Patent Publication Number: US-2023162526-A1

Title: Fingerprint recognition device, readout circuit and operating method of fingerprint recognition device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of U.S. Provisional application Ser. No. 63/282,661, filed on Nov. 23, 2021. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND 
     Technical Field 
     The invention relates to an electronic device, an electronic circuit and an operating method, more specifically, a fingerprint recognition device, a readout circuit and an operating method of the fingerprint recognition device. 
     Description of Related Art 
     Fingerprint recognition is a commonly used technology for security protection. A complete sensing cycle of an optical fingerprint sensor includes actions such as reset, exposure (exposure of the photosensitive element in the optical fingerprint sensor), and readout. Different lengths of exposure time should be applied for different fingerprint sensing conditions. For example, a fingerprint sensing under strong light requires a short exposure time to avoid overexposure of the fingerprint sensor. In the case of dry fingers, longer exposure times are required to accumulate images. 
     Therefore, in the traditional operation, when the fingerprint image obtained by the first fingerprint sensing is not suitable for identification. For example, it is determined to be an overexposed image, a host circuit will adjust the exposure time length, and perform fingerprint sensing again. The fingerprint image obtained by the second fingerprint sensing is re-identified. As a result, the time required for unlocking the electronic device becomes longer. 
     SUMMARY 
     The invention is direct to a fingerprint recognition device, a readout circuit and an operating method of the fingerprint recognition device, capable of saving an unlocking time. 
     An embodiment of the invention provides a fingerprint recognition device including a sensor panel, a readout circuit and a host circuit. The sensor panel includes a plurality of sensor pixels arranged in an array. The sensor pixels are configured to output sensing signals. The readout circuit is coupled to the sensor pixels. The readout circuit is configured to read out the sensing signals from the sensor pixels after a first exposure period to obtain a first fingerprint image, and read out the sensing signals after a second exposure period to obtain a second fingerprint image. The sensor pixels are reset before the first exposure period and after the second exposure period. The host circuit is coupled to the readout circuit. The host circuit is configured to perform a fingerprint recognition operation according to the first fingerprint image or the second fingerprint image. 
     In an embodiment of the invention, the readout circuit includes an analog-to-digital converter (ADC) circuit. The ADC circuit is coupled to the sensor pixels. The ADC circuit is configured to convert the sensing signals from an analog format to a digital format, and output the sensing signals of the digital format. The sensing signals of the digital format include a first data and a second data. The first data corresponds to the first fingerprint image, and the second data corresponds to the second fingerprint image. 
     In an embodiment of the invention, the ADC circuit has different parameter settings for different lengths of exposure periods. 
     In an embodiment of the invention, the host circuit is configured to set the parameter settings of the analog-to-digital converter circuit according to the lengths of the exposure periods. 
     In an embodiment of the invention, the readout circuit further includes a first storage device. The first storage device is coupled to the ADC circuit. The first storage device is configured to receive and store the sensing signals of the digital format. 
     In an embodiment of the invention, the host circuit includes a second storage device. The second storage device is coupled to the readout circuit. The second storage device is configured to receive and store the sensing signals of the digital format. 
     In an embodiment of the invention, the fingerprint recognition further includes a driver circuit. The driver circuit is coupled to the sensor pixels. The driver circuit is configured to reset the sensor pixels and scan the sensor pixels to drive the sensor pixels to output sensing signals. The driver circuit resets the sensor pixels before the first exposure period and after the second exposure period. 
     In an embodiment of the invention, the driver circuit includes a scan circuit and a gate driving circuit. The scan circuit is coupled to the sensor pixels. The scan circuit is configured to reset the sensor pixels and scan the sensor pixels to drive the sensor pixels to output the sensing signals. The gate driving circuit is coupled to the scan circuit. The gate driving circuit is configured to drive the scan circuit to perform a reset operation and a scan operation. 
     In an embodiment of the invention, the scan circuit is disposed on the sensor panel, and the gate driving circuit is disposed in the readout circuit. 
     In an embodiment of the invention, the readout circuit is implemented in a semiconductor chip which has having a display driving function, a touch sensing function and a fingerprint sensing function. 
     An embodiment of the invention provides a readout circuit configured to read out sensing signals from sensor pixels of a sensor panel. The readout circuit includes an ADC circuit and a driver circuit. The ADC circuit is coupled to the sensor pixels. The ADC circuit is configured to receive the sensing signals from the sensor pixels after a first exposure period and after a second exposure period. The ADC circuit is further configured to convert the sensing signals from an analog format to a digital format, and output the sensing signals of the digital format. The driver circuit is coupled to the sensor pixels. The driver circuit is configured to reset the sensor pixels and scan the sensor pixels to drive the sensor pixels to output the sensing signals. The driver circuit resets the sensor pixels before the first exposure period and after the second exposure period. 
     In an embodiment of the invention, the readout circuit further includes a storage device. The storage device is coupled to the analog-to-digital circuit. The storage device is configured to receive and store the sensing signals of the digital format. 
