Patent Publication Number: US-2009225207-A1

Title: Optical pointing device and method of adjusting exposure time and comparison voltage range of the optical pointing device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part application of U.S. application Ser. No. 10/744,887 filed Dec. 23, 2003, which claims the benefit of Korean Patent Application No. 2002-82889, filed on Dec. 23, 2002, the disclosures of which are hereby incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical pointing device and a method of adjusting an exposure time and a comparison voltage range of the optical pointing device and, more particularly, to an optical pointing device capable of obtaining a high-resolution image using a low-resolution analog-to-digital (A/D) converter to calculate an accurate movement value, and a method of adjusting an exposure time and a comparison voltage range of the optical pointing device. 
     2. Description of the Related Art 
       FIG. 1  shows an internal block diagram of a conventional optical pointing device. 
     Referring to  FIG. 1 , the conventional optical pointing device includes a light source  100 , an image sensor  110 , an analog-to-digital (A/D) converter  120 , an image data processor  130  and a shutter control circuit  140 . 
     The light source  100  emits light toward a subject (e.g. the surface of a worktable), and the image sensor  110  outputs analog signals IMO of pixels according to a quantity of the light reflected from the subject. The image sensor  110  is provided with a plurality of pixels, each of which responds to a shutter control signal CSHT to charge voltage in proportion to a quantity of light input when a shutter is turned on. Then, when the shutter is turned off, the image sensor  110  outputs the analog signals IMO of the pixels, each of which has a charge voltage. 
     The A/D converter  120  receives the analog signals IMO of the image sensor  110 , and converts the received analog signals IMO into N-bit digital signals ADCO. 
     The image data processor  130  receives the N-bit (N is natural number) digital signals ADCO of the pixels which are output from the A/D converter  120 , calculates a movement value MV using the received N-bit digital signals ADCO, and outputs the calculated movement value MV. 
     Here, the movement value MV can be obtained using correlation between the previously received N-bit digital signals ADCO and currently received N-bit digital signals ADCO. 
     Further, the image data processor  130  determines an exposure time of the image sensor  110  which allows the quantity of light input into the image sensor  110  to be maintained within a predetermined range using the received N-bit digital signals ADCO, generates a first shutter control signal CSHT 1  having an m-bit (m is natural number) code value corresponding to the determined exposure time, and provides the generated the first shutter control signal CSHT 1  to the shutter control circuit  140 . 
     The shutter control circuit  140  receives the first shutter control signal CSHT 1  which is provided from the image data processor  130 , generates a second shutter control signal CSHT 2  having a pulse width corresponding to the first shutter control signal CSHT 1 , and provides the generated the second shutter control signal CSHT 2  to an electronic shutter (not shown) composed of a complementary metal oxide semiconductor (CMOS) transistor in the image sensor  110 . In other words, the second shutter control signal CSHT 2  is an exposure time TS of the image sensor  110  corresponding to a shutter-on time section during which the electronic shutter is turned on in response to an activated pulse width section. 
     In the above-mentioned configuration, the image data processor  130  has been described as functionally separate from the shutter control circuit  140 . However, the image data processor  130  may include the function of the shutter control circuit  140  according to necessity. 
       FIG. 2  is a graph explaining how the image data processor of  FIG. 1  determines an exposure time of the image sensor. 
     In  FIG. 2 , plotted lines represents charge voltages PS 1  to PS 3  according to an exposure time of each unit pixel of the image sensor  110 , an X axis represents an exposure time of the image sensor  110 , and a Y axis represents a code corresponding to a charge voltage. 
     Here, it is assumed that the optical pointing device includes the 4-bit A/D converter  120  having a code value range between “0” and “15” and a central code value of “7,” and the image sensor  110  having three pixels. 
     Under this assumption, referring to  FIG. 2  again, the image data processor  130  selects an exposure time TS such that an average value of three charge voltages PS 1  to PS 3  output from the image sensor  110  is distributed on “7,” which is the central code value of the A/D converter  120 . 
     In detail, the image data processor  130  selects an exposure time TS such that an average voltage between the charge voltage PS 1  having the minimum voltage value and the charge voltage PS 3  having the maximum voltage value on the basis of the same time is distributed on the central code value of the A/D converter  120 , “7”. 
     Further, the image data processor  130  generates the first shutter control signal CSHT 1  having a code value corresponding to the selected exposure time TS. 
     Although the image sensor  110  may have pixels ranging from one to several millions, all of the pixels are controlled by shutter control signals CSHT 2  having the same value. 
     Thus, when too much or too little light is incident on the image sensor  110 , the average value of the analog signals IMO provided from the image sensor  110  excessively converge on the upper limit codes CODE 10  to CODE 15  or the lower limit codes CODE 0  to CODE 5  of the A/D converter  120 . 
     In this case, the A/D converter  120  fails to normally recognize these signals, thus failing to perform normal A/D conversion. In other words, the A/D converter  120  does not generate digital signals required by the optical pointing device to calculate a movement value. 
     Thus, as in  FIG. 2 , the conventional optical pointing device is equipped with the image data processor  130  controlling the exposure time of the image sensor  110 , thereby allowing the analog signals IMO generated through the image sensor  110  to be always distributed on the central code of the A/D converter  120 . 
     Here, the analog signals output by the image sensor  110  are analog image signals, while the digital signals output by the A/D converter  120  are digital image signals. 
       FIG. 3  shows one embodiment of a circuit diagram of the A/D converter of  FIG. 1 , in which the A/D converter  120  includes a comparison voltage generator  121  and an N-bit comparator  123 . 
     The comparison voltage generator  121  receives fixed reference voltage values from first and second reference voltages Vref 1  and Vref 2 , and determines a comparison voltage range. Here, the comparison voltage generator  121  divides the comparison voltage range into 2 N  units, and generates and outputs 2 N  comparison voltages. Further, differences between the comparison voltages generated from the comparison voltage generator  121  are uniformly maintained at all times. 
     The N-bit comparator  123  compares the 2 N  comparison voltages transmitted from the comparison voltage generator  121  with the voltages of the analog signals IMO transmitted from the image sensor  110 , and outputs the compared results in the form of an N-bit digital signal ADCO. 
     In this manner, the conventional A/D converter  120  converts the analog signals IMO into the N-bit digital signals ADCO using the 2 N  comparison voltages, the differences between which are uniform at all times. 
