Wireless identification device system based on 51 single chip microcomputer

1 Introduction
Radio frequency identification is a contactless automatic identification technology that automatically identifies the target object and obtains relevant data through radio frequency signals. Radio frequency identification does not require human intervention, is contactless, has a fast reading speed, is wear-free, is not affected by the environment, has a long life, and is easy to use. At present, radio frequency identification technology has developed very rapidly abroad, and there are many types of radio frequency identification products. It has been widely used in many fields such as industrial automation, commercial automation, transportation control and management, such as automobile, train and other traffic monitoring; highway automatic toll collection system; parking management system; item management; warehouse management; vehicle anti-theft, etc. Due to the late start of radio frequency identification technology in China, in addition to being used in the automatic identification system of the vehicle number of China Railway, it is limited to the application of radio frequency bus cards.
Here, a method to realize a simple radio frequency identification system is given. The reader and the transponder are both included in the single-chip control system. Using the 2ASK modulation and demodulation circuit and the matching network circuit, the effective identification distance of the entire system is about 10 cm, which can meet the needs of general applications.

2 System Design Overview
The system design is mainly divided into three parts: reader, transponder, and coil. The reader uses crystal oscillator 1 and crystal oscillator 2 to provide the power drive signal and digital modulation signal of the transponder respectively. The oscillation signal generated by crystal oscillator 1 is processed by a bandpass filter, amplified at the power level, and impedance transformed through a matching network, and then transmitted from the coil with maximum efficiency to provide the required working energy for the transponder. The signal generated by crystal oscillator 2 is sent to the switch circuit after filtering out high-frequency clutter through a low-pass filter; the manually set information is converted into a corresponding control signal by the South microcontroller to control the on and off of the switch, thereby forming a 2ASK modulation signal to communicate with the transponder. In addition, the reader needs to filter, amplify and detect the response signal received on the coil, and finally obtain valid information, which is read by the serial port. Figure 1 shows the structure of the reader.

After the crystal oscillator generates an oscillation signal, it is removed by a low-pass filter to remove high-frequency noise and sent to the switch circuit as the carrier signal of 2ASK. The signal that controls the on and off of the switch circuit is manually set by the operator through the DIP switch, and the corresponding control signal is generated after being read by the microcontroller. At the same time, the transponder receives the power drive signal through the coil, and after rectification and filtering, a DC level is obtained, and then a DC-DC conversion is performed to obtain the final required DC level for the entire transponder part to work. Figure 2 shows the transponder structure.

3 Hardware Circuit
3.1 2ASK Modulation and Demodulation Circuit of Reader and Transponder
The carrier signal of 2ASK is a 2MHz sine wave, which is obtained by passing the square wave generated by the active crystal oscillator through a low-pass filter. The digital modulation signal is output from the serial port of the CPU, and the analog switch MX7501 controls the signal on and off to generate a 2ASK signal. L1, L2, C1 and C2 form a second-order Butterworth low-pass filter, and the output is a 2 MHz signal that is approximately a sine wave. R1 and R4 match the impedance of the LC filter. When EN is 0, OUT is 0; otherwise, it is the S1 channel signal. Figure 3 shows the 2ASK modulation circuit of the reader.

Figure 4 shows the demodulation circuit. The 2ASK signal passing through the matching network has low amplitude and high noise, and needs to go through three levels of processing to demodulate the digital signal. First, the signal is amplified 5 times by the high-speed operational amplifier in OP37, and then passes through the comparator A in the high-speed comparator MAX910 to widen the burr-like signal and reduce the noise. The D/A output passes through the low-pass filter composed of L1, C1 and C2 to take the DC component, which is equivalent to envelope detection. The cutoff frequency of the LC low-pass filter is 480 HZ. Finally, the signal is compared and shaped to obtain digital modulation, and then input to the microcontroller serial port through QB.