     An embodiment of the invention provides an operating method of a fingerprint recognition device. The fingerprint recognition device includes a sensor panel, and the sensor panel includes a plurality of sensor pixels configured to output sensing signals. The operating method of the fingerprint recognition device includes: reading out the sensing signals from the sensor pixels after a first exposure period to obtain a first fingerprint image; reading out the sensing signals from the sensor pixels after a second exposure period to obtain a second fingerprint image; and performing a fingerprint recognition operation according to the first fingerprint image or the second fingerprint image, wherein the sensor pixels are reset before the first exposure period and after the second exposure period. 
     In an embodiment of the invention, the fingerprint recognition device includes a readout circuit. The operating method of the fingerprint recognition device further includes: converting, via an ADC converter of the readout circuit, the sensing signals from an analog format to a digital format, and outputting the sensing signals of the digital format. The sensing signals of the digital format include a first data and a second data. The first data corresponds to the first fingerprint image, and the second data corresponds to the second fingerprint image. 
     In an embodiment of the invention, the operating method of the fingerprint recognition device further includes: setting parameter settings of the ADC converter circuit according to lengths of exposure periods. 
     In an embodiment of the invention, the operating method of the fingerprint recognition device further includes: storing the sensing signals of the digital format in a first storage device of the readout circuit. 
     In an embodiment of the invention, the fingerprint recognition device includes a host circuit. The operating method of the fingerprint recognition device further includes: storing the sensing signals of the digital format in a second storage device of the host circuit. 
     In an embodiment of the invention, the operating method of the fingerprint recognition device further includes resetting the sensor pixels before the first exposure period and after the second exposure period, and scanning the sensor pixels to drive the sensor pixels to output sensing signals. 
     To make the aforementioned more comprehensible, several embodiments accompanied with drawings are described in detail as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
         FIG.  1    is a schematic diagram of a fingerprint recognition device according to an embodiment of the invention. 
         FIG.  2    is a schematic diagram of a readout circuit according to an embodiment of the invention. 
         FIG.  3    is a schematic diagram of a fingerprint recognition device according to another embodiment of the invention. 
         FIG.  4    is a waveform diagram of signals for operating the fingerprint recognition device of  FIG.  3    according to an embodiment of the invention. 
         FIG.  5    is a schematic diagram of a sensor pixel according to an embodiment of the invention. 
         FIG.  6    is a waveform diagram of control signals and sensing signals of the sensor pixel of  FIG.  5    according to an embodiment of the invention. 
         FIG.  7    illustrates a fingerprint sensing and recognition process according to an embodiment of the invention. 
         FIG.  8    illustrates a fingerprint sensing and recognition process according to another embodiment of the invention. 
         FIG.  9    illustrates a fingerprint sensing and recognition process according to another embodiment of the invention. 
         FIG.  10    illustrates a fingerprint sensing and recognition process according to another embodiment of the invention. 
         FIG.  11    is a schematic block diagram illustrating an electronic circuit depicted according to an embodiment of the invention. 
         FIG.  12    is a schematic diagram of a fingerprint recognition device according to another embodiment of the invention. 
         FIG.  13    illustrates a fingerprint sensing and recognition process of  FIG.  12    according to an embodiment of the invention. 
         FIG.  14    is a waveform diagram of signals for operating the fingerprint recognition device of  FIG.  12    according to an embodiment of the invention. 
         FIG.  15    illustrates a signal transmission operation between the readout circuit and the host circuit according to an embodiment of the invention. 
         FIG.  16    is a waveform diagram of signals transmitted between the readout circuit and the host circuit of  FIG.  15    according to an embodiment of the invention. 
         FIG.  17    is a flowchart illustrating an operating method of a fingerprint recognition device according to an embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Embodiments are provided below to describe the disclosure in detail, though the disclosure is not limited to the provided embodiments, and the provided embodiments can be suitably combined. The term “coupling/coupled” or “connecting/connected” used in this specification (including claims) of the application may refer to any direct or indirect connection means. For example, “a first device is coupled to a second device” should be interpreted as “the first device is directly connected to the second device” or “the first device is indirectly connected to the second device through other devices or connection means.” In addition, the term “signal” can refer to a current, a voltage, a charge, a temperature, data, electromagnetic wave or any one or multiple signals. 
       FIG.  1    is a schematic diagram of a fingerprint recognition device according to an embodiment of the invention. Referring to  FIG.  1   , the fingerprint recognition device  100  of the present embodiment includes a sensor panel  110 , a readout circuit  120  and a host circuit  130 . The sensor panel  110  and the readout circuit  120  may serve as a fingerprint sensing device  200  to sense fingerprint images and output sensing data D 1  to the host circuit  130 . The sensing data D 1  may include fingerprint image information for fingerprint recognition. 
     To be specific, the sensor panel  110  is configured to output sensing signals S 1  to the readout circuit  120 . The readout circuit  120  is coupled to the sensor panel  110  via sensing lines, for example. The readout circuit  120  is configured to read out the sensing signals S 1  from the sensor panel  110  during a readout period. The host circuit  130  is coupled to the readout circuit  120 . The host circuit  130  is configured to perform a fingerprint recognition operation, e.g. an unlock operation of an electronic device, according to at least one fingerprint image. For example, the host circuit  130  may be an application processor of a smartphone and configured to perform an unlock operation of the smartphone according to a correct fingerprint image. 
