CMOS image sensor with shared multiplexer and method of operating the same

A CMOS image sensor includes a pixel array unit, a row selection unit, and a logic circuit. The pixel array unit is used for sensing an object. The pixel array unit includes M pixels and P multiplexers and each of the M pixels is electrically connected to one of the P multiplexers, wherein M is a positive integer and P is a positive integer smaller than M. The row selection unit and the logic circuit are electrically connected to the P multiplexers. The row selection unit is used for generating a row selection signal. The logic circuit is used for determining a sensing region corresponding to the object wherein the sensing region includes N of the M pixels. Furthermore, the logic circuit controls Q multiplexers, which are electrically connected to the N pixels, to transmit the row selection signal to the N pixels.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a complementary metal oxide semiconductor (CMOS) image sensor and a method of operating the same and, more particularly, to a CMOS image sensor and a method of operating the same capable of improving fabrication yield rate through self-calibration.

2. Description of the Prior Art

Since image sensors have been developed well and image processing speed has been improved, people have paid much attention to optical touch screen. So far the image sensor is divided into a charge coupled device (CCD) image sensor and CMOS image sensor. In general, the CCD image sensor has less noise and better image quality than the CMOS sensor. However, signal processing circuits of the CMOS image sensor can be integrated into single chip so as to minimize electronic device. Furthermore, the CMOS image sensor has lower power consumption than the CCD image sensor, so it has become more and more popularly.

Referring toFIG. 1,FIG. 1is a schematic diagram illustrating an optical touch screen1of the prior art. As shown inFIG. 1, the optical touch screen1comprises a touch panel10and two CMOS image sensors12and14. The CMOS image sensors12and14are respectively disposed at both sides of the touch panel10. When a user uses an object16, such as finger, stylus, or the like, to operate the touch panel10, the CMOS image sensors12and14will sense a projection of the object16. Afterward, if an angle between the projection and the touch position is known and a distance between the two CMOS image sensors12and14can be obtained, a coordinate of the touch position can be then calculated.

Referring toFIGS. 2 and 3,FIG. 2is a schematic diagram illustrating a moving trajectory160of the object16projected on the CMOS image sensor12shown inFIG. 1, andFIG. 3is a schematic diagram illustrating a moving trajectory160′ of the object16projected on the CMOS image sensor12shown inFIG. 1. For example, if the CMOS image sensor12is attached to the touch panel10without deviation or obliqueness, the moving trajectory160of the object16, which is projected on a pixel array unit120of the CMOS image sensor12, has a rectangular shape, as shown inFIG. 2. However, if the CMOS image sensor12is attached to the touch panel10with deviation or obliqueness due to assembly tolerance, the moving trajectory160′ of the object16, which is projected on the pixel array unit120of the CMOS image sensor12, has an oblique shape, as shown inFIG. 3. If the moving trajectory160′ of the object16is oblique, the read-out circuit122needs to read more pixel data for following algorithm so as to eliminate the influence of assembly tolerance. Consequently, operating frequency and power consumption of the system will increase a lot.

SUMMARY OF THE INVENTION

An objective of the invention is to provide a CMOS image sensor and a method of operating the same capable of improving fabrication yield rate through self-calibration so as to solve the aforesaid problems.

According to an embodiment of the invention, the CMOS image sensor comprises a pixel array unit, a row selection unit and a logic circuit. The pixel array unit is used for sensing an object. The pixel array unit comprises M pixels and P multiplexers. Each of the M pixels is electrically connected to one of the P multiplexers wherein M is a positive integer and P is a positive integer smaller than M. The row selection unit and the logic circuit are electrically connected to the P multiplexers. The row selection unit is used for generating a row selection signal. The logic circuit is used for determining a sensing region corresponding to the object. The sensing region comprises N of the M pixels and N is a positive integer smaller than or equal to M. The logic circuit further controls Q multiplexers, which are electrically connected to the N pixels, to transmit the row selection signal to the N pixels wherein Q is a positive integer smaller than or equal to N and smaller than or equal to P.

