Abstract:
A 5-wire touch screen system includes a touch screen ( 10 ) including a wiper ( 11 ) and a resistive layer ( 16 ) aligned with the wiper and first (UL), second (UR), third (LR), and fourth (LL) resistive layer contacts, wherein a touch on the screen presses a small portion of the wiper against the resistive layer, producing a touch resistance (R Z ) between them at a touch point on the resistive layer. The wiper and various contacts are selectively coupled to first (V DD ) and second (GND) reference voltages, respectively, to generate an analog touch voltage (V Z ) at the touch point. The wiper and various contacts are selectively coupled to an analog input ( 56 ) and a reference voltage input of an ADC ( 48 ) for converting the touch voltage (V Z ) to a digital representation. Analog voltages (V X ) and (V Y ) at the touch point are converted to corresponding digital representations by the ADC.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to 5-wire touch screens, and more particularly to systems and methods for accurately determining touch pressure/force applied on 5-wire touch screens. 
       FIG. 1  shows an exploded isometric diagram of a conventional 5-wire resistive touch screen  10  including a transparent bottom layer  14 , coated with resistive film  16  and four conductive corner pads  15 - 1 ,  15 - 2 ,  15 - 3  and  15 - 4  that can be connected to an outside contact terminal, and a top layer  12 . (The layers need to be transparent to allow display or LCD (liquid crystal display) backlighting to pass through.)  FIG. 2  shows a section view of an implementation of the assembled version of the exploded view of touch screen  10  in  FIG. 1 , wherein top layer  12  typically is formed of polyester or polyethylene terephthalate (PET) and is coated underneath with highly conductive (e.g., metal) transparent material to form wiper layer  11  (also referred to simply as “wiper  11 ”). 
     Transparent bottom layer  14  also is formed of PET, coated with transparent resistive film  16 , which usually is ITO (indium tin oxide). 
     Elastic and insulative spacers  22  separate top layer  12  from bottom layer  14  so as to maintain a thin air gap  23  between them. Spacers  22  are typically very thin, and are used to avoid a large difference in the touch point contact resistance, which is dependent on where the touch is located relative to the locations of the spacers, and also to avoid substantial variation in the “feel” for various locations of the touch point relative to the spacers. 
     Applying a touch pressure to the outer surface of top layer  12  pushes a small touch contact area of wiper  11  against resistive ITO layer  16 . When no touch pressure is present on top layer  12 , it is separated from the bottom resistive layer  14  by spacers  22  and air gap  23 . 
     The pressure of a touch on the upper surface of touch screen  10  typically is detected by a conventional 5-wire touch screen controller that controls various drive signals applied to the passive resistance of resistive layer  16  so as to facilitate measurement of various voltages resulting from touching various locations on the top surface of touch screen  10 . 
       FIG. 3  shows an equivalent circuit of the idealized 5-wire resistive touch screen  10  depicted in  FIGS. 1 and 2 . Transparent resistive layer  16  of  FIG. 2  is represented in  FIG. 3  as a rectangular grid of equivalent resistors having conductive terminals UL, UR, LL, and LR on its upper left, upper right, lower left, and lower right corners corresponding to conductive pads  15 - 1 ,  15 - 2 ,  15 - 4 , and  15 - 3 , respectively, in  FIG. 1 . Wiper layer  11  thus is directly over resistive layer  16  and is connected to wiper contact terminal  35 . Conductors or corner terminals  15 - 1 ,  15 - 2 ,  15 - 4 ,  15 - 3 , and wiper contact terminal  35  are the 5 accessible conductors or “wires” of 5-wire touch screen  10 . If corner terminals UL and LL are connected by a conductor  27  and terminals UR and LR are connected by a conductor  29  as indicated in the lower portion of  FIG. 3 , then resistive layer  16  appears as a resistor connected between conductors  27  and  29 , as shown in the simplified equivalent circuit representation in the lower portion of  FIG. 3 . Similarly, if terminals UL and UR are connected together by conductor  26  and terminals LL and LR are connected together by conductor  28 , resistive layer  16  appears as a resistor connected between conductors  26  and  28 . A touch point area  31  on top conductive wiper  11  conducts touch pressure to a point or area  30  on the resistive grid when a touch is applied on touch screen  10 . 