     Thus, when the voltage differences of the analog signals of the pixels input into the A/D converter  120  are large enough to be distinguished by the 2 N  comparison voltages, the A/D converter  120  recognizes the voltage differences of the analog signals IMO, and generates the N-bit digital signals ADCO corresponding to the respective voltages. 
     Accordingly, the conventional A/D converter  120  can provide an image that accurately reflects the shape of a subject to the image data processor. 
     By contrast, when the voltage differences of the analog signals IMO of the pixels input into the A/D converter  120  are too small to be distinguished by the 2 N  comparison voltages, the A/D converter  120  fails to distinguish the voltage differences of the analog signals IMO, and the A/D converter  120  cannot provide an image that accurately reflects the shape of a subject to the image data processor. 
     Generally, the optical pointing device has to accurately recognize the image of the subject in order to calculate an accurate movement value. Thus, the optical pointing device is designed to calculate the accurate movement value using a high-resolution A/D converter capable of distinguishing even very small voltage differences of the analog signals IMO. 
     However, with the high-resolution A/D converter, there are problems in that a layout area of the A/D converter is increased, and in that manufacturing cost and consumption power of the optical pointing device are inevitably increased. 
     This is because, in view of characteristics of the circuit of the A/D converter, when the number of bits increases by 1, a chip size of the A/D converter increases doubly, and the consumption power of the A/D converter increases twice. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to provide an optical pointing device capable of obtaining a high-resolution image of a surface using a low-resolution analog-to-digital (A/D) converter, thereby calculating an accurate movement value. 
     The present invention is also directed to provide a method of adjusting an exposure time and a comparison voltage range of the optical pointing device. 
     According to an aspect of the present invention, there is provided an optical pointing device, which includes a light source emitting light, a first image sensor on which the light reflected from a subject is incident, receiving an image of the subject in the form of the light to generate first analog signals of pixels, a first analog-to-digital (A/D) converter varying a comparison voltage range according to a conversion control signal, and converting the first analog signals of the pixels into first digital signals of the pixels according to the varied comparison voltage range, and an image data processor calculating a movement value in response to the first digital signals of the pixels, and generating and outputting a first shutter control signal for controlling an exposure time of the first image sensor. 
     According to another aspect of the present invention, there is provided a method of adjusting an exposure time and a comparison voltage range of an optical pointing device, in which the optical pointing device includes a light source emitting light, a first image sensor on which the light reflected from a subject is incident, receiving an image of the subject in the form of the light to generate first analog signals of pixels, a first analog-to-digital (A/D) converter varying a comparison voltage range according to a conversion control signal, and converting the first analog signals of the pixels into first digital signals of the pixels according to the varied comparison voltage range, and an image data processor calculating a movement value in response to the first digital signals of the pixels, and generating and outputting a first shutter control signal for controlling an exposure time of the first image sensor, the method including: emitting light; receiving the light reflected from the subject, and receiving the image of the subject in the form of the light to generate first analog signals of the pixels; varying the comparison voltage range according to the conversion control signal, and converting the first analog signals of the pixels into the first digital signals of the pixels according to the varied comparison voltage range; calculating the movement value using the correlation between the first digital signals of the pixels stored during previous operation and the first digital signals of the pixels applied at present; and adjusting and outputting the conversion control signal and the first shutter control signal for controlling the exposure time of the first image sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  shows a block circuit diagram of a conventional optical pointing device; 
         FIG. 2  is a graph explaining how the image data processor of  FIG. 1  determines an exposure time of the image sensor; 
         FIG. 3  shows one embodiment of a circuit diagram of the A/D converter of  FIG. 1 ; 
         FIG. 4  is a block diagram showing an optical pointing device according to a first embodiment of the present invention; 
         FIG. 5  is an internal block diagram of the A/D converter of  FIG. 4 ; 
         FIG. 6  is a graph explaining how the A/D converter of  FIG. 5  generates N-bit digital signals; 
         FIG. 7  shows an example of the A/D converter of  FIG. 5 ; 
         FIG. 8  shows another example of the A/D converter of  FIG. 5 ; 
         FIG. 9  is a block diagram showing an optical pointing device according to a second embodiment of the present invention; 
         FIG. 10  is an internal block diagram of the A/D converter of  FIG. 9 ; 
         FIG. 11  is a block diagram showing an optical pointing device according to a third embodiment of the present invention; 
         FIGS. 12 and 13  are flowcharts explaining how the image data processor of  FIG. 11  adjusts the exposure time of the image sensor and the comparison voltage range; 
         FIG. 14  is a view explaining a method of rapidly determining the exposure time of an image sensor in an optical pointing device according to an exemplary embodiment of the present invention; 
         FIGS. 15 and 16  are flowcharts explaining how an optical pointing device adjusts the exposure time of an image sensor and a comparison voltage range using the method of  FIG. 14 ; 
         FIGS. 17 and 18  are flowcharts explaining how the optical pointing device of  FIG. 9  adjusts the exposure time of the image sensor and the comparison voltage range using the method of  FIG. 13 ; and 
         FIGS. 19 and 20  show an operating difference between a conventional optical pointing device and a proposed optical pointing device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An optical pointing device and a method of adjusting an exposure time and a comparison voltage range of the optical pointing device according to exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
       FIG. 4  is a block diagram showing an optical pointing device according to a first embodiment of the present invention. 
     In the optical pointing device shown in  FIG. 4  in accordance with a first embodiment of the present invention, a light source  200 , an image sensor  210 , an image data processor  230  and a shutter control circuit  240  are functionally identical to those of the optical pointing device shown in  FIG. 1 . However, unlike the A/D converter  120  of  FIG. 1 , an A/D converter  220  of  FIG. 4  receives a first shutter control signal CSHT 1 , varies a comparison voltage range in response to the first shutter control signal CSHT 1 , and converts analog signals IMO received from the image sensor  210  into digital signals ADCO according to the varied comparison voltage range. 
       FIG. 5  is an internal block diagram of an A/D converter of  FIG. 4 . 
     As shown in the drawing, the A/D converter  220  includes a variable comparison voltage generator  221  and an N-bit comparator  223 . The A/D converter  220  receives the first shutter control signal CSHT 1  from the image data processor  230 . 
     The variable comparison voltage generator  221  receives fixed reference voltage values from first and second reference voltages Vref 1  and Vref 2 , and varies the comparison voltage range applied to the N-bit comparator  223  in response to the first shutter control signal CSHT 1 . 