     In an embodiment, the fingerprint recognition device  100  may be an electronic device having a display function, a touch sensing function and/or a fingerprint sensing function. In an embodiment, the fingerprint recognition device  100  may be, but not limited to, a smartphone, a non-smart phone, a wearable electronic device, a tablet computer, a personal digital assistant, a notebook and other portable electronic devices that can operate independently and have the display function, the touch sensing function and the fingerprint sensing function. In an embodiment, the fingerprint recognition device  100  may be, but not limited to, a portable or un-portable electronic device in a vehicle intelligent system. In an embodiment, the fingerprint recognition device  100  may be, but not limited to, intelligent home appliances such as, a television, a computer, a refrigerator, a washing machine, a telephone, an induction cooker, a table lamp and so on. 
       FIG.  2    is a schematic diagram of a readout circuit according to an embodiment of the invention. Referring to  FIG.  2   , the readout circuit  120  of the present embodiment includes an analog-to-digital converter (ADC) circuit  122  and a driver circuit  124 . 
     To be specific, the ADC circuit  122  is coupled to the sensor panel  110  via the sensing lines. The ADC circuit  122  is configured to receive the sensing signals S 1  from the sensor panel  110 , convert the sensing signals S 1  from an analog format to a digital format, and output the sensing signals in the digital format, e.g. the sensing data D 1 . The driver circuit  124  is coupled to the sensor panel  110 . The driver circuit  124  is configured to reset sensor pixels of the sensor panel  110 , and scan the sensor pixels to drive the sensor pixels to output the sensing signals S 1 . 
     In an embodiment, the readout circuit  120  may further include a digital circuit, an analog front end (AFE) circuit and/or other functional circuits for performing a fingerprint sensing operation. In addition, enough teaching, suggestion, and implementation illustration for hardware structures of the ADC circuit  122  and the driver circuit  124  can be obtained with reference to common knowledge in the related art. 
       FIG.  3    is a schematic diagram of a fingerprint recognition device according to another embodiment of the invention.  FIG.  4    is a waveform diagram of signals for operating the fingerprint recognition device of  FIG.  3    according to an embodiment of the invention. Referring to  FIG.  3    and  FIG.  4   , the sensor panel  110  includes a plurality of sensor pixels  112  arranged in an array. The sensor pixels  112  include optical fingerprint sensors to perform the fingerprint sensing operation. The sensor pixels  112  are configured to output the sensing signals S 1  to the readout circuit  120  via sensing lines RO[ 1 ] and RO[ 2 ] to RO[M], where M is an integer larger than 2. For example, the sensor pixels  112  output the sensing signals S 1  to respective AFE circuits  126 . 
     In the present embodiment, the readout circuit  120  further includes a plurality of AFE circuits  126 . The AFE circuits  126  are coupled to respective pixel columns. The AFE circuits  126  are configured to receive the sensing signals S 1  from the sensor pixels  112 . The AFE circuit  122  may be include an analog circuit and/or a digital analog hybrid circuit, and it is responsible for performing many tasks, including signal capture, analog filtering, power amplification, etc. The AFE circuits  126  output the processed sensing signals S 1  of the analog format to the ADC circuit  122 . In addition, enough teaching, suggestion, and implementation illustration for hardware structures of the AFE circuit  126  can be obtained with reference to common knowledge in the related art. 
     The driver circuit  124  is coupled to the sensor pixels  112  via first driving lines RG[ 1 ] and RG[ 2 ] to RG[N] and second driving lines WG[ 1 ] and WG[ 2 ] to WG[N], where N is an integer larger than 2. The driver circuit  124  is configured to reset the sensor pixels  112  and scan the sensor pixels  112  to drive the sensor pixels  112  to output sensing signals S 1 . 
     The driver circuit  124  includes a gate driving circuit  310  and a scan circuit  320 . The gate driving circuit  310  is coupled to the scan circuit  320 . The scan circuit  320  is coupled to the sensor pixels  112  via the first driving lines RG[ 1 ] and RG[ 2 ] to RG[N] and the second driving lines WG[ 1 ] and WG[ 2 ] to WG[N]. In the present embodiment, the scan circuit  320  is disposed on the sensor panel  110 , and the gate driving circuit  310  is disposed in the readout circuit  120 , but the invention is not limited thereto. In an embodiment, the scan circuit  320  may be disposed in the readout circuit  120 . 
     The gate driving circuit  310  is configured to drive the scan circuit  320  to perform a reset operation and a scan operation. For example, the gate driving circuit  310  may output a control signal Ctrl_ 1  to drive the scan circuit  320 . The control signal Ctrl_ 1  includes start pulse signals STV 1  and STV 2  and clock signals CKV 1  and CKV 2 . 
     The scan circuit  320  is configured to reset the sensor pixels  112  and scan the sensor pixels  112  to drive the sensor pixels  112  to output the sensing signals S 1  according to the start pulse signals STV 1  and STV 2  and the clock signals CKV 1  and CKV 2 . 