       FIG. 4  shows an equivalent circuit similar to the equivalent circuit shown in  FIG. 3  but further including the “touch resistance”  33  having a value R Z  of a touch between contact area  31  on wiper  11  and contact area  30  on ITO resistive layer  16 . Touch contact areas  30  and  31  are small contact areas that occur as a result of touch pressure applied on top layer  12  that presses small area  31  of wiper layer  11  against small area  30  of resistive layer  16 . Note that wiper layer  11  is assumed to be of zero resistance in the equivalent circuit of  FIG. 4 . 
     Unfortunately, it is not presently practical to provide a highly conductive (e.g., metal) contact wiper layer  11  that is sufficiently transparent for the LCD backlighting applications in which touch screens often are utilized. The wiper layer coat  11  on the lower surface of top layer  12  is presently composed of nearly-transparent ITO resistive material, the same as resistive layer  16  on the upper surface of bottom layer  14 . As the result, the equivalent circuit of a practical 5-wire touch screen  10  may be as shown in  FIG. 5 , where resistance  34  having a value R wiper  represents the resistance of ITO resistive wiper layer  11  between the touch area  31  and wiper contact terminal  35 . 
     Typically, each of the two ITO resistive layers  11  and  16  is approximately 90% transparent. Therefore, the top and bottom layers  12  and  14  together are 90%×90%=81% transparent, theoretically. This is very important, because lower transparency of the touch screen causes more power to be dissipated in the LCD backlighting circuitry in order to provide sufficient light intensity. 
       FIG. 6A  is an equivalent circuit that is useful in explaining the process of determining the y-coordinate of a touch on a conventional 5-wire resistive touch screen. Measurement of the y-coordinate includes applying a voltage V DD  of voltage source  38  between conductor  26 , which is connected to terminals UL ( 15 - 1 ) and UR ( 15 - 2 ), and conductor  28 , which is connected to terminals LL ( 15 - 4 ) and LR ( 15 - 3 ). Sensing the y-coordinate location of the electrical contact at the touch point (not shown) is accomplished through conductive terminal  35  of wiper  11 . Similarly,  FIG. 6B  is an equivalent circuit useful in explaining the process of determining the x-coordinate of a touch on the touch screen. Measurement of the x-coordinate includes applying a voltage V DD  between conductor  29 , which is connected to terminals LR and UR, and conductor  27 , which is connected to terminals UL and LL. Sensing the location of the electrical contact at the touch point is accomplished through conductive point  35  of wiper  11 . 
     More specifically, the above-mentioned touch screen controller to which touch screen  10  is coupled first applies the screen driving voltage V DD  of voltage source  38  between conductors  26  and  28 , causing current to flow uniformly across the screen from top to bottom in  FIG. 6A . The y-coordinate voltage V Y  is read from contact terminal  35  of wiper  11 , and is given by the expression 
                       V   Y     =         V   DD       R   Y       ×     R     Y   ⁢           ⁢   2           ,           Eq   .           ⁢   1               
where the y-direction resistance R Y  between conductors  26  and  28  is a known value that can be easily measured. R Y2  is the resistance between the touch point  30  and the negative (−) terminal of voltage source  38 . (R Y  and R Y2  are illustrated in  FIG. 7A .)
 
     Similarly, the touch screen controller applies the screen driving voltage V DD  of voltage source  38  between conductors  29  and  27  in  FIG. 6B , causing current to flow uniformly across the screen from right to left. The x-coordinate voltage V X  is read from contact terminal  35  of wiper  11 , and is given by the expression 
                       V   X     =         V   DD       R   X       ×     R     X   ⁢           ⁢   2           ,           Eq   .           ⁢   2               
where the x-direction resistance R X  between conductors  27  and  29  is a known value that can be easily measured. R X2  is the resistance between the touch point  30  and the negative (−) terminal of voltage source  38 . (R X  and R Y2  are illustrated in  FIG. 7B .)
 
     In addition to the foregoing touch screens, the closest prior art is believed to also include U.S. Pat. Nos. 6,246,394 and 7,215,330. U.S. Pat. No. 6,246,394 “Touch screen Measurement Circuit and Method”, issued Jun. 12, 2001 to Kalthoff et al., discloses a 4-wire touch screen digitizing system, and presents a method that measures the x and y coordinates of a touch location. U.S. Pat. No. 7,215,330 “Touch-Sensitive Surface Which Is Also Sensitive to Pressure Levels”, issued May 8, 2007 to Rantet, discloses a 4-wire touch screen that includes orthogonal conductive tracks  6  and  8  connected to resistive strips along edges of the two screens that make up the touch-sensitive screen, such that the x and y coordinates and the applied pressure can be measured. The method measures the pressure or third or “z” coordinate of a touch point on a 4-wire resistive touch screen. 
