NFC card detection method with low power consumption and high sensitivity

Abstract

The invention provides an NFC (Near Field Communication) card detection method with low power consumption and high sensitivity. The method comprises the following steps: setting switching frequencies of a variable-frequency DC-DC (Direct Current-Direct Current) power supply under different working time in a card detection period; performing matching setting on the frequency and the impedance of the NFC antenna; entering an NFC mode and carrying out initialization; periodically detecting the card, and sending a carrier signal by the NFC antenna; carrying out signal processing on the carrier signal, and converting the carrier signal into a digital signal to obtain an AD sampling value; setting a detection threshold value, judging whether a near field communication device exists or not according to the detection threshold value, and if yes, establishing a magnetic field and sending a radio frequency signal to search a card; if not, returning to execute the step S4; and S4, judging whether card searching succeeds or not, if so, performing identity authentication, and if not, returning to execute the step S4. According to the invention, ultra-low power consumption during NFC sleep is realized through power supply of the variable-frequency power source, and the sensitivity of the NFC module is improved through frequency and impedance matching of the NFC antenna and environment adaptive setting.

Description

Technical Field

[0001] The present invention relates to the technical field of near field communication (NFC), and in particular to an NFC card detection method with low power consumption and high sensitivity. Background Art

[0002] The NFC function uses NFC technology and short-range high-frequency wireless communication to achieve direct interaction between mobile phones and vehicles. When the mobile phone is close to the sensing area of ​​the vehicle, the vehicle can be unlocked, started, and other operations can be achieved without the need for traditional physical keys. At present, the polling mechanism is generally used to implement the NFC function. Polling is the process in which the NFC reader searches for the card by periodically sending a card search instruction. The NFC reader needs to activate the magnetic field first, which takes at least 8ms, and then send a card search instruction to allow nearby NFC tags or devices to respond. The card sensing cycle is usually 1 second. It can be seen that during the polling process, the NFC reader needs to wait for a while before detecting the presence of the tag. This process may take tens of milliseconds, which is a significant disadvantage for NFC applications that require fast response. In addition, the NFC card reader chip has a large emission current, the module power consumption is as high as 5mA, and the polling mechanism requires the NFC reader to be continuously activated and wait for the tag to respond, resulting in high device energy consumption.

[0003] In addition, the chip's built-in low-power card detection function (LPCD) can also be used to implement a low-power card search function. Low-power card detection can intermittently transmit a radio frequency field at extremely low power consumption to detect whether there is a card nearby. However, it is susceptible to external interference during implementation and may cause false triggering. In addition, due to the lack of a coupling design for matching the NFC antenna with the card / mobile phone frequency, and the inability to implement NFC detection calibration and module environmental adaptation, the sensitivity of the detection card when the distance is too close does not reach the expected effect, which will seriously affect the user experience.

[0004] Therefore, a card detection method with low power consumption, high sensitivity and environmental adaptability for realizing an NFC module is needed. Summary of the invention

[0005] The present invention provides an NFC card detection method with low power consumption and high sensitivity, which is mainly used to solve the problems of high energy consumption and low sensitivity of existing NFC devices, thereby achieving the effects of low power consumption, high sensitivity and environmental adaptability of the NFC module.

[0006] The present invention achieves the above-mentioned purpose through the following technical solutions:

[0007] A NFC card detection method with low power consumption and high sensitivity, comprising:

[0008] S1: Powering the NFC module via a variable frequency DC-DC power supply, and setting a switching frequency of the variable frequency DC-DC power supply at different working times within a card detection cycle.

[0009] S2: The NFC antenna is matched and debugged with the frequency of the near field communication device and the antenna resonant frequency is set, and the output impedance of the NFC antenna is matched with the internal impedance of the chip.

[0010] S3: Enter NFC mode and initialize.

[0011] S4: Perform periodic card detection. During the NFC working time of a card detection cycle, the NFC antenna sends a carrier signal.

[0012] S5: Process the carrier signal and convert it into a digital signal to obtain an AD sampling value.

[0013] S6: Setting a detection threshold, and judging whether there is a near field communication device according to the detection threshold, if there is, establishing a magnetic field and sending a radio frequency signal to search for the card; if there is not, returning to step S4.