According to another embodiment of the invention, the method of operating the CMOS image sensor comprises steps of: sensing an object by a pixel array unit wherein the pixel array unit comprises M pixels and P multiplexers, each of the M pixels is electrically connected to one of the P multiplexers, M is a positive integer, and P is a positive integer smaller than M; determining a sensing region corresponding to the object wherein the sensing region comprises N of the M pixels and N is a positive integer smaller than or equal to M; generating a row selection signal; and controlling Q multiplexers, which are electrically connected to the N pixels, to transmit the row selection signal to the N pixels wherein Q is a positive integer smaller than or equal to N and smaller than or equal to P.

According to another embodiment of the invention, the CMOS image sensor comprises a pixel array unit, a row selection unit, a read-out circuit and a logic circuit. The read-out circuit and the row selection unit are electrically connected to the pixel array unit. The logic circuit is electrically connected to the read-out circuit. The pixel array unit is used for sensing an object and comprises M pixels, wherein M is a positive integer. The row selection unit is used for generating a row selection signal wherein the row selection signal controls the M pixels to output signals. The read-out circuit is used for reading signals generated by the M pixels. The logic circuit is used for determining a sensing region corresponding to the object wherein the sensing region comprises N of the M pixels and N is a positive integer smaller than or equal to M. Afterward, the logic circuit determines a first pixel and a last pixel for each row within the sensing region and controls the read-out circuit to read the first pixel through the last pixel of each row in row-major order, so as to output signals generated by the N pixels.

According to another embodiment of the invention, the method of operating the CMOS image sensor comprises steps of: sensing an object by a pixel array unit wherein the pixel array unit comprises M pixels and M is a positive integer; determining a sensing region corresponding to the object wherein the sensing region comprises N of the M pixels and N is a positive integer smaller than or equal to M; generating a row selection signal for controlling the M pixels to output signals; determining a first pixel and a last pixel for each row within the sensing region; reading the first pixel through the last pixel of each row in row-major order; and outputting signals generated by the N pixels.

Therefore, according to the CMOS image sensor and the method of operating the same mentioned in the above, the invention only needs to output pixel data within the sensing region to eliminate the influence of assembly tolerance, so as to reduce operating frequency and power consumption substantially.

DETAILED DESCRIPTION

Referring toFIG. 4,FIG. 4is a schematic diagram illustrating a CMOS image sensor3according to an embodiment of the invention. As shown inFIG. 4, the CMOS image sensor3comprises a pixel array unit30, a row selection unit32, a logic circuit34and a read-out circuit36. The row selection unit32, the logic circuit34and the read-out circuit36are electrically connected to the pixel array unit30.

The pixel array unit30is used for sensing an object (not shown) or a moving trajectory thereof. In this embodiment, the pixel array unit30comprises M pixels300and P multiplexers320wherein each of the M pixels300is electrically connected to one of the P multiplexers302, M is a positive integer, and P is a positive integer smaller than or equal to M. For further description, if P is equal to M, an amount of the pixels300is the same as that of the multiplexers302and each multiplexer302is electrically connected to a unique pixel300. On the other hand, if P is smaller than M, an amount of the multiplexers302is less than that of the pixels300and each multiplexer302is electrically connected to at least one pixel300. The pixel array unit30shown inFIG. 4comprises the same amount of pixels300and multiplexers302for illustrative purpose. For example, if the pixel array unit30has a 640*480 pixel array and an amount of the pixels300is the same as that of multiplexers302, both M and P are equal to 640*480. In other words, the pixel array unit30comprises 640*480 pixels300and 640*480 multiplexers302. In addition, the pixel300can absorb light reflected from an object and then transform the absorbed light into an electric signal. The pixel300usually consists of transistors and photo diodes. It should be noted that the structure and principle of the pixel300can be easily achieved by one skilled in the art and thus will not be described in detail here.

The row selection unit32receives a time sequence signal and a control signal from a controller (not shown) and then generates a row selection signal. The row selection signal is used for controlling the pixels300of the pixel array unit30to output data. The logic circuit34is used for determining a sensing region corresponding to an object or a moving trajectory sensed by the pixel array unit30. The sensing region comprises N of the M pixels300wherein N is a positive integer smaller than or equal to M. Afterward, the logic circuit34controls Q multiplexers302, which are electrically connected to the N pixels300, to transmit the row selection signal to the N pixels300wherein Q is a positive integer smaller than or equal to N and smaller than or equal to P. For example, in the pixel array unit30shown inFIG. 4, Q is equal to N and smaller than P. The read-out circuit36is used for reading signals generated by the N pixels300of the sensing region.

