Patent Publication Number: US-2021173523-A1

Title: Capacitive touch device and operating method thereof

Description:
BACKGROUND 
     1. Field of the Disclosure 
     This disclosure generally relates to an interactive input device and, more particularly, to a capacitive touch device and an operating method thereof capable of reducing the power consumption in the sleep mode. 
     2. Description of the Related Art 
     As the capacitive touch panel can provide a better user experience, it has been broadly applied to various electronic devices, e.g. applied to a display device so as to form a touch display device. 
     Generally speaking, if a capacitive touch panel is not operated by a user for a predetermined period of time, a sleep mode is entered to save power. In the sleep mode, the capacitive touch panel continuously to perform the scanning to confirm whether the sleep mode should be left. The power saving purpose can be achieved by reducing the scan frequency, scanning a part of electrodes or changing to a self-capacitance mode in the sleep mode. However, no matter which of the above mentioned method is adopted, the driving circuit will be used to input driving signals into the capacitive touch panel to generate detecting signals for the signal processing in the downstream circuit. Said driving circuit still consumes significant electricity. 
     Accordingly, the present disclosure provides a capacitive touch device and an operating method thereof that can further reduce the power consumption in a sleep mode or low power mode. 
     SUMMARY 
     The present disclosure provides a capacitive touch device and an operating method thereof that identify whether a touch event occurs according to the noise magnitude (including background DC value of the capacitive touch device without driving signal) of the null scanning to accordingly leave a sleep mode (or referred to low power mode). 
     The present disclosure further provides a capacitive touch device and an operating method thereof that double check whether a touch event occurs according to a null frame obtained in a sleep mode and a driven frame obtained in a normal mode to improve the identification accuracy. 
     The present disclosure further provides a capacitive touch device and an operating method thereof that double check whether a touch event occurs according to the comparison result of comparing noise magnitudes in the null frame of different frequency bands with noise thresholds to improve the identification accuracy. 
     The present disclosure provides a capacitive touch device including a touch panel, a plurality of driving circuits, an analog front end and a processor. The plurality of driving circuits is configured to output driving signals to the touch panel in a normal mode, and not to output the driving signals to the touch panel in a sleep mode. The analog front end is configured to scan the touch panel in the sleep mode, and sample and output a null frame. The processor is configured to identify a touch event according to the null frame to accordingly leave the sleep mode and return to the normal mode. 
     The present disclosure further provides an operating method of a capacitive touch device including a plurality of driving circuits, a touch panel, an analog front end and a processor. The operating method includes the steps of: stopping outputting driving signals from the plurality of driving circuits to the touch panel; scanning, by the analog front end, the touch panel within an interval that the touch panel does not receive the driving signals to sample and output a null frame; and comparing, by the processor, noises of the null frame with a noise threshold to confirm whether to control the plurality of driving circuits to output the driving signals to the touch panel. 
     The present disclosure further provides an operating method of a capacitive touch device including a control chip and a touch panel. The operating method includes the steps of: controlling a plurality of switches of the control chip to bypass driving signals that are inputted into the touch panel; receiving, by the control chip, background noises outputted from the touch panel within an interval that the driving signals are bypassed, and amplifying the received background noises with a first gain value; and comparing, by the control chip, the amplified background noises with a noise threshold to control switching of the plurality of switches. 
     In the present disclosure, the normal mode is a mode in which the driving circuits output driving signals to the touch panel to identify a touch position; whereas, the sleep mode is a mode in which the driving circuits do not output the driving signals to the touch panel or the driving signals outputted by the driving circuits are bypassed and unable to enter the touch panel. 
     That is, an interval during which the driving circuits output driving signals is referred to a touching interval, and an interval during which the driving circuits stop outputting the driving signals is referred to a sleep interval or a frequency scanning interval. The frequency scanning interval is entered to select a suitable channel when the SNR value is not good enough, but the sleep interval is entered to save power when there is nobody operating the capacitive touch device. 
     In the present disclosure, compared with the normal mode, it is able to scan a part of electrodes (or regions) of the touch panel, extend the scanning period or reduce a number of times of scanning the touch panel in the sleep mode to further reduce the power consumption. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, advantages, and novel features of the present disclosure will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic block diagram of a capacitive touch system according to one embodiment of the present disclosure. 
         FIG. 2  is a schematic block diagram of a capacitive touch system according to another embodiment of the present disclosure. 
         FIG. 3  is a schematic diagram of an analog front end of a capacitive touch system according to one embodiment of the present disclosure. 
         FIG. 4  is schematic diagram of a frequency selection method of a capacitive touch system according to one embodiment of the present disclosure. 
         FIG. 5  is a flow chart of a frequency selection method of a capacitive touch system according to one embodiment of the present disclosure. 
         FIG. 6  is a schematic block diagram of a capacitive touch device according to one embodiment of the present disclosure. 
         FIG. 7  is a flow chart of an operating method of a capacitive touch device according to one embodiment of the present disclosure. 
         FIG. 8  is an operational schematic diagram of a capacitive touch device according to one embodiment of the present disclosure. 
         FIG. 9  is a flow chart of an operating method of a capacitive touch device according to another embodiment of the present disclosure. 