[0014] S7: Determine whether the card search is successful. If successful, perform identity authentication. If unsuccessful, return to step S4.

[0015] A further solution is that in step S1:

[0016] During an NFC working time of a card detection cycle, the switching frequency of the variable frequency DC-DC power supply is set so that its conversion efficiency is higher than 80%.

[0017] During the NFC sleep time of a card detection cycle, the switching frequency of the variable frequency DC-DC power supply is set so that the standby current of the NFC module is within 20uA.

[0018] A further solution is that in step S2:

[0019] The frequencies of the NFC antenna and the near field communication device are matched and debugged by a network analyzer, so that the frequencies of the NFC antenna and the near field communication device are in a tightly coupled relationship.

[0020] A further solution is that the frequency tight coupling setting of the NFC antenna and the near field communication device satisfies:

[0021] When the near field communication device does not enter the magnetic field, the carrier amplitude of the NFC antenna is the largest and its variation amplitude is stable within 2%; when the near field communication device enters the magnetic field, the carrier amplitude of the NFC antenna is positively correlated with its distance from the near field communication device.

[0022] A further solution is that in step S4:

[0023] In a card detection cycle, the NFC working time is about 4ms, and the rest of the time is the sleep time of 60ms-1000ms.

[0024] A further solution is that in step S5, the signal processing method of the carrier signal includes:

[0025] S51: Convert the carrier signal into a sampling voltage signal through a detection circuit.

[0026] S52: Perform AD conversion on the sampled voltage signal to obtain the AD sampled value.

[0027] A further solution is that the signal conversion method in step S51 includes:

[0028] S511: The sampling capacitor is connected to the NFC antenna via a Schottky diode, and the Schottky diode is controlled to discharge the sampling capacitor.

[0029] S512: After the NFC antenna resonates, the carrier signal charges the sampling capacitor through the Schottky diode within a set time.

[0030] S513: After the carrier signal is turned off, the voltage of the sampling capacitor is maintained.

[0031] A further solution is that the method for setting the detection threshold in step S6 includes:

[0032] S61: Repeat steps S4-S5 to continuously perform multiple cycles of card detection, and calculate the average value of the AD sampling values ​​obtained through multiple cycles.

[0033] S62: Taking the average value of the AD sampling values ​​as a reference value, and setting the detection threshold with a reference range of 5%.

[0034] A further solution is that in step S6:

[0035] When the AD sampling value is within the detection threshold, it is determined that there is no near field communication capable device; when it is greater than the detection threshold, it is determined that there is a near field communication capable device.

[0036] Further solutions include:

[0037] S8: If the presence of the near field communication capable device is not detected after a number of consecutive card detection cycles, an average value of the AD sampling values ​​obtained after the number of card detection cycles is calculated.

[0038] S9: Calculate a new reference value based on the above average value and reset the detection threshold.

[0039] It can be seen that the present invention has the following beneficial effects:

[0040] 1. The present invention supplies power to the NFC module through a variable frequency DC-DC power supply, and sets different switching frequencies during the NFC working time and the sleep time in a detection cycle, thereby greatly improving the power conversion efficiency during the NFC working time, and reducing the standby current of the NFC module to less than 20uA during the NFC sleep time, thereby achieving the effect of NFC low power consumption and avoiding the problem of high module power consumption caused by large emission current of the traditional NFC card reader chip.

[0041] 2. The present invention detects the card periodically, making the detection period less than 64ms, and enables the NFC module to detect the card 16 times within 1s. Compared with the traditional polling mechanism, the card search time of the NFC module is greatly shortened and the card search efficiency is improved.

[0042] 3. The present invention matches and couples the NFC antenna with NFC cards, mobile phones and other near-field communication devices, and performs impedance matching. Compared with the traditional low-power card detection solution, the present invention improves the sensitivity and impedance matching of the NFC module in detecting the card by enhancing the coupling degree between the antenna and the card, thereby avoiding that the sensitivity cannot reach the expected effect when the distance to the card is too close, which seriously affects the user experience.

[0043] 4. The present invention calibrates the reference value of the detection threshold and adjusts the module environment adaptively, so that the NFC module detects the card judgment threshold and changes accordingly, making the NFC module more compatible, thereby achieving adaptation to environmental changes.

