Contact detection circuit for charging and discharging with different currents and method of operation thereof

Abstract

A contact detection circuit includes a charging circuit, a discharging circuit, a counter, and a processor. The charging circuit charges a detection capacitor using different currents during charging. The discharging circuit discharges the detection capacitor using different currents during discharging. The counter is used to count the charging period and the discharging period. The processor is used to subtract a baseline time from the counted charging time and the counted discharging time to eliminate the interference of noise.

Description

TECHNICAL FIELD

[0001] The present invention relates to a contact detection circuit, and more particularly to a contact detection circuit using different current to charge and discharge a detection capacitor. BACKGROUND

[0002] Capacitive touch circuit detects contact by detecting the change of capacitance when a conductor approaches. Therefore, capacitive touch circuit can be used as a capacitive switch, for example, to sense whether a human body contacts a door handle.

[0003] When capacitive switch is applied in outdoor environment, the environmental parameters, including temperature and humidity, etc., can have significant variations, which results in the change of the capacitance of the capacitive switch also has significant changes and reduces the detection accuracy.

[0004] In addition, when capacitive switch is applied in a high noise environment, the time of counting the change of the capacitance can be affected by the noise and cause misjudgment. For example, refer to Figure 1 which is a schematic diagram of the charge and discharge of a known capacitive switch. The charging period Tr is defined as the period when the voltage of the capacitor reaches the reference voltage V H . As can be seen from the figure, when the capacitive switch receives external noise, the period to reach the reference voltage V H is shortened, which also causes misjudgment.

[0005] Therefore, a contact detection circuit capable of eliminating environmental changes and noise interference is needed. SUMMARY

[0006] The present invention provides a contact detection circuit that charges and discharges a detection capacitor with two different current values and subtracts a baseline count when judging contact, thereby improving detection accuracy.

[0007] The present invention also provides a contact detection circuit that changes the charge and discharge current to avoid noise frequency, thereby improving detection accuracy.

[0008] The present application provides a contact detection circuit including a detection capacitor, a charging circuit, a discharging circuit, a counter and a processor. The detection capacitor is used to generate a capacitor voltage. The charging circuit is used to charge the detection capacitor with a first charging current during a first charging period and with a second charging current less than the first charging current during a second charging period. The discharging circuit is used to discharge the detection capacitor with a first discharging current during a first discharging period and with a second discharging current less than the first discharging current during a second discharging period. The counter is used to sequentially count the first charging period, the second charging period, the first discharging period and the second discharging period as a detection period. The processor is used to determine a contact event according to the second charging period and the second discharging period, but not according to the first charging period and the first discharging period.

[0009] The present application also provides a contact detection circuit including a detection capacitor, a charging circuit, a discharging circuit, a counter and a processor. The detection capacitor is used to generate a capacitor voltage. The charging circuit is used to sequentially charge the detection capacitor with a first charging current and a second charging current less than the first charging current during a charging period. The discharging circuit is used to sequentially discharge the detection capacitor with a first discharging current and a second discharging current less than the first discharging current during a discharging period. The counter is used to sequentially count the charging period and the discharging period as a detection period. The processor is used to subtract a charging reference time and a discharging reference time from the detection period to generate a key time, and output a control signal according to a variation of the key time between successive detection periods.

[0010] The present application also provides a method for operating a contact detection circuit. The contact detection circuit includes a detection capacitor, a charging circuit, a discharging circuit and a counter. The method includes the following steps: charging the detection capacitor with a first charging current using the charging circuit and counting a first charging period with the counter; charging the detection capacitor with a second charging current less than the first charging current using the charging circuit and counting a second charging period with the counter; discharging the detection capacitor with a first discharging current using the discharging circuit and counting a first discharging period with the counter; discharging the detection capacitor with a second discharging current less than the first discharging current using the discharging circuit and counting a second discharging period with the counter; and determining a contact event according to a time variation of a total of a plurality of the second charging periods and the second discharging periods.

[0011] So that the manner in which the above recited and other advantages and features of the application can be understood in detail, a brief description of the drawings is made, followed by a detailed description of the application, and its various implementations, and embodiments. Furthermore, in the course of the description, reference will be made to the accompanying drawings, which form a part of the disclosure. Attached Figure Description

[0012] Figure 1 This is a schematic diagram of the charging and discharging of a known capacitor switch;

[0013] Figure 2 This is a block diagram of the contact detection circuit according to an embodiment of the present invention;

[0014] Figure 3 This is a schematic diagram of the current source circuit of the contact detection circuit according to an embodiment of the present invention;

[0015] Figure 4 This is a schematic diagram of the charging and discharging of the contact detection circuit according to an embodiment of the present invention; and

[0016] Figure 5 This is a flowchart of the operation method of the contact detection circuit according to an embodiment of the present invention.