     Touch screen users may occasionally bump a nearby article that imparts mechanical vibration to a touch screen that can cause the associated touch screen system to erroneously interpret touch location or erroneously interpret the vibration as an intentional touch. Also, users may inadvertently touch the screen surface. If the touch screen and associated controller have the capability of measuring the touch resistance between the wiper layer and the resistive layer of the touch screen, then a “sensitivity” threshold value can be established which prevents erroneous touch interpretation due to mechanical vibration or light extraneous touches on the touch screen surface. In some applications, for example, interpreting Chinese characters being written on a touch screen or drawing of graphical features, varying amounts of force/pressure applied to the touch screen surface by a stylus can be interpreted as representing lines of varying width or darkness. Also, there are applications in which the above-mentioned sensitivity threshold value can be utilized to prevent electrical noise, such as EMI (electro-magnetic interference), from causing touch interpretation errors. 
     The prior 5-wire touch screen systems can only measure x- and y-coordinates, lack any method for obtaining third-coordinate or pressure data, and have limited capability for performing certain functions, such as signature verification, in which the pressure applied to provide a valid signature can be very significant. Without the pressure measurement of the present invention for a 5-wire touch screen system, the 5-wire touch screen system can generate only 2-dimensional coordinates, and therefore supports only 2-dimensional applications on the touch screen surface. 
     Thus, there is an unmet need for a system that measures 3 touch point coordinate voltages developed in a touch screen panel to represent x coordinates, y coordinates, and a touch point contact resistance coordinate, respectively, between a wiper layer and a resistive layer of a 5-wire touch screen. 
     There also is an unmet need for a system that measures 3 touch point coordinate voltages developed in a touch screen panel to represent x coordinates, y coordinates, and a touch point contact resistance coordinate between a wiper layer and a resistive layer of a 5-wire touch screen, wherein the touch point contact resistance is utilized to determine a touch point contact pressure or force. 
     There also is an unmet need for a touch screen system capable of providing improved signature verification by utilizing the touch pressure contact resistance in a 5-wire touch screen. 
     There also is an unmet need for a touch screen system capable of providing touch intensity measurements by utilizing the touch point contact resistance on a 5-wire touch screen. 
     There also is an unmet need for a touch screen system capable of providing touch sensitivity measurements by utilizing the touch point contact resistance on a 5-wire touch screen, wherein EMI (electro-magnetic interference) from the touch screen can be distinguished from real touches or pressures. 
     There also is an unmet need for a touch screen system capable of providing touch sensitivity measurements by utilizing the touch point contact resistance in a 5-wire touch screen. wherein touch point size information can be determined. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a system that measures 3 touch point coordinate voltages developed in a 5-wire touch screen panel to represent x coordinates, y coordinates, and a touch point contact resistance coordinate, respectively, between a wiper layer and a resistive layer. 
     It is another object of the invention to provide a system that measures 3 touch point coordinate voltages developed in a 5-wire touch screen panel to represent x coordinates, y coordinates, and a touch point contact resistance, respectively, between a wiper layer and a resistive layer, wherein the touch point contact resistance is utilized to determine a touch point contact pressure or force. 
     It is another object of the invention to provide a touch screen system capable of providing improved signature verification by utilizing touch point contact resistance in a 5-wire touch screen. 
     It is another object of the invention to provide touch sensitivity measurements by utilizing touch point contact resistance in a 5-wire touch screen. 
     It is another object of the invention to provide touch sensitivity measurements by utilizing touch point contact resistance in a 5-wire touch screen, wherein EMI (electro-magnetic interference) from the touch screen can be distinguished from real touches or pressures. 
     It is another object of the invention to provide touch sensitivity measurement by utilizing touch point contact resistance in a 5-wire touch screen, wherein touch point size can be determined. 
     Briefly described, and in accordance with one embodiment, the present invention provides a 5-wire touch screen system that includes a touch screen ( 10 ) including a wiper ( 11 ) and a resistive layer ( 16 ) aligned with the wiper and first (UL), second (UR), third (LR), and fourth (LL) resistive layer contacts, wherein a touch on the screen presses a small portion of the wiper against the resistive layer, producing a touch resistance (R Z ) between them at a touch point on the resistive layer. The wiper and various contacts are selectively coupled to first (V DD ) and second (GND) reference voltages, respectively, to generate an analog touch voltage (V Z ) at the touch point. The wiper and various contacts are selectively coupled to an analog input ( 56 ) and a reference voltage input of an ADC ( 48 ) for converting the touch voltage (V Z ) to a digital representation. Analog voltages (V X ) and (V Y ) at the touch point are converted to corresponding digital representations by the ADC. 