[0044] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 It is a flow chart of an NFC card detection method according to an embodiment of the present invention.

[0046] Figure 2 The figure is a circuit diagram of a variable frequency DC-DC power supply according to an embodiment of the present invention.

[0047] Figure 3 It is a schematic diagram of frequency and impedance matching of an NFC antenna according to an embodiment of the present invention.

[0048] Figure 4 Schematic diagram of a detection circuit according to an embodiment of the present invention.

[0049] Figure 5 Schematic diagram of an AD conversion circuit according to an embodiment of the present invention.

[0050] Figure 6 It is a flow chart of a method for adaptively adjusting an NFC module to an environment according to an embodiment of the present invention. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0052] An embodiment of an NFC card detection method with low power consumption and high sensitivity

[0053] See also Figure 1 The present invention relates to a NFC card detection method with low power consumption and high sensitivity, comprising:

[0054] S1: Powering the NFC module via a variable frequency DC-DC power supply, and setting a switching frequency of the variable frequency DC-DC power supply at different working times within a card detection cycle.

[0055] S2: The NFC antenna is matched and debugged with the frequency of the near field communication device and the antenna resonant frequency is set, and the output impedance of the NFC antenna is matched with the internal impedance of the chip.

[0056] S3: Enter NFC mode and initialize.

[0057] S4: Perform periodic card detection. During the NFC working time of a card detection cycle, the NFC antenna sends a carrier signal.

[0058] S5: Process the carrier signal and convert it into a digital signal to obtain an AD sampling value.

[0059] S6: Setting a detection threshold, and judging whether there is a near field communication device according to the detection threshold, if there is, establishing a magnetic field and sending a radio frequency signal to search for the card; if there is not, returning to step S4.

[0060] Specifically, in this embodiment, when the near field communication device approaches, the resonant voltage of the NFC antenna will drop significantly, and the voltage of the sampling capacitor will also drop accordingly. By comparing the detection threshold, it can be determined whether there is a near field communication device close to the NFC antenna.

[0061] S7: Determine whether the card search is successful. If successful, perform identity authentication. If unsuccessful, return to step S4.

[0062] Specifically, the near field communication device in this embodiment includes but is not limited to an NFC card reader, a mobile phone with a built-in NFC function, an NFC tag, and a card.

[0063] In this embodiment, in step S1:

[0064] During an NFC working time of a card detection cycle, the switching frequency of the variable frequency DC-DC power supply is set so that its conversion efficiency is higher than 80%.

[0065] During the NFC sleep time of a card detection cycle, the switching frequency of the variable frequency DC-DC power supply is set so that the standby current of the NFC module is within 20uA.

[0066] See also Figure 2 Specifically, the variable frequency DC-DC power supply of this embodiment includes a DC-DC conversion chip and its peripheral circuits. The power input end of the DC-DC conversion chip inputs 12V DC power, and the output end pin outputs 5V DC power after filtering through a resonant filter circuit to supply the NFC module. The MODE pin of the DC-DC conversion chip is grounded through resistors R7 and R9, and the working mode of the DC-DC conversion chip is selected by changing the resistance values ​​of resistors R7 and R9. The working mode includes a variable frequency mode and a fixed frequency mode. The FB pin of the DC-DC conversion chip is connected to the resonant filter circuit through a voltage divider circuit, which is used to match the output voltage by changing the voltage divider voltage value.

[0067] Specifically, in this embodiment, the switching frequency of the variable frequency DC-DC power supply is set to 1 MHz during the NFC working time of a card detection cycle, that is, the switching frequency is 1M times per second, so that the variable frequency DC-DC power supply is at the optimal operating frequency and its power conversion efficiency is higher than 80%.

[0068] Specifically, the switching frequency setting of the variable frequency DC-DC power supply described in this embodiment is only exemplary. The optimal switching frequency can be set according to the loss of the switching tube of the conversion chip, the loss of the magnetic component, the circuit design, etc. in the actual application, so that the switching loss and the magnetic component loss are balanced to obtain the highest conversion efficiency.

[0069] During the NFC sleep time of a card detection cycle, the switching frequency of the variable frequency DC-DC power supply is reduced to once every tens of milliseconds, so that the current of the NFC module is reduced from the mA level to less than 20uA.

