Abstract:
An image sensor is proposed, which can output different amount of data according to different resolution modes. The image sensor employs a plurality of photo diodes to convert received optical signals into charges and uses a plurality of sets of transfer gates to move out the charges on the photo diodes. The image sensor uses a plurality of shift registers to receive the charges moved out from the transfer gates and to remove the charges according to two sets of control signals. A floating diffusion node is used to receive the charges on the shift registers for generating electrical signals. The image sensor uses a charge control unit to control whether the output charges from the shift registers are passed onto the floating diffusion node. Because the image sensor uses the charge control unit to control all, ½ or ¼ parts of charges into the floating diffusion node, a scanner with the image sensor can obtain a high quality image when scanning in high resolution modes and run at a high speed when scanning in low resolution modes.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The invention relates to an image sensor and a scanner using the image sensor. In particular, the invention relates to an image sensor that can output different amount of data according to different resolutions.  
           [0003]    2. Related Art  
           [0004]    Due to the progress in PC (Personal Computer) technology, the CPU (central processing unit) operation speed, major memory (RAM) and auxiliary memory (HDD and CD-ROM) capacities have been increased a lot. The peripheral devices of the computer are also improved very much from the prior art. Taking image scanner as an example, its resolution has been increased from 300˜600 dpi (dots per inch) to more than 2400 dpi. However, with the continuous increase in resolution, if one uses a high-resolution image scanner to scan document in low-resolution formats, the limit of the scanning speed is not on the interface with the PC (there are already high speed interfaces such as 1394 and USB2.0, etc), but on the image sensor (such as CCD, Charge coupled device) with a large amount of pixels. Thus, the high-resolution image scanner needs to wait for the whole data of a scanning line to be transferred out from a shift register of the image sensor even the scanner is running at the low-resolution mode. The scanner then samples the pixels to form a low-resolution image. The scanning speed is, of course, not as fast as conventional low-resolution scanners.  
           [0005]    As shown in FIG. 1, a normal linear CCD  10  includes a plurality of photo diodes  11 , a transfer gate  12 , a shift register  13 , a floating diffusion node  14 , a clamp  15 , and an output buffer amplifier  16 . The number of photo diodes  11  is determined by the required resolution. The higher the resolution is, the more photo diodes there are. When a photo diode  11  is exposed, its charges are transferred to the shift register  13  via the transfer gate  12 . The CCD  10  uses control signals Φ 1  and Φ 2  to dispense the charges of the shift register  13  into the floating diffusion node  14 . FIG. 2 is a timing diagram showing the control signal Φ 1 , Φ 2 , CP and RS. With reference to FIG. 2, the control signal RS is used to clear the charges of the floating capacitor at the floating diffusion node  14 , and the control signal CP is used to restrict the electric potential of the clamp  15 . Therefore, the time for the CCD  10  to transmit the whole data of a scan line is (number of the photo diodes  11 )*(transmission time for each data). For example, a CCD of a scanner with 1200 dpi for an A4-size medium has 10K photo diodes  11 . If the transmission time for each data is 600 ns, then it takes about 6 ms (10K*600 ns) to transmit whole data in each scan line. The transmission time for each scan line is the same for the resolution of 300 dpi, 600 dpi, or 1200 dpi. Therefore, when one uses a 1200 dpi scanner to scan documents in the 600 dpi resolution mode, the speed cannot be as fast as a scanner with a highest resolution of 600 dpi.  
           [0006]    As shown in FIG. 3, a double shift register linear CCD  20  includes a plurality of photo diodes  11 , two transfer gates  22 ,  22 ′, two shift registers  23 ,  23 ′, a floating diffusion node  14 , a clamp  15 , and an output buffer amplifier  16 . The number of photo diodes  11  is determined by the required resolution. The higher the resolution is, the more photo diodes there are. The control method of the CCD  20  is similar with the CCD  10 , but the CCD  20  uses two shift registers  23 ,  23 ′ to shift charges. FIG. 4 is a timing diagram showing the control signal Φ 1 , Φ 2 , CP and RS. With reference to FIG. 4, if the data rates of output signal at FIG. 4 are same with the data rates of output signal at FIG. 2, the frequency of the control signal Φ 1 , Φ 2  at FIG. 4 is half the frequency of the control signal Φ 1 , Φ 2  at FIG. 2. For example, a CCD of a scanner with 2400 dpi for an A4-size medium has 20K photo diodes  11 . If the transmission time for each data is 600 ns, then it takes about 12 ms (20K*600 ns) to transmit whole data in each scan line. The transmission time for each scan line is the same for the resolution of 300 dpi, 600 dpi, or 1200 dpi. Therefore, when one uses a 2400 dpi scanner to scan documents in the 600 dpi resolution mode, the speed cannot be as fast as a scanner with a highest resolution of 600 dpi.  