     Then, the varied comparison voltage range is divided into 2 N  units. The 2 N  comparison voltages are generated and outputted. 
     Thus, voltage differences between the comparison voltages generated from the variable comparison voltage generator  221  are variously adjusted according to the first shutter control signal CSHT 1 . 
     Here, the variable comparison voltage generator  221  can be realized as a plurality of subdivided comparators or by analog continuous voltage control. 
     The N-bit comparator  223  receives the 2 N  comparison voltages from the variable comparison voltage generator  221  and the analog signals IMO from the image sensor  210  to compare voltage magnitudes, and generates and outputs N-bit digital signals ADCO corresponding to the compared results. Here, the A/D converter  220  is assumed to a full flash type architecture, but it is natural to use other A/D converter architecture. 
       FIG. 6  is a graph explaining how the A/D converter of  FIG. 5  generates N-bit digital signals. 
     As described above, the A/D converter of  FIG. 5  varies the comparison voltage range in response to the first shutter control signal CSHT 1 , and converts the analog signals IMO received from the image sensor into the digital signals according to the varied comparison voltage range. Since the comparison voltage range is varied in response to the first shutter control signal CSHT 1  received from the image data processor  230 , although the same analog signals IMO are received from the image sensor  210 , the N-bit digital signals ADCO having different code values are output according to the first shutter control signal CSHT 1 . For example, in  FIG. 6 , CASE 1  represents a case in which the first shutter control signal CSHT 1  has a medium value, and CASE 2  and CASE 3  represent cases in which the first shutter control signals CSHT 1  have high and low values, respectively. More precisely, when the first shutter control signal CSHT 1  has the high value due to a small quantity of incident light, an exposure time TS becomes long. Further, differences between the comparison voltages become narrow as in CASE 2 , and all of the N-bit comparison voltages are located on the area of a low voltage level so as to correspond to low luminance distribution. In contrast, when the first shutter control signal CSHT 1  has the low value due to a large quantity of incident light, the exposure time TS becomes short. Further, the differences between the comparison voltages become narrow as in CASE 3 , but all of the N-bit comparison voltages are located on the area of a high voltage level so as to correspond to high luminance distribution. When the differences between the comparison voltages are medium, the differences between the comparison voltages are distributed widely as in CASE 1 . 
     In the above description, the exposure time TS is varied in proportion to the value of the first shutter control signal CSHT 1 , and the N-bit comparison voltages are varied so as to correspond to the high and low luminance distribution in inverse proportion to the value of the first shutter control signal CSHT 1 . However, the optical pointing device may be designed such that the exposure time TS is operated in inverse proportion to the value of the first shutter control signal CSHT 1 . Furthermore, the comparison voltages may also be varied so as to correspond to the luminance distribution in proportion to the value of the first shutter control signal CSHT 1 . 
       FIG. 7  shows an example of the A/D converter of  FIG. 5 . 
     Referring to  FIG. 7 , the variable comparison voltage generator  221  includes a variable resistance circuit VR 1  connected with the first reference voltage Vref 1  and varying a resistance value VR in response to a comparison voltage control signal, 2 N  comparison voltage generating circuits R 11  to R 1 ( 2   N ) dependently connected in series between the variable resistance circuit VR 1  and the second reference voltage Vref 2 , and a control signal generating unit  222  generating the comparison voltage control signal having an adjustable voltage value in response to the first shutter control signal CSHT 1 . 
     The N-bit comparator  223  includes 2 N  comparators CMP 11  to CMP 1 ( 2   N ), which are connected with the variable comparison voltage generating circuits R 11  to R 1 ( 2   N ), respectively. 
     The first shutter control signal CSHT 1  is a digital signal, a code value of which is varied according to an exposure time. Thus, the control signal generating unit  222  generates a voltage value of the comparison voltage control signal corresponding to the code value of the first shutter control signal CSHT 1 . The variable resistance circuit VR 1  varies its resistance value VR according to the voltage value of the comparison voltage control signal. 
     Hereinafter, an operation of the A/D converter will be described with reference to  FIG. 5 . 
     Here, it is assumed that all of the 2 N  comparison voltage generating circuits R 11  to R 1 ( 2   N ) of the A/D converter have the same resistance value R. 
     The control signal generating unit  222  adjusts the voltage value of the comparison voltage control signal according to the code value of the received first shutter control signal CSHT 1 , and the variable resistance circuit VR 1  varies the resistance value VR in response to the comparison voltage control signal having the adjusted voltage value. 
     The variable resistance circuit VR 1  having the varied resistance value VR outputs a comparison voltage having a value of “Vref 1 −(Vref 1 −Vref 2 )×VR/(VR+2 N ×R)” to a first output node N 1 . 
     Then, the first comparison voltage generating circuit R 11  outputs the comparison voltage having a value of “Vref 1 −(Vref 1 −Vref 2 )×(VR+R)/(VR+2 N ×R)” to a second output node N 2 , and the second comparison voltage generating circuit R 12  outputs a comparison voltage having a value of “Vref 1 −(Vref 1 −Vref 2 )×(VR+2R)/(VR+2 N ×R)” to a third output node N 3 . 
     In this manner, the third to 2 N -th comparison voltage generating circuits R 13  to R 1 ( 2   N ) output the respective comparison voltages to fourth to 2 N -th output nodes N 4  to N( 2   N ). 
     The 2 N  comparators CMP 11  to CMP 1 ( 2   N ) receiving the respective comparison voltages compare magnitudes of the comparison voltages with those of the voltages of the analog signals IMO output from the image sensor  210 , and output the compared results. 
     The N-bit comparator  233  generates and outputs N-bit digital signals ADCO corresponding to values of the 2 N  compared results of the 2 N  comparators CMP 11  to CMP 1 ( 2   N ). The N-bit comparator  233  may comprise a decoder (not shown) to generate N-bit digital signals ADCO from the values of the 2 N  compared results. 
     In this manner, the A/D converter of  FIG. 7  outputs the N-bit digital signals as in the conventional A/D converter, and continuously varies the comparison voltage range according to the first shutter control signal CSHT 1  to thus increase its resolution. 
     In the above-mentioned configuration, the variable resistance circuit VR 1  is disposed between the first reference voltage Vref 1  and the first comparison voltage generating circuit R 11 , and the voltage differences between the comparison voltages provided by the A/D converter are varied. However, if necessary, the variable resistance circuit VR 1  may be disposed between the second reference voltage Vref 2  and the 2 N -th comparison voltage generating circuit R 1 ( 2   N ), and the voltage differences between the comparison voltages provided by the A/D converter may also be varied. 