[0017] Explanation of reference numerals in the attached figures

[0018] 200 Contact Detection Circuit

[0019] 20. Detection Capacitance

[0020] 21c charging circuit

[0021] 21c1 Variable Current Source

[0022] 21c3 Switching Element

[0023] 21d discharge circuit

[0024] 21d1 Variable Current Source

[0025] 21d3 Switching element

[0026] 23 Comparator Circuit

[0027] 25 counters

[0028] 27 processors Detailed Implementation

[0029] The contact detection circuit of this invention, for example, is applied to a capacitive switch. It can eliminate noise interference and is especially suitable for applications with large variations in environmental parameters and high external noise. By eliminating the baseline voltage, the detection accuracy can be improved.

[0030] Please refer to Figure 2The diagram shown is a block illustration of a contact detection circuit 200 according to an embodiment of the present invention. The contact detection circuit 200 includes a detection capacitor 20, a charging circuit 21c, a discharging circuit 21d, a comparator circuit 23, a counter 25, and a processor 27. The processor 27 is, for example, a digital signal processor (DSP) or an application-specific integrated circuit (ASIC), which uses hardware and / or firmware to perform its computational functions. In one embodiment, the charging circuit 21c, the discharging circuit 21d, the comparator circuit 23, the counter 25, and the processor 27 form a detection chip and are electrically connected to the detection capacitor 20.

[0031] The detection capacitor 20 is typically formed of electrodes and has a capacitance value Csef to generate a capacitive voltage Vc across its terminals when energized. The detection capacitor 20 is disposed on a component used to detect a conductor (e.g., a hand), such as a doorknob, electrical switch, light switch, etc., but is not limited thereto. When the conductor approaches or contacts the detection capacitor 20, the capacitance value Csef changes, which serves as the detection mechanism of the inductive switch.

[0032] like Figure 2 As shown, one end of the detection capacitor 20 is connected to ground voltage, while the other end is connected to the charging circuit 21c, the discharging circuit 21d, and the comparator circuit 23.

[0033] The capacitor voltage Vc changes during the charging and discharging process. Figure 4 (Displayed as ΔVc). When the capacitance value Csef changes, the charging and discharging time will change simultaneously, which serves as a mechanism for determining contact events.

[0034] The charging circuit 21c includes a variable current source 21c1 and a switching element 21c3 connected in series, wherein the switching element 21c3 is, for example, a transistor switch. In a non-limiting embodiment, the variable current source 21c1 includes a plurality of current sources 31 and a plurality of current switches 33 to form a current pool, such as... Figure 3 As shown, the current switch 33 is, for example, a transistor switch.

[0035] Please refer to the following at the same time Figure 4 The diagram shows a charging and discharging schematic of the contact detection circuit 200 according to an embodiment of the present invention. The charging circuit 21c charges the detection capacitor 20 with a first charging current Ic1 during a first charging period t1, and charges the detection capacitor 20 with a second charging current Ic2, which is less than the first charging current Ic1, during a second charging period t2. For example, the first charging period t1 and the second charging period t2 form a complete charging period. During this complete charging period, the charging circuit 21c first charges the detection capacitor 20 with the first charging current Ic1, and then charges the detection capacitor 20 with the second charging current Ic2.

[0036] Figure 2and Figure 4 In the embodiment, the charging and discharging current is denoted as Icd. When the detection capacitor 20 is being charged, Icd is positive; when the detection capacitor 20 is being discharged, Icd is negative; and vice versa. The positive and negative are merely used to indicate the current flow direction, and are not used to limit the present application.

[0037] Charging and discharging the detection capacitor 20 with a large current can shorten the charging period, but has lower sensitivity and noise immunity; while charging and discharging the detection capacitor 20 with a small current has higher sensitivity, but requires a longer charging period and thus prolongs the scanning time. The present application uses the features of both, and regards the first charging period t1 as a charging reference time, which is substantially maintained when the capacitor value Csef is changed by an external conductor. The processor 27 controls the variable current source 21c1 of the charging circuit 21c (i.e. controls the first charging current Ic1 and the second charging current Ic2) and the switching element 21c3, so that the second charging period t2 is greater than the first charging period t1. That is, the processor 27 controls the charging reference time (i.e. t1) to be less than half of the charging period. Preferably, under circuit limitations, the shorter the first charging period t1 is, the better; and the longer the second charging period t2 is, the better.

