Abstract:
A voltage is generated after a touch screen is touched, and the voltage is transformed into a digital signal for detecting a touch point which the touch screen is touched. A touch sensing system includes a first switch having a first node coupled to a first voltage source, a second switch having a node coupled to a second voltage source, a first resistor coupled between the first switch and the second switch in a series connection, a comparator having a first input node coupled to a second node of the first switch, a successive approximation register (SAR) having an input node coupled to an output node of the comparator, and a simulation circuit. An output voltage of the simulation circuit is continuously adjusted by the comparator and the simulation circuit to transform the voltage generated on the touch screen into a digital signal.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a screen measurement system, and more particularly, to a resistive touch screen measurement system. 
   2. Description of the Prior Art 
   Primary resistive screen measurement systems in use recently comprise single-ended mode resistive screen measurement systems and differential mode resistive screen measurement systems. 
   Please refer to  FIG. 1 , which is a diagram of an equivalent circuit of a resistive touch screen measurement system utilizing the differential mode. Please also refer to  FIG. 2 , which is a status table of the elements of the resistive touch screen measurement system  100  measuring a first coordinate factor and a second coordinate factor for determining a coordinate of a touch point corresponding to a touch screen. The coordinate is a two-dimensional coordinate utilized for representing a planar position. Therefore, the first coordinate factor represents the X-coordinate of the coordinate, and the second coordinate factor represents the Y-coordinate of the coordinate. The resistive touch screen system  100  comprises a touch-sensing system  101 , an analog-to-digital converter  103 , and a touch screen  120  disposed on the touch-sensing system  101  for receiving a signal generated from an artificial touch and transmitting the signal to the touch-sensing system  101 . The touch screen  120  is not included in the touch-sensing system  101  and overlaps with the touch-sensing system  101 . The resistive touch screen measurement system  100  further comprises a first voltage source  151  and a second voltage source  153  for providing a bias voltage of the touch-sensing system  101 . The touch sensing system  101  comprises a first transistor  105 , a second transistor  107 , a third transistor  109 , a fourth transistor  111 , a first resistor  113 , a second resistor  115 , a third resistor  117 , a fourth resistor  119 , and a touch point  129 . The first resistor  113  is coupled to the first transistor  105  at a first output  121 . The second resistor  115  is coupled to the second transistor  107  at a second output  123 . The third resistor  117  is coupled to the third transistor  109  at a third output  125 . The fourth resistor  119  is coupled to the fourth transistor  111  at a fourth output  127 . The touch point  129  is coupled to the first resistor  113 , the second resistor  115 , the third resistor  117 , and the fourth resistor  119 . The analog-to-digital converter  103  comprises an analog input  131 , a reference upper bound input  133 , a reference lower bound input  135 , and a digital output  137 . The analog input  131  is coupled to the first output  121  through a switch  171  and is coupled to the third output  125  through a switch  173 . The reference upper bound input  133  is coupled to the first output  121  through a switch  175  and is coupled to the third output  125  through a switch  177 . The reference lower bound  135  is coupled to the second output  123  through a switch  179  and is coupled to the fourth output  127  through a switch  181 . The first resistor  113  and the second resistor  115  are physically the same resistor, and the third resistor  117  and the fourth resistor  119  are physically the same resistor. However, the artificial touch on the touch screen  120  generates a corresponding signal at the touch point  129 . Therefore, the artificial touch temporarily separates the first resistor  113  and the second resistor  115 . The artificial touch also temporarily separates the third resistor  117  and the fourth resistor  119 . 
   While measuring the first coordinate factor of the coordinate of the touch point  129  on the touch screen  120 , the third transistor  109  and the fourth transistor  111  are both turned on, and the first transistor  105  and the second transistor  107  are both turned off. A first output voltage is generated at the touch point  129 , and a second output voltage is generated at the first output  121  from the first output voltage through the first resistor  113 . At this time, the switch  171  coupled to the analog input  131  and the first output  121  is turned on, therefore, the second output voltage is inputted to the analog-to-digital converter  103  through the analog input  131 . Moreover, the switch  177  coupled to the reference upper bound input  133  and the third output  125  is turned on, and the switch  175  coupled to the reference upper bound input  133  and the first output  121  is turned off. Therefore, the voltage at the third output  125  is inputted to the analog-to-digital converter  103  through the reference upper bound input  133  and is regarded as an upper bound reference voltage. At this time, the switch  181  coupled to the reference lower bound input  135  and the fourth output  127  is turned on, and the switch  179  coupled to the reference lower bound input  135  and the second output  123  is turned off. Therefore, the voltage at the fourth input  127  is inputted to the analog-to-digital converter  103  through the reference lower bound input  135 . Through the functions of the analog-to-digital converter  103 , the inputted second output voltage is transformed into a corresponding digital signal for representing the first coordinate factor of the coordinate and outputting the first coordinate factor through the digital output  137 . 