     Similarly, a first variable resistance circuit may be disposed between the first reference voltage Vref 1  and the first comparison voltage generating circuit R 11 , and a second variable resistance circuit may be disposed between the second reference voltage Vref 2  and the 2 N -th comparison voltage generating circuit R 1 ( 2   N ). The voltage differences between the comparison voltages provided by the A/D converter may be varied through these variable resistance circuits. 
       FIG. 8  shows another example of the A/D converter of  FIG. 5 . In  FIG. 8 , only a variable comparison voltage generator  221 - 1  is shown as a part of the A/D converter. An N-bit comparator is not separately shown because it has the same configuration as the N-bit comparator  223  shown in  FIG. 7 . 
     The A/D converter of  FIG. 8  includes m resistance circuits  51   a  to  5 ( m ) a  and m switches  51   b  to  5 ( m ) b  instead of the variable resistance circuit VR 1  of the variable comparison voltage generator  221  of  FIG. 5 . 
     A detailed description will be omitted regarding the A/D converter of  FIG. 8  which has the same configuration and operation as that of  FIG. 5 . 
     The m resistance circuits  51   a  to  5 ( m ) a  are connected to the first reference voltage Vref 1  in parallel, and the m switches  51   b  to  5 ( m ) b  are disposed between the respective m resistance circuits  51   a  to  5 ( m ) a  and the reference voltage Vref 1 . 
     The m switches  51   b  to  5 ( m ) b  control connection between the m resistance circuits  51   a  to  5 ( m ) a  and the 2 N  comparison voltage generating circuits R 21  to R 2 ( 2   N ) in response to the m-bit code value of the first shutter control signal CSHT 1 . 
     If the bit number of the first shutter control signal CSHT 1  is more than m, the m switches  51   b  to  5 ( m ) b  may be controlled using m bits of the first shutter control signal CSHT 1 . 
     Thus, a variable comparison voltage generator  221 - 1  controls connection between a specified at least one of the resistance circuits and the 2 N  comparison voltage generating circuits R 21  and R 2 ( 2   N ) according to the exposure time of the first shutter control signal CSHT 1 , varies a comparison voltage range of the A/D converter, divides the varied comparison voltage range into 2 N  units, and generates and outputs 2 N  comparison voltages, which become references of the N codes provided by the A/D converter, respectively. 
     In this manner, the A/D converter of  FIG. 8  sets multiple comparison voltage ranges, and selects a specified one of the comparison voltage ranges according to the first shutter control signal CSHT 1 , so that the A/D converter increases its resolution. 
     In the above-mentioned configuration, the m resistance circuits  51   a  to  5 ( m ) a  and the m switches  51   b  to  5 ( m ) b  are disposed between the first reference voltage Vref 1  and the first comparison voltage generating circuit R 21 , and the voltage differences between the comparison voltages provided by the A/D converter are varied. However, if necessary, the m resistance circuits  51   a  to  5 ( m ) a  and the m switches  51   b  to  5 ( m ) b  may be disposed between the second reference voltage Vref 2  and the 2 N -th comparison voltage generating circuit R 2 ( 2   N ), and the voltage differences between the comparison voltages provided by the A/D converter may be varied. 
     Similarly, first resistance circuits and switches may be disposed between the first reference voltage Vref 1  and the first comparison voltage generating circuit R 11 , and second resistance circuits and switches may be disposed between the second reference voltage Vref 2  and the 2 N -th comparison voltage generating circuit R 2 ( 2   N ), and the voltage differences between the comparison voltages provided by the A/D converter may be varied. 
       FIG. 9  is a block diagram showing an optical pointing device according to a second embodiment of the present invention. 
     Unlike the optical point device of  FIG. 4 , the optical point device of  FIG. 9  further includes a second image sensor  350  for detecting luminance, and a second A/D converter  360 . A first image sensor  310  is identical to the image sensor  210  of  FIG. 4 . 
     The second image sensor  350  is for detecting only the luminance, and thus has pixels smaller than those of the first image sensor  310 . Generally, the optical pointing device compares a previously obtained image with a currently obtained image in order to calculate a movement value. To this end, each image requires a predetermined level of resolution, so that the first image sensor  310  requires a large number of pixels (e.g. one hundred thousand pixels). In contrast, the second image sensor  350  can sufficiently detect the luminance even with a very small number of pixels (e.g. ten pixels or fewer). 
     The second A/D converter  360  receives second analog signals IMO 2  from the second image sensor  350 , and converts the second analog signals IMO 2  into second digital signals ADCO 2 . Since the second image sensor  350  is for detecting the luminance, the second A/D converter  360  compares a fixed comparison voltage with the second analog signals IMO 2 , thereby generating the second digital signals ADCO 2 , like the A/D converter  120  of  FIG. 1 . The second digital signals ADCO 2  generated from the second A/D converter  360  may have the same number of N bits or a different number of bits, compared to a first A/D converter  320 . 
     Meanwhile, like the image data processor  230  of  FIG. 4 , an image data processor  330  of  FIG. 9  outputs a first shutter control signal CSHT 1  in response to first digital signals ADCO 1  output by the first A/D converter  320  as well as an luminance control signal CLUM for controlling a comparison voltage range of the first A/D converter  320  in response to the second digital signals ADCO 2  received from the second A/D converter  360  aside from the first shutter control signal CSHT 1 . 
     Unlike the A/D converter  220  of  FIG. 4 , the first A/D converter  320  varies a comparison voltage range in response to the luminance control signal CLUM, and converts the first analog signals IMO 1  received from the first image sensor  310  into the first digital signals ADCO 1  according to the varied comparison voltage range. 
     Since the first A/D converter  320  varies the comparison voltage range in response to the luminance control signal CLUM generated in response to the second digital signals ADCO 2 , the first A/D converter  320  can more accurately adjust the comparison voltage range to actual luminance, compared to the A/D converter  220  of  FIG. 4  which varies the comparison voltage range in response to the first shutter control signal CSHT 1  which the image data processor  230  generates in response to the previously adjusted digital signals ADCO. 