         FIG. 10  is a flow chart of an operating method of a capacitive touch device according to an alternative embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT 
     It should be noted that, wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     Referring to  FIG. 1 , it is a schematic block diagram of the capacitive touch system according to one embodiment of the present disclosure. The capacitive touch system  1  includes a plurality of driving units  11 , a touch panel  12 , an analog front end  13 , an analog-to-digital conversion (ADC) circuit  14  and a digital back end  15 . In some embodiments, the ADC circuit  14  may be included in the analog front end  13 . 
     The analog front end  13  is configured to pre-process the analog signal outputted from the touch panel  12 . Then, the pre-processed analog signal is converted to the digital signal by the ADC circuit  14  for the post-processing of the digital back end  15 . Said pre-processing includes, for example, the amplification, downconversion, accumulation and filtering of the analog signal, but not limited thereto. Said post-processing includes, for example, identifying a touch position and/or a touch position variation (i.e. displacement) with respect to the touch panel  12  according to the digital signal, and identifying the noise level of the digital signal, but not limited thereto. 
     The touch panel  12  is, for example, a capacitive touch panel which includes a plurality of driving electrodes  121  and a plurality of sensing electrodes  122  configured to form inductive capacitance therebetween, wherein the inductive capacitance may be a self-capacitance and a mutual capacitance without particular limitations. For example, one driving electrode  121  may intersect with one sensing electrode  122  so as to form a sensing unit Cm, wherein  FIGS. 1 to 2  only show one sensing unit Cm but for simplifying the drawings other sensing units Cm formed by other pairs of the driving electrodes  121  and the sensing electrodes  122  are not shown. The method of forming a plurality of driving electrodes and a plurality of sensing electrodes on a touch panel is well known and thus details thereof are not described herein. 
     When a driving signal Sd is inputted to the driving electrode  121 , at least one detecting signal Si is induced on the sensing electrode  122  due to the inductive capacitance. When at least one finger or a conductor approaches the touch panel  12 , the capacitance of the sensing units Cm nearby is changed to accordingly change the detecting signal  51 . Accordingly, the processing unit  15  may detect at least one touch position according to the capacitance variation. The method of a capacitive touch system inducing at least one detecting signal Si corresponding to a driving signal Sd through the inductive capacitance is well known and thus details thereof are not described herein. The present disclosure is to provide a capacitive touch system and a frequency selection method thereof capable of shortening a frequency scanning interval and reducing the power consumption of the frequency scanning interval. 
     The driving units  11  are respectively coupled to the driving electrodes  121  and configured to output a driving signal Sd at one of a plurality of predetermined driving frequencies to the driving electrode  121  coupled thereto within a driving interval, and not to output the driving signal Sd to the driving electrode  121  coupled thereto within a frequency scanning interval. Referring to  FIG. 4 , it is a schematic diagram of a frequency selection method of a capacitive touch system according to one embodiment of the present disclosure. The capacitive touch system  1  is arranged with, for example, a plurality of predetermined driving frequencies such as 75 KHZ, 100 KHZ, 200 KHZ, 300 KHZ, 400 KHZ and 500 KHZ, but not limited thereto. The driving unit  11  output a driving signal Sd having, for example, periodic driving waveforms or non-periodic driving waveforms to the driving electrode  121  coupled thereto, wherein said driving waveforms are, for example, square waves, sinusoidal waves, triangular waves or trapezoid waves and so on without particular limitations. 
     Preferably, each of the driving electrodes  121  is coupled to one driving unit  11 . For simplification,  FIGS. 1 and 2  only show one driving unit  11 , but it is not to limit the present disclosure. In some embodiments, the driving units  11  may be coupled to the driving electrodes  121  respectively through a change-over switch (not shown) so as to control the connection or breakup between the driving units  11  and the driving electrodes  121 . Each of the driving units  11  also can be coupled to more than one driving electrodes  121 , that is to say more than one driving electrodes  121  can be driven with one driving signal Sd at the same time. 
     When the driving signal Sd is inputted to the driving electrode  121 , the associated sensing electrode  122  then outputs at least one detecting signal Si to the analog front end  13 . In this embodiment, the analog front end  13  includes a plurality of amplification units  131  configured to perform the signal amplification and a plurality of filters  132  configured to perform the signal filtering. In one embodiment, the sensing electrodes  122  are coupled to the amplification units  131  respectively through a change-over switch (not shown) so as to control the output of the detecting signal Si through the change-over switches. 
     The amplification units  131  are, for example, integrated programmable gain amplifier (IPGA) and respectively coupled to the sensing electrodes  122 . In one embodiment, each of the amplification units  131  is coupled to one of the sensing electrodes  122  and configured to amplify the detecting signal Si outputted from the sensing electrode  122  coupled thereto and output an amplified detecting signal Sia. In this embodiment, the amplification units  131  have the characteristic of the high-pass filter and have a high-pass cutoff frequency. 
     The filters  132  are, for example, anti-aliasing filters and respectively coupled to the amplification units  131 . In one embodiment, each of the filters  132  is coupled to one of the amplification units  131  and configured to filter the amplified detecting signal Sia and output an amplified and filtered detecting signal Siaf. In this embodiment, the filters  132  have the characteristic of the low-pass filter and have a low-pass cutoff frequency. 
     For example referring to  FIG. 3 , it is a schematic diagram of the amplification unit  131  and the filter  132  of the capacitive touch system  1  according to one embodiment of the present disclosure. The filter  132  outputs the amplified and filtered detecting signal Siaf to the ADC circuit  14  to be converted to the digital signal. 