[0038] For example Figure 3 In the embodiment, the processor 27 changes the first charging current Ic1 and the second charging current Ic2 by changing the on or connection state of the plurality of current switches 33 and the plurality of current sources 31. For example, the more current switches 33 are turned on, the higher the charging current is generated.

[0039] The discharging circuit 21d includes a variable current source 21d1 and a switching element 21d3 connected in series with each other, wherein the switching element 21c3 is, for example, a transistor switch. Similarly, in an embodiment, the variable current source 21d1 includes a plurality of current sources 31 and a plurality of current switches 33, as shown in Figure 3 .

[0040] Please refer to Figure 4 , the discharging circuit 21d is used to discharge the detection capacitor 20 with a first discharging current Id1 during a first discharging period t3, and discharge the detection capacitor 20 with a second discharging current Id2 smaller than the first discharging current Id1 during a second discharging period t4. For example, the first discharging period t3 and the second discharging period t4 form a complete discharging period. In the complete discharging period, the discharging circuit 21d first discharges the detection capacitor 20 with the first discharging current Id1, and then discharges the detection capacitor 20 with the second discharging current Id2.

[0041] In the present application, the first discharge period t3 is considered as a discharge reference time, which is substantially maintained when the capacitance value Csef is changed by the external conductor. The processor 27 controls the variable current source 21d1 of the discharge circuit 21d (i.e. controls the first discharge current Idl and the second discharge current Id2) and the switching element 21d3 to make the second discharge period t4 greater than the first discharge period t3. That is, the processor 27 controls the discharge reference time (i.e. t3) to be less than half of the discharge period. Preferably, under the circuit limitation, the shorter the first discharge period t3 is, the longer the second discharge period t4 is.

[0042] For example Figure 3 In an embodiment, the processor 27 changes the first discharge current Idl and the second charge current Id2 by changing the on / off or connection state of the plurality of current switches 33 and the plurality of current sources 31. For example, the more current switches 33 are turned on, the higher the discharge current is generated.

[0043] The comparison circuit 23 is used to compare the capacitance voltage Vc with the first reference voltage V H and the second reference voltage V L (e.g. less than the first reference voltage V H ) to turn on / connect the charge circuit 21c and the detection capacitor 20 or turn on / connect the discharge circuit 21d and the detection capacitor 20. For example, when the charge circuit 21c charges the detection capacitor 20 so that the capacitance voltage Vc reaches the first reference voltage V H , the output signal of the comparison circuit 23 controls the switching element 21c3 to be turned off and controls the switching element 21d3 to be turned on so that the discharge circuit 21d starts to discharge the detection capacitor 20; and when the capacitance voltage Vc is discharged to the second reference voltage V L , the output signal of the comparison circuit 23 controls the switching element 21d3 to be turned off and controls the switching element 21c3 to be turned on so that the charge circuit 21c starts to charge the detection capacitor 20; and so on and so forth to charge and discharge the detection capacitor 20.

[0044] In an embodiment, the comparison circuit 23 comprises two comparators, each of which has the first reference voltage V H and the second reference voltage V L as an input signal of one of the two input terminals and has the capacitance voltage Vc connected to the other of the two input terminals. The output of one of the two comparators is used to control the on / off of the switching element 21c3 and the output of the other comparator is used to control the on / off of the switching element 21d3.

[0045] In another embodiment, the comparison circuit 23 comprises one comparator and one multiplexer, one input terminal of the comparator receives the first reference voltage V H through the multiplexer.H or a second reference voltage V L The other input of the comparator is connected to the capacitor voltage Vc. The output of the comparator is used to control the on / off of the switching elements 21c3 and 21d3.

[0046] It must be noted that the comparison circuit 23 is not limited to the example shown in the present application, as long as it can be used to compare the capacitor voltage Vc with the first reference voltage V H and the second reference voltage V L The circuit architecture for changing the charging / discharging by controlling the on / off of the switching elements 21c3 and 21d3 can be used as the comparison circuit 23 of the present application.

[0047] Figure 2 In the above example, the inverter of the discharging circuit 21d is used to ensure that the switching elements 21c3 and 21d3 are not turned on at the same time or turned off at the same time, which is not intended to limit the present application. For example, the inverter can be provided in the charging circuit 21c, or the comparison circuit 23 outputs opposite signals to control the switching elements 21c3 and 21d3 without using the inverter in the charging / discharging circuit.