   While measuring the second coordinate factor of the coordinate of the touch point  129  on the touch screen  120 , the first transistor  105  and the second transistor  107  are turned on, and the third transistor  109  and the fourth transistor  111  are turned off. Therefore, a third output voltage is generated at the touch point  129 , and a fourth output voltage is generated from the third output voltage at the third output  125  through the third resistor  117 . At this time, the switch  173  coupled to the analog input  131  and the third output  125  is turned on, and the switch  171  coupled to the analog input  131  and the first output  121  is turned off. Therefore, the fourth output voltage is inputted to the analog-to-digital converter  103  through the analog input  131 . Moreover, the switch  177  coupled to the reference upper bound input  133  and the third output  125  is turned off, and the switch  175  coupled to the reference upper bound input  133  and the first output  121  is turned on. Therefore, the voltage at the first output  121  is inputted to the analog-to-digital converter  103  through the reference upper bound input  133  and is regarded as an upper bound reference voltage. At this time, the switch  181  coupled to the reference lower bound input  135  and the fourth output  127  is turned off, and the switch  179  coupled to the reference lower bound input  135  and the second output  123  is turned on. Therefore, the voltage at the second output  123  is inputted to the analog-to-digital converter  103  through the reference lower bound input  135  and is regarded as a lower bound reference voltage. Through the functions of the analog-to-digital converter  103 , the inputted fourth output voltage is transformed into a corresponding digital signal for representing the second coordinate factor of the coordinate and outputting the second coordinate factor through the digital output  137 . 
   However, while measuring the first coordinate factor of the coordinate of the touch point  129  on the touch screen  120  and after the second output voltage is inputted to the analog-to-digital converter  103 , the reference upper bound input  133  must be continuously supplied with the voltage input at the third output  125 , and the reference lower bound input  135  must be continuously supplied with the voltage input at the fourth output  127 , therefore, the third transistor  109  and the fourth transistor  111  cannot be turned off. If the third transistor  109  and the fourth transistor  111  are turned off at this time, the upper bound reference voltage and the lower bound reference voltage cannot be maintained anymore, and larger offsets are thus generated. Since the third transistor  109  and the fourth transistor  111  cannot be turned off, the third transistor  109  and the fourth transistor  111  must work for a longer time, and a large power consumption caused by not turning off the third transistor  109  and the fourth transistor  111  is thus generated. It means that a resistive touch screen measurement system utilizing the differential mode is in company with a large power consumption. 
   Please refer to  FIG. 3 , which is a diagram of the resistive touch screen measurement system  200  utilizing a single-ended mode. Please refer to  FIG. 4  also.  FIG. 4  is a status table of the elements of the resistive touch screen measurement system  200  measuring a first coordinate factor and a second coordinate factor for determining a coordinate of a touch point on a touch screen. The coordinate is a two-dimensional coordinate for representing a planar coordinate as well as in  FIG. 1 . The first coordinate factor of the coordinate represents the X-coordinate of the coordinate. The second coordinate factor of the coordinate represents the Y-coordinate of the coordinate. The resistive touch screen measurement system  200  comprises a touch-sensing system  201 , an analog-to-digital converter  203 , and a touch screen  220 . The touch screen  220  is disposed above the touch-sensing system  201  for receiving a signal generated from an artificial touch and transmitting the signal to the touch-sensing system  201 . In  FIG. 3 , the touch screen  220  is not comprised by the touch-sensing system  201  and overlaps with the touch-sensing system  201 . The resistive touch screen measurement system  200  further comprises a first voltage source  251  and a second voltage source  253  for providing bias voltages to the touch-sensing system  201 . The touch-sensing system  201  comprises a first transistor  205 , a second transistor  207 , a third transistor  209 , a fourth transistor  211 , a first resistor  213 , a second resistor  215 , a third resistor  217 , a fourth resistor  219 , and a touch point  229 . The first resistor  213  is coupled to the first transistor  205  through a first output  221 . The second resistor  215  is coupled to the second transistor  207  through a second output  223 . The third resistor  217  is coupled to the third transistor  209  through a third output  225 . The fourth resistor  219  is coupled to the fourth transistor  211  through a fourth output  227 . The touch point  229  is coupled to the first resistor  213 , the second resistor  215 , the third resistor  217 , and the fourth resistor  219 . The analog-to-digital converter  203  comprises an analog input  231 , a reference upper bound input  233 , a reference lower bound input  235 , and a digital output  237 . The analog input  231  is coupled to the first output  221  through a switch  271  and is coupled to the third output  225  through a switch  273 . The reference upper bound input  233  is coupled to a direct current (DC) voltage source VDD. The reference lower bound input  235  is coupled to the ground. The first resistor  213  and the second resistor  215  are physically the same resistor. The third resistor  217  and the fourth resistor  219  are physically the same resistor. The artificial touch generated on the touch screen  220  generates a signal on the touch point  229  of the touch-sensing system  201 . The artificial touch thus separates the first resistor  213  and the second resistor  215 . The artificial touch also separates the third resistor  217  and the fourth resistor  219 . 