[0070] In this embodiment, in step S2:

[0071] The frequencies of the NFC antenna and the near field communication device are matched and debugged by a network analyzer, so that the frequencies of the NFC antenna and the near field communication device are in a tightly coupled relationship.

[0072] Specifically, the method for frequency matching and debugging the NFC antenna in this embodiment includes:

[0073] S21: Select a suitable network analyzer according to the frequency range and debugging requirements of the NFC antenna and complete the preparations before debugging, including wiring the network analyzer to the NFC antenna and other equipment required for debugging.

[0074] S22: Setting parameters such as a start frequency and an end frequency on a network analyzer according to the NFC antenna.

[0075] S23: Select a parameter measurement mode, test the parameter, and record the test data.

[0076] S24: Analyze and calculate the test data, including the gain, standing wave ratio, loss, etc. of the NFC antenna.

[0077] S25: adjusting the matching frequency between the NFC antenna and the near field communication device according to the test result, and repeating step S23 until an ideal matching frequency value is obtained.

[0078] See also Figure 3 Specifically, the upper semicircle above the symmetry axis of the test image obtained by the network analyzer in this embodiment indicates that the impedance is biased towards inductive, and the lower semicircle below the symmetry axis indicates that the impedance is biased towards capacitive. The resonant impedance of the NFC antenna and the internal impedance of its chip are both set to about 40 ohms, and the antenna resonant frequency matching is obtained to be about 14.1 MHz. The relationship between the carrier amplitude of the NFC antenna and its distance from the near field communication device is shown in Table 1:

[0079] Table 1 Carrier change amplitude and distance relationship

[0080] distance 10-6cm 5cm 4cm 3cm 2cm 1cm 0cm Carrier variation range 2％ 7％ 12％ 30％ 60％ 80％ 90％

[0081] It can be seen from the table that the frequency tight coupling setting of the NFC antenna and the near field communication device satisfies: when the near field communication device does not enter the magnetic field, the carrier amplitude of the NFC antenna is the largest and its variation amplitude is stable within 2%; when the near field communication device enters the magnetic field, the carrier amplitude of the NFC antenna is positively correlated with its distance from the near field communication device.

[0082] Specifically, in this embodiment, the resonant impedance of the NFC antenna is set to be consistent with the internal impedance of the chip. For example, if the internal impedance of the chip is 40 ohms, the antenna impedance is also adjusted to 40 ohms. If the impedance is too large, such as 50 ohms, the carrier amplitude of the near field communication device will not change significantly when it is close to the antenna, and the sensitivity will be low; if the impedance is too small, such as 15 ohms, the card reading distance will be too close and there will be a card reading blind spot.

[0083] In this embodiment, in step S4:

[0084] In a card detection cycle, the NFC working time is about 4ms, and the rest of the time is the sleep time of 60ms-1000ms.

[0085] Specifically, the NFC module of this embodiment detects the card 16 times within 1 second, the detection period is less than 64 ms, the module power consumption is less than 0.2 mA, and the shortest detection distance of various mobile phones or NFC cards can reach 5 cm.

[0086] In this embodiment, the signal processing method of the carrier signal in step S5 includes:

[0087] S51: Convert the carrier signal into a sampling voltage signal through a detection circuit.

[0088] S52: Perform AD conversion on the sampled voltage signal to obtain the AD sampled value.

[0089] See also Figure 4 Specifically, the detection circuit of this embodiment includes a sampling capacitor circuit composed of multiple sampling capacitors and the Schottky diode D4. The sampling capacitor circuit is connected to the NFC antenna module ANT1 through the Schottky diode D4 and outputs a sampling voltage signal. When the sine wave high voltage of the NFC antenna is higher than the voltage of the sampling capacitor, the Schottky diode D4 is turned on and the sampling capacitor is charged; when the sine wave high voltage of the NFC antenna is lower than the voltage of the sampling capacitor, the Schottky diode D4 is reversed and is not cut off, and the voltage of the sampling capacitor is maintained.