     In the above-mentioned configuration, the image data processor  330  generates the first shutter control signal CSHT 1  in response to the first digital signals ADCO 1 . However, the image data processor  330  may generate the first shutter control signal CSHT 1  in response to the second digital signals ADCO 2 . Since the second image sensor  350  has a very small number of pixels compared to the first image sensor  310 , the image data processor  330  can reduce an amount of data to be calculated in order to generate the first shutter control signal CSHT 1  when generating the first shutter control signal CSHT 1  in response to the second digital signals ADCO 2 . Accordingly, the image data processor  330  can rapidly generate the first shutter control signal CSHT 1 . 
     Further, since the first A/D converter  320  varies the comparison voltage range in response to the luminance control signal CLUM, the image data processor  330  can directly generate a second shutter control signal CSHT 2  having a predetermined pulse width in response to the first digital signals ADCO 1 . In this manner, when the image data processor  330  is configured to generate the second shutter control signal CSHT 2 , a shutter control circuit  340  can be omitted. 
       FIG. 10  is an internal block diagram of the A/D converter of  FIG. 9 . 
     Referring to  FIG. 10 , like the A/D converter  220  of  FIG. 5 , the A/D converter  320  includes a variable comparison voltage generator  321  and an N-bit comparator  323 . The N-bit comparator  323  is identical to the N-bit comparator  223  of  FIG. 5 . However, the variable comparison voltage generator  321  varies the comparison voltage range provided to the N-bit comparator  323  in response to the luminance control signal CLUM, unlike the variable comparison voltage generator  221  of  FIG. 5  which receives the first shutter control signal CSHT 1  to vary the reference voltage. The variable comparison voltage generator  321  divides the varied comparison voltage range into N units, and generates and outputs N comparison voltages. Thus, voltage differences between the comparison voltages generated from the variable comparison voltage generator  321  are variously adjusted according to the luminance control signal CLUM. 
     The AD converter  320  of  FIG. 10  has the same configuration as the AD converter  220  of  FIG. 5 , except that it varies the comparison voltage range in response to the luminance control signal CLUM rather than the first shutter control signal CSHT 1 . Thus, the A/D converters  220  of  FIGS. 7 and 8  may be configured so that the variable comparison voltage generators  221  and  221 - 1  receive the luminance control signal CLUM instead of the first shutter control signal CSHT 1 . Thereby, the A/D converters  220  of  FIGS. 7 and 8  can be implemented into the A/D converter  320  of  FIG. 10 , and thus they are not shown separately. 
     According to circumstances, one of the first shutter control signal CSHT 1  and the luminance control signal CLUM may be selected to vary the comparison voltage range. In order to select one of the first shutter control signal CSHT 1  and the luminance control signal CLUM to thereby vary the comparison voltage range, a selection switch (not shown) for receiving the first shutter control signal CSHT 1  and the luminance control signal CLUM is additionally provided, and the image data processor  330  applies a selection signal (not shown) to the selection switch, so that the selection switch can select one of the first shutter control signal CSHT 1  and the luminance control signal CLUM to apply it to the comparison voltage generator  321 . Here, the selection switch may be installed on the A/D converter  320 . However, without the separate selection switch, the image data processor  330  may select one of the first shutter control signal CSHT 1  and the luminance control signal CLUM to directly apply it to the comparison voltage generator  321 . 
       FIG. 11  is a block diagram showing an optical pointing device according to a third embodiment of the present invention. 
     As described above, the optical pointing device of  FIG. 4  adjusts the exposure time, i.e. the shutter-on time in order to adjust the quantity of light incident on the image sensor  210 . Further, the A/D converter  220  varies the comparison voltage range in response to the first shutter control signal CSHT 1 , and converts the analog signals IMO into the digital signals according to the varied comparison voltage range. This adjustment of the exposure time and the comparison voltage range is directed to obtain the high-resolution image and calculate the accurate movement value MV using the obtained image. 
     A conventional optical pointing device can adjust only the exposure time TS of an image sensor in order to obtain a high-resolution image. In contrast, the optical pointing device of  FIG. 4  can vary the comparison voltage range. However, since the A/D converter  220  varies the comparison voltage range in response to the first shutter control signal CSHT 1  that determines the exposure time TS, the optical pointing device of  FIG. 4  cannot separately adjust the exposure time TS of the image sensor and the comparison voltage range in a practical sense. 
     If the exposure time TS of the image sensor and the comparison voltage range can be separately adjusted, it is possible to obtain a high-resolution image as well as an area capable of obtaining such an image. Since the exposure time TS of the image sensor  410  can generally be adjusted only within a designated range, minimum and maximum exposure times are preset. Thus, when a large quantity of light is incident on the image sensor  410  despite the exposure time TS of the image sensor  410  being minimum, or when a small quantity of light is incident on the image sensor  410  despite the exposure time TS of the image sensor  410  being maximum, it is difficult to obtain an accurate image. Further, when a current quantity of light incident on the image sensor  410  becomes much or less than a previous quantity of light within a narrow range, it is not necessary to adjust both the exposure time TS and the comparison voltage range. 
     In the optical pointing device of  FIG. 11 , an image data processor  430  applies a first shutter control signal CSHT 1  to a shutter control circuit  440 , and a separate conversion control signal CTR to an A/D converter  420 . The first shutter control signal CSHT 1  and the conversion control signal CTR are different from each other, but they are both generated by the image data processor  430  in response to a digital signals ADCO. Thus, the image data processor  430  may adjust one of the first shutter control signal CSHT 1  and the conversion control signal CTR to which priority is given, and then adjust the other. The light source  400 , image sensor  410  and shutter control circuit  440  are identical to the light source  200 , image sensor  210  and shutter control circuit  240  of the optical pointing device of  FIG. 4 , and so description thereof will be omitted. Further, the A/D converter  420  is identical to the A/D converter  220  of  FIG. 4 , except that it receives the conversion control signal CTR instead of the first shutter control signal CSHT 1 , and so description thereof will not be made separately. 
       FIGS. 12 and 13  are flowcharts explaining how the image data processor of  FIG. 11  adjusts the exposure time of the image sensor and the comparison voltage range. 
       FIG. 12  shows a method in which the image data processor gives priority to the comparison voltage range and adjusts the exposure time of the image sensor and the comparison voltage range. 
