, RFID has the advantages of fast identification speed, strong anti-interference ability, high security and non-contact. Therefore, this technology has been widely used in access control system, logistics distribution, campus card and other daily life, as the key technology of Internet of things, and has great development potential in the future. However, the existing card readers generally have the disadvantages of low main frequency, slow processing speed and poor portability, which are difficult to meet the growing needs. In view of these shortcomings, based on the S3C6410 embedded microprocessor with high main frequency and strong performance, this paper selects a new cr95hf RF chip to develop and design a handheld RFID reader working at high frequency 13.56 MHz. At the same time, it innovatively carries the emerging Android embedded system, supports iso14443, iso15693 and other protocols, with fast processing speed and high accuracy.
1 system structure and working principle
The embedded RFID card reading system designed in this paper is composed of microprocessor, touch screen, power supply, RF, storage, antenna and so on. The overall structure is shown in Figure 1.
The card reader requires fast processing speed and real-time display of information. Therefore, S3C6410 embedded microprocessor with strong performance and very low power consumption is selected as the core, which is based on the advanced ARM11 core and burns the Android embedded system. The RF chip adopts the new non-contact chip cr95hf of ST company, which communicates with the microprocessor through serial port. The working principle is as follows: when reading the tag data, put the tag close to the card reader, the touch screen control end transmits the read command to S3C6410, its control RF module sends the command to the tag, and the tag returns the required data to the card reader for display after receiving it; When performing the write operation, S3C6410 controls the RF module to write data to the tag after receiving the touch screen write command.
2 system hardware design
2.1 microprocessor peripheral and power circuit design
This design uses Samsung's S3C6410 embedded microprocessor as the main control chip, and its main frequency is up to 667 MHz. It is a high-performance RISC processor based on ARM11 core. S3c 6410 includes various hardware interfaces such as power management, serial port, SPI, I2C bus, USB and I / O. it has the advantages of strong performance, fast processing speed and low power consumption, and can meet the design needs of the system. Two 128M DDR chips k4x1g163pc are used in parallel to realize 256M RAM circuit. The main control chip communicates with the RF module through serial port, and communicates with the host computer through USB interface.
The system designs two power supply modes: 5 V DC and 3.7 V lithium battery to meet the handheld requirements of card reader, and uses jumper cap to select. The card reader can be used by hand when powered by lithium battery. The system power supply requires 5 V and 3.3 v. The 3.7 V lithium battery is converted to 5 V through the boost chip, and then converted to 3.3 V through the voltage stabilizing chip lm1117 to supply power to the microprocessor and RF chip. When DC power is supplied, the power supply can be completed through lm1117. Lm1117 can support high current output close to 1 A, and its circuit is shown in Figure 2. The input is vcc5 and the output is vcc3.3. C1 ~ C4 are decoupling capacitors, which are used to eliminate the self excitation of the power pin and maintain the stability of the power supply.
2.2 RF circuit and matching network design
The RF chip of the card reader designed in this paper adopts cr95hf, which is a new high-frequency 13.56 MHz transceiver chip with SPI and serial port of ST company. It supports a variety of protocols such as iso14443, iso15693 and ISO 18092. It is mainly used for RFID and NFC near-field communication. The RF circuit is shown in Figure 3. Cr95hf uses serial port to communicate with microprocessor, and its serial port pin is UART_ TX and UART_ RX。 SSI_ 0、SSI_ 1 is grounded and set to 0 to realize serial port mode. Pull up resistors R5 and R6 clamp the level at the high level and play the role of current limiting protection. C2 and C5 are decoupling capacitors. Two TX and Rx pins connect the matching network and antenna.
In the design of matching network, a low-pass filter is formed by 0 Ω resistance and non welded grounding capacitor to suppress high-order harmonics, and then the matching capacitor is designed. Using PCB rectangular antenna, because it forms a 13.56 MHz LC resonant circuit with the equivalent capacitance of matching network, the inductance parameters can be obtained for antenna design. When designing PCB, pay attention to widen the power line and isolate it from the RF part, shorten the connection length between RF circuits as far as possible, and reduce the loop area to prevent signal crosstalk and electromagnetic interference (EMI) of PCB circuits and improve the stability of board making.
2.3 touch screen and storage circuit
The card reader is 4.3 inches with a resolution of 272 × 480 LCD, which can achieve good interface display. The screen is a resistive touch screen, which uses 24 I / O ports to communicate with the microprocessor to display information. The microprocessor is connected with the screen through 10 control I / O ports to realize the control function.