         SUMMARY OF THE INVENTION  
         [0007]    In view of the foregoing, an objective of the invention is to provide an image sensor that can provide different amount of data, so that a scanner with the image sensor can obtain a high quality image when scanning in high resolution modes and run at a high speed when scanning in low resolution modes.  
           [0008]    To achieve the above objective, the disclosed image sensor includes: a plurality of photo diodes, which converts received optical signals into electrical signals; two sets of transfer gates, which transfer charges on the photo diodes; two shift registers, including a first and a second shift registers to receive charges transferred out by the transfer gates, respectively, and pass out the charges according to control signals; a floating diffusion unit, which receives the charges from the first shift register and the second shift register to produce electrical signals; a charge control unit, which controls whether the output charges of the second shift register is to be passed to the floating diffusion unit; a clamp, which receives the electrical signals from the floating diffusion unit to maintain its potential level; and an output buffer unit, which receives the signals of the clamp and produces output signals.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    These and other features, aspects and advantages of the invention will become apparent by reference to the following description and accompanying drawings which are given by way of illustration only, and thus are not limitative of the invention, and wherein:  
         [0010]    [0010]FIG. 1 shows a structure of a conventional CCD with a single shift register.  
         [0011]    [0011]FIG. 2 shows a timing diagram of the control signals Φ 1 , Φ 2 , RS, CP and the output signal OUTPUT of the CCD in FIG. 1.  
         [0012]    [0012]FIG. 3 shows a structure of a conventional CCD with two single shift registers. FIG. 4 shows a timing diagram of the control signals Φ 1 , Φ 2 , RS, CP and the output signal OUTPUT of the CCD in FIG. 3.  
         [0013]    [0013]FIG. 5 is a structure of the CCD with double shift registers of the present invention.  
         [0014]    [0014]FIG. 6 is a timing diagram of the control signals Φ 1 , Φ 2 , RS, CP and the output signal when the charge control switch of the CCD in FIG. 5 is OFF.  
         [0015]    [0015]FIG. 7 is a timing diagram of the control signals Φ 1 , Φ 2 , RS, CP and the output signal when the charge control switch of the CCD in FIG. 5 is ON.  
         [0016]    [0016]FIG. 8 is a structure of the CCD with three shift registers of the present invention.  
         [0017]    [0017]FIG. 9 is a timing diagram of the control signals Φ 1 , Φ 2 , Φ 3 , Φ 4 , RS, CP and the output signal when the first, second and third charge control switches of the CCD in FIG. 8 are ON.  
         [0018]    [0018]FIG. 10 is a timing diagram of the control signals Φ 1 , Φ 2 , Φ 3 , Φ 4 , RS, CP and the output signal when the first charge control switch of the CCD in FIG. 8 is ON and the second and third charge control switches of the CCD in FIG. 8 are OFF.  
         [0019]    [0019]FIG. 11 is a timing diagram of the control signals Φ 1 , Φ 2 , Φ 3 , Φ 4 , RS, CP and the output signal when the first and second charge control switches of the CCD in FIG. 8 are OFF and the third charge control switch of the CCD in FIG. 8 is ON.  
         [0020]    [0020]FIG. 12 is a flowchart of the scanner control method for a CCD that can provide different amount of data according to different resolution modes. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0021]    The present invention will be apparent from the following detailed description, which proceeds with reference to the accompanying drawings, wherein the same references relate to the same elements.  
         [0022]    As shown in FIG. 5, the CCD  30  of the present invention is similar to the conventional CCD  20  (see FIG. 3). The CCD  30  also contains several photo diodes  31 , two transfer gates  321 ,  322 , two shift registers  331 ,  332 , a floating diffusion node  14 , a clamp  15 , and an output buffer amplifier  16 . The functions and structures of these elements are the same as the same elements in the conventional device and, therefore, are not further described herein. Nonetheless, the disclosed CCD  30  further contains a charge control switch  38  to control the action of moving out charges in the shift register  332 . The charge control switch  38  is controlled by the control signal SW. When the control signal SW is enabled, the charge control switch  38  is ON; when the control signal SW is disabled, the charge control switch  38  is OFF.  