     Referring to  FIG. 1  again, the digital back end  15  includes a processing unit  151 , which may be a digital signal processor (DSP), configured to perform the touch identification and determine whether to enter a frequency scanning mode, wherein the processing unit  15  may identify whether a conductor approaches the touch panel  12  according to the digital signal (e.g. obtained by digitizing the amplified and filtered detecting signal Siaf) detected within a predetermined detection interval (for example, but not limited to, 32 cycles of driving waveforms), and identify the signal-to-noise ratio (SNR) of the digital signal. For example in one embodiment, the driving unit  11  outputs the driving signal Sd at a current driving frequency to the touch panel  12 , and the analog front end  13  further includes, for example, an accumulation capacitor  133  configured to accumulate charges of the amplified and filtered detecting signal Siaf within the predetermined detection interval. The ADC circuit  14  samples the voltage of the accumulation capacitor  133  and converts sampled values to the digital signal to be inputted to the processing unit  151 . When the processing unit  151  identifies that an SNR value of the obtained digital signal is smaller than a threshold, the frequency scanning interval is entered, wherein the threshold may be determined according to the durable noise of the system without particular limitations. 
     In this embodiment, the processing unit  151  may further include a scan control unit  16  configured to control, in the frequency scanning interval, the high-pass cutoff frequency and the low-pass cutoff frequency so as to form an equivalent bandpass filter, and to adjust a center frequency of the equivalent bandpass filter to correspond to the predetermined driving frequencies. In addition, the scan control unit  16  is further configured to control, in the frequency scanning interval, the driving unit  11  to stop outputting the driving signal Sd to the touch panel  12  as well. 
     In one embodiment, the scan control unit  16  sequentially adjusts, in the frequency scanning interval, a center frequency Fc of the equivalent bandpass filter to be equal to each of the predetermined driving frequencies. For example in  FIG. 4 , the center frequency Fc of the equivalent bandpass filter is sequentially adjusted to substantially be equal to 75 KHZ, 100 KHZ, 200 KHZ, 300 KHZ, 400 KHZ and 500 KHZ, or vice versa. When the center frequency Fc is adjusted to each predetermined driving frequency, the scan control unit  16  detects the amplified and filtered detecting signal Siaf within a scan detection period (e.g. identical to or different from the predetermined detection interval of the driving interval, e.g. 32 cycles of driving waveforms). In the descriptions of the present disclosure, the frequency scanning interval is referred to an interval in which the touch panel  12  does not receive any driving signal Sd and the scan control unit  16  adjusts the cutoff frequencies, and the driving interval is referred to an interval in which the driving unit  11  inputs the driving signal Sd to the touch panel  12  and the processing unit  15  identifies the touch event according to the detected results. 
     In some embodiments, the scan control unit  16  identifies an amplified and filtered detecting signal having a smallest energy value among the amplified and filtered detecting signals Siaf associated with all the predetermined driving frequencies to accordingly determine a selected driving frequency. For example, the rectangular areas filled with slant lines in  FIG. 4  indicate the detected energy values corresponding to each of the predetermined driving frequencies in the frequency scanning interval, and 200 KHZ is shown as the selected driving frequency herein. In some embodiments, said energy value may be an energy sum of the amplified and filtered detecting signals associated with at least a part of the sensing electrodes  122  outputted in the frequency scanning interval, e.g. adding amplified and filtered detecting signals Siaf associated with all the sensing electrodes  122  to be served as the energy value. 
     In another embodiment, after entering the frequency scanning interval, the scan control unit  16  may sequentially adjust the center frequency Fc of the equivalent bandpass filter to substantially be equal to rest predetermined driving frequencies among the predetermined driving frequencies other than the current driving frequency and two adjacent driving frequencies of the current driving frequency. As the frequency scanning interval is generally entered due to the high noise level in driving at the current driving frequency, the current driving frequency and its adjacent driving frequencies may be directly ignored in frequency scanning, e.g. two immediately adjacent driving frequencies thereof, but not limited thereto. In some embodiments, when the number of the predetermined driving frequencies is larger, a plurality of predetermined driving frequencies close to the current driving frequency may be ignored in the frequency scanning interval. Next, the scan control unit  16  may identify an amplified and filtered detecting signal having a smallest energy value among the amplified and filtered detecting signals Siaf associated with the rest predetermined driving frequencies so as to accordingly determine a selected driving frequency. 
     Referring to  FIG. 2 , it is a schematic block diagram of a capacitive touch system according to another embodiment of the present disclosure. The capacitive touch system  1 ′ also includes a plurality of driving units  11 , a touch panel  12 , an analog front end  13 , an ADC circuit  14  and a digital back end  15 . Similarly, the ADC circuit  14  may be included in the analog front end  13 . The difference between this embodiment and  FIG. 1  is that in this embodiment the scan control unit  16  is disposed in the analog front end  13  and configured to perform the frequency selection directly according to the energy value of the amplified and filtered detecting signal Siaf associated with the predetermined driving frequencies. 
     In one embodiment, the analog front end  13 , for example, further includes an accumulation capacitor  133  configured to accumulate the amplified and filtered detecting signal Siaf within a predetermined detection interval. When the driving unit  11  outputs the driving signal Sd at a current driving frequency and the processing unit  151  identifies an SNR value of the obtained amplified and filtered detecting signal Siaf (e.g. obtained by sampling the accumulation capacitor  133  with the ADC circuit  14 ) is smaller than a threshold, a frequency scanning interval is entered. In the frequency scanning interval, the scan control unit  16  determines a selected driving frequency directly according to an amplified and filtered detecting signal having a smallest energy value among the amplified and filtered detecting signals Siaf associated with all the predetermined driving frequencies or the rest predetermined driving frequencies. It is appreciated that the method that the ADC circuit  14  samples the amplified and filtered detecting signal Siaf is not limited to sample the voltage of a capacitor as disclosed in the present disclosure. 