Referring toFIG. 5,FIG. 5is a schematic diagram illustrating the pixel array unit30shown inFIG. 4having a 3*3 pixel array. The 3*3 pixel array shown inFIG. 5is used, for example, to depict features of the invention. In this embodiment, the pixel array unit30comprises the same amount of pixels300and multiplexers302. That is to say, the aforesaid M and P both are equal to 9. Also, referring toFIG. 6,FIG. 6is a schematic diagram illustrating a circuit of the CMOS image sensor3shown inFIG. 5.

When a user uses an object (not shown), such as finger, stylus or the like, to operate an optical positioning system (not shown) equipped with the CMOS image sensor3, the pixel array unit30will sense the object or a moving trajectory thereof. Afterward, the logic circuit34determining a sensing region304according to the object or the moving trajectory thereof sensed by the pixel array unit30. As shown inFIG. 5, the sensing region304comprises five pixels P1, P2, P5, P6and P9. That is to say, the aforesaid N and Q both are equal to 5. Then, the logic circuit34controls the multiplexers302, which are electrically connected to the aforesaid five pixels, to transmit the row selection signal to the five pixels and enables the read-out circuit36to read signals generated by the five pixels within the sensing region304in row-major order. In other words, the red-out circuit36will read the pixels P2, P6, P1, P5and P9within the sensing region304in order. That is to say, the first row read by the read-out circuit36includes the pixels P2and P6and the second row read by the read-out circuit36includes the pixels P1, P5and P9. In this embodiment, the sensing region304is variable and can be set through self-calibration while booting. Furthermore, when the object or the moving trajectory thereof has a random shape, the logic circuit34may determine the sensing region as a parallelogram including the object or the moving trajectory thereof so as to prevent the following algorithm from getting complicated.

It should be noted that because the sensing region304shown inFIG. 5exceeds a real region of the pixel array unit30, the scanning time may become unstable every time and thus the calculation of exposure time may increase. To solve this problem, the read-out circuit36of the invention can add dummy pixel(s) to the sensing region304while reading pixel data so as to keep the scanning time inconstant every time and simplify the calculation of exposure time. Referring toFIG. 7,FIG. 7is a schematic diagram illustrating the sensing region304shown inFIG. 5with a dummy pixel P0. As shown inFIG. 7, after adding the dummy pixel P0, the pixels within the sensing region304are arranged as a parallelogram and an amount of pixels in each row is the same. Accordingly, the scanning time can be kept in constant every time.

Referring toFIG. 8,FIG. 8is a flowchart illustrating a method of operating the CMOS image sensor according to the invention. Also, referring toFIGS. 4 to 6along with the aforesaid CMOS image sensor3, the method of operating the CMOS image sensor of the invention comprises the following steps.

In the beginning, step S100is performed to sense an object or a moving trajectory thereof by the pixel array unit30. Afterward, step S102is performed to determine the sensing region304corresponding to the object or the moving trajectory thereof. Step S104is then performed to generate a row selection signal. Step S106is then performed to control the multiplexers, which are electrically connected to the pixels P2, P6, P1, P5and P9within the sensing region304, to transmit the row selection signal to the pixels P2, P6, P1, P5and P9. Finally, step S108is performed to read signals generated by the pixels P2, P6, P1, P5and P9of the sensing region304in row-major order.

Referring toFIG. 9,FIG. 9is a flowchart illustrating a method of operating the CMOS image sensor according to another embodiment of the invention. Also, referring toFIG. 7along with the aforesaid CMOS image sensor3, the method of operating the CMOS image sensor of the invention comprises the following steps once the sensing region304exceeds a real region of the pixel array unit30.

In the beginning, step S200is performed to sense an object or a moving trajectory thereof by the pixel array unit30. Afterward, step S202is performed to determine the sensing region304corresponding to the object or the moving trajectory thereof. Step S204is then performed to generate a row selection signal. Step S206is then performed to control the multiplexers, which are electrically connected to the pixels P2, P6, P1, P5and P9within the sensing region304, to transmit the row selection signal to the pixels P2, P6, P1, P5and P9. Step S208is then performed to judge whether the sensing region304exceeds a real region of the pixel array unit30, and step S210is then performed if it is YES, otherwise, step S212is performed. Step210is performed to add the dummy pixel P0to the sensing region304. Step S212is performed to read signals generated by the dummy pixel P0, if any, and the pixels P2, P6, P1, P5and P9of the sensing region304in row-major order.