     To be specific, the gate driving circuit  310  includes a first driving circuit  312  and a second driving circuit  314 , and the scan circuit  320  includes a reset circuit  322  and a writing circuit  324 . The reset circuit  322  and the writing circuit  324  may be gate on array (GOA) circuits disposed on the sensor panel  110 . The reset circuit  322  is coupled to the sensor pixels  112  via the first driving lines RG[ 1 ] and RG[ 2 ] to RG[N]. The writing circuit  324  is coupled to the sensor pixels  112  via the second driving lines WG[ 1 ] and WG[ 2 ] to WG[N]. 
     The first driving circuit  312  outputs the start pulse signal STV 1  and the clock signal CKV 1  to the reset circuit  322 . The reset circuit  322  outputs reset signals  410  to the first driving lines RG[ 1 ] and RG[ 2 ] to RG[N] according to the start pulse signal STV 1  and the clock signal CKV 1  during a reset period, so as to reset the sensor pixels  112  row by row. 
     The second driving circuit  314  outputs the start pulse signal STV 2  and the clock signal CKV 2  to the writing circuit  324 . The writing circuit  324  outputs readout signals  420  to the second driving lines WG[ 1 ] and WG[ 2 ] to WG[N] according to the start pulse signal STV 2  and the clock signal CKV 2  during a readout period, such that the readout circuit  120  reads out the sensing signals S 1  from the sensor pixels  112  row by row. 
     Taking the first pixel row of the sensor pixels  112  for example, the reset signal  410 _ 1  resets the first pixel row of the sensor pixels  112 , and then the sensor pixels  112  located on the first pixel row are exposed to sense fingerprint images. Next, the readout signal  420 _ 1  drives the first pixel row of the sensor pixels  112  to output the sensing signals S 1  to the readout circuit  120 . A time interval between a rising edge of the reset signal  410 _ 1  and a rising edge of the readout signal  420 _ 1  is an exposure period, as depicted in  FIG.  4   . The exposure period means the time span for which the sensor pixels  112  are exposed to the light so as to record the fingerprint images. 
       FIG.  5    is a schematic diagram of a sensor pixel according to an embodiment of the invention.  FIG.  6    is a waveform diagram of control signals and sensing signals of the sensor pixel of  FIG.  5    according to an embodiment of the invention. Referring to  FIG.  5    and  FIG.  6   , the sensor pixel  112  operates between a first operating voltage SVDD_O and a second operating voltage SVSS_O. The reset signal  610  transmitted on the first driving lines RG[N] turns on a transistor T 1  during a reset period, and thus a voltage at a node N 1  is reset by the first operating voltage SVDD_O. After the reset period, a photodiode PD is exposed to the light so as to record fingerprint images during an exposure period. After the exposure period, the readout signal  620  transmitted on the second driving lines WG[N] turns on a transistor T 2  during a readout period, and thus the voltage at the node N 1  is transmitted to a node N 2 . The voltage at the node N 2  serves as the sensing signal S 1  and is outputted to the readout circuit  120  via the sensing line RO[M]. 
     In the present embodiment, the readout signal  620  includes a plurality of readout pulses  610 _ 1 ,  610 _ 2  and  610 _ 3 , such that the voltage at the node N 2  can be readout at different time points after different exposure periods ET 1 , ET 2  and ET 3 .  FIG.  6    illustrates the sensing signal S 1  has different discharging rates when the photodiode PD is exposed to different light intensity. For example, the sensing signal S 1  has a slow discharging rate as a slope  631  when the photodiode PD does not sense light or senses very weak light. The sensing signal S 1  has a fast discharging rate as a slope  633  when the photodiode PD senses strong light. In addition, the sensing signal S 1  has a discharging rate between the slops  631  and  633 , such as a slope  632 , when the photodiode PD senses normal light. 
       FIG.  7    illustrates a fingerprint sensing and recognition process according to an embodiment of the invention. Referring to  FIG.  3    and  FIG.  7   , after the gate driving circuit  310  controls the scan circuit  320  to perform the first reset operation S 710  on the sensor pixels  112 , the sensor pixels  112  are continuously exposed. Based on different exposure time length settings, the sensing signals S 1  are read out at the first readout time point t 1  and converted into digital signals by the ADC circuit  122 , and then the sensing signals S 1  can be read out again at least at the second readout time point t 2  and converted into digital signals by the ADC circuit  122  without performing a second reset operation. The sensing signals S 1  of the digital format include a first data and a second data. The first data corresponds to a first fingerprint image, and the second data corresponds to a second fingerprint image. In addition, a third fingerprint image may be obtained at the third readout time point t 3  in a similar manner. 
