Overview
RFID (Wireless RF Recognition Technology) As an emerging non-contact automatic identification technology, the target object is automatically identified by the RF signal, and the relevant data can be obtained, and the high-speed moving object can be identified, which can simultaneously identify multiple tags, which can operate in harsh environments. Quick and convenient, in many fields such as logistics, transportation, charges, identification. RFID technology has developed abroad, there are many kinds of RFID products, like Ti, Motorola, Philips, etc. produce RFID products, and their products have its own products. At present, China RFIT technology and application are in the primary development stage. As an emerging automatic identification technology, RFID technology has begun in domestic applications, and the application areas are getting wider.
This article applies RFID technology to the logistics system, which realizes the hardware and software prototype of logistics system based on RFID technology.
System hardware design
In order to enhance the versatility and scalability of the read and write module, the modular design ideas are followed in hardware design. The entire read and write module consists of three major parts:
The master MCU mainly provides control operations for radio frequency read and write chips.
The radio frequency read and write chip is responsible for receiving the control information of the master MCU and completes communication with the RFID card. For normally, the RF read-write chip must be connected to the MCU connection with a suitable parallel interface. In order to transmit, a stable high frequency signal is received, the radio frequency read and write chip is connected to the antenna part by a high frequency filtering circuit.
Part of antenna, including coils and matching circuits.
In the designed TRH031M evaluation board, the master MCU consists mainly of the microcontroller atmega64L and the power supply circuit, the reset circuit, the crystal circuit, the JTAG interface, and the RS-232 serial interface; and increase the human-machine interface display circuit, using EDM12864 LCD Display controller; RF read-write chip uses TRH031M multi-protocol reader chip. The microcontroller ATMEGA64L and TRH031m are connected by latch SN74HC373N. The overall design of the TRH031M evaluation board is shown in Figure 1.
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Figure 1 TRH031M evaluation board overall design block diagram
RFID interface circuit
TRH031M is a three-in-one chip, compatible with ISO14443 Type A & B, and ISO15693 protocol. The TRH031M operating voltage range is 2.7V-3.6V, which is characterized by extremely low power consumption, and the packaging method of chips is also particularly suitable for use in a handset. The control of the MCU to TRH031M is achieved by reading and writing of its internal registers. There are 64 registers in TRH031M, divided into 8 pages, 8 registers per page.
Atmega64L is a low power 8-bit CMOS microcontroller based on enhanced AVR RISC structure. Due to its advanced instruction set and single clock cycle instruction execution time, the data throughput of Atmega64 is as high as 1 MIPS / MHz, which can slow down the system between power consumption and processing speed.
Atmega64L achieves control and data transmission of TRH031M chips through TRH031M parallel interface. Atmega64L uses a separate read, write signal line connection to the TRH031M, and the two I / 0 pins are used to control the read and write signal lines of TRH031M, respectively. In order to save I / O port, the way address / data line multiplexing is used here, which does not require a special I / O port to control the address line. ATMEGA64L and TRH031M connection are shown in Figure 2.
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Figure 2 ATMEGA64L and TRH031M connection diagram
Atmega64L's PTA0 ~ PTA7 connects TRH031M DATA0 ~ DATA7 as data / address lines, transmitting data and address information. Due to the connection mode of the data / address multiplexing, the PTA0 to PTA7 of Atmega64L is connected to the address line pins A0, Al, A2 of TRH031M by the latch SN74HC373DBLE and TRH031M.
Antenna circuit
The antenna part of TRH031M is composed, mainly divided into transmit and receiving portions, and the transmitting portion is also divided into EMC low pass filters, antenna matching portion, and antenna coil. The antenna is directly connected to TRH031M. Figure 3 is the structural schematic of the antenna.
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Figure 3 Structure Schematic Schemration
According to the data manual of RTH031M, the chip simulation portion (excluding receiver portion) is used as a load, and the load impedance is 15W. This is because the optimization setting output impedance is 15W, and the noise, gain, and output power can be reached at this time.
The impedance of the antenna should match 500W.
EMC low pass filter
The TRH031M system operates at 13.56 MHz frequencies, which is generated by quartz crystal oscillator. However, in addition to 13.56 MHz, it is possible to transmit outward in a high-harmonic manner. In order to comply with the international EMC, three five times and high harmonics in 13.56 MHz should be well suppressed, so there must be a suitable filter filter output signal to meet this provision. In order to reduce interference on the signal line, the EMC high frequency filter circuit is used. The principle diagram of the EMC filter circuit and the receiving circuit is shown in Figure 4. The low-pass filter consists of L0 and C0, and their values see Table 1.
Table 1
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Figure 4 Schematic diagram of EMC filtering circuit and receiving circuit
According to F =, when we choose = 1MH, the internal receiving circuit of TRH031M uses the card's response signal to modulate this concept on the bilateral band of the subcarrier. The VMID generated inside the chip is used as the RX pin input. In order to stabilize the output of the VMID, a capacitor C4 must be connected between the VMID and GND. The receiving circuit needs to connect a partial voltage circuit between the RX and VMID. In addition, it is recommended to connect one capacitor between the antenna coil and the repleaser. This receiving circuit consists of R1, R2, C3, and C4, and the value is shown in Table 1.