     In one embodiment, the invention provides a 5-wire touch screen system ( 40 ) including a 5-wire touch screen sensor ( 10 ), a substantially transparent, substantially conductive wiper layer ( 11 ), a substantially transparent resistive layer ( 16 ) aligned with the wiper layer ( 11 ) wherein the resistive layer ( 16 ) includes first (UL), second (UR), third (LL), and fourth (LR) contact terminals, and a plurality of thin spacers ( 22 ) separating the wiper layer ( 11 ) and the resistive layer ( 16 ), wherein a touch on the wiper layer ( 11 ) presses a small portion ( 31 ) of the wiper layer ( 11 ) against the resistive layer ( 16 ) to form a resistive contact area ( 30 ) having a touch resistance (R Z ) between the wiper layer ( 11 ) and the resistive layer ( 16 ), the touch resistance (R Z ) being inversely proportional to an intensity (P touch ) of the touch. A controller ( 41 ) coupled to the touch screen sensor ( 10 ) includes touch screen driver circuitry ( 42 ) for selectively coupling the wiper layer ( 11 ) and the various contact terminals (UL,UR,LR,LL) to first (V DD ) and second (GND) reference voltages, respectively, to generate first (V X ) and second (V Y ) analog touch location voltages and an analog touch voltage (V Z ) on the resistive layer ( 16 ) at the resistive contact area ( 30 ). Analog to digital conversion circuitry ( 48 ) has an input ( 56 ) coupled to the touch screen driver circuitry ( 42 ). Multiplexing circuitry ( 44 ) in the controller ( 41 ) selectively couples the wiper layer ( 11 ) and various contact terminals (UL,UR,LR,LL) to the input ( 56 ) of the analog to digital conversion circuitry ( 48 ) so as cause it to convert the first (V X ) and second (V Y ) analog touch location voltages and the analog touch voltage (V Z ) to digital representations ( 60 ) thereof, respectively. 
     In one embodiment, the first (UL), second (UR), third (LR), and fourth (LL) contact terminals are corner contact terminals. The touch screen driver circuitry ( 42 ) couples the second (UR) and third (LR) contact terminals to the first reference voltage (V DD ), the first (UL) and fourth (LL) contact terminals to the second reference voltage (GND), and the wiper layer ( 11 ) to the input ( 56 ) of the analog to digital conversion circuitry ( 48 ) so as to produce an analog x-coordinate voltage (V X ) on the input ( 56 ) of the analog to digital conversion circuitry ( 48 ). The touch screen driver circuitry ( 42 ) couples the first (UL) and second (UR) contact terminals to the first reference voltage (V DD ), the third (LR) and fourth (LL) contact terminals to the second reference voltage (GND), and the wiper layer ( 11 ) to the input ( 56 ) of the analog to digital conversion circuitry ( 48 ) to produce an analog y-coordinate voltage (V Y ) on the input ( 56 ) of the analog to digital conversion circuitry ( 48 ). The touch screen driver circuitry ( 42 ) couples the wiper layer ( 11 ) to the first reference voltage (V DD ), the third (LR) and fourth (LL) contact terminals to the second reference voltage (GND), and the first (UL) and second (UR) contact terminals to the input ( 56 ) of the analog to digital conversion circuitry ( 48 ) to produce the analog touch voltage (V Z ) as an analog z-coordinate voltage (V Z ) on the input ( 56 ) of the analog to digital conversion circuitry ( 48 ). 
     In a described embodiment, a digital output of the controller ( 41 ) is coupled by means of at least a digital bus ( 64 ) to a host processor ( 66 ), wherein the host processor ( 66 ) computes a value of the touch resistance (R Z ) which corresponds to the analog y-coordinate voltage (V Y ), the analog z-coordinate voltage (V Z ), and a predetermined value of a touch screen resistance (R Y ). The host processor ( 66 ) computes a value of the touch intensity (P touch ) from the value of the touch resistance (R Z ) based on a predetermined relationship between the touch resistance (R Z ) and the touch intensity (P touch ). 