     As illustrated in  FIG.  7   , the readout circuit  120 , e.g. a readout integrated circuit (ROIC), reads out the sensing signals S 1  at three readout time points t 1 , t 2  and t 3  respectively (operation S 720 ) and converts the sensing signals S 1  into digital signals by the ADC circuit  122  (operation S 730 ), but only performs one reset operation. That is to say, the readout circuit  120  reads out the sensing signals S 1  from the sensor pixels  112  after a first exposure period ET 1  to obtain the first fingerprint image, and read out the sensing signals S 1  after a second exposure period ET 2  to obtain the second fingerprint image. The sensor pixels  112  are reset before the first exposure period ET 1  and after the second exposure period ET 2 , or even after the third exposure period ET 3 . The reset operation are not performed between the first readout time point tl and the third readout time point t 3 . 
     The digital fingerprint image signals of multiple fingerprint images are all stored, which can be stored in the readout circuit  120  or the host circuit  130  (operation S 740 ). For example, referring to  FIG.  1    and  FIG.  2   , the readout circuit  120  may include a first storage device (not shown) coupled to the analog-to-digital circuit  126 . The first storage device is configured to receive and store the sensing signals S 1  of the digital format. Alternatively, the host circuit  130  may include a second storage device (not shown) coupled to the readout circuit  120 . The second storage device is configured to receive and store the sensing signals S 1  of the digital format. 
     Next, the host circuit  130  can determine which read data to be used for the fingerprint recognition operation S 750 . For example, the host circuit  130  can perform the fingerprint recognition operation S 750  according to the first fingerprint image, the second fingerprint image and/or the third fingerprint image. In other words, even in a strong light environment or a dry finger sensing condition, the fingerprint recognition device  100  does not need to run a complete fingerprint sensing cycle including a reset operation every time, and thus the unlocking time is saved. 
       FIG.  8    illustrates a fingerprint sensing and recognition process according to another embodiment of the invention. As illustrated in  FIG.  8   , the host circuit  130  performs the fingerprint recognition operation S 840  based on the second fingerprint image. The fingerprint recognition operation S 840  includes a calculation operation and an unlock operation. The second fingerprint image is obtained according to the fingerprint sensing operation S 840  with the exposure period ET 2  and a preset ADC setting (a second setting). The ADC setting is, for example, an input voltage range of the ADC circuit  122 . 
     The obtained second fingerprint image is used for the fingerprint recognition (operation S 840 ). The readout circuit  120  reads out the sensing signals S 1  and converts the sensing signals S 1  into a digital fingerprint image signal not only after the exposure period ET 2 , but also after the other two exposure time lengths, i.e. the exposure periods ET 1  and ET 3 . The exposure time length of the exposure period ET 1  is shorter than the exposure time length of the exposure period ET 2 , and the exposure time length of the exposure period ET 3  is longer than the exposure time length of the exposure period ET 2 . 
     In detail, as illustrated in operation S 810 , when the exposure period ETI elapses from a specified pixel row in the sensor panel  110  after the reset operation of the specified pixel row is completed, the readout circuit  120  reads out the sensing signals S 1  from the specified pixel row for the first time, and then the sensing signals S 1  are converted by the ADC circuit  122  having a first setting to generate the first read data. The first read data is stored in the first storage device, e.g. a Static Random Access Memory (SRAM), of the readout circuit  120  and transmitted to the host circuit  130  through an interface, e.g. a Serial Peripheral Interface (SPI), and stored in the second storage device of the host circuit  130 . A complete fingerprint image signal can be obtained by performing the above reset-exposure-readout operations row by row on the sensor panel  110 . The digital fingerprint image signal transmission from the readout circuit  120  to the host circuit  130  can be transmitted to the host circuit  130  row by row in real time from the readout circuit  120 , or a complete fingerprint image is stored on the readout circuit  120  and then transmitted to the host circuit  130 . 
     When the readout circuit  120  reads out the sensing signals S 1  of the specified pixel row for the first time and converts the sensing signal S 1  with the ADC circuit  122 , the specified pixel row continues to be exposed. As illustrated in operation S 820 , when the exposure period ET 2  elapses from the specified pixel row in the sensor panel  110  after the reset operation of the specified pixel row is completed, the readout circuit  120  reads out the sensing signals S 1  from the specified pixel row for the second time, and then the sensing signals S 1  are converted by the ADC circuit  122  having the second setting to generate the second read data. The second read data is stored in the first storage device of the readout circuit  120  and transmitted to the host circuit  130  through the interface, and stored in the second storage device of the host circuit  130 . 
     When the readout circuit  120  reads out the sensing signals S 1  of the specified pixel row for the second time and converts the sensing signal S 1  with the ADC circuit  122 , the specified pixel row continues to be exposed. Next, as illustrated in operation S 830 , the readout circuit  120  reads out the sensing signal S 1  of the specified pixel row for the third time after the exposure period ET 3  and converts the sensing signal S 1  through the ADC circuit  122  having a third setting to generate the third read data, which is transmitted to the host circuit  130 , and then stored in the second storage device of the host circuit  130 . 
     It should be noted that since the readout is performed row by row and each pixel row is read out through the same fingerprint sensing lines, the time interval between the end points of each exposure period must be long enough. After the fingerprint image obtained by the exposure period ET 1  is read out row by row, the fingerprint image obtained by the exposure period ET 2  can be read row by row. 