     First, in the event of initial operation of the optical pointing device, the image data processor  430  applies a conversion control signal CTR having a preset initial value to the A/D converter  420 , and the first shutter control signal CSHT 1  to the shutter control circuit  440 . The shutter control circuit  440  controls an electronic shutter of the image sensor  410  in response to the first shutter control signal CSHT 1  to determine the exposure time TS of the image sensor. The image sensor  410  outputs analog signals IMO of pixels thereof to the A/D converter  420  according to a quantity of light that is emitted from the light source  400  and reflected from a subject. The A/D converter  420  converts the analog signals IMO into digital signals ADCO in response to the conversion control signal CTR, and outputs the digital signals to the image data processor  430  (S 101 ). 
     When receiving the digital signals ADCO of the pixels of the image sensor, the image data processor  430  calculates an average value of the received digital signals ADCO of the pixels (S 103 ). It is determined whether or not the calculated average value is greater to a set maximum value (S 105 ). If the calculated average value is less than or equal the maximum value, it is determined whether the average value is less than a set minimum value (S 107 ). If the average value is greater than or equal the set minimum value, the average value results in a value between the set maximum and minimum values. Thus, the first shutter control signal CSHT 1  for adjusting the exposure time and the conversion control signal CTR for varying the comparison voltage range are maintained without change (S 109 ). 
     Here, the set maximum and minimum values are target maximum and minimum values of the average value for keeping the quantity of light incident on the image sensor maintained within a proper range. 
     When the average value is greater than the set maximum value, it means that a large quantity of light is incident on the image sensor  410 . Thus, the conversion control signal CTR for adjusting the comparison voltage range is adjusted to increase its value so as to correspond to the analog signals IMO having a relatively high voltage level due to high luminance (S 11 ). Then, it is determined whether or not a current value of the first shutter control signal CSHT 1  is less than or equal to a set maximum value of the first shutter control signal CSHT 1  (S 113 ). If a current value of the first shutter control signal CSHT 1  is less than or equal to a set maximum value of the first shutter control signal CSHT 1 , the first shutter control signal CSHT 1  is maintained as it is (S 115 ). However, if a current value of the first shutter control signal CSHT 1  is greater than a set maximum value of the first shutter control signal CSHT 1 , the value of the first shutter control signal CSHT 1  is reduced such that the exposure time TS is reduced (S 117 ). 
     Here, it is assumed that the value of the first shutter control signal CSHT 1  is proportional to the exposure time TS. Further, it is assumed that the A/D converter  420  adjusts the comparison voltage range so as to correspond to the analog signals IMO having a high voltage level when the conversion control signal CTR has a high value, and so as to correspond to the analog signals IMO having a low voltage level when the conversion control signal CTR has a low value. 
     Thus, in order to reduce the exposure time TS, it is necessary to reduce the value of the first shutter control signal CSHT 1 , and to increase the value of the conversion control signal CTR such that the comparison voltage range corresponds to the analog signals IMO having the high voltage level. 
     In contrast, when the average value is less than the set minimum value, it means that a small quantity of light is incident on the image sensor  410 . Thus, the conversion control signal CTR for adjusting the comparison voltage range is adjusted to reduce its value so as to correspond to the analog signals IMO having a relatively low voltage level due to low luminance (S 119 ). Then, it is determined whether or not a current value of the first shutter control signal CSHT 1  is greater than or equal to a set minimum value of the first shutter control signal CSHT 1  (S 121 ). If a current value of the first shutter control signal CSHT 1  is greater than or equal to a set minimum value of the first shutter control signal CSHT 1 , the first shutter control signal CSHT 1  is maintained as it is (S 123 ). However, if a current value of the first shutter control signal CSHT 1  is less than a set minimum value of the first shutter control signal CSHT 1 , the value of the first shutter control signal CSHT 1  is increased such that the exposure time TS is increased (S 125 ). 
       FIG. 13  shows a method in which the image data processor gives priority to the exposure time of the image sensor and adjusts the exposure time and the comparison voltage range. 
     When the digital signals ADCO of the pixels of the image sensor are applied to the image data processor  430  by initial operation of the optical pointing device, the image data processor  430  calculates an average value of the applied digital signals ADCO of the pixels (S 203 ). It is determined whether or not the calculated average value is greater than to a set maximum value (S 205 ). If the calculated average value is less than or equal the maximum value, it is determined whether the average value is less than a set minimum value (S 207 ). If the average value is greater than or equal the set minimum value, the average value results in a value between the set maximum and minimum values. Thus, the first shutter control signal CSHT 1  for adjusting the exposure time and the conversion control signal CTR for varying the comparison voltage range are maintained without change (S 209 ). In other words, the image data processor has the same operation as that of  FIG. 12 . 
     When the average value is greater than the set maximum value, the first shutter control signal CSHT 1  is adjusted to reduce its value such that the exposure time TS is reduced (S 211 ). Then, it is determined whether or not the value of the conversion control signal CTR for adjusting the comparison voltage range is less than or equal to a set maximum value of the conversion control signal CTR (S 213 ). If the value of the conversion control signal CTR is less than or equal to a set maximum value of the conversion control signal CTR, the conversion control signal CTR is maintained as it is (S 215 ). However, if the value of the conversion control signal CTR is greater than the set maximum value of the conversion control signal CTR, the value of the conversion control signal CTR is reduced (S 217 ). 
     In contrast, when the average value is less than the set minimum value, it means that a small quantity of light is incident on the image sensor  410 . Thus, the first shutter control signal CSHT 1  is adjusted to increase its value such that the exposure time TS is increased (S 219 ). Then, it is determined whether or not the value of the conversion control signal CTR for adjusting the comparison voltage range is less than or equal to a set minimum value of the conversion control signal CTR (S 221 ). If the value of the conversion control signal CTR is greater than or equal to a set minimum value of the conversion control signal CTR, the conversion control signal CTR is maintained as it is (S 223 ). However, if the value of the conversion control signal CTR is less than the set minimum value of the conversion control signal CTR, the value of the conversion control signal CTR is increased (S 225 ). 
     Consequently, in  FIGS. 12 and 13 , the image data processor  430  adjusts the exposure time of the image sensor and the comparison voltage range by giving priority to one of the exposure time and the comparison voltage range. In practical use, the methods of  FIGS. 12 and 13  are sequentially alternated. Thus, the determining steps such as step S 113  of  FIG. 12  and steps S 213  and S 221  of  FIG. 13  are required. 
       FIG. 14  is a view explaining a method of rapidly determining the exposure time of an image sensor in an optical pointing device according to an exemplary embodiment of the present invention. 
