Electronic label decryption device based on 1 T single chip microcomputer

0 Introduction
Electronic tags are increasingly used in anti-theft systems of modern supermarkets, libraries, warehouses and other public circulation departments. The essence of electronic tags and their decryption codes is a radio frequency identification (RFID) system, which works in a contactless manner. It uses radio frequency signals and their spatial coupling and transmission characteristics to automatically identify stationary or moving items to be identified, and has the advantages of being fast and convenient. This design uses the single-chip microcomputer STC12C2052AD as the core, and uses chips such as NE546 phase-locked loop, as well as frequency synthesis, high-frequency small signal detection and other technologies. The electronic tag decryption device has high reliability and practical application value.

1 System hardware design
RFID systems generally consist of two parts: electronic tags and readers. In applications, electronic tags are attached to items to be identified. When the items to be identified with electronic tags pass through its reading range, the reader is used to automatically retrieve the agreed identification information in the electronic tags in a contactless manner, thereby realizing the function of automatically identifying items or automatically collecting item identification information.
RFID works using an LC oscillation circuit. The oscillation circuit is tuned to a specified resonant frequency fH. In modern systems, the transponder coil is etched on the conductor of the product nameplate film, and the insulating film in the middle of the produced capacitor sheet is 10μm thick. The oscillation circuit is moved near the alternating magnetic field. If the frequency fG of the alternating magnetic field coincides with the resonant frequency fH of the oscillation circuit, the oscillation circuit resonates and obtains energy from the alternating magnetic field. Therefore, the oscillation process on the oscillation coil can be obtained based on the short-term voltage or current change of the oscillation coil in the alternating magnetic field. This short-term rise in coil current (or short-term drop in coil voltage) is intuitively called a dip.
The relative intensity of this dip depends mainly on the speed at which the two coils approach each other. In order to ensure reliable identification of the transponder oscillation circuit attached to the product, it is necessary to obtain a dip as obvious as possible. The frequency of the generated magnetic field can be scanned instead of constant. The oscillator frequency is continuously swept through the range between the maximum and minimum frequencies. If the scanned oscillator frequency hits
fH (of the oscillation circuit in the transponder), the oscillation circuit begins to oscillate, and thus an obvious dip is generated in the power supply current of the oscillator coil, thereby detecting the electronic tag. In order to avoid removing the electronic tag at the checkout counter, the cashier will place the protected product on a device (electronic tag de-coder) after receiving the payment. The device generates a sufficiently strong magnetic field, and its induced voltage can
break down the film capacitor of the electronic tag, destroy the oscillation circuit, and make the oscillation circuit not resonate within the range of the scanning frequency, so that the protected product cannot be detected.
The main technical indicators of the electronic tag de-coder are: 1) It can identify the electronic tag and deactivate the electronic tag. 2) The resonance frequency of the electronic tag: 8.2MHz+10%. 3) Scanning frequency: 90Hz. 4) Maximum working speed: 40 pieces/s. 5) Maximum working distance: 50cm.
The block diagram of the electronic tag de-coder is shown in Figure 1. It consists of a scanning frequency generation circuit, a power amplifier and transmission circuit, a detection circuit, a single-chip microcomputer control circuit and an alarm circuit.

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1.1 Scanning circuit
The scanning circuit is composed of a high-frequency analog phase-locked loop NE564. The maximum operating frequency of NE564 can reach 50MHz. It is powered by a single +5V power supply. It is particularly suitable for modulation and demodulation of FM signals and FSK (frequency shift keying) signals in high-speed digital communications, and does not require external complex filters. Using the frequency modulation function of NE564, it is first allowed to work at a fixed frequency, and then its frequency modulation characteristics are used to scan the frequency. The PWM voltage with a duty cycle period is output through the P3.7 pin of the microcontroller. After filtering by resistors R30, R31 and capacitors C30, C31, the modulation signal voltage generated by the differential amplifier composed of the operational amplifier LM324 is input to the FM terminal of NE564, thereby controlling the output frequency of NE564. The scanning frequency is set at 8.2MHz as the center frequency, with a frequency deviation of 10% on the left and right, and through the scanning mode (90Hz scanning), the frequency of 7.38~9.02MHz is generated by 128 steps. The scanning circuit is shown in Figure 2.