     In the embodiment of  FIG.  8   , the host circuit  130  performs the fingerprint recognition operation according to the second fingerprint image, which is obtained by fingerprint sensing with the exposure period ET 2  and the second ADC setting. Under normal circumstances (no over-exposure, or no dry finger touching), the fingerprint recognition operation by the host circuit  130  based on the second fingerprint image obtained by the exposure period ET 2  should be able to unlock the fingerprint recognition device  100  after the calculation operation. If the fingerprint recognition device  100  can be successfully unlocked, the first fingerprint image obtained by the exposure period ET 2  can be discarded, and the third fingerprint image obtained according to the exposure period ET 3  is not required to be used and sent to the host circuit  130 . 
     In the present embodiment, the ADC circuit  122  has different parameter settings for different lengths of exposure periods, and the host circuit  130  is configured to set the parameter settings of the ADC circuit  122  according to the lengths of the exposure periods. 
     To be specific, depending on the exposure period, the discharge range of the optical fingerprint sensor will be different, that is, the voltage range of the read sensing signal will be different, and the input voltage range of the ADC circuit will be different. For example, after a long time of discharge, the output voltage of the fingerprint sensor read through the sensing line may drop from 5V to 1V. If the ADC setting is to process the input signal with a voltage range of 0-2V, the ADC circuit  122  can accurately capture the input sensing signal and convert it into a digital data. 
     However, if the discharge time is not enough, the output voltage of the fingerprint sensor only drops from 5V to 4V (outside the voltage range of 0-2V), and then the ADC circuit  122  cannot accurately capture the input sensing signal according to the previous setting. In order to enable the processing input voltage range of the ADC circuit to include the sensed signal level read out, it is necessary to adjust the setting of the ADC. In other words, different exposure time lengths are required to correspond to different ADC settings. The setting of the ADC circuit can be determined by the host circuit  130 , and the host circuit  130  can store several groups of ADC settings in advance. 
       FIG.  9    illustrates a fingerprint sensing and recognition process according to another embodiment of the invention. In the embodiment of  FIG.  9   , the host circuit  130  performs the fingerprint recognition operation according to the second fingerprint image obtained by fingerprint sensing with the exposure period ET 2  and the second ADC setting. However, in a strong light environment, the second fingerprint image may be an overexposed image, which causes the host circuit  130  to perform the fingerprint recognition operation according to the fingerprint image obtained by exposure period ET 2 , but it is found that the fingerprint recognition device  100  cannot be unlocked after the calculation operation. At this time, the host circuit  130  may unlock the fingerprint recognition device  100  by changing another fingerprint image. As illustrated in  FIG.  9   , the host circuit  130  uses the first fingerprint image obtained according to the exposure period ET 1  for the fingerprint recognition operation, and after the calculation operation is completed, the unlocked operation is successful. The third fingerprint image obtained according to the exposure period ET 3  does not need to be used. 
     As illustrated in  FIG.  9   , it is assumed that the exposure period ET 1  is  110  milliseconds (ms), the exposure period ET 2  is 150 ms, and the time required for the calculation operation is 100 ms. In this case, it takes 150+100+110+100=460 ms to unlock using the traditional method. 
     However, the unlocking time using the method of the embodiment is only 150+100+100=350 ms. Alternatively, in other embodiments, when the host circuit  130  performs the fingerprint recognition operation according to the second fingerprint image obtained by the exposure period ET 2 , and it is found that the fingerprint recognition device  100  cannot be unlocked, the host circuit  130  may still maintain the second fingerprint image, and perform an image process on the first fingerprint image and the second fingerprint image to obtain a post-processed fingerprint image containing two image components, which is then used for the fingerprint recognition operation. The noise in the post-processed fingerprint image may be reduced, thereby improving the probability of successful unlocking. 
     If the color information of the image data (for example, 12 bits) of the complete fingerprint image exceeds a certain percentage (for example, 40%) and is above the maximum value (for example, 4095), it can be determined that the fingerprint image is overexposed. In addition, the data read from the fingerprint sensor array can be stored in the second memory of the host circuit  130 , or registered in the first memory of the readout circuit  120 , and then stored the second memory of the host circuit  130  through the interface when the host circuit  130  needs the data. 
       FIG.  10    illustrates a fingerprint sensing and recognition process according to another embodiment of the invention. In the embodiment of  FIG.  10   , the host circuit  130  performs the fingerprint recognition operation according to the second fingerprint image obtained by fingerprint sensing with the exposure period ET 2  and the second ADC setting. The readout circuit  120  reads out the sensing signals S 1  and converts the sensing signals S 1  into a digital fingerprint image signal not only after the exposure period ET 2 , but also after the other two exposure time lengths, i.e. the exposure periods ET 1  and ET 3 . 
     In the embodiment of  FIG.  10   , the host circuit  130  performs the fingerprint recognition operation based on the second fingerprint image obtained by fingerprint sensing with the exposure period ET 2  and the second ADC setting. However, in a dry environment, the surface of the finger shrinks, and thus the resolution between peaks and valleys is not good in a fingerprint image, so that the host circuit  130  cannot unlocked the fingerprint recognition device  100  based on the second fingerprint image obtained by the exposure period ET 2 . The third fingerprint image obtained according to the exposure period ET 3  (longer than the exposure period ET 2 ) is used for the fingerprint recognition operation, and the fingerprint recognition device  100  may be unlocked successfully after the calculation operation. 