[0048] The counter 25 (or timer) sequentially counts / times the lengths of the first charging period tl, the second charging period t2, the first discharging period t3 and the second discharging period t4, and the sum of tl to t4 is used as the detection period.

[0049] In one embodiment, the processor 27 determines the contact event based on the second charging period t2 and the second discharging period t4, but not based on the first charging period tl and the first discharging period t3. As mentioned above, since the charging reference time (i.e. tl) and the discharging reference time (i.e. t3) do not change when the conductor approaches, they are considered as baseline times, which reflect the baseline voltage of the detection capacitor 20. Therefore, although the counter 25 counts the entire detection period (tl+t2+t3+t4), the processor 27 subtracts the charging reference time tl and the discharging reference time t3 from the detection period (tl+t2+t3+t4) to generate the time of interest, i.e. calculates the sum of the second charging period and the second discharging period (t2+t4).

[0050] The processor 27 then determines whether a contact event occurs based on the change of the time of interest (t2+t4) between consecutive detection periods. For example, when the change of the time of interest (t2+t4) is greater than a change threshold, the processor 27 confirms that a contact event occurs, and sends a control signal Sc to open a door or turn on / off an electric appliance or a light, etc. according to different applications; otherwise, it indicates that no conductor approaches.

[0051] For example Figure 4The change of the capacitance voltage ΔVc of the detection capacitor 20 when the contact event occurs is changed from the solid line to the dashed line, and the critical time (t2+t4) is prolonged to (t2'+t4'). Therefore, when (t2'+t4')-(t2+t4) exceeds the change threshold, the processor 27 confirms that the contact event occurs.

[0052] In addition, the processor 27 can also determine whether the contact event occurs according to the comparison result of the change of the detection period (t1+t2+t3+t4) with the predetermined threshold, i.e. calculating (t1+t2'+t3+t4')-(t1+t2+t3+t4).

[0053] In addition, when the change amount of the single critical time or detection period due to the change of the capacitance value Csef is too small, the processor 27 can also determine whether the contact event occurs according to the change of the multiple critical times N×(t2+t4) or multiple detection periods N×(t1+t2+t3+t4).

[0054] In one embodiment, the processor 27 also changes the first charging current Ic1 and the first discharging current Id1 (or simultaneously changes the second charging current Ic2 and the second discharging current Id2) when determining that the detection period (t1+t2+t3+t4) is equal to or close to the noise period (or the detection frequency is equal to or close to the noise frequency) to change the detection period, so as to avoid the frequency band of the noise frequency to improve the detection accuracy. For example, the processor 27 additionally builds-in a time-frequency conversion algorithm for calculating the noise frequency. The way of calculating the noise frequency or period is known, and thus is not described here.

[0055] Please refer to Figure 5 Fig. 6 shows a flowchart of the operation method of the touch detection circuit 200 according to an embodiment of the present application, which comprises the following steps: charging the detection capacitor 20 with the first charging current Ic1 using the charging circuit 21c and counting the first charging period t1 with the counter 25 (step S51); charging the detection capacitor 20 with the second charging current Ic2 smaller than the first charging current Ic1 using the charging circuit 21c and counting the second charging period t2 with the counter 25 (step S52); discharging the detection capacitor 20 with the first discharging current Id1 using the discharging circuit 21d and counting the first discharging period t3 with the counter 25 (step S53); discharging the detection capacitor 20 with the second discharging current Id2 smaller than the first discharging current Id1 using the discharging circuit 21d and counting the second discharging period t4 with the counter 25 (step S54); and determining the contact event according to the time change of the total of multiple second charging periods t2 and second discharging periods t4. The time change of the total of multiple second charging periods t2 and second discharging periods t4 is, for example, N×(t2'+t4')-N×(t2+t4).

[0056] As mentioned earlier, using multiple charge-discharge periods avoids variations during a single charge-discharge period that are less than the detection sensitivity. In this invention, N is greater than or equal to 1.

[0057] The details of this method have been explained above, so they will not be repeated here.

[0058] As previously stated, the processor 27 can determine a contact event based solely on the change of the sum (t2+t4) of the second charging period t2 and the second discharging period t4 over a continuous detection cycle, and the first charging period t1 and the first discharging period t3 are used as baseline times and not for determining the contact event.