Referring toFIG. 10,FIG. 10is a schematic diagram illustrating a circuit of a CMOS image sensor3′ according to another embodiment of the invention. As shown inFIG. 10, the CMOS image sensor3′ comprises a pixel array unit30′, a row selection unit32, a logic circuit34and a read-out circuit36wherein the principles of the row selection unit32, the logic circuit34and the read-out circuit36are the same as those mentioned in the above and will not be described in detail here. In this embodiment, the pixel array unit30′ has a 4*5 pixel array. In other words, the pixel array unit30′ comprises twenty pixels P1-P20. Compared to the pixel array unit30shown inFIG. 6, an amount of the multiplexers of the pixel array unit30′ is less than that of the pixels. As shown inFIG. 10, the pixel array unit30′ comprises seventeen multiplexers wherein the pixels P1and P2are electrically connected to one single multiplexer302a, the pixels P3and P4are electrically connected to one single multiplexer302b, and the pixels P11and P12are electrically connected to one single multiplexer302c. That is to say, the invention may utilize one multiplexer to control more than one pixel simultaneously so as to reduce the amount of multiplexers. The amount of pixels, which are electrically to one single multiplexer, can be determined based on practical applications and is not limited to two as shown inFIG. 10. It should be noted that if there are at least two pixels electrically connected to one single multiplexer, the at least two pixels are located at different columns of the pixel array unit30′. As shown inFIG. 10, the pixels P1and P2are located at different columns, the pixels P3and P4are located at different columns, and the pixels P11and P12are located at different columns. Preferably, the pixels, which are electrically connected to one single multiplexer, may be, but not limited to, located at one row of the pixel array unit30′. As shown inFIG. 10, the pixels P1and P2are located at one row, the pixels P3and P4are located at one row, and the pixels P11and P12are located at one row.

Referring toFIGS. 11 and 12,FIG. 11is a schematic diagram illustrating a CMOS image sensor5according to another embodiment of the invention, andFIG. 12is a time sequence diagram illustrating reading order of pixel data. As shown inFIG. 11, the CMOS image sensor5comprises a pixel array unit50, a column selection unit52, a logic circuit54, a read-out circuit56and a frame buffer58. The column selection unit52, the logic circuit54and the read-out circuit56are electrically connected to the pixel array unit50, and the frame buffer58is electrically connected to the read-out circuit56. The 4*3 pixel array shown inFIG. 11is used for illustrative purpose only and the invention is not limited to that manner. The pixels P1-P12can absorb light reflected from an object and then transform the absorbed light into an electric signal. The pixels P1-P12usually consist of transistors and photo diodes. It should be noted that the structure and principle of the pixels P1-P12can be easily achieved by one skilled in the art and thus will not be described in detail here.

The column selection unit52receives a time sequence signal and a control signal from a controller (not shown) and generates a column selection signal. The column selection signal is used for controlling the pixels P1-P12of the pixel array unit50to output data. The logic circuit54is used for determining a sensing region corresponding to an object or a moving trajectory thereof sensed by the pixel array unit50. The read-out circuit56reads signals generated by the pixels of the sensing region in column-major order. Afterward, the frame buffer transforms the output data from column-major order to row-major order.

For example, when a user uses an object (not shown), such as finger, stylus or the like, to operate an optical positioning system (not shown) equipped with the CMOS image sensor5, the pixel array unit50will sense the object or a moving trajectory thereof. Afterward, the logic circuit54determines a sensing region504corresponding to the object or the moving trajectory thereof sensed by the pixel array unit50. As shown inFIG. 11, the sensing region504comprises five pixels P2, P3, P7, P8and P12. It should be noted that because the sensing region504shown inFIG. 11exceeds a real region of the pixel array unit50, the scanning time may become unstable every time and thus the calculation of exposure time may increase. To solve this problem, the read-out circuit56of the invention can add a dummy pixel P0to the sensing region504while reading pixel data so as to keep the scanning time in constant every time and simplify the calculation of exposure time.