     As illustrated in  FIG.  10   , it is assumed that the exposure period ET 2  is 150 ms, the exposure period ET 3  is 200 ms, and the time required for the calculation operation is 100 ms. In this case, it takes 150+100+200+100=550 ms to unlock using the traditional method. However, the unlocking time using the method of the embodiment is only 150+100+100=350 ms. In other embodiments, when the host circuit  130  performs the fingerprint recognition operation according to the second fingerprint image obtained by the exposure period ET 2 , and it is found that the fingerprint recognition device  100  cannot be unlocked, the host circuit  130  may perform an image process on the third fingerprint image and at least one image obtained earlier, i.e. at least one of the first fingerprint image and the second fingerprint image, to obtain a post-processed fingerprint image containing (more than) two image components, which is then used for the fingerprint recognition operation. The noise in the post-processed fingerprint image may be reduced, thereby improving the probability of successful unlocking. 
     It should be noted that, although it takes three different exposure time lengths as an example in the above-mentioned embodiments, two readout time points can be determined based on only two different exposure time lengths in other embodiments of the disclosure, so as to perform the readout operation twice in a time-division manner, and store the obtained digital data. That is to say, the number of readings can be designed according to actual requirements. In the embodiment where the two readout time points are determined based on two different exposure time lengths, the two different exposure time lengths can be set to be suitable for operation in a normal environment and operation in a strong light environment. 
       FIG.  11    is a schematic block diagram illustrating an electronic circuit depicted according to an embodiment of the invention. Referring to  FIG.  11   , the electronic circuit  510  may is adapted to drive a display panel having a display function, a touch sensing function and a fingerprint sensing function. The electronic circuit  510  includes a display driving circuit  512 , a touch sensing circuit  514 , and a fingerprint sensing circuit  516 . 
     The display driving circuit  512  is configured to drive and control display pixels of the display panel to display images. The display driving circuit  512  generates display driving signals for driving the display panel  120 . The display driving circuit  512  may include a timing controller  521 , a display driver and other functional circuits for a display operation. The display driving circuit  512  may also include other controllers or processors  523  for other control activities of the display operation. 
     The touch sensing circuit  514  is configured to drive and control touch sensors of the display panel to sense the touch event of the display panel. The touch sensing circuit  514  may include a touch controller  541 , an AFE circuit, an ADC circuit and other functional circuits for a touch sensing operation. 
     The fingerprint sensing circuit  516  is configured to drive and control fingerprint sensors of the display panel, e.g. the sensor pixels  112  of  FIG.  3   , to sense fingerprint images on the display panel via the first driving lines RG[ 1 ] and RG[ 2 ] to RG[N], the second driving lines WG[ 1 ] and WG[ 2 ] to WG[N], and the sensing lines RO[ 1 ] and RO[ 2 ] to RO[M]. The readout circuit  120  receives the sensing signals S 1  corresponding to fingerprint images from the sensor pixels  112 . The fingerprint sensing circuit  516  may include the readout circuit  120 . The readout circuit  120  is implemented in a semiconductor chip which has having a display driving function, a touch sensing function and a fingerprint sensing function. The fingerprint sensing circuit  516  may also process the sensing signals S 1  to obtain the fingerprint images. The fingerprint sensing circuit  516  may include a digital circuit, an AFE circuit, an ADC circuit and other functional circuits for the fingerprint sensing operation. 
     In an embodiment, when the electronic circuit  510  is implemented as a single chip integrated circuit that can drive and control the display panel to perform the display operation, the touch sensing operation and the fingerprint sensing operation, the electronic circuit  510  may include a control circuit. The control circuit  530  may be a micro-controller based core to perform all of control activities of the display operation, the touch sensing operation and the fingerprint sensing operation. The control circuit  530  may include at least one of the timing controller  521 , the touch controller  541 , the digital circuit of the fingerprint sensing circuit  516 , and the other controllers or processors  523  of the display driving circuit  512 . 
     The display driving circuit  512 , the touch sensing circuit  514  and the fingerprint sensing circuit  516  communicate with one another via signal transmission interfaces, such as Mobile Industry Processor Interface (MIPI), Inter-Integrated Circuit (I2C) Interface, Serial Peripheral Interface (SPI) and/or other similar or suitable interfaces. 
     Regarding hardware structures of the components in the embodiment of  FIG.  3   , the timing controller  521 , the touch controller  541  and the digital circuit  561  may be a processor having computational capability. Alternatively, the timing controller  521 , the touch controller  541  and the digital circuit  561  may be designed through hardware description languages (HDL) or any other design methods for digital circuits familiar to people skilled in the art and may be hardware circuits implemented through a field programmable gate array (FPGA), a complex programmable logic device (CPLD), or an application-specific integrated circuit (ASIC). 