   While measuring the first coordinate factor of the coordinate of the touch point  229  on the touch screen  220 , the third transistor  209  and the fourth transistor  211  are turned on, and the first transistor  205  and the second transistor  207  are turned off. Therefore, a first output voltage is generated on the touch point  229 , and a second output voltage is generated from the first output voltage at the first output  221  through the first resistor  213 . At this time, the switch  271  coupled to the analog input  231  and the first output  221  is turned on, and the switch  273  coupled to the analog input  231  and the third output  225  is turned off so that the second output voltage is inputted to the analog-to-digital converter  203  through the analog input  231 . Moreover, the DC voltage source VDD at the reference upper bound input  233  is regarded as an upper bound reference voltage, and the ground GND at the reference lower bound input  235  is regarded as a lower bound reference voltage. Through the functions of the analog-to-digital converter  203 , the inputted second output voltage coupled is transformed into a corresponding digital signal for representing the first coordinate factor of the coordinate of the touch point and outputting the digital signal from the digital output  237 . 
   While measuring the second coordinate factor of the touch point  229  on the touch screen  220 , the first transistor  205  and the second transistor  207  are turned on, and the third transistor  209  and the fourth transistor  211  are turned off. Therefore, a third output voltage is generated at the touch point  229 , and a fourth output voltage is generated from the third output voltage at the third output  225  through the third resistor  217 . At this time, the switch  273  coupled to the analog input  231  and the third output  225  is turned on, and the switch  271  coupled to the analog input  231  and the first output  221  is turned off. Therefore, the fourth output voltage is inputted to the analog-to-digital converter  203  through the analog input  231 . Besides, the DC voltage source VDD at the reference upper bound input  233  is regarded as an upper bound reference voltage, and the ground GND at the reference lower bound input  235  is regarded as a reference lower bound voltage. Through the functions of the analog-to-digital converter  203 , the inputted fourth output voltage is transformed into a corresponding digital signal for representing the second coordinate factor of the coordinate of the touch point and outputting the digital signal from the digital output  237 . 
   While measuring the first coordinate factor of the coordinate of the touch point  229  on the touch screen  220  and after the second output voltage is inputted to the analog-to-digital converter  203 , the analog-to digital converter  203  does not have to maintain the voltages at the third output  225  and the fourth output  227  in comparison with the analog-to-digital converter  103 . Therefore, the power consumption of the resistive touch screen measurement system  200  utilizing the single-ended mode is less than the power consumption of the resistive touch screen measurement system  100  utilizing the differential mode. However, since the upper bound reference voltage of the analog-to-digital converter  203  is the DC voltage source VDD, the lower bound reference voltage of the analog-to-digital converter  203  is the ground GND, the actual voltage at the first output  221  is VDD-Vsd, and the actual voltage at the second output  223  is GND+Vds, therefore, a larger offset is generated in the bias voltage and the gain of the analog-to-digital converter  203 . Vsd represents the voltage difference between the source and the drain of the first transistor. Vds represents the voltage difference between the drain and the source of the second transistor  207 . The values of Vsd and Vds are both related to bias voltages and temperature of the transistors, and even to the ratio of the width to the length of the transistors. Therefore, the values of Vds and Vsd vary a lot. Moreover, in comparison with the analog-to-digital converter  203 , the bias voltage and the gain of the analog-to-digital converter  103  are more precise since the analog-to-digital converter  103  utilizes the voltages outputted at the third output  125  and the fourth output  127  as the upper bound reference voltage and the lower bound reference voltage. While measuring the second coordinate factor of the coordinate of the touch point  229  on the touch screen  220 , the same situation takes place. 