     In the above embodiments, as in the frequency scanning interval the driving unit  11  does not input any driving signal Sd to the touch panel  12 , the amplified and filtered detecting signal Siaf outputted by the filters  132  only contain background noise, and thus the amplified and filtered detecting signal Siaf in the frequency scanning interval is sometimes referred to the amplified and filtered background signal for distinguishing. 
     In other words, according to  FIGS. 1 and 2 , the scan control unit  16  may be disposed in the analog front end  13  or in the digital back end  15  without particular limitations. The scan control unit  16  may identify a smallest energy sum according to the amplified and filtered detecting signal before being digitized (i.e. analog signal) or according to the amplified and filtered detecting signal after being digitized (i.e. digital signal) so as to accordingly determine a selected driving frequency. 
     Referring to  FIG. 5 , it is a flow chart of a frequency selection method of a capacitive touch system according to one embodiment of the present disclosure, which includes the steps of: entering a driving interval (Step S 61 ); comparing an SNR value with a threshold (Step S 62 ); entering a frequency scanning interval when the SNR value is smaller than the threshold (Step S 63 ); deactivating driving signals (Step S 64 ); controlling cutoff frequencies to perform a frequency scanning (Step S 65 ); and searching a driving frequency having a lowest output energy value (Step S 66 ). The frequency selection method of this embodiment is adaptable to both the capacitive touch systems of  FIGS. 1 and 2 . 
     Referring to  FIGS. 1 to 5 , details of the frequency selection method of this embodiment are described hereinafter. 
     Step S 61 : In a driving interval the driving unit  11  drives the touch panel  12  at a current driving frequency, and the driving signal Sd is induced as at least one detecting signal Si through the sensing unit Cm between the driving electrode  121  and the sensing electrode  122 . The detecting signal Si sequentially passes through the amplification units  131  and the filters  132  to allow the filters  132  to respectively output an amplified and filtered detecting signal Siaf. The amplified and filtered detecting signal Siaf is, for example, accumulated in an accumulation capacitor  133  for a predetermined detection interval (e.g. 32 cycles of driving waveforms, but not limited thereto) and then converted to the digital signal by the ADC circuit  14 . For simplification, the amplified and filtered detecting signal after being digitized is also referred as the amplified and filtered detecting signal herein. 
     Step S 62 : The processing unit  151  identifies a touch event according to the amplified and filtered detecting signal Siaf and a noise level of the amplified and filtered detecting signal Siaf. When an SNR value of the amplified and filtered detecting signal Siaf exceeds a threshold, the driving interval (or touch detection mode) is maintained and the Step S 61  is returned; whereas when the SNR value is smaller than the threshold, a frequency scanning interval (or frequency scanning mode) is entered and the Step S 63  is entered. 
     Steps S 63 -S 64 : In the frequency scanning interval, the scan control unit  16  controls the driving unit  11  to stop driving the touch panel  12  or control the change-over switches between the driving units  11  and the driving electrodes  121  to break off. Accordingly, the touch panel  12  only outputs the background signal to the amplification units  131  such that the filters  132  output amplified and filtered background signals. 
     Step S 65 : After the driving signal Sd is ceased, the scan control unit  16  controls a high-pass cutoff frequency of the amplification units  131  and a low-pass cutoff frequency of the filters  132  to form an equivalent bandpass filter, and adjusts a center frequency Fc of the equivalent bandpass filter to correspond to a plurality of predetermined driving frequencies so as to determine a selected driving frequency according to the amplified and filtered background signal obtained by adjusting the center frequency Fc of the equivalent bandpass filter, as shown in  FIG. 4 . In one embodiment, a band of the equivalent bandpass filter may be 50-100 KHZ, but not limited thereto. 
     Step S 66 : In one embodiment, the scan control unit  16  reads the amplified and filtered background signal, which is an analog signal or a digital signal according to the disposed position of the scan control unit  16 , outputted from the filters  132 . For example in  FIG. 1 , the scan control unit  16  is in the digital back end  15  and thus the amplified and filtered background signal is the digital background signal converted by the ADC circuit  14 . For example in  FIG. 2 , the scan control unit  16  is in the analog front end  13  and thus the amplified and filtered background signal is the analog background signal not being converted by the ADC circuit  14 . In one embodiment, the scan control unit  16  identifies an amplified and filtered background signal having a smallest energy value among the amplified and filtered background signals associated with all the predetermined driving frequencies so as to accordingly determine a selected driving frequency. In another embodiment, the scan control unit  16  identifies an amplified and filtered background signal having a smallest energy value among the amplified and filtered background signals associated with the rest predetermined driving frequencies (i.e. other than the current driving frequency and its adjacent predetermined driving frequencies) so as to accordingly determine a selected driving frequency. 