Kernel code, display data, application program and read label information need to be stored, so flash and SD card are designed in the card reader. Flash uses k9g8g08u0a chip with 1GB capacity and is controlled by chip selection signal CSN2 to store kernel code and application software. The SD card has a capacity of 8GB and is connected with the microprocessor through a high-speed MMC interface to store display data and label information. Its circuit is shown in Figure 4. The clock pin is mmc0_ CLK，MMC0_ CDN、MMC0_ WPN、MMC0_ CMD is the control pin, which is used to control the reading and writing of SD card. R17 ~ R24 are pull-up resistors, mm C0_ DATA0～MMC0_ Data3 is the data communication pin, which is used to transmit read-write data.
3 system software design
3.1 embedded system transplantation
The card reader transplants the embedded Android 2.3 operating system and develops RFID application software. Android is a mobile operating system based on Linux kernel launched by Google. It is a truly open-source and powerful embedded mobile system, which adopts software heap layer architecture. The transplantation of embedded system is shown in Figure 5. The PC builds the development environment through the arm linux GCC cross programming tool, compiles uboot to generate the flash boot program, compiles the Linux kernel to generate the underlying image zimage required for Android, cuts and compiles the Android source code to generate the root system rootfs. Yaffs2, and imports the above files into the SD card to realize the system transplantation and burning.
3.2 card reader drive design
The card reader software is mainly composed of bottom driver, data processing and interactive interface. Microprocessor and RF chip communicate through serial port, and Android is used for the development of serial port driver NDK generates the corresponding. So dynamic link library from the C function of Linux through the JNI interface for Android software developed in Java language. Because the data is transmitted in hexadecimal, the serial communication needs to be configured as the original data input / output. In order to improve the communication rate, this paper adopts the high-speed baud rate of 921600 B / s, effectively reduces the bit error rate through parity check, modifies the register to realize the transmission of 8 data bits, which is 1 / 8 higher than the traditional 7 data bit rate. The main C codes are as follows:
Cr95hf supports iso14443, 693 and other wireless protocols. It calls serial port function and delay function according to protocol standards and loads data to realize RF drive. The format of RF command is shown in Figure 6.
Different protocols are selected by different CMD commands. See Table 1 for protocol commands.
3.3 data processing
The card reader shall have the functions of card searching, anti-collision and card reading and writing. Set the serial port baud rate and select the protocol to prevent conflict to judge whether there are one or more label cards nearby. The wireless communication between the card reader and the tag is realized by processing the data according to the selected CMD and the returned results. The processing flow is shown in Fig. 7.
After card searching and anti-collision, select the address to read and write the card. Each address can store 32-bit data. The default is iso15693 protocol. Due to the input / output of raw data, after receiving the information, it is converted to hex string through the function bytearrtohex (byte []).
After the tag card is found, the unique identifier uid data of the card must be globally universal in the Android card reader software, so that the card reading interface and card writing interface can jointly control the data.
Because the data of each activity interface is independent, and the traditional intent class is used to transmit the same group of data between multiple interfaces, the operation is cumbersome and prone to data conflict. In this paper, the received label card information is stored in the array rcvbuffer through application class to realize global sharing. The Java program for global sharing is as follows:
3.4 human computer interaction interface software design
Android interface development is realized by mixing XML layout file and Java program. During design, the required interface is registered in the androidmanifest.xml file, and the picture button display of the main interface is realized through the grid view grid view. The main interface includes function buttons such as setting, card searching, card reading and card writing. After selection, the corresponding sub interface component activity will be called through the intent class, which starts the corresponding XML through the setcontentview() function.
4 system test
The physical object of the developed card reader is shown in Figure 8. The left side is the main body and the right side is the antenna. The test results after downloading the RFID software to the card reader are shown in Figure 9 and figure 10. Figure 9 shows the card search results when four tags supporting iso15693 protocol are selected and placed near the card reader. Each tag corresponds to a unique uid identifier. It can be seen that the card reader has found all nearby tags. Figure 10 shows the card reading test results. When selecting uid and entering address, the tag data "aa1107ff" of the address can be successfully read. It can be seen that the card reading system can successfully communicate with the tag wirelessly.
The card reader has been tested 400 times for card searching and 400 times for card reading and writing, and only one time of data loss occurs, indicating that the stability of the card reader is very high. After testing, the reading and writing distance of the card reader is at least 6.4 cm when there are no obstacles, and at least 5 cm when there are obstacles such as wood, books and leather. It can be seen that the recognition distance can meet the needs. At the same time, the label test of iso14443 protocol also shows that the card reader can read and write it stably.
epilogue
This paper introduces the solution of a new embedded RFID card reading system based on S3C6410 and cr95hf in detail. The card reader works in the high frequency band of 13.56MHz and supports a variety of protocols. After testing, the card reader can stably read and write the tags in accordance with the protocol, and has the characteristics of portability, strong performance and low power consumption. The design has a wide range of applications and can be used in aviation logistics industry and other occasions requiring strong portability and fast processing speed.
Reprinted from - Electronics World“
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