   SUMMARY OF THE INVENTION 
   The claimed invention discloses a resistive touch screen measurement system comprises a first switch having a first terminal coupled to a first voltage source, a second switch having one terminal coupled to a second voltage source, a first resistor coupled to the first switch and the second switch in a series connection, a comparator having a first input coupled to a second terminal of the first switch, a successive-approximation register having an input coupled to an output of the comparator, and a simulation circuit. The simulation circuit comprises a second resistor, a third switch having a first terminal coupled to the first voltage source, and a second terminal coupled to the second resistor and a second input of the comparator, a fourth switch coupled to the second resistor and the second voltage source, a fifth switch set having a plurality of switches, each switch coupled to the second resistor and the first voltage source in a parallel connection with the third switch, and a control terminal of each switch of the fifth switch set coupled to an output of a set of outputs of the successive-approximation register, and a sixth switch set having a plurality of switches, each switch coupled to the second resistor and the second voltage source in a parallel connection with the fourth switch, and a control terminal of each switch of the sixth switch set coupled to an output of the set of outputs of the successive-approximation register. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of an equivalent circuit of a resistive touch screen measurement system utilizing the differential mode. 
       FIG. 2  is a status table of the elements of the resistive touch screen measurement system of  FIG. 1  for measuring a first coordinate factor and a second coordinate factor for determining a coordinate of a touch point corresponding to a touch screen. 
       FIG. 3  is a diagram of the resistive touch screen measurement system utilizing a single-ended mode. 
       FIG. 4  is a status table of the elements of the resistive touch screen measurement system of  FIG. 3  for measuring a first coordinate factor and a second coordinate factor for determining a coordinate of a touch point on a touch screen. 
       FIG. 5  is a diagram of the equivalent circuit of a touch-sensing system of the resistive touch screen measurement system of the present invention. 
       FIG. 6  is a diagram of the equivalent circuit of a signal generating system of the resistive touch screen measurement system of the present invention. 
       FIG. 7  is the status table of the elements of the resistive touch screen measurement system of the present invention under the first phase. 
       FIG. 8  is a diagram the equivalent circuit of the simulation circuit of the resistive touch screen measurement system of the present invention. 
       FIG. 9  is a diagram of an analog-to-digital converter of the resistive touch screen measurement system of the present invention. 
       FIG. 10  is a status table of the elements of the resistive touch screen measurement system of the present invention under the second phase. 
   

   DETAILED DESCRIPTION 
   The operations of the resistive touch screen measurement system of the present invention are predominantly classified into a first phase and a second phase. In the first phase, a digital signal corresponding to the first coordinate factor of the coordinate of the touch point on the touch screen is generated, and a signal corresponding to the second coordinate factor of the coordinate of the touch point on the touch screen is also generated. In the second phase, both the digital signals are utilized for controlling the statuses of two sets of transistors. Therefore, an upper bound reference voltage and a lower bound reference voltage are generated by the both sets of transistors while measuring both coordinate factors of the coordinate of the touch point on the touch screen. The upper bound reference voltage and the lower bound reference voltage are also inputted to an analog-to-digital converter for utilizing the analog-to-digital converter for measuring the coordinate factors of the coordinates of the other touch points on the touch screen. In other words, in comparison to the resistive touch screen measurement system utilizing the single-ended mode, the resistive touch screen measurement system of the present invention neutralizes the offsets of the transistors in advance. Therefore, the offset as shown in  FIG. 3  is not generated and reduced power consumption is achieved utilizing the resistive touch screen measurement system  200  of  FIG. 3 . The procedures in  FIG. 5 ,  FIG. 6 , and  FIG. 7  are related to the first phase, and the procedures in  FIG. 5 ,  FIG. 8 , and  FIG. 9  are related to the second phase. 