Based on the column selection signal generated by the column selection unit52, the read-out circuit56reads signals generated by the pixels of the sensing region504in column-major order wherein the reading order is as follows, P0, P2, P3, P7, P8and P12. Afterward, the frame buffer58transforms the output data form column-major order to row-major order. As shown inFIG. 12, after being transformed by the frame buffer58, the reading order is changed to be as follows, P0, P3, P8, P2, P7and P12. Furthermore, in this embodiment, since the scanning line is not orthogonal to the contour of the object, the scanning result may show an oblique contour. The invention utilizes the frame buffer58to rearrange the pixel data so as to improve the aforesaid problem.

Referring toFIG. 13,FIG. 13is a schematic diagram illustrating a CMOS image sensor7according to another embodiment of the invention. As shown inFIG. 13, the CMOS image sensor7comprises a pixel array unit70, a row selection unit72, a logic circuit74and a read-out circuit76. The row selection unit72and the read-out circuit76are electrically connected to the pixel array unit70. The logic circuit74is electrically connected to the read-out circuit76.

The pixel array unit70is used for sensing an object (not shown) or a moving trajectory thereof. In this embodiment, the pixel array unit70comprises M pixels700wherein M is a positive integer. In addition, the pixel700can absorb light reflected from an object and then transform the absorbed light into an electric signal. The pixel700usually consists of transistors and photo diodes. It should be noted that the structure and principle of the pixel700can be easily achieved by one skilled in the art and thus will not be described in detail here.

The row selection unit72receives a time sequence signal and a control signal from a controller (not shown) and then generates a row selection signal. The row selection signal is used for controlling the pixels700of the pixel array unit70to output data. The read-out circuit76reads signals generated by the pixels700of the pixel array unit70. The logic circuit74is used for determining a sensing region corresponding to the object or the moving trajectory thereof wherein the sensing region comprises N of the M pixels700and N is a positive integer smaller than or equal to M. Afterward, the logic circuit74determines a first pixel and a last pixel for each row within the sensing region and controls the read-out circuit76to read the first pixel through the last pixel of each row in row-major order, so as to output signals generated by the N pixels.

Referring toFIGS. 14 and 15,FIG. 14is a schematic diagram illustrating the pixel array unit70shown inFIG. 13having a 3*3 pixel array, andFIG. 15is a schematic diagram illustrating a circuit of the CMOS image sensor7shown inFIG. 14. The 3*3 pixel array shown inFIGS. 14 and 15is used, for example, to depict features of the invention.

When a user uses an object (not shown), such as finger, stylus or the like, to operate an optical positioning system (not shown) equipped with the CMOS image sensor7, the pixel array unit70will sense the object or a moving trajectory thereof. Afterward, the logic circuit74determining a sensing region704according to the object or the moving trajectory thereof sensed by the pixel array unit70. As shown inFIG. 14, the sensing region704comprises five pixels P1, P2, P5, P6and P9. That is to say, the aforesaid N is equal to 5. Furthermore, the pixels P1and P2are located at the first row, the pixels P5and P6are located at the second row, and the pixel P9is located at the third row. Then, the logic circuit74determines a first pixel and a last pixel for each row within the sensing region704. As shown inFIG. 14, for the first row within the sensing region704, the first pixel is P1and the last pixel is P2; for the second row within the sensing region704, the first pixel is P5and the last pixel is P6; and for the third row within the sensing region704, the first pixel is P9and the last pixel is also P9. Then, the logic circuit74controls the read-out circuit76to read the first pixel through the last pixel of each row in row-major order, so as to output signals generated by the five pixels within the sensing region704. In this embodiment, the red-out circuit76will read the pixels P1, P2, P5, P6and P9within the sensing region704in order.

In this embodiment, the sensing region704is variable and can be set through self-calibration while booting. Furthermore, when the object or the moving trajectory thereof has a random shape, the logic circuit74may determine the sensing region as a parallelogram including the object or the moving trajectory thereof so as to prevent the following algorithm from getting complicated.

It should be noted that because the sensing region704shown inFIG. 14exceeds a real region of the pixel array unit70, the scanning time may become unstable every time and thus the calculation of exposure time may increase. To solve this problem, the read-out circuit76of the invention can add dummy pixel(s) to the sensing region704while reading pixel data so as to keep the scanning time in constant every time and simplify the calculation of exposure time. Referring toFIG. 16,FIG. 16is a schematic diagram illustrating the sensing region704shown inFIG. 14with a dummy pixel P10. As shown inFIG. 16, after adding the dummy pixel P10, the pixels within the sensing region704are arranged as a parallelogram and an amount of pixels in each row is the same. Accordingly, the scanning time can be kept in constant every time. At this time, for the third row within the sensing region704, the first pixel is P9and the last pixel is the dummy pixel P10.