[0059] The operation method of this embodiment also includes: comparing the capacitor voltage Vc of the detection capacitor 20 with the first reference voltage V using the comparison circuit 23. H and the second reference voltage V L This determines whether to charge or discharge the detection capacitor 20.

[0060] In some implementations, the comparator circuit 23 may also include a flip-flop to generate a "1" or "0" level based on the output of the comparator in the comparator circuit 23 for counting by the counter 25.

[0061] It must be noted that the numerical values ​​in the above embodiments, for example... Figure 4 The displayed charging and discharging times are merely illustrative and are not intended to limit the scope of the invention.

[0062] It should be noted that in the above embodiments, the charging period involves charging the detection capacitor 20 with two different currents, and the discharging period involves discharging the detection capacitor 20 with two different currents, as an example. However, the present invention is not limited to this. In other embodiments, the detection capacitor 20 may be charged sequentially with two or more different currents during the charging period, and the detection capacitor 20 may be discharged sequentially with two or more different currents during the discharging period. The processor 27 can determine the contact event based on the charging and discharging period of the relatively minimum charging and discharging current.

[0063] It should be noted that in the above embodiments, the contact detection circuit 200 is described using a single self-capacitive electrode (e.g., forming a detection capacitor 20) as an example, but the present invention is not limited thereto. In other embodiments, the contact detection circuit 200 may include multiple self-capacitive electrodes that are parallel to each other, and each self-capacitive electrode is connected to its own charging / discharging circuit, comparator circuit, and counter, such as... Figure 2 As shown. The operation of each self-capacitive electrode is the same as described above. The counting results of multiple counters are transmitted to the same processor 27. When the processor 27 determines that the counting time change associated with at least one self-capacitive electrode or a predetermined number of self-capacitive electrodes exceeds a change threshold, a touch event is confirmed to have occurred.

[0064] It must be noted that although the present application is described with respect to a contact detection circuit, the contact detection circuit is not limited to detecting contact only. When a conductor is close to the detection capacitor 20 (affecting the detection capacitor), the contact detection circuit can detect the proximity of the conductor even though the conductor does not actually contact the detection capacitor 20 (or the member on which the detection capacitor 20 is disposed) as long as the change in the charging and discharging period (i.e. the change in the capacitance value) exceeds a threshold value. That is, the contact event detected by the contact detection circuit 200 of the present application includes contact and proximity of an object.

[0065] In summary, it is known that a capacitive switch is susceptible to environmental changes and noise, which can reduce the detection accuracy. Therefore, the present application further provides a contact detection circuit (e.g. refer to Figure 2 ) and a method of operating a contact detection circuit (e.g. refer to Figure 5 ), which uses a large current to charge and discharge the detection capacitor, and considers the charging and discharging period associated with the large current as a baseline time and eliminates it when determining a contact event. In addition, when the frequency of charging and discharging the detection capacitor is close to the noise frequency, the noise band can be avoided by changing the charging and discharging current value to change the charging and discharging frequency.

[0066] Although the present application has been disclosed by the foregoing examples, it is not intended to limit the present application, and any person skilled in the art can make various modifications and changes without departing from the spirit and scope of the present application. Therefore, the scope of protection of the present application should be determined by the scope defined by the appended claims.

Claims

1. A contact detection circuit, the contact detection circuit comprising: a detection capacitor for generating a capacitor voltage; a charging circuit for charging the detection capacitor with a first charging current during a first charging period and charging the detection capacitor with a second charging current less than the first charging current during a second charging period; a discharging circuit for discharging the detection capacitor with a first discharging current during a first discharging period and discharging the detection capacitor with a second discharging current less than the first discharging current during a second discharging period; a counter for sequentially counting the first charging period, the second charging period, the first discharging period and the second discharging period as a detection cycle; and a processor for determining a contact event based on a sum of the second charging period and the second discharging period and regarding the first charging period and the first discharging period as baseline times that do not change with proximity to a conductor and not determining the contact event based on the first charging period and the first discharging period.

2. The contact detection circuit of claim 1, further comprising a comparison circuit for comparing the capacitor voltage with a first reference voltage and a second reference voltage to turn on the charging circuit and the detection capacitor or to turn on the discharging circuit and the detection capacitor.

3. The contact detection circuit of claim 2, wherein the charging circuit and the discharging circuit each comprise a variable current source and a switching element, the processor further for: controlling the variable current source and the switching element of the charging circuit to make the second charging period greater than the first charging period, and controlling the variable current source and the switching element of the discharging circuit to make the second discharging period greater than the first discharging period.