   Please refer to  FIG. 5 ,  FIG. 6 , and  FIG. 7  simultaneously.  FIG. 5  is a diagram of the equivalent circuit of a touch-sensing system  301  of the resistive touch screen measurement system of the present invention.  FIG. 6  is a diagram of the equivalent circuit of a signal generating system  303  of the resistive touch screen measurement system of the present invention.  FIG. 7  is the status table of the elements of the resistive touch screen measurement system under the first phase in the present invention. The resistive touch screen measurement system of the present invention further comprises a first voltage source  351  and a second voltage source  353 , and both of the voltage sources are shown in  FIG. 5  and  FIG. 6 . The resistive touch screen measurement system of the present invention further comprises a touch screen  320  disposed above the touch-sensing system  301  for receiving a signal generated from an artificial touch and transmitting the signal to the touch-sensing system  301 . In  FIG. 5 , the touch screen  320  is not comprised in the touch-sensing system  301  and overlaps with the touch-sensing system  301 . The touch-sensing system  301  comprises a first transistor  305 , a second transistor  307 , a third transistor  309 , a fourth transistor  311 , a first resistor  313 , a second resistor  315 , a third resistor  317 , and a fourth resistor  319 . The first transistor  305  has a first terminal coupled to the first voltage source  351 . The second transistor  307  has a first terminal coupled to the second voltage source  353 . The third transistor  309  has a first terminal coupled to the first voltage source  353 . The fourth transistor  311  has a first terminal coupled to the second voltage source  353 . The first resistor  313  is coupled to the first transistor  305  through a first output  321 . The second resistor  315  is coupled to the second transistor  307  through a second output  323 . The third resistor  317  is coupled to the third transistor  309  through a third output  325 . The fourth resistor  319  is coupled to the fourth transistor  311  through a fourth output  327 . The first resistor  313  and the second resistor  315  are physically the same resistor. The third resistor  317  and the fourth resistor  319  are also physically the same resistor. While a touch point  329  on the touch screen  320  of the touch-sensing system  301  is triggered by an artificial touch, the first resistor  313  and the second resistor  315  are temporarily separated, and the third resistor  317  and the fourth resistor  319  are also temporarily separated. Therefore, the equivalent circuit of the touch-sensing system  301  is thus formed as shown in  FIG. 5 . As shown in  FIG. 5 , while measuring a digital signal and triggering the touch point  329 , the touch point  329  is coupled to the first resistor  313 , the second resistor  315 , the third resistor  317 , and the fourth resistor  319 . The signal generating system  303  comprises a simulation circuit  331 , a comparator  333 , and a successive-approximation register  335 . The comparator  333  has a first input coupled to the first output  321  through a switch  371  and coupled to the second output  325  through a switch  373 . The comparator  333  also has a second input coupled to the simulation circuit  331 . The successive-approximation register  335  has an output  361 . The simulation circuit  331  comprises a fifth resistor  337 , a fifth output  347 , a fifth transistor  339 , a sixth transistor  341 , a set of first transistors  343 , a set of second transistors  345 , and a set of inverters  363 . The fifth output  347  is coupled to the fifth resistor  337  and a second input of the comparator  333 . The fifth transistor  339  has a first terminal coupled to the first voltage source  351  and a second terminal coupled to the fifth output  347 . The sixth transistor  341  is coupled to the fifth resistor  337  and the second voltage source  353 . The set of first transistor  343  is coupled to the fifth output  347  and the first voltage source  351  in parallel connection with the fifth transistor  339 . The set of second transistors  345  is coupled to the fifth resistor  337  and the second voltage source  353  in parallel connection with the sixth transistor  341 . The set of inverters  363  is coupled to the set of first transistors  339  and the set of second transistors  341 . Each of the set of first transistors  343  has a control terminal coupled to an output of the set of outputs of the successive-approximation register  335 . Each of the set of second transistors  345  also has a control terminal coupled to an output of the set of outputs of the successive-approximation register  335 . Each of the set of first transistors  343  corresponds to each of the set of second transistors  345 . The set of inverters  363  is utilized to transmit signals from the set of outputs of the successive-approximation register  335  to the control terminal of each of the set of first transistors  343 . 