     In one embodiment, the analog front end  13  and the digital back end  15  may form a readout circuit configured to couple to a touch panel  12  and read a plurality of detecting signals Si outputted by the touch panel  12 . The readout circuit includes a plurality of amplification units  131 , a plurality of filters  132  and a scan control unit  16 . The amplification units  131  are coupled to the touch panel  12  and configured to amplify the detecting signals Si outputted by the touch panel  12 , and have a high-pass cutoff frequency. The filters  132  are respectively coupled to the amplification units  131  and configured to output an amplified and filtered detecting signal Siaf, and have a low-pass cutoff frequency. The scan control unit  16  is configured to control the high-pass cutoff frequency of the amplification units  131  and the low-pass cutoff frequency of the filters  132  to form an equivalent bandpass filter, and adjust a center frequency Fc of the equivalent bandpass filter to correspond to at least a part of a plurality of predetermined driving frequencies of the touch panel  12 , as shown in  FIG. 4 . As mentioned above, the scan control unit  16  may determine a selected driving frequency according to one amplified and filtered detecting signal having a smallest energy value among the amplified and filtered detecting signals Siaf associated with all or at least a part of the predetermined driving frequencies. 
     As mentioned above, the processing unit determines whether to enter a frequency scanning interval from a driving interval (or referred to normal mode) according to the SNR value. In addition, the processing unit further determines to enter a sleep mode to reduce the system power when identifying no touch event occurs for a predetermined time interval. In order to further reduce the system power within a sleep interval, a null scan is performed within the sleep interval in the present disclosure to confirm whether a touch event occurs and determine whether the sleep mode should be left. It should be mentioned that although the above driving interval and the sleep interval both can identify a touch event, they have different purposes. The driving interval is used to identify at least one touch position and/or a displacement to perform a corresponding control, but the sleep interval is used to identify whether a touch event occurs to return to the driving interval. 
     Please referring to  FIG. 6 , it is a schematic block diagram of a capacitive touch device  600  according to one embodiment of the present disclosure. The capacitive touch device  600  includes a control chip  60  and a touch panel  62  connected to each other via a bus line or multiple signal lines for the communication therebetween, wherein an example of the touch panel  62  is referred to the touch panel  12  mentioned in the previous embodiment and thus details thereof are not repeated herein. The control chip  60  is used to drive and scan the touch panel  62  to identify a current mode that the capacitive touch device  600  is being operated such as a normal mode, a frequency scanning mode or a sleep mode, wherein the normal mode and the frequency scanning mode have been illustrated above, and thus details thereof are not repeated herein. Details of the sleep mode (or referred to sleep interval) are illustrated hereinafter. 
     The control chip  60  is formed as a package structure that has multiple pins as input/output paths of signals. The control chip  60  includes a plurality of driving circuits  601 , an analog front end  603  and a digital back end  605 , wherein operations of the driving circuit  601 , the analog front end  603  and the digital back end  605  are all considered to be executed by the control chip  60 . In this embodiment, the plurality of driving circuits  601  output, in the normal mode, driving signals Sd to the touch panel  62  via the driving electrodes thereof to cause the the touch panel  62  to detect the capacitance variation, and the plurality of driving circuits  601  stops outputting, in the sleep mode, the driving signals Sd to the touch panel  62  via the driving electrodes thereof. In one aspect, the plurality of driving circuits  601  includes a signal generator and respectively coupled to the driving electrodes of the touch panel  62  via a plurality of switches SW. The plurality of switches SW are used to bypass the driving signals Sd from or conduct the driving signals Sd to the corresponding driving electrodes. 
     The analog front end  603  is connected to the sensing electrodes of the touch panel  62 . In addition to scanning the touch panel  62  in the normal mode, the analog front end  603  is further used to scan the touch panel  62  in the sleep mode to sample and output a null frame, wherein the null frame herein is referred to a frame being generated by scanning and sampling the touch panel  62  when the touch panel  62  does not receive any driving signal. Said null frame contains the background noises or background signals mentioned above. In the present disclosure, said scanning is performed by, for example, control signals outputted from a row decoder  64  and a column decoder  66 , and said sampling is performed, for example, by a correlated doubling sampling. Said scanning and sampling the touch panel  62  may be implemented by conventional techniques without particular limitations. 
     As mentioned above, the capacitive touch device  600  includes an ADC for converting the null frame into a digital frame. For simplification purposes, the digital frame is also called null frame herein. 
     The digital back end  605  includes a processor  6051 , e.g., an application specific integrated circuit (ASIC), a digital signal processor (DSP) or a microcontroller unit (MCU), and is used to identify a touch event according to the digitized null frame to accordingly confirm whether to return to the normal mode from the sleep mode. 
     In another aspect, the analog front end  603  further includes an identifying circuit (not shown) used to identify the touch event according to the non-digitized null frame. In this case, the processor  6051  controls the capacitive touch device  600  to change an operation mode thereof after receiving a notification from the identifying circuit of the analog front end  603 . 
     It is noticed that when a human body approaches the touch panel  62 , the common mode noise is generated and contained in the background noises or background signals to increase a total noise level of the null frame. Accordingly, the present disclosure utilizes this common mode noise as a way to identify a touch event in the sleep mode. The identifying method includes: 
     1. Comparing a noise of every sensing unit (e.g., referring to  FIGS. 1-2 ) of the null frame with a noise threshold to confirm the touch event. For example, the processor  6051  calculates a number of sensing units that have the noise exceeding the noise threshold. When the number exceeds a number threshold, the occurrence of a touch event is identified. 