EMC filter circuit simulation
Simulation of EMC filtering circuit for 500W antenna impedance:
1 Set Feature Impedance Z. = 500W, the input signal frequency is 13.56MHz;
2 Licening ZL = 500W + 0.00JW, determine the starting point 1;
3 Co-capacitance C0 on ZL, get point 2;
4 is again connected in C0, and obtain points 3;
The point 3 should be located at a 15W matching point. If the point 3 cannot be in this point, the component parameters should be finered. This adjustment is not difficult to achieve by the intuitiveness of the circle. Thereby, the impedance circle shown in Fig. 5 (a) is obtained. When C0 = 136pf, L0 = 1Mh is obtained from Smith Chart, the equivalent impedance of 1, 3 is 15.24 + J 0.00W, close to the load impedance 15Ω, indicating that the component parameters we have design are correct. . Its error is from unlimited components.
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(A)
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(B)
Figure 5 EMC filter circuit smith circle
Thereby, the impedance circle shown in Fig. 5 (b) is obtained. When C0 = 136pf, L0 = 1Mh is obtained from Smith Chart, the equivalent impedance between 1, 3 is 500.42 + J 50.47 ohms, close to the antenna impedance 500 ohms, indicating that the component parameters we designed in front is correct.
Antenna matching circuit design
The antenna itself is a low-resistance device that connects the antenna to TRH031M to a matching circuit. There are two ways to design antennas: 50W matched antenna and use direct matching antenna configuration. Direct matching antenna configuration is used in this design.
Calculate the inductance of antenna coil
The inductance value of the antenna coil is very difficult, usually estimates the following formula:
L [NH] = 2 × L [cm] × (LN (L [mm] / d [mm] -k)) (1)
Where L is the length of the antenna coil, N is the number of antenna coils, typically 3 turns, D is an index factor of N, which is a width of the antenna coil diameter or conductor, and the index factor defined by the technique of antenna coil (see Table 2 ).
Estimation of coil resistance
The first antenna tuning without impedance analyzers can be used to use the following formula:
Rant = 5rdc (2)
In order to provide sufficient energy to the RFID card, the antenna and the card must be achieved, the coupling coefficient is 0.3 (when the coupling coefficient is 0, that is, the distance is too far or magnetic shield results in complete decoupling, the coupling coefficient is 1 ie fully coupled ). Therefore, the antenna coil is a wire having a diameter of 1 mm, and is designed as a three-circle 76mm × 49mm rectangular antenna. At this time, the inductance generated by the antenna coil can calculate the inductance value of the antenna coil by the formula 1 to be about L = 1.7 MH. Antenna resistor R = 1.4W.
Since the actual antenna coil inductive antenna circuit board is always slightly different, when the actual PCB design, the design process of the component of the antenna matching network is adjusted in Figure 6. The resonant capacitor is replaced by a fixed capacitor = 150pf and a tonal capacitor CV2. By adjusting the adjustable capacitor Cv2, the oscillation frequency of the antenna is 13.35 MHz, by adjusting C1, the impedance of the antenna is 500W, and the reading and writing distance of each antenna plate is adjusted by adjusting the adjustable capacitor.
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Figure 6 Antenna Matching Circuit Adjustment Process
The antenna matching network simulation step is as follows:
1 Set Feature Impedance Z. = 500 W, the input signal frequency is 13.56MHz;
2 Licening ZL = 0.00W + 0.00JW, determine the starting point 1;
3 On ZL series resistance RcOIL = 0.7W, get point 2;
4, then in the rcoil, the substance L = 0.85Mh, get point 3;
5. Co-capacitance C2, get point 4;
6 Re-serial capacitor C1 to get point 5.
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Figure 7 Antenna Matching Network Circuit SMITH
Thereby, the impedance circle shown in Fig. 7 is obtained. In the figure, RCOIL is 0.7W, L is 0.85 michen, so that the antenna is equivalent to RCOIL and L. C2 = 163pf, C1 = 15pf is obtained from Smith Chart, whereby the equivalent impedance between 1, 5 is 2577.79 + J11W, is close to half the impedance 250W of the symmetric antenna.
Through the results obtained, the component parameters designed by the author are basically consistent, this shows that the antenna circuit is correct.
Conclusion
Based on the ISO / IEC 15693 standard, an RFID reader system based on the working frequency of the ATMEGA64L microcontroller and TRH031M card card is designed to be 13.56MHz. The hardware and software prototype of the logistics system based on RFID technology has been designed. After actual use prove, the overall plan of the system is designed, and the main functions are basically realized, and the system's performance indicators: the design of the card reader system The recognition distance of the 15693 protocol card can reach 7.5 cm. At the same time, the system can read and write the ISO15693 protocol. The system is operating correctly, the display is accurate, easy to use, and has strong anti-interference performance.
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Source: Wiku Electronic Market Network
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