   In the first phase of the present invention, while a digital signal corresponding to the first factor of the coordinate of the touch point  329  on the touch screen  320  is to be generated, the third transistor  309  and the fourth transistor  311  are turned on, and the first transistor  305  and the second transistor  307  are turned off. Therefore, an output voltage is generated at the third output  325 . At this time, the switch  373  coupled to the third output  325  and the comparator  333  is turned on, and the switch  371  coupled to the first output  321  and the comparator  333  is turned off. Therefore, the output voltage is inputted to the first input of the comparator  333 . At this time, since no voltage is generated at the fifth output  347 , therefore, no voltage is inputted at the second input of the comparator  333  also, and the comparator thus outputs a signal for representing the voltage at the first input is higher than the voltage at the second input. After the signal outputted from the comparator  333  is received at an input of the successive-approximation register  335 , a control signal is inputted at each output of the set of the first transistors  343  and the set of the second transistors  345  for turning on or turning off the set of the first transistors  343  and the set of the second transistors  345 . The statuses of a transistor of the set of the first transistors  343  and a corresponding transistor of the set of the second transistors  345  are the same. The voltage outputted at the fifth output  347  is also determined by the statuses of the set of the first transistors  343  and the set of the transistors  345  by the successive-approximation register  335  for approximating the output voltage outputted at the third output  325 . At this time, a digital signal corresponding to the statuses of the set of the first transistors  343  and the set of the second transistors  345  is recorded and outputted at the output  361  of the successive-approximation register  335  for representing the first coordinate factor of the coordinate of the touch point  329  on the touch screen  320 . The digital signal representing the first coordinate factor of the coordinate of the touch point  329  is also denoted as a first digital signal. After outputting the first digital signal, the switch  373  coupled to the third output  325  and the comparator  333  is turned off for preparing for the procedure of measuring the second coordinate factor of the coordinate of the touch point  329  on the touch screen  320 . 
   In the first phase, while measuring the second coordinate factor of the coordinate of the touch point  329  on the touch screen  320 , the third transistor  325  and the fourth transistor  327  are turned off, and the first transistor  321  and the second transistor  323  are turned on. Therefore, an output voltage is generated at the first output  321 . At this time, the switch  373  coupled to the third output  325  and the comparator  333  is turned off, and the switch  371  coupled to the first output  321  and the comparator  333  is turned on. Therefore, the output voltage is inputted at the first input of the comparator  333 . At this time, since no voltage is generated at the fifth output  347 , no voltage is generated at the second output of the comparator  333  either, and the comparator outputs a signal for representing the voltage at the first input of the comparator  333  is higher than the voltage at the second input of the comparator  333 . After the outputted signal from the comparator  333  is received at an input of the successive-approximation register  335 , a control signal is inputted at each input of the set of the first transistors  343  and the set of the second transistors  345  for controlling the statuses of the set of the first transistors  343  and the set of the second transistors  345 . The status of a transistor of the set of the first transistors  343  is the same with the status of a corresponding transistor of the set of the second transistors  345 . A voltage at the fifth output  347  is thus determined by the statuses of the set of the first transistors  343  and the set of the second transistor  345  by the successive-approximation register  335  for approximating the inputted voltage at the first input of the comparator  333 . It also represents that the voltage at the fifth output  347  is thus converged to the output voltage at the first output  321 . At this time, a digital signal representing the statuses of the set of the first transistors  343  and the set of the second transistors  345  is recorded and outputted at the output  361  of the successive-approximation register  335  for representing the second coordinate factor of the coordinate of the touch point  329  on the touch screen  320 . The recorded digital signal is also denoted as a second digital signal. After outputting the second digital signal, the switch  371  coupled to the first  321  and the comparator  333  is turned off for preparing for the second phase of the present invention. After the first digital signal and the second digital signal are outputted, since the first transistor  305  and the second transistor  307  are turned off right away as well as the third transistor  309  and the fourth transistor  311 , the power consumption of the transistors is decreased as well as the single-ended mode. 
   The voltages Vds and Vsd mentioned in  FIG. 3  are easily affected by the temperature, bias voltages, and the ratio of the width to the length of the transistors. In  FIG. 5 , each transistor of the set of the first transistors  343  and the set of the second transistors  345  is in parallel connection and close to each other, therefore, the transistors in parallel connection almost work at the same temperature. Since each transistor of the set of the first transistors  343  is coupled to the first voltage source  351 , each transistor of the set of the first transistors  343  works under the same bias voltage. And since each transistor of the set of the second transistors  345  is coupled to the second voltage source  353 , each transistor of the set of the second transistors  345  works under the same bias voltage. Since the variable factors such as temperature and bias voltage are eliminated by such designs, the values of the voltages Vsd and Vds can be determined by the ratios of the width to the length of the set of the transistors. However, the lengths of the set of the first transistors  343  are the same, and the lengths of the set of the second transistor  345  are the same. Therefore, the values of the voltages Vsd and Vds can merely be determined by the sum of the widths of the transistors being turned on. In other words, the outputted voltage at the fifth output  347  is determined by the number of the transistors turned on. Furthermore, while the voltages at both the inputs of the comparators are nearly equal to each other, the digital signal corresponding to the statuses of the transistors in parallel connection is utilized for representing the first coordinate factor and the second coordinate factor of the coordinate of the touch point  329  on the touch screen  320 . 