     2. Comparing a summation of noises of at least one row or at least one column of the null frame with a noise threshold to confirm the touch event. For example, the processor  6051  calculates a number of sensing unit rows or sensing unit columns that have a summation of noises exceeding a noise threshold. When the number of sensing unit rows or sensing unit columns exceeds a number threshold, the occurrence of a touch event is identified. The summation of noises is added directly by a circuit in the touch panel  62 , or implemented in the processor  6051  of the digital back end  605  without particular limitations. 
     3. Comparing a summation of all frame noises of the null frame with a noise threshold to confirm the touch event. Similarly, the summation of frame noises is added in the touch panel  62 , or implemented in the processor  6051  of the digital back end  605  without particular limitations. 
     It is appreciated that the noise thresholds in the above three identifying methods are not identical. 
     Please referring to  FIG. 7 , it is a flow chart of an operating method (or called awaking method) of a capacitive touch device  600  according to one embodiment of the present disclosure, including the steps of: entering a sleep mode (Step S 70 ); null scanning a predetermined channel (Step S 71 ); comparing a noise level with a noise threshold (Step S 72 ) to determine whether to leave the sleep mode (Step S 73 ). 
     Referring to  FIGS. 6 to 8 , details of this embodiment is illustrated below.  FIG. 8  shows that the touch panel  62  respectively generates one frame at times t 1  to t 7 . 
     Step S 70 : When the processor  6051  does not detect any touch event for a predetermined time interval within the driving interval, a sleep mode is entered. 
     In this embodiment, after the sleep mode is entered, the touch panel  62  stops receiving driving signals Sd from the plurality of driving circuits  601 . In one aspect, said stopping is implemented by controlling a plurality of switches SW of the control chip  60  to bypass the driving signals Sd that are inputted into the touch panel  62  via the driving electrodes thereof in the normal mode. In another aspect, the stopping is implemented by directly controlling the plurality of driving circuits  601  not to output any signal to the coupled driving electrodes. In this case, the capacitive touch device  600  may not include the plurality of switches SW. 
     Step S 71 : In an interval that the touch panel  62  does not receive the driving signals Sd (as mentioned above the driving signals Sd being bypassed or not outputted at all), the analog front end  603  scans the touch panel  62  to sample and output, using a predetermined scanning period, a null frame that contains background noises. For example, the analog front end  603  scans a predetermined channel (e.g., channel I or channel II in  FIG. 8  each corresponding to one predetermined driving frequency mentioned above) of the touch panel  62  to sample and output a null frame, wherein the predetermined channel is one frequency selected from multiple driving frequencies for driving the touch panel  62 , referring to  FIG. 4 . 
     As mentioned above, the analog front end  603  includes amplifiers  6031  and filters  6033  for respectively amplifying and filtering the null frame (or referred to background noises). For example, in the sleep mode, the amplifiers  6031  amplify the null frame with a first gain value. The amplifiers  6031  and the filters  6033  are respectively similar to the amplification units  131  and filters  132  mentioned above, and thus details thereof are not described herein. 
     Step S 72 : Next, the processor  6051  compares noises of the null frame with a noise threshold to confirm whether to control the plurality of driving circuits  601  to output driving signals Sd to the touch panel  62  and leave the sleep mode. For example, when the noises (e.g., the noise of at least one sensing unit as mentioned above) are larger than a noise threshold (e.g., different thresholds corresponding to different ways of calculating noises), the Step S 73  is entered to leave the sleep mode; on the contrary, the sleep mode and the null scanning are maintained. The null scanning is referred to a scanning procedure while the touch panel  62  is not receiving any driving signal Sd. 
     In other words, by comparing noises of the null frame with the noise threshold, it is able to control ON/OFF of the plurality of switches SW or driving circuits  601 . For example, when the amplified background noises (e.g., by a first gain value as mentioned above) is larger than the noise threshold (e.g., the frame at time t 6  in  FIG. 8  larger than a threshold TH 1 ), the plurality of switches SW are controlled to conduct the driving signals Sd or the plurality of driving circuits  601  are controlled to output driving signals Sd to the touch panel  62 . When the amplified background noises are smaller than the noise threshold (e.g., the frames at times t 1  to t 3  in  FIG. 8  smaller than the threshold TH 1 ), the Step S 70  is returned and the touch panel  62  is null scanned continuously. 
     Step S 73 : After the sleep mode is left and the normal mode is entered, the capacitive touch device  600  identifies touch positions or displacement, and the operation thereof is described in the previous embodiment. 
     In addition, after entering the sleep mode, the capacitive touch device  600  preferably records and stores a reference frame corresponding to every predetermined channel (e.g., storing in a buffer) for eliminating background noises without human body approaching in a differential process. That is, the reference frame is a null frame without human body close to the touch panel  62 . Then, after obtaining the null frame and before comparing the null frame with the noise threshold, the processor  6051  firstly calculates a difference between the null frame and the reference frame, and then compares the differential frame with the noise threshold to improve the identification accuracy. However, this step is optionally executed. 
     In one aspect, the differential process is performed between multiple pairs of pixels of a current null frame, and the calculated differential noise (i.e. obtained by subtracting the noise of one pixel from the noise of another pixel) is then compared with a differential noise threshold to determine whether a touch event is occurred or not. Preferably, the one pixel that is subtracted from another pixel is selected from those pixels having smaller magnitude of noises used as the reference background noise. 