     In a described embodiment, the analog to digital conversion circuitry ( 48 ) converts the analog x-coordinate voltage (V X ) to a digital x-coordinate location number representative of an x-coordinate of the resistive contact area ( 30 ). The analog to digital conversion circuitry ( 48 ) also converts the analog y-coordinate voltage (V Y ) to a digital y-coordinate location number representative of a y-coordinate of the resistive contact area ( 30 ). The analog to digital conversion circuitry ( 48 ) also converts the analog z-coordinate voltage (V Z ) to a z-coordinate location number representative of the touch resistance (R Z ) on the contact area ( 30 ). 
     In a described embodiment, the host processor ( 66 ) converts the digital x-coordinate location number to a digital x-coordinate voltage value (V X ) and converts the digital y-coordinate location number to a digital y-coordinate voltage value (V Y ). The host processor ( 66 ) converts the digital z-coordinate location number to a digital z-coordinate voltage value (V Z ) and also converts the digital z-coordinate voltage value (V Z ) to a digital value of the touch resistance (R Z ). The host processor ( 66 ) computes a value of the touch intensity (P touch ) based on the digital value of the touch resistance (R Z ). 
     In a described embodiment, the touch screen driver circuitry ( 42 ) includes first (Q 1 ), second (Q 2 ), third (Q 3 ) and fourth (Q 5 ) P-channel switching transistors having sources coupled to the first reference voltage (V DD ) and drains coupled to the wiper layer ( 11 ), the second contact terminal (UR), the third contact terminal (LR), and the first contact terminal (UL), respectively. Fifth (Q 4 ) and sixth (Q 6 ) N-channel switching transistors have sources coupled to the second reference voltage (GND) and drains coupled to the third contact terminal (LR) and the fourth contact terminal (LL), respectively. The gates of the first, second, third, fourth, fifth, and sixth switching transistors are coupled to a touch screen driver control circuit ( 68 ) for controlling operation of the touch screen driver circuitry ( 42 ) to measure the analog x-coordinate voltage (V X ), the analog y-coordinate voltage (V Y ), and the analog z-coordinate voltage (V Z ). In a described embodiment, a pre-processing circuit ( 50 ) is coupled between an output ( 60 ) of the analog to digital conversion circuitry ( 48 ) and the digital bus ( 64 ) to perform filtering of digital signals on the output ( 60 ) of the analog to digital conversion circuitry ( 48 ). 
     In one embodiment, the invention provides a method for operating a 5-wire touch screen system ( 40 ), including providing a 5-wire touch screen sensor ( 10 ) that includes a wiper layer ( 11 ) and a resistive layer ( 16 ) aligned with the wiper layer ( 11 ) and also includes first (UL), second (UR), third (LR), and fourth (LL) contact terminals, wherein a touch on the wiper layer ( 11 ) presses a small portion ( 31 ) of the wiper layer ( 11 ) against the resistive layer ( 16 ) thereby causing or substantially changing a touch resistance (R Z ) between a contact area ( 31 ) of the wiper layer ( 11 ) and a resistive contact area ( 30 ) of the resistive layer ( 16 ), the touch resistance (R Z ) being inversely proportional to an intensity (P touch ) of the touch; selectively coupling the wiper layer ( 11 ) and various contact terminals (UL,UR,LR,LL) to first (V DD ) and second (GND) reference voltages, respectively, to generate an analog touch voltage (V Z ) on the resistive layer ( 16 ) at the resistive contact area ( 30 ), the analog touch voltage (V Z ) being a function of the touch resistance (R Z ); and selectively coupling the wiper layer ( 11 ) and various contact terminals (UL,UR,LR,LL) to an input ( 56 ) of analog to digital conversion circuitry ( 48 ) and converting the analog touch voltage (V Z ) to a digital representation ( 60 ) thereof by means of the analog to digital conversion circuitry ( 48 ). 
     In one embodiment, the method includes coupling the first (UL) and second (UR) contact terminals to the first reference voltage (V DD ), coupling the fourth (LL) and third (LR) contact terminals to the second reference voltage (GND), and coupling the wiper layer ( 11 ) to the input ( 56 ) of the analog to digital conversion circuitry ( 48 ) to produce an analog y-coordinate voltage (V Y ) on the input ( 56 ) of the analog to digital conversion circuitry ( 48 ); coupling the second (UR) and third (LR) contact terminals to the first reference voltage (V DD ), coupling the first (UL) and fourth (LL) contact terminals to the second reference voltage (GND), and coupling the wiper layer ( 11 ) to the input ( 56 ) of the analog to digital conversion circuitry ( 48 ) to produce an analog x-coordinate voltage (V X ) on the input ( 56 ) of the analog to digital conversion circuitry ( 48 ); and coupling the third (LR) and fourth (LL) contact terminals to the second reference voltage (GND), coupling the wiper layer ( 11 ) to the first reference voltage (V DD ); and coupling the first (UL) and second (UR) contact terminals to the input ( 56 ) of the analog to digital conversion circuitry ( 48 ) to produce the analog touch voltage (V Z ) as an analog z-coordinate voltage (V Z ) on the input ( 56 ) of the analog to digital conversion circuitry ( 48 ). 