4. The contact detection circuit of claim 3, wherein in the detection cycle, the first charging period precedes the second charging period, and the first discharging period precedes the second discharging period.

5. The contact detection circuit of claim 1, wherein the processor is further for changing the first charging current and the first discharging current to change the detection cycle when the detection cycle is equal to a noise cycle.

6. The contact detection circuit of claim 5, wherein the charging circuit and the discharging circuit each comprise a plurality of current sources and a plurality of current switches, and the processor changes the first charging current and the first discharging current by changing a turn-on state of the plurality of current switches and the plurality of current sources.

7. The contact detection circuit of claim 1, wherein the processor determines that the contact event occurs when the sum of the second charging period and the second discharging period changes between successive detection cycles by more than a change threshold.

8. A capacitive switch, the capacitive switch comprising: a detection capacitor for generating a capacitor voltage; ​ a charging circuit for charging the detection capacitor sequentially with a first charging current and a second charging current smaller than the first charging current during a charging period, wherein the first charging current charges the detection capacitor in a charging reference time; a discharging circuit for discharging the detection capacitor sequentially with a first discharging current and a second discharging current smaller than the first discharging current during a discharging period, wherein the first discharging current discharges the detection capacitor in a discharging reference time; a counter for sequentially counting the charging period and the discharging period to take a sum of the charging period and the discharging period as a detection period; a processor for subtracting the charging reference time and the discharging reference time, which are baseline times not changing with conductor proximity, from the detection period to generate a critical time, and outputting a control signal according to a change in the critical time between successive detection periods.

9. The capacitive switch of claim 8, further comprising a comparison circuit for comparing the capacitor voltage with a first reference voltage and a second reference voltage to turn on the charging circuit and the detection capacitor or to turn on the discharging circuit and the detection capacitor.

10. The capacitive switch of claim 9, wherein the charging circuit and the discharging circuit each comprise a switching element, and the processor is further for controlling the switching element of the charging circuit to make the charging reference time smaller than half of the charging period, and controlling the switching element of the discharging circuit to make the discharging reference time smaller than half of the discharging period.

11. The capacitive switch of claim 8, wherein the processor is further for changing the first charging current and the first discharging current to change the detection period when the detection period is equal to a noise period.

12. The capacitive switch of claim 11, wherein the charging circuit and the discharging circuit each comprise a plurality of current sources and a plurality of current switches, and the processor changes the first charging current and the first discharging current by changing on / off states of the plurality of current switches and the plurality of current sources.

13. A method for operating a contact detection circuit comprising a detection capacitor, a charging circuit, a discharging circuit, and a counter, the method comprising: charging the detection capacitor with a first charging current using the charging circuit and counting a first charging period with the counter; charging the detection capacitor with a second charging current smaller than the first charging current using the charging circuit and counting a second charging period with the counter; discharging the detection capacitor with a first discharging current using the discharging circuit and counting a first discharging period with the counter; discharging the detection capacitor with a second discharging current smaller than the first discharging current using the discharging circuit and counting a second discharging period with the counter; regarding the first charging period and the first discharging period as baseline times not changing with conductor proximity and not judging a contact event according to the first charging period and the first discharging period; and ​ ​ A contact event is determined based on a time variation of a total of the second charging periods and the second discharging periods.

14. The operation method of claim 13, wherein the contact detection circuit further comprises a comparison circuit, and the operation method further comprises: comparing, by the comparison circuit, the capacitance voltage of the detection capacitor with a first reference voltage and a second reference voltage to determine whether to charge or discharge the detection capacitor.

15. The operation method of claim 13, wherein the charging circuit and the discharging circuit respectively comprise a variable current source and a switching element, and the operation method further comprises: controlling the variable current source and the switching element of the charging circuit to make the second charging period greater than the first charging period; and controlling the variable current source and the switching element of the discharging circuit to make the second discharging period greater than the first discharging period.

16. The operation method of claim 13, wherein the first charging period, the second charging period, the first discharging period and the second discharging period form a detection period, and the operation method further comprises: changing the first charging current and the first discharging current to change the detection period when the detection period is equal to a noise period.

17. The operation method of claim 16, wherein the charging circuit and the discharging circuit respectively comprise a plurality of current sources and a plurality of current switches, and the operation method further comprises: changing the on-off states of the plurality of current switches and the plurality of current sources to change the first charging current and the first discharging current.

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