     In an alternative aspect, the digital backend  605  calculates a differential noise between every pair of pixels at adjacent two rows or adjacent two columns, and then the calculated differential noise (i.e. obtained by subtracting the noise of one pixel from the noise of the adjacent pixel thereof) is then compared with a differential noise threshold to determine whether a touch event is occurred or not. For example, the digital backend  605  calculates the differential noise between first and second pixel rows (or columns), between third and fourth pixel rows (or columns), . . . , between the last pixel row (or column) and the pixel row (or column) previous to the last pixel row (or column). 
     In addition, in order to further improve the identification accuracy,  FIG. 7  further includes the following steps after awaking (i.e., leaving the sleep mode) the device: performing a driving scan with the predetermined channel (Step S 74 ); identifying a touch (Step S 75 ) to determine whether to maintain a normal mode (Step S 76 ) or return to the sleep mode. These steps are used to improve the identification accuracy in case the noises in the null frame larger than the noise threshold are not caused by the common mode noise induced by a human body. However, these steps are also optionally executed steps. 
     Step S 74 : After entering the normal mode, the touch panel  62  starts to receive the driving signals Sd, and the analog front end  603  scans the predetermined channel (e.g., same channel as the sleep mode) of the touch panel  62  to sample and output a driven frame, which is a frame outputted when the touch panel  62  is being driven by the driving signals Sd. 
     Step S 75 : The processor  6051  then double checks occurrence of the touch event (i.e. the touch event detected in the sleep mode) according to the driven frame. When the touch event is true, the normal mode is maintained (Step S 76 ) and the corresponding control is performed; whereas when the touch event is not true, the capacitive touch device  600  is controlled to return to the sleep mode (returning to Step S 70 ). For example,  FIG. 8  schematically shows that at time t 7  at least one sensing unit of the touch panel  62  has the capacitance variation larger than a capacitance threshold THc, and the occurrence of a touch event is confirmed. The method of identifying whether a touch panel  62  is touched in the driving interval has been illustrated above, and thus details thereof are not repeated herein. Besides, within an interval that the driving signals Sd are conducted, the control chip  60  amplifies the detecting signal Si outputted by the touch panel  62  with a second gain value smaller than the first gain value. 
     That is, after returning to the normal from the sleep mode, the processor  6051  preferably uses one or two driven frames to confirm whether the touch event is true or not so as to improve the accuracy of mode transformation. In one aspect, identical scans are performed in the normal mode and the sleep mode, and the difference is whether the driving signals Sd are inputted into the touch panel  62 . In another aspect, the scanning period of the sleep mode is longer than the scanning period of the normal mode. 
     Please referring to  FIG. 9 , it is a flow chart of an operating method of a capacitive touch device  600  according to another embodiment of the present disclosure, including the steps of: entering a sleep mode (Step S 80 ); performing null scanning and frequency scanning to find a channel having the smallest noises (Step S 81 ): comparing noises of the channel with a noise threshold (Step S 82 ) to leave a sleep mode (Step S 82 ) or maintain the sleep mode (Step S 821 ), wherein said frequency scanning is identical to the frequency scanning mode mentioned above. That is, in the embodiment of  FIG. 5  a frequency scanning mode is entered when the SNR value is not good enough; and in this embodiment the frequency scanning mode is performed right after entering the sleep mode. 
     The Steps S 80  and S 83  in  FIG. 9  are similar to the Steps S 70  and S 73  in  FIG. 7 , and thus details thereof are not repeated herein. The difference of this embodiment and  FIG. 7  is that in  FIG. 7  the control chip  60  confirms whether to awaken the capacitive touch device  600  according to the scanned result of a predetermined channel, whereas in  FIG. 9 , a frequency scanning is firstly performed to determine a channel having the smallest noises to replace the predetermined channel in  FIG. 7 , wherein the frequency scanning method has been illustrated in  FIGS. 4 and 5 . 
     In this embodiment, the frequency scanning is performed once or twice right after entering the sleep mode so as to find a channel having the smallest noises. As the sleep mode is an interval without user operation, performing the frequency selection procedure in the sleep mode does not influence the user experience. 
     After finding a null frame having the smallest noises by the frequency selection procedure, the processor  6051  uses the found channel having the smallest noises as the predetermined channel for scanning the touch panel  62 , Step S 81 . As mentioned above, the analog front end  603  includes IPGA and AAF to form equivalent bandpass filter corresponding to the channel for scanning the touch panel  62 . 
     In Step S 82 , the processor  6051  calculates a noise level of the selected channel having the smallest noises to be compared with the corresponding noise threshold (e.g., different channels having identical or different noise thresholds), wherein the method of calculating the noises has been illustrated above and thus details thereof are not repeated herein. When the noises of the channel having the smallest noises are larger than the corresponding noise threshold, the Step S 83  is entered to leave the sleep mode; whereas when the noises of the channel having the smallest noises are smaller than the corresponding noise threshold, the noises of the channel having the smallest noises are continuously monitored in the sleep mode, Step S 821 , i.e. continuously performing the null scanning. 
     In the embodiment of  FIG. 9 , a difference between the null frame of the channel having the smallest noises and a reference frame is selected to be performed before the Step S 82  to improve the identification accuracy, and details thereof are similar to  FIG. 7  and thus are not repeated herein, wherein the reference frame may be different depending on the selected channel having the smallest noises. Similarly, after leaving the sleep mode in Step S 83 , the Steps S 74  to S 76  in  FIG. 7  are further executed as  FIG. 7  only the predetermined channel is changed to a channel having the smallest noises, and thus details thereof are not repeated herein. 