     In the optical pointing device as described above, the light incident on the image sensor is light that is reflected by a subject and input. Thus, the quantity of light incident on the image sensor varies according to whether the subject has a bright surface or a dark surface, and the exposure time TS is adjusted according to the quantity of incident light. Consequently, the exposure time TS is varied according to whether the subject has a bright surface or a dark surface. However, the surface of the subject may not have constant brightness. For example, when the surface of the subject has a pattern in which white and black colors are alternately arranged, the exposure time has to be sharply varied as the optical pointing device moves. This sharp variation is increased as the movement of the optical pointing device becomes faster. Further, even when the movement of the optical pointing device is slow, such variation is increased when the surface of the subject has a pattern in which the white and black colors are alternately arranged at very dense intervals. In this manner, when the optical pointing device quickly moves on the surface of the subject having the alternating bright and dark patterns, it is necessary to rapidly adjust the exposure time. 
     In order to rapidly determine the exposure time, the optical pointing device may include a separate image sensor as shown in  FIG. 9 . As a concrete example for the separate image sensor, a part of the image sensor can be used as the image sensor. The easiest method is to use circumferential pixels of the image sensor, which are irrelevant to the image used in the optical pointing device, as pixels for the image sensor. The optical pointing device of  FIG. 9  requiring the separate image sensor makes its design difficult, and requires additional production cost. 
     For this reason,  FIG. 14  shows an example in which the exposure time TS is set as a unit of section according to a code range so as to rapidly determine the exposure time without such a separate image sensor. Here, as in  FIG. 2 , it is assumed that the code range is defined as a value between 0 bits and 15 bits and that a central code value is 7 bits. 
     The method of rapidly determining the exposure time in the optical pointing device will be described with reference to  FIG. 14 . First, when a current average value of digital signals ADCO output by the A/D converter ranges from 0 to 3, and when a previous exposure TS belongs to section A FA, a current exposure time TS is set to four times the previous exposure time. Of course, when this set value exceeds a maximum exposure time TSmax, the current exposure time is set to the maximum exposure time TSmax. Further, the exposure time TS is actually set to a minimum value aside from 0 (zero). When the average value of digital signals ADCO ranges from 0 to 3, and when the previous exposure TS belongs to section C FC, the current exposure time TS is set to three fourths the previous exposure time. 
     If the average value of digital signals ADCO ranges from 6 to 9, the previous exposure time TS is maintained as it is. In other words, since the average value of digital signals ADCO is already located at the center of the code range, it is not necessary to adjust the exposure time. Meanwhile, when the average value of digital signals ADCO ranges from 12 to 15, the current exposure time is set to half the previous exposure time. 
     In  FIG. 14 , it is shown that the exposure time can be set to at least three sections FA, FB and FC for the code range from 3 to 6 and the code range from 9 to 12. Further, the sections for the exposure time may be differently set for each code range. In  FIG. 14 , a numeral added to the exposure time TS is relevant to a system clock value. When a system clock is varied or an operating clock of the shutter control circuit is controlled in order to reduce consumption power and increase an operating speed in the optical pointing device, it is natural that the numeral added to the exposure time TS and the sections of  FIG. 14  can be varied. 
     Consequently, the exposure time of  FIG. 14  is adjusted by dividing the average value of the digital signals ADCO and the exposure time TS into multiple sections, discriminating the current exposure time, the range corresponding to the average value of the digital signals ADCO determined according to the current exposure time, and determining the next exposure time. Since the current exposure time is designated according to a section, the exposure time may not be varied when the surface of the subject is uniform. Even if the exposure time is varied, the exposure time is set according to the section, so that the exposure time can be rapidly set. 
     Here, if the average value of the digital signals ADCO and the exposure time TS continue to be varied in the proximity of a boundary of each section, the exposure time TS has to be continuously varied, which makes it difficult to rapidly set the exposure time. Thus, the sections of the average value of the digital signals ADCO and the exposure time TS are set with a predetermined level of margin (e.g. a code range between 0.0 and 0.3) at a boundary zone of each section, so that the number of times of the variation of the exposure time TS can be reduced. This is based on a method of adding a kind of hysteresis characteristic on determining the exposure time. This method of adding the hysteresis characteristic is well known in various fields, and so detailed description thereof will be omitted. 
       FIGS. 15 and 16  are flowcharts explaining how an optical pointing device adjusts the exposure time of an image sensor and a comparison voltage range using the method of  FIG. 14 . 
       FIG. 15  shows a method in which the image data processor gives priority to the comparison voltage range and adjusts the exposure time of the image sensor and the comparison voltage range. 
     As in  FIG. 12 , when the digital signals ADCO of the pixels of the image sensor are applied to the image data processor  430  by initial operation of the optical pointing device, the image data processor  430  calculates an average value of the applied digital signals of the pixels (S 303 ). Then, a code range corresponding to the calculated average value of the digital signals ADCO is checked (S 304 ). It is determined whether or not the calculated average value is greater than to a set maximum value (S 305 ). If the calculated average value is less than or equal the set maximum value, it is determined whether the calculated average value is less than a set minimum value (S 307 ). When the average value is greater than or equal the set minimum value, the average value results in a value between the set maximum and minimum values. Thus, the first shutter control signal CSHT 1  for adjusting the exposure time and the conversion control signal CTR for varying the comparison voltage range are maintained without change (S 309 ). 
     When the average value is greater than the set maximum value, the conversion control signal CTR for adjusting the comparison voltage range is adjusted to increase its value so as to correspond to the analog signals IMO having a relatively high voltage level due to high luminance (S 311 ). Then, it is determined whether or not a current value of the first shutter control signal CSHT 1  is less than or equal to a set maximum value of the first shutter control signal CSHT 1  (S 313 ). If a current value of the first shutter control signal CSHT 1  is less than or equal to a set maximum value of the first shutter control signal CSHT 1 , the first shutter control signal CSHT 1  is maintained as it is (S 315 ). However, if a current value of the first shutter control signal CSHT 1  is greater than a set maximum value of the first shutter control signal CSHT 1 , the value of the first shutter control signal CSHT 1  is reduced so as to correspond to a smaller section of the exposure time TS (S 317 ). 
     As described with reference to  FIG. 12 , it is assumed that the value of the first shutter control signal CSHT 1  is proportional to the exposure time TS. However, since the exposure time TS is divided according to section in  FIG. 15 , the value of the first shutter control signal CSHT 1  is discretely varied in response to the exposure time TS divided according to section. Further, it is assumed that the A/D converter  420  adjusts the comparison voltage range so as to correspond to the analog signals IMO having a high voltage level when the conversion control signal CTR has a high value, and so as to correspond to the analog signals IMO having a low voltage level when the conversion control signal CTR has a low value. 