[0090] Specifically, the calibration source of the AD conversion in this embodiment uses an internal fixed voltage to prevent the conversion result from being affected by fluctuations in the variable frequency DC-DC power supply. If the power supply fluctuates in the range of 4.85V-5.15V, the conversion result will have a 6% random deviation.

[0091] In this embodiment, the signal conversion method in step S51 includes:

[0092] S511: The sampling capacitor is connected to the NFC antenna via a Schottky diode, and the Schottky diode is controlled to discharge the sampling capacitor.

[0093] Specifically, the MCU of the NFC module of this embodiment controls the voltage of the sampling voltage signal output pin ADC-T to ground the Schottky diode D4 to discharge the sampling capacitor.

[0094] Specifically, in this embodiment, after the sampling capacitor in the sampling circuit is completely discharged in step S511, the MCU of the NFC module controls the card reader chip to transmit a short carrier wave, and the carrier wave is 20-100us.

[0095] S512: After the NFC antenna resonates, the carrier signal charges the sampling capacitor through the Schottky diode within a set time.

[0096] S513: After the carrier signal is turned off, the voltage of the sampling capacitor is maintained.

[0097] Specifically, the charging time of the sampling capacitor circuit in this embodiment is set to be about 20 us, that is, after a charging cycle of about 20 us, the voltage of the sampling capacitor can be increased to about 1V.

[0098] See also Figure 5 Specifically, step S52 of this embodiment further includes:

[0099] S521: When the voltage of the sampling capacitor is stable in step S513, the AD conversion program is started through software control to perform sampling and record the AD sampling value.

[0100] S522: Control the sampling voltage signal output pin ADC-T to be pulled low, and discharge the sampling capacitor again through the Schottky diode.

[0101] In this embodiment, the method for setting the detection threshold in step S6 includes:

[0102] S61: Repeat steps S4-S5 to continuously perform multiple cycles of card detection, and calculate the average value of the AD sampling values ​​obtained through multiple cycles.

[0103] S62: Taking the average value of the AD sampling values ​​as a reference value, and setting the detection threshold with a reference range of 5%.

[0104] Specifically, in this embodiment, the average value of the AD sampling values ​​is used as the center point, and the sensitivity is set to a size deviation such as 5% as a threshold.

[0105] In this embodiment, in step S6:

[0106] When the AD sampling value is within the detection threshold, it is determined that there is no near field communication capable device; when it is greater than the detection threshold, it is determined that there is a near field communication capable device.

[0107] See also Figure 6 , in this embodiment, further comprising:

[0108] S8: If the presence of the near field communication capable device is not detected after a number of consecutive card detection cycles, an average value of the AD sampling values ​​obtained after the number of card detection cycles is calculated.

[0109] S9: Calculate a new reference value based on the above average value and reset the detection threshold.

[0110] Specifically, this embodiment also includes:

[0111] S81: Obtain the current AD sampling value, and determine whether the current AD sampling value exceeds the current detection threshold.

[0112] S82: If not, repeat step S81 to determine whether the AD sampling values ​​after multiple card detection cycles have not exceeded the current detection threshold. If so, execute steps S8-S9.

[0113] S83: If exceeded, execute steps S6-S7. If the card search fails, execute steps S8-S9.

[0114] Specifically, this embodiment fine-tunes the current detection threshold by performing threshold comparisons multiple times to adapt to environmental changes, and fine-tunes the current detection threshold by detecting a card and failing to find the card, to prevent continuous triggering of misreading of the card due to the proximity of external metal objects.

[0115] Specifically, in step S82 of this embodiment, after a certain number of consecutive card-free detections, the average value H of the AD sampling values ​​after the number of consecutive card-free detections is calculated, and the new reference value is:

[0116] 0.75*K+0.25*H

[0117] Wherein, K is the reference value of the previous detection threshold.

[0118] Specifically, the calculation weights of the new reference values ​​in this embodiment are only exemplary and not unique. The best calibration solution can be selected according to actual test results.

[0119] It can be seen that by calibrating NFC detection through the above method, when the external environment changes slowly, such as when the temperature changes, the reference value of the detection threshold will also change accordingly, and the NFC module detection card judgment threshold will also change accordingly, so that the NFC module has better compatibility, thereby achieving self-adaptation to environmental changes.