         [0023]    The output terminal of the first shift register is connected to the floating diffusion node  14 , and the output terminal of the second shift register  332  is connected to the floating diffusion node  14  via the charge control switch  38 . Thus, when the charge control switch  38  is OFF, only the charges on the first shift register  311  are output to the floating diffusion node  14 . When the charge control switch  38  is set ON, the charges on both the first shift register  311  and the second shift register  332  are output into the floating diffusion node  14 . When one uses a scanner with the CCD  30  to scan documents, the scanner turns on the charge control switch  38  when the scanning resolution is over {fraction (1/2)}, obtaining all data. If the resolution is below {fraction (1/2)}, the charge control switch  38  is set OFF and the scanner just obtains half of the data for each scanning line to save half of the data reading time.  
         [0024]    With reference to FIG. 6, the control signal SW is disabled in this case. At the case, only the charges on the first shift register  331  will be moved into the floating diffusion node  14  according to the control signals Φ 1  and Φ 2 . Since only the charges on the first shift register  331  are processed, the frequency of the control signals Φ 1  and Φ 2  is the same the frequency of the output signal.  
         [0025]    With reference to FIG. 7, the control signal SW is enabled in this case. At the case, the charges both on the first and second shift registers  331 ,  332  will be moved into the floating diffusion node  14  according to the control signals Φ 1  and Φ 2 . Since the charges on both the first and second shift registers  331 ,  332  are processed, the frequency of the control signals Φ 1  and Φ 2  is half of the frequency of the output signal.  
         [0026]    [0026]FIG. 8 is the structure of the CCD with three shift registers of the second embodiment of the present invention. As shown in FIG. 8, the CCD  40  includes two sets of separate pluralities of photo diodes  411 ,  412 , three sets of transfer gates  421 ,  422 ,  423 , three shift registers  431 ,  432 ,  433 , a floating diffusion node  14 , a clamp  15 , an output buffer amplifier  16 , and a charge control unit  48 . Aside from the charge control unit  48 , the functions and structures of the other elements are the same as those in the prior art and therefore are not further described herein. The charge control unit  48  contains charge control switches  481 ,  482 ,  483  and a charge shift register  484 . The charge control switches  481 ,  482 ,  483  are controlled by switch signals SW 1 , SW 2 , SW 3 . In this embodiment, the control signals for the shift registers  431 ,  432 ,  433 ,  484  can be grouped into two sets, one being the control signals Φ 1  and Φ 2 , and the other being the control signals Φ 3  and Φ 4 . The charge shift register  484  is controlled by the first set of control signals Φ 1  and Φ 2 .  
         [0027]    The first shift register  431  is connected to the floating diffusion node  14  via the first charge control switch  481  and is controlled by the first set of control signals Φ 1  and Φ 2 . The second shift register  432  is connected to the charge shift register  484 , and the third shift register  433  is connected to the charge shift register  484  via the second charge control switch  482 . The charge shift register  484  is connected to the floating diffusion node  14  via the third charge control switch  483 . The second shift register  432  and the third shift register  433  are controlled by the second set of control signals Φ 3  and Φ 4 .  
         [0028]    Therefore, when a scanner uses the CCD  40  as its image sensor and the scanning resolution of the scanner is above {fraction (1/2)}, the scanner sets the first, second and third charge control switches  481 ,  482 , and  483  ON. In this case, all data in each scan line are obtained. When the scanning resolution is set between {fraction (1/4)} to {fraction (1/2)}, the scanner turns the third charge control switch  483  off and turns the first and second charge control switches  481  and  482  on. In this case, only half of the data in each scan line are obtained, therefore the scanner can save half of the data reading time. Furthermore, when the resolution of the scanner goes below {fraction (1/4)}, the scanner turns the first and second charge control switches  481  and  482  off, and leaves the third charge control switch on. In this case, only {fraction (1/4)} of the data in each scan line are obtained, therefore the scanner can save {fraction (1/4)} of the data reading time.  