     Referring to  FIG. 10 , it is a flow chart of an operating method of a capacitive touch device  600  according to an alternative embodiment of the present disclosure, including the steps of: entering a sleep mode (Step S 90 ); null scanning a first channel (Step S 91 ); comparing a noise level with at least one noise threshold (Step S 92 -S 921 ) to determine whether to leave the sleep mode (Step S 93 ); null scanning a second channel (Step S 922 ); comparing a noise level with another noise threshold (Step S 923 ) to determine whether to leave the sleep mode (Step S 93 ), wherein the Steps S 90  and S 93  are similar to the Steps S 70  and S 73  in  FIG. 7 , and thus details thereof are not repeated herein. 
     The difference between this embodiment and  FIGS. 7 and 9  is that in  FIG. 10  a first channel (e.g., identical to or different from the predetermined channel in  FIG. 7  or the smallest noise channel in  FIG. 9 ) of the touch panel  62  is null scanned at first to obtain a null frame. When noises of the null frame is between two predetermined noise thresholds TH 1  and TH 2 , a second channel is used to scan the touch panel  62  to obtain another null frame. Then, noises of said another null frame is compared with another noise threshold TH 3  to confirm whether to leave the sleep mode again thereby increasing the identification accuracy. In this embodiment, the first channel (shown as channel I in  FIG. 8 ) and the second channel (shown as channel II in  FIG. 8 ) are, for example, respectively one of predetermined driving frequencies in  FIG. 4 . 
     In other words, in the embodiments of  FIGS. 7 and 9 , the processor  6051  compares amplified background noises obtained by scanning a single channel of the touch panel  62  with a noise threshold. However in  FIG. 10 , the processor  6051  compares amplified background noises obtained by scanning different channels of the touch panel  62  respectively with different noise thresholds to perform the double check. 
     For example, in Steps S 92 -S 921 , when the processor  6051  identifies that noises of the null frame associated with the first channel (e.g., a frame at time t 6  in  FIG. 8 ) are larger than a first noise threshold TH 1 , a touch event is confirmed and the Step S 93  is entered, and the processor  6051  controls the plurality of driving circuits  601  to output driving signals Sd to the touch panel  62 . When identifying that noises of the null frame associated with the first channel are smaller than the first noise threshold and larger than a second noise threshold (e.g., a frame at time t 4  in  FIG. 8  between thresholds TH 1  and TH 2 ), the processor  6051  controls the analog front end  603  to scan another predetermined channel (e.g., second channel) of the touch panel  62  to sample and output another null frame (e.g., a frame at time t 5  associated with a second channel in  FIG. 8 ), Step S 922 . 
     The processor  6051  then compares noises of the another null frame associated with the second channel with another noise threshold (Step S 923 ), e.g., TH 3  in  FIG. 8 , to confirm a touch event according to the another null frame associated with the second channel. For example, when the noises of the another null frame associated with the second channel are larger than TH 3 , the Step S 93  is entered to leave the sleep mode; whereas when the noises of the another null frame associated with the second channel are smaller than TH 3 , the Step S 91  is returned to continuously monitor a touch event in the sleep mode. In this embodiment, as the noise threshold TH 3  is corresponding to a different channel, TH 3  is preferably different from TH 1  and TH 2 , but not limited thereto. 
     In this embodiment, the analog front end  603  is previously arranged to scan multiple channels (e.g., the multiple predetermined driving frequencies in  FIG. 4 ) of the touch panel  62 . Preferably, said another predetermined channel (e.g., the second channel) is a channel in the multiple channels farthest from the predetermined channel (e.g., the first channel), but the present disclosure is not limited thereto. Preferably, the second channel is not adjacent channels of the first channel. 
     In the embodiment of  FIG. 10 , a difference between the null frame associated with the first channel and a corresponding reference frame is selected to be performed before the Step S 92  to improve the identification accuracy. Similarly, a difference between the null frame associated with the second channel and a corresponding reference frame is selected to be performed before the Step S 923  to improve the identification accuracy. Details thereof are similar to  FIG. 7  and thus are not repeated herein. 
     In  FIG. 10 , after leaving the sleep mode in Step S 93 , the Steps S 74  to S 76  in  FIG. 7  are further executed as  FIG. 7  only the predetermined channel is changed to a first channel (e.g., entering Step S 93  from S 92 ) or a second channel (e.g., entering Step S 93  from S 923 ), and thus details thereof are not repeated. 
     As mentioned above, in the conventional capacitive touch system, although the power consumption in a sleep mode can be reduced by extending the scanning period or reducing a number of sensing units being driven, a significant power is still consumed because the control chip or driving chip still outputs driving signals to a touch panel. Therefore, the present disclosure further provides a capacitive touch device ( FIG. 6 ) and operating methods thereof ( FIGS. 7-10 ) that sample and output a null frame by null scanning the touch panel in a sleep mode to identify whether a touch event occurs according to a noise level in the null frame and confirm whether to end the sleep mode. As the null scanning in the sleep mode is only used to confirm whether to leave the sleep mode without identifying a touch position, a summation of noises may be used for the identification. 
     Although the disclosure has been explained in relation to its preferred embodiment, it is not used to limit the disclosure. It is to be understood that many other possible modifications and variations can be made by those skilled in the art without departing from the spirit and scope of the disclosure as hereinafter claimed.