[0120] The above-mentioned embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and substitutions made by technicians in this field on the basis of the present invention shall fall within the scope of protection required by the present invention.

Claims

1. A NFC card detection method with low power consumption and high sensitivity, characterized in that: include: S1: Powering the NFC module through a variable frequency DC-DC power supply, and setting the switching frequency of the variable frequency DC-DC power supply at different working times in a card detection cycle; S2: Matching and debugging the NFC antenna with the frequency of the near field communication device and setting the antenna resonant frequency, and matching and setting the output impedance of the NFC antenna with the internal impedance of its chip; S3: Enter NFC mode and initialize; S4: Periodically detect the card. During the NFC working time of a card detection cycle, the NFC antenna sends a carrier signal. S5: Process the carrier signal and convert it into a digital signal to obtain an AD sampling value; S6: Setting a detection threshold, and judging whether there is a near field communication device according to the detection threshold, if there is, establishing a magnetic field and sending a radio frequency signal to search for the card; if there is no card, returning to step S4; S7: Determine whether the card search is successful. If successful, perform identity authentication. If unsuccessful, return to step S4.

2. The NFC card detection method with low power consumption and high sensitivity according to claim 1, characterized in that: In step S1: During an NFC working time of a card detection cycle, the switching frequency of the variable frequency DC-DC power supply is set so that its conversion efficiency is higher than 80%; During the NFC sleep time of a card detection cycle, the switching frequency of the variable frequency DC-DC power supply is set so that the standby current of the NFC module is within 20uA.

3. The NFC card detection method with low power consumption and high sensitivity according to claim 1, characterized in that: In step S2: The frequencies of the NFC antenna and the near field communication device are matched and debugged by a network analyzer, so that the frequencies of the NFC antenna and the near field communication device are in a tightly coupled relationship.

4. The NFC card detection method with low power consumption and high sensitivity according to claim 3, characterized in that: The frequency tight coupling arrangement between the NFC antenna and the near field communication device satisfies: When the near field communication device does not enter the magnetic field, the carrier amplitude of the NFC antenna is the largest and its variation amplitude is stable within 2%; when the near field communication device enters the magnetic field, the carrier amplitude of the NFC antenna is positively correlated with its distance from the near field communication device.

5. The NFC card detection method with low power consumption and high sensitivity according to claim 2, characterized in that: In step S4: In a card detection cycle, the NFC working time is about 4ms, and the rest of the time is the sleep time of 60ms-1000ms.

6. The NFC card detection method with low power consumption and high sensitivity according to claim 1, characterized in that: In step S5, the signal processing method of the carrier signal includes: S51: converting the carrier signal into a sampling voltage signal through a detection circuit; S52: Perform AD conversion on the sampled voltage signal to obtain the AD sampled value.

7. The NFC card detection method with low power consumption and high sensitivity according to claim 6, characterized in that: The signal conversion method in step S51 includes: S511: The sampling capacitor is connected to the NFC antenna via a Schottky diode, and the Schottky diode is controlled to discharge the sampling capacitor; S512: After the NFC antenna resonates, the carrier signal charges the sampling capacitor through the Schottky diode within a set time; S513: After the carrier signal is turned off, the voltage of the sampling capacitor is maintained.

8. The NFC card detection method with low power consumption and high sensitivity according to claim 1, characterized in that: The method for setting the detection threshold in step S6 includes: S61: Repeat steps S4-S5 to continuously perform multiple cycles of card detection, and calculate the average value of the AD sampling values ​​obtained in the multiple cycles; S62: Taking the average value of the AD sampling values ​​as a reference value, and setting the detection threshold with a reference range of 5%.

9. The NFC card detection method with low power consumption and high sensitivity according to claim 8, characterized in that: In step S6: When the AD sampling value is within the detection threshold, it is determined that there is no near field communication capable device; when it is greater than the detection threshold, it is determined that there is a near field communication capable device.

10. The NFC card detection method with low power consumption and high sensitivity according to any one of claims 1 to 9, characterized in that: Also includes: S8: if the presence of the near field communication device is not detected after a number of consecutive card detection cycles, calculating an average value of the AD sampling values ​​obtained after the number of card detection cycles; S9: Calculate a new reference value based on the above average value and reset the detection threshold.

Images