         [0029]    [0029]FIG. 9 is a timing diagram of the control signals Φ 1 , Φ 2 , Φ 3 , Φ 4 , RS, CP and the output signal when the first, second and third charge control switches of the CCD in FIG. 8 are ON. In FIG. 9, the control signals SW 1 , SW 2 , SW 3  are enabled. In this case, the charges on the first, second and third shift registers  431 ,  432  and  433  are moved to the floating diffusion node  14  according to the control signals Φ 1 , Φ 2 , Φ 3 , and Φ 4 . Since the charges on the first, second and third shift registers  431 ,  432  and  433  are to be processed, the frequency of the first set of control signals Φ 1  and Φ 2  is set to half that of the output signal, while the frequency of the second set of control signals Φ 3  and Φ 4  is set to {fraction (1/4)} of the frequency of the output signal.  
         [0030]    [0030]FIG. 10 is a timing diagram of the control signals Φ 1 , Φ 2 , Φ 3 , Φ 4 , RS, CP and the output signal when the first charge control switch of the CCD in FIG. 8 is ON and the second and third charge control switches of the CCD in FIG. 8 are OFF. In FIG. 10, the control signals SW 1  is enabled and the control signals SW 3  is disabled. In this case, only the charges on the first shift registers  431  are moved into the floating diffusion node  14  according to the control signals Φ 1 , Φ 2 , Φ 3 , and Φ 4 . Since only the charges on the first shift registers  431  are to be processed, the frequency of the first set of control signals Φ 1  and Φ 2  are same with the frequency of the output signal, while the frequency of the second set of control signals Φ 3  and Φ 4  is set to half of the frequency of the output signal.  
         [0031]    [0031]FIG. 11 is a timing diagram of the control signals Φ 1 , Φ 2 , Φ 3 , Φ 4 , RS, CP and the output signal when the first and second charge control switches of the CCD in FIG. 8 are OFF and the third charge control switch of the CCD in FIG. 8 is ON. In FIG. 11, the control signals SW 1  and SW 2  are disabled and the control signals SW 3  is enabled. In this case, only the charges on the second shift registers  432  are moved into the floating diffusion node  14  according to the control signals Φ 1 , Φ 2 , Φ 3 , and Φ 4 . Since only the charges on the second shift registers  432  are to be processed, the frequency of the first and second sets of control signals Φ 1 , Φ 2 , Φ 3  and Φ 4  are same with the frequency of the output signal.  
         [0032]    Since the disclosed CCD  30 ,  40  can provide different amount of data according to different resolution requirements, therefore the data processing time is shorter when scanning in the lower resolution mode to increase the scanning speed.  
         [0033]    As shown in FIG. 12, the disclosed control method adjusts the frequencies of the control signals according to different resolution modes to achieve the high speed scanning in lower resolution modes. The control method includes the following steps:  
         [0034]    Step S 1202 : Read the scanning resolution inputted by user.  
         [0035]    S 1204 : Set the resolution mode. In accordance with the scanning resolution and the highest optical resolution of the CCD, a resolution mode is determined. When the scanning resolution is greater than {fraction (1/2)} of the highest optical resolution, the scanner is set at the highest resolution mode. When the scanning resolution is between {fraction (1/4)} to {fraction (1/2)} of the highest optical resolution, the scanner is set at the {fraction (1/2)} resolution mode. When the scanning resolution is smaller than {fraction (1/4)} of the highest optical resolution, the scanner is set at the {fraction (1/4)} resolution mode.  
         [0036]    Step S 1206 : Generate control signals. The control signals are generated according to different resolution modes. The control signals include the control signals for controlling the shift register of the linear image sensor, the switch control signals SW 1 , SW 2 , SW 3 , and other related control signals known in the prior art. The frequencies of these control signals are already described in the previous paragraphs and not further detailed hereinafter.  
         [0037]    Step S 1208 : Scan a document and transmit data according to the control signals. This step is similar to a conventional scanner, and thus is not repeated herein.  
         [0038]    Step S 1210 : Finish.  
         [0039]    Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. For example, the embodiments use the structures of two and three shift registers, the scanner can be designed to have more than three shift registers. The disclosed specification uses the control signals Φ 1 , Φ 2 , Φ 3 , and Φ 4  to control the movement of the shift registers, but the invention is not limited to these control signals. Any signal that can be used to control the shift register can be applied to the invention. For example, a double shifted control method which combines the charges of adjacent two points into a single charge can be utilized in the invention to achieve an even smaller data capacity requirement.