   Please refer to  FIG. 5 ,  FIG. 8 ,  FIG. 9 , and  FIG. 10 .  FIG. 8  is a diagram the equivalent circuit of the simulation circuit  431  of the resistive touch screen measurement system of the present invention.  FIG. 9  is a diagram of an analog-to-digital converter  403  of the resistive touch screen measurement system of the present invention.  FIG. 10  is a status table of the elements of the resistive touch screen measurement system of the present invention under the second phase. The simulation circuit  431  comprises a fifth resistor  437 , a fifth output  447 , a sixth output  449 , a fifth transistor  439 , a sixth transistor  441 , a set of first transistors  443 , a set of second transistors  445 , and a set of inverters  463 . The fifth output  447  is coupled to a first terminal of the fifth resistor  437 . The sixth output  449  is coupled to a second terminal of the fifth resistor  437 . The fifth switch  439  is coupled to the first voltage source  351  and the fifth output  447 . The sixth transistor  441  is coupled to the second voltage source  353  and the sixth output  449 . The set of first transistors  443  is coupled to the first voltage source  351  and the fifth output  447  in parallel connection with the fifth transistor  439 . The set of the second transistors  445  is coupled to the second voltage  353  and the sixth output  449  in parallel connection with the sixth transistor  441 . The set of inverters  463  is coupled to the set of the first transistors  439  and the set of the second transistors  441 . A control terminal of each transistor of the set of the first transistors  443  is coupled to an output of a set of control signals. A control terminal of each transistor of the set of the second transistors  445  is also coupled to an output of the set of control signals. The set of control signals is the first digital signal and the second digital signal generated in the first phase and outputted by the successive-approximation register  335  in  FIG. 6 , the first digital signal utilized for representing the first coordinate factor of the coordinate of the touch point  329  on the touch screen  320 , the second digital utilized for representing the second coordinate factor of the coordinate of the touch point  329  on the touch screen  320 . Each transistor of the set of the first transistors  443  corresponds to each transistor of the set of the second transistors  445 . The set of inverters  463  is utilized for transmitting the set of digital signals outputted at the output  361  of the signal generating system  303  shown in  FIG. 6  to a control terminal of each transistor of the set of the first transistors  443 . The analog-to-digital converter  403  comprises an analog input  455 , a reference upper bound input  457 , a reference lower bound input  459 , and a digital output  461 . The analog input  455  is coupled to the first output  321  through a switch  471  and is coupled to the third output  325  through a switch  473 . The reference upper bound input  457  is coupled to the fifth output  447 . The reference lower bound input  459  is coupled to the sixth output  449 . 
   In the second phase, while measuring the first coordinate factor of the coordinate of the touch point  329  on the touch screen  320 , the third transistor  309  and the fourth transistor  311  are turned on, and the first transistor  305  and the second transistor  307  are turned off. Therefore, an output voltage is generated at the first output  321 . At this time, the switch  471  coupled to the first output  321  and the analog input  455  of the analog-to-digital converter  403  is turned on, and the switch  473  coupled to the third output  325  and the analog input  455  of the analog-to-digital converter  403  is turned off. Therefore, the output voltage can be directly inputted at the analog input  455 . At this time, a control signal coupled to a control terminal of each transistor of the set of the first transistors  443  and the set of the second transistors  445  is utilized for controlling the statuses of both the sets of transistors. Therefore, an upper bound reference voltage a little higher than the output voltage is generated at the fifth output  447 , and a lower bound reference voltage a little lower than the output voltage is also generated at the sixth output  449 . The control signal is the first digital signal utilized for representing the first coordinate factor of the coordinate  329  on the touch screen  320  and outputted by the successive-approximation register  335  in the first phase of the present invention. The reference upper bound voltage is then inputted at the reference upper bound input  457  of the analog-to-digital converter  403  and is regarded as an upper bound of the reference voltage utilized by the analog-to-digital converter  403 . The reference lower bound voltage is also inputted at the reference lower bound input  459  of the analog-to-digital converter  403  and is regarded as a lower bound of the reference voltage utilized by the analog-to-digital converter  403 . Through the functions of the analog-to-digital converter  403  and both the reference upper bound voltage and the reference lower bound voltage, the inputted output voltage is transformed into a digital signal for representing the first coordinate factor of the coordinate of the touch point  329  on the touch screen  320  and outputted at the digital output  461 . After outputting the digital signal representing the first coordinate factor, the switch  471  coupled to the first output  321  and the analog-to-digital converter  403  is turned off for preparing for the procedure of measuring the second coordinate factor of the coordinate of the touch point  329  on the touch screen  320 . 