     In contrast, when the average value is less than the set minimum value, it means that a small quantity of light is incident on the image sensor  410 . Thus, the conversion control signal CTR for adjusting the comparison voltage range is adjusted to reduce its value so as to correspond to the analog signals IMO having a relatively low voltage level due to low luminance (S 319 ). Then, it is determined whether or not a current value of the first shutter control signal CSHT 1  is greater than or equal to a set minimum value of the first shutter control signal CSHT 1  (S 321 ). If a current value of the first shutter control signal CSHT 1  is greater than or equal to a set minimum value of the first shutter control signal CSHT 1 , the first shutter control signal CSHT 1  is maintained as it is (S 323 ). However, if a current value of the first shutter control signal CSHT 1  is less than a set minimum value of the first shutter control signal CSHT 1 , the value of the first shutter control signal CSHT 1  is increased so as to correspond to greater section of the exposure time TS (S 325 ). 
       FIG. 16  shows a method in which the image data processor gives priority to the exposure time of the image sensor and adjusts the exposure time and the comparison voltage range. 
     When the digital signals ADCO of the pixels of the image sensor are applied to the image data processor  430  by initial operation of the optical pointing device, the image data processor  430  calculates an average value of the applied digital signals ADCO of the pixels (S 403 ). Then, a code range corresponding to the calculated average value of the digital signals ADCO is checked (S 404 ). It is determined whether or not the calculated average value is greater than to a set maximum value (S 405 ). If the calculated average value is less than or equal the maximum value, it is determined whether the average value is less than a set minimum value (S 407 ). If the average value is greater than or equal the set minimum value, the average value results in a value between the set maximum and minimum values. Thus, the first shutter control signal CSHT 1  for adjusting the exposure time and the conversion control signal CTR for varying the comparison voltage range are maintained without change (S 409 ). 
     When the average value is greater than the set maximum value, the first shutter control signal CSHT 1  is adjusted to reduce its value so as to correspond to a smaller section of the exposure time TS (S 411 ). Then, it is determined whether or not the value of the conversion control signal CTR for adjusting the comparison voltage range is less than or equal to a set maximum value (S 413 ) of the conversion control signal CTR. If the value of the conversion control signal CTR is less than or equal to a set maximum value of the conversion control signal CTR, the conversion control signal CTR is maintained as it is (S 415 ). However, if the value of the conversion control signal CTR is greater than the set maximum value of the conversion control signal CTR, the value of the conversion control signal CTR is reduced (S 417 ). 
     In contrast, when the average value is less than the set minimum value, the conversion control signal CTR for adjusting the comparison voltage range is adjusted to increase its value so as to correspond to a greater section of the exposure time TS (S 419 ). Then, it is determined whether or not the value of the conversion control signal CTR for adjusting the comparison voltage range is greater than or equal to a set minimum value of the conversion control signal CTR (S 421 ). If the value of the conversion control signal CTR is greater than or equal to a set minimum value of the conversion control signal CTR, the conversion control signal CTR is maintained as it is (S 423 ). However, if the value of the conversion control signal CTR is less than the set minimum value of the conversion control signal CTR, the value of the conversion control signal CTR is increased (S 425 ). 
     In practical use, it is natural that the methods of  FIGS. 15 and 16  are sequentially alternated. In this case, the conversion control signal CTR or the first shutter control signal CSHT 1  may be beyond the set maximum or minimum value, so that the determining steps such as step S 313  of  FIG. 15  and steps S 413  and S 421  of  FIG. 16  are required. 
       FIGS. 17 and 18  are flowcharts explaining how the optical pointing device of  FIG. 9  adjusts the exposure time of the image sensor and the comparison voltage range using the method of  FIG. 13 . 
     The optical pointing device of  FIG. 9  includes the second image sensor  350  and the second A/D converter  360  generating the second digital signals ADCO 2  in response to the second analog signals IMO 2  output by the second image sensor  350 , in addition to the first image sensor  310  and the first A/D converter  320 . The image data processor  330  outputs the luminance control signal CLUM to the first A/D converter  320  in response to the second digital signals ADCO 2 , as well as the first shutter control signal CSHT 1  to the shutter control signal  340  in response to the first digital signals ADCO 1 . The methods of  FIGS. 17 and 18  are similar to those of  FIGS. 15 and 16 , except that the luminance control signal CLUM generated in response to the second digital signals ADCO 2  is used instead of the conversion control signal CTR. The luminance control signal CLUM and the conversion control signal CTR have been described as different signals. However, since both of the two signals are for varying the comparison voltage range of the A/D converter, the luminance control signal CLUM may be interpreted as the conversion control signal CTR. 
     Further, as described above, the first shutter control signal CSHT 1  may be generated in response to the second digital signals ADCO 2 . Thus, the first shutter control signal CSHT 1  may be generated in response to one of the first digital signals ADCO 1  and the second digital signals ADCO 2 . At this time, the first shutter control signal CSHT 1  may be generated so as to correspond to the exposure time section. If the first shutter control signal CSHT 1  is generated in response to the second digital signals ADCO 2  so as to correspond to the exposure time section, the first shutter control signal CSHT 1  can be generated as rapidly as possible. Further, as described above, the image data processor  330  may directly generate the second shutter control signal CSHT 2  in response to the first and second digital signals ADCO 1  and ADCO 2 . 
       FIGS. 19 and 20  show an operating difference between a conventional optical pointing device and a proposed optical pointing device. 
     As shown in  FIGS. 19 and 20 , the proposed optical pointing device can not only determine the exposure time at a higher speed when using the methods of  FIGS. 15 through 18 , but can also obtain the high-resolution image and the resulting accurate movement value even when using a low-resolution A/D converter. Thus, even when the surface of the subject is mixed with the bright and dark patterns, and even when the optical pointing device rapidly moves, the accurate movement value can be calculated. 
     The case of varying the comparison voltage range of the N-bit comparator has been described above. However, the DC offset of an input signal may be varied. In this case, similar effects can be obtained. 
     As set forth above, the optical pointing device and the method of adjusting exposure time and comparison voltage range of the optical pointing device can obtain a high-resolution image using a low-resolution A/D converter, so that they can calculate an accurate movement value, and reduce production cost and power consumption. 
     Although exemplary embodiments of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.