1.2 Power amplifier and transmitting circuit
The power amplifier circuit uses a broadband transformer coupling circuit. The broadband transformer uses a high-frequency transformer and a transmission line transformer wound with a high-frequency magnetic core. The broadband power amplifier does not require a tuning circuit and can obtain linear amplification in a wide frequency range. The transmitting circuit transmits radio waves through the antenna, and the transmission and reception of radio waves are completed by the antenna. The signal output by the sweeping circuit with a center frequency of 8.2MHz and a left and right frequency deviation of 10% is input by the WAVE end of the power amplifier and transmitting circuit, and is sent to the transmitting antenna after being amplified by Q1 and coupled by the high-frequency transformer T1, and transmits radio waves in the specified direction. At the same time, the antenna is also used to receive radio waves and restore the received radio waves to high-frequency current. The power amplifier and transmitting circuit is shown in Figure 3.

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1.3 Detection Circuit
When the electronic tag resonates, a dip point will appear on the antenna. The task of the detection circuit is to detect this special frequency among the many frequencies in the antenna and convert it into the external interrupt level of the microcontroller. However, in engineering, it is difficult to detect the dip point, but it is easier to detect the second harmonic generated by the emission when the electronic tag resonates. Put an electronic tag into a range with a frequency that can make it resonate (8.2MHz). When the electronic tag resonates, the second harmonic it generates will be emitted in the opposite direction, which can be detected by the receiver. The signal generated by the second harmonic frequency can activate the alarm device. The detection circuit is mainly composed of a receiving antenna, a bandpass filter circuit, an amplifier circuit, and a level conversion circuit. The receiving antenna is used to receive high-order harmonics, the bandpass filter circuit is used to extract the second harmonic component, and the level conversion circuit is used to convert the AC component into a DC component and send it to the microcontroller as an interrupt request signal. During system detection, the microcontroller will send a set of control signals to control the HC4066 to turn on and off the transmitting circuit and the detection circuit. The detection signal enters the detection circuit through FI0 and FI1, and passes through the second-order bandpass filter composed of diode IN4148 and OP37, and the second harmonic component output bandwidth is 14-18MHz. The signal is then amplified by the resonant amplifier circuit composed of Q6 to obtain the required harmonic component, and the output DC voltage is shaped by the level conversion circuit to control the generation of the single-chip detection interrupt. The detection circuit is shown in Figure 4.

1.4 Single-chip microcomputer control circuit
The STC12C2052AD single-chip microcomputer adopts an enhanced 8051 core, 1 clock and 1 machine cycle (1T single-chip microcomputer), which is 8 to 12 times faster than the ordinary 8051. The operating frequency is 0 to 35MHz, the chip has 8k bytes of Flash program memory, the number of erase and write times is more than 1097 times, the chip has 256 bytes of RAM data memory, 2 hardware 16-bit timers, 1 full-duplex asynchronous serial port, 2 PWM outputs, and 8 A/D conversions. It has the advantages of high speed, low power consumption, and super strong anti-interference, and is a product with a high cost performance among similar technologies. The timer interrupt of the timer is used inside the single-chip microcomputer system to control the analog switch HC4066 to realize the control of emission and detection. The P3.7 pin of the single-chip microcomputer outputs a PWM voltage with a duty cycle change to generate a periodic scanning frequency; once the system detects the presence of an alarm object, the single-chip microcomputer generates an interrupt through INT0, the P1.7 pin of the single-chip microcomputer outputs a low level, turns on the power tube Q2, increases the transmission power to break through the capacitor in the electronic tag, and outputs a high level at the P1.2 pin to make the buzzer sound an alarm. The single-chip microcomputer control circuit is shown in Figure 2.

2 Software Design
The design of the STC12C2052AD software part is based on embedded C language and adopts a modular program structure. It includes the main program, system initialization subroutine, control function subroutine, detection subroutine, and PWM subroutine with a duty cycle change.
The main program is the core program of the electronic tag decoder. After the test system starts working, the program keeps running in the main program in a loop, and calls other function subroutines according to different needs. After the call is completed, the program returns to the main program to continue the loop. The main program flow chart is shown in Figure 5. The system initialization subroutine mainly completes the system initialization work, including pin configuration initialization, timer initialization, interrupt initialization, system parameter initialization, etc. The control function subroutine enables the control system to work normally according to the functional requirements. The detection subroutine completes the detection and breakdown of the electronic tag. The PWM subroutine with duty cycle variation is used to control the generation of scanning frequency. The main program flow chart is shown in Figure 5.

3 Conclusion
RFID uses contactless reading and writing, can identify multiple objects at the same time, and has good anti-interference ability and confidentiality performance, all of which are unmatched by barcodes. With the continuous improvement of the level of informatization, RFID technology has broad application prospects and huge business opportunities.