3.2 Transmitter Circuit
The 8 MHz square wave generated by the active crystal oscillator is taken out as an 8 MHz sinusoidal signal through a bandpass filter, and then output to the coil after passing through a power amplifier. In Figure 5, L3, L4, C7 and C8 form a second-order 8 MHz Butterworth bandpass filter to filter out harmonic components. VQ1 is a collector-emitter follower, which is used to adjust R3 so that its static operating current is about 1.5 mA. This level of circuit plays a role in isolating from the signal source. VQ2 is a Class C amplifier. Adjust the base bias of R7 and VQ2 to make it work in the Class C amplification state. Change C2 to make it resonant, at which time the power supply current is minimum. Then adjust the base bias of R4 and VQ2 to make the power supply current smaller and the output amplitude larger, so as to be in the Class C amplification state. L1 is a coil and also acts as a resonant inductor. C6 is the equivalent capacitance of the coil, which is about 34 pF after measurement; C3 is an external capacitor, which can make the parallel circuit resonate at 8 MHz. When resonating, the voltage on L1 can reach 45V. Another signal transmitted by the reader and transponder is a 2ASK signal, which is directly connected to the coil through a matching network after being operationally amplified.
3.3 Transponder power supply circuit design
The transponder's rectifier filter circuit adopts a single-phase bridge rectifier filter circuit. T1 is two coils, and C1 is connected in parallel at both ends of the transponder coil. Under 8 MHz parallel resonance, its input amplitude is the largest. VD1~VD4 use IN5817 type rectifier diodes, which are required to rectify 8 MHz signals. Under normal circumstances, the voltage across C2 is basically stable, that is, the rectifier current passes through the load R1, so the larger RL is, the larger its voltage is. This requires that the load is as small as possible and the load input impedance is as large as possible. The capacitance value of C2 should not be too large or too small. If it is too large, the more charge is absorbed, the smaller the output voltage is; resulting in poor filtering effect. Therefore, C2=47μF is taken here. When the coupling signal amplitude is constant, the power driving capability of the circuit is fixed. It is found in the actual measurement that when the input frequency is 8 MHz, the output voltage is 5 V; when the load frequency is 1 kHz, the output voltage is 4.25 V. It can be seen that its driving power P = U2/RL = 18 mW, and the transponder power consumption must be less than this value. L1 and L2 further filter, and can also use the effect of induced potential to prevent large current mutations. The circuit is shown in Figure 6.

4 System software design
4.1 Software concept
The basic principle of the system software design is: first, the reader is controlled by the single-chip microcomputer to send a digital baseband signal, which is modulated by 2ASK and then transmitted by the antenna. After the transponder receives the modulated signal through the antenna, it performs 2ASK demodulation, and the demodulated signal is sent to the single-chip microcomputer through the serial port. After the single-chip microcomputer verifies the signal and processes it, it reads out the information stored in the EEPROM and sends it to the modulator, and the modulated signal is then sent out through the antenna. After receiving the returned signal, the reader demodulates the signal and sends it back to the single-chip microcomputer. The single-chip microcomputer decodes the signal and other operations to analyze the returned signal to achieve the purpose of identifying the object.
4.2 Program flow chart
After reset, the reader side checks whether there is a corresponding item. If so, the reader side controls the generation of the modulated signal. At the same time, the signal is allowed to be sent out. The transponder demodulates the corresponding signal received, checks whether the received information is wrong, and makes corresponding processing. The program flow is shown in Figure 7.

5 System Test and Conclusion
This system is based on 51 single-chip microcomputer control and wireless communication to build a simple radio frequency identification system. After debugging, the system runs well and works normally, with an average recognition distance of 5 cm.
From the operation of the system, it can be seen that the system has the following advantages: ① No contact is required during recognition; ② Short recognition time; ③ The probability of false recognition is relatively small; ④ Good scalability. This system can be used in various occasions such as parking lots, traffic road management, and intelligent property management after functional expansion.