   In the second phase, while measuring the second coordinate factor of the coordinate of the touch point  329  on the touch screen  320 , the third transistor  309  and the fourth transistor  311  are turned off, and the first transistor  305  and the second transistor  307  are turned on, therefore, a second output voltage is generated at the third output  325 . At this time, the switch  471  coupled to the first output  321  and the analog input  455  of the analog-to-digital converter  403  is turned off, and the switch  473  coupled to the third output  325  and the analog input  455  of the analog-to-digital converter  403  is turned on, therefore, the second output voltage is directly inputted at the analog input  455 . At this time, a control signal coupled to a control terminal of each transistor of the set of the first transistors  443  and the set of the second transistors  445  is utilized for controlling the statuses of both the sets of the transistors. Therefore, a reference upper bound voltage a little higher than the second output voltage is generated at the fifth output  447 , and a reference lower bound voltage a little lower than the second output voltage is generated at the sixth output  449 . The control signal is the second digital signal outputted from the successive-approximation register  335  at the first phase of the present invention and utilized for representing the second coordinate factor of the coordinate of the touch point  329  on the touch screen  320 . The reference upper bound voltage is inputted at the reference upper bound input  457  of the analog-to-digital converter  403 , and the reference lower bound voltage is inputted at the reference lower bound input  459  of the analog-to-digital converter  403 . Through the functions of the analog-to-digital converter  403 , the reference upper bound voltage, and the reference lower bound voltage, the inputted second output voltage is transformed into a corresponding digital signal representing the second coordinate factor of the coordinate of the touch point  329  on the touch screen  320 , and the digital signal is then outputted from the digital output  461 . After outputting the digital signal representing the second coordinate factor, the switch  473  coupled to the third output  325  and the analog input  455  of the analog-to-digital converter  403  is turned off. Then the second phase of the present invention is executed again for measuring a first coordinate factor and a second coordinate factor of the coordinate of another touch point on the touch screen  320 . 
   In the present invention, the principle of determining the first coordinate factor and the second coordinate factor of the touch point on the touch screen is summarized as follows. First, in the first phase, a set of reference digital signals is determined according to the statuses of the transistors in parallel connection as shown in  FIG. 6 . Second, in the second phase, the set of reference digital signals is utilized for controlling the statuses of both the sets of the transistors in parallel connection and for generating a reference upper bound voltage at the fifth output  447  and a reference lower bound voltage at the sixth output  449 . The reference upper bound voltage and the reference lower bound voltage are utilized for compensating for the offsets of the transistors in the single-ended mode in advance. The present invention also utilizes the output voltage generated from the touch-sensing system as well as the differential mode, therefore, the precision of the present invention is the same with the differential mode. The set of digital signals for generating reference voltages is maintained in the simulation circuit so that the resistive touch screen measurement system of the present invention may repeatedly measure various coordinates of consecutive touch points. 
   The resistive touch screen measurement system of the present invention is a little similar with the resistive touch screen measurement system of the single-ended mode. The difference between the resistive touch screen measurement systems of the present invention and the single-ended mode is the reference upper bound voltage and the reference lower bound voltage. The reference upper bound voltage of the single-ended mode is the DC voltage source VDD whereas the reference upper bound voltage in the present invention is determined from the output voltage at the touch point. The reference lower bound voltage of the single-ended mode is the ground GND whereas the reference lower bound voltage in the present invention is also determined from the output voltage at the touch point. After inputting the reference upper bound voltage and the reference lower bound voltage determined by the set of digital signals outputted in the first phase to the analog-to-digital converter, the first transistor and the second transistor of the touch-sensing system can be immediately turned off as well as the third transistor and the fourth transistor of the touch-sensing system. Therefore, the power consumption of the present invention is decreased to the same level as the single-ended mode and is much less than the differential mode having a large power consumption. In summary, the resistive touch screen measurement system of the present invention has the precision of the differential mode and the low power consumption of the single-ended mode simultaneously. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.