     In addition, enough teaching, suggestion, and implementation illustration for hardware structures of the display driving circuit  512 , the touch sensing circuit  514  and the fingerprint sensing circuit  516  can be obtained with reference to common knowledge in the related art, which is not repeated hereinafter. 
       FIG.  12    is a schematic diagram of a fingerprint recognition device according to another embodiment of the invention. Referring to  FIG.  12   , the fingerprint recognition device  100  of the present embodiment includes the sensor panel  110 , the electronic circuit  510  and the host circuit  130 . The sensor panel  110  has the display function, the touch sensing function and the fingerprint sensing function. The electronic circuit  510  is configured to drive and control the sensor panel  110  to perform the display operation, the touch sensing operation and the fingerprint sensing operation. The sensor panel  110  and the electronic circuit  510  may serve as the fingerprint sensing device  200  to sense fingerprint images and output the sensing data D 1  to the host circuit  130 . 
     In the present embodiment, a scan circuit  526  may be a GOA circuit disposed on the sensor panel  110  for driving a pixel array  114  to perform the display operation. The electronic circuit  510  drives and controls an operation of the scan circuit  526 . In an embodiment, the scan circuits  320  and  526  may be disposed in the electronic circuit  510 . 
       FIG.  13    illustrates a fingerprint sensing and recognition process of  FIG.  12    according to an embodiment of the invention.  FIG.  14    is a waveform diagram of signals for operating the fingerprint recognition device of  FIG.  12    according to an embodiment of the invention. 
     Referring to  FIG.  12    to  FIG.  14   , display scanning operations S 130  are performed between the first reset operation S 710  and the read operation S 720 _ 1 , or between two of the read operations S 720 _ 1 , S 720 _ 2  and S 730 _ 3 . In  FIG.  14   , the scan circuit  526  may scan the pixel array  114  via driving lines DG[ 1 ] and DG[ 2 ] to DG[N] according to a start pulse signal STV 3  and a clock signal CKV 3  after the reset operation of the pixel array  114  is completed. 
       FIG.  15    illustrates a signal transmission operation between the readout circuit and the host circuit according to an embodiment of the invention.  FIG.  16    is a waveform diagram of signals transmitted between the readout circuit and the host circuit of  FIG.  15    according to an embodiment of the invention. 
     Referring to  FIG.  15    and  FIG.  16   , an interface  121  between the readout circuit  120  and the host circuit  130  may be a serial peripheral interface (SPI), but the invention is not limited thereto. When the readout circuit  120  finishes receiving the data from the display panel  110  via the AFE circuit  126  of  FIG.  3   , a trigger circuit  128  outputs a signal S 2  to trigger the host circuit  130  to inform that host circuit  130  can start to fetch data. In an embodiment, the data received from the display panel  110  may be stored in first storage device  123 , e.g. SRAM. Next, the host circuit  130  pulls a voltage V 1  at a signal line SPI CS to be low, and outputs a clock signal CLK via a signal line SPI_CLK. At the same time, the host circuit  130  outputs the read command COM 1  on a signal line SPI_DI to the readout circuit  120 , and the readout circuit  120  uploads the data D 1  on a signal line SPI_DO. Therefore, the host circuit  130  can start receiving data from the readout circuit  120 . 
       FIG.  17    is a flowchart illustrating an operating method of a fingerprint recognition device according to an embodiment of the invention. Referring to  FIG.  1    and  FIG.  17   , the operating method of the present embodiment is at least adapted to the fingerprint recognition device  100  depicted in  FIG.  1   , but the disclosure is not limited thereto. 
     Taking the fingerprint recognition device  100  for example, in step S 100 , the readout circuit  120  reads out the sensing signals S 1  from the sensor pixels  110  after the first exposure period ET 1  to obtain a first fingerprint image. In step S 110 , the readout circuit  120  reads out the sensing signals S 1  from the sensor pixels  110  after the second exposure period ET 2  to obtain a second fingerprint image. In step S 120 , the host circuit  130  performs a fingerprint recognition operation according to the first fingerprint image or the second fingerprint image. The sensor pixels  112  are reset before the first exposure period ET 1  and after the second exposure period ET 2 . 
     The operating method of the fingerprint recognition device described in the embodiment of the invention is sufficiently taught, suggested, and embodied in the embodiments illustrated in  FIG.  1    to  FIG.  16   , and therefore no further description is provided herein. 
     In summary, in the embodiments of the invention, the readout circuit reads out the sensing signals at some readout time points respectively and converts the sensing signals into digital signals by the ADC circuit, but only performs one reset operation. The ADC circuit has different parameter settings for different lengths of exposure periods. The digital fingerprint image signals of multiple fingerprint images are all stored, which can be stored in the readout circuit or the host circuit. The host circuit can determine which read data to be used for the fingerprint recognition operation. Therefore, even in a strong light environment or a dry finger sensing condition, the fingerprint recognition device does not need to run a complete fingerprint sensing cycle including a reset operation every time, and thus the unlocking time is saved. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the disclosure covers modifications and variations provided that they fall within the scope of the following claims and their equivalents.