Referring toFIG. 17,FIG. 17is a schematic diagram illustrating a sensing region704′ according to another embodiment of the invention. Besides the aforesaid parallelogram, the read-out circuit76of the invention may also read and output pixel data within the sensing region704′ shown inFIG. 17through appropriate setting. It should be noted that since the second row within the sensing region704′ includes six pixels P10, P11, P12, P14, P15and P16and there is a break between the pixels P10, P11, P12and the other pixels P14, P15, P16, the logic circuit74will determine a first pixel and a last pixel for the pixels P10, P11, P12and determine another first pixel and another last pixel for the pixels P14, P15, P16. In other words, for the pixels P10, P11, P12, the first pixel is P10and the last pixel is P12; and for the pixels P14, P15, P16, the first pixel is P14and the last pixel is P16. Therefore, for the second row shown inFIG. 17, the read-out circuit76will read the first pixel P10through the last pixel P12first and then read another first pixel P14through another last pixel P16. That is to say, the pixels within each row read by the read-out circuit76can be continuous or non-continuous, and it depends on the sensing region.

Referring toFIG. 18,FIG. 18is a flowchart illustrating a method of operating the CMOS image sensor according to another embodiment of the invention. Also, referring toFIGS. 13 to 15along with the aforesaid CMOS image sensor7, the method of operating the CMOS image sensor of the invention comprises the following steps.

In the beginning, step S300is performed to sense an object or a moving trajectory thereof by the pixel array unit70. Afterward, step S302is performed to determine the sensing region704corresponding to the object or the moving trajectory thereof. Step S304is then performed to generate a row selection signal for controlling the pixels of the pixel array unit70to output signals. Step S306is then performed to determine a first pixel and a last pixel for each row within the sensing region704. Step S308is then performed to read the first pixel through the last pixel of each row in row-major order. Finally, step S310is performed to output signals generated by the pixels P1, P2, P5, P6and P9of the sensing region704.

Referring toFIG. 19,FIG. 19is a flowchart illustrating a method of operating the CMOS image sensor according to another embodiment of the invention. Also, referring toFIG. 16along with the aforesaid CMOS image sensor7, the method of operating the CMOS image sensor of the invention comprises the following steps once the sensing region704exceeds a real region of the pixel array unit70.

In the beginning, step S400is performed to sense an object or a moving trajectory thereof by the pixel array unit70. Afterward, step S402is then performed to determine the sensing region704corresponding to the object or the moving trajectory thereof. Step S404is then performed to generate a row selection signal for controlling the pixels of the pixel array unit70to output signals. Step S406is then performed to judge whether the sensing region704exceeds a real region of the pixel array unit70, and step S408is then performed if it is YES, otherwise, step S410is performed. Step408is performed to add the dummy pixel P10to the sensing region704. Step S410is performed to determine a first pixel and a last pixel for each row within the sensing region704. Step S412is then performed to read the first pixel through the last pixel of each row in row-major order. Finally, step S414is performed to output signals generated by the pixels P1, P2, P5, P6, P9and the dummy pixel P0, if any, of the sensing region704.

Compared to the prior art, the invention utilizes multiplexers to control output of pixel data. The logic circuit can control the multiplexers to define a slope of scanning line selected by each row selection signal. Furthermore, the invention can read the pixel data of the sensing region in column-major order first and then transform the output data from column-major order to row-major order by using the frame buffer. Moreover, the invention may add a logic circuit to the read-out circuit and utilize the logic circuit to determine a starting point (i.e. the aforesaid first pixel) and a terminal point (i.e. the aforesaid last pixel) for each row within the sensing region corresponding to the sensed object or the moving trajectory thereof, so as to control the read-out circuit to output the pixel data within the sensing region. Since the invention only needs to output pixel data within the sensing region corresponding to the object or the moving trajectory thereof, the operating frequency and power consumption can be reduced substantially. Moreover, once the logic circuit judges that the sensing region exceeds the real region of the pixel array unit, the read-out circuit can add dummy pixel(s) to the sensing region so as to keep the scanning time in constant every time and simplify the calculation of exposure time.