     In one embodiment, the method includes coupling an output ( 60 ) of the analog to digital conversion circuitry ( 48 ) by means of at least a digital bus ( 64 ) to a host processor ( 66 ), and operating the host processor ( 66 ) to compute a value of the touch resistance (R Z ) which corresponds to the analog y-coordinate voltage (V X ), the analog z-coordinate (V Z ), and a predetermined value of a touch screen resistance (R Y ). 
     In one embodiment, the method includes operating the analog to digital conversion circuitry ( 48 ) to convert the analog x-coordinate voltage (V X ) to a digital x-coordinate location number representative of an x-coordinate of the resistive contact area ( 30 ), to convert the analog y-coordinate voltage (V Y ) to a digital y-coordinate location number representative of a y-coordinate of the resistive contact area ( 30 ), and to convert the analog z-coordinate voltage (V Z ) to a digital y-coordinate location number representative of a z-coordinate of the resistive contact area ( 30 ), wherein the host processor ( 66 ) computes the value of the touch resistance (R Z ) on the basis of the z-coordinate location numbers. In one embodiment, the host processor ( 66 ) computes a value of the touch intensity (P touch ) from the value of the touch resistance (R Z ) based on a predetermined relationship between the touch resistance (R Z ) and the touch intensity (P touch ). 
     In one embodiment, the invention provides a 5-wire touch screen system ( 40 ) including a 5-wire touch screen sensor ( 10 ) that includes a wiper layer ( 11 ) and a resistive layer ( 16 ) aligned with the wiper layer ( 11 ) and includes first (UL), second (UR), third (LR), and fourth (LL) contact terminals, wherein a touch on the wiper layer ( 11 ) presses a small portion of the wiper layer ( 11 ) against the resistive layer ( 16 ) to produce a touch resistance (R Z ) between a contact area ( 31 ) of the wiper layer ( 11 ) and a resistive contact area ( 30 ) of the resistive layer ( 16 ), the touch resistance (R Z ) being inversely proportional to an intensity (P touch ) of the touch; means ( 42 ) for selectively coupling the wiper layer ( 11 ) and various contact terminals (UL,UR,LR,LL) to first (V DD ) and second (GND) reference voltages, respectively, to generate an analog touch voltage (V Z ) on the resistive layer ( 16 ) at the resistive contact area ( 30 ), the analog touch voltage (V Z ) being a function of the touch resistance (R Z ); and means ( 44 ) for selectively coupling the wiper layer ( 11 ) to various contact terminals (UL,UR,LR,LL) to an input ( 56 ) of analog to digital conversion means ( 48 ) for converting the analog touch voltage (V Z ) to a digital representation ( 60 ) thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded isometric diagram of a conventional 5-wire resistive touch screen. 
         FIG. 2  is a section view of a conventional 5-wire resistive touch screen of  FIG. 1 . 
         FIG. 3  is a diagram illustrating an equivalent circuit of a conventional 5-wire resistive touch screen as shown in  FIGS. 1 and 2 . 
         FIG. 4  is a diagram illustrating an equivalent circuit of a conventional 5-wire resistive touch screen as shown in  FIGS. 1 and 2 , where the touch point contact resistance Rz is displayed. 
         FIG. 5  is a diagram illustrating an equivalent of a conventional 5-wire resistive touch screen as shown in  FIG. 4 , wherein wiper resistance is indicated. 
         FIG. 6A  is a diagram of an equivalent circuit useful in explaining in the measurement of the y-coordinate of a touch for the conventional 5-wire resistive touch screens depicted in  FIGS. 1-5 . 
         FIG. 6B  is a diagram of an equivalent circuit useful in explaining the measurement of the x-coordinate of a touch for the conventional 5-wire resistive touch screens depicted in  FIGS. 1-5 . 
         FIG. 7A  is a diagram of an equivalent circuit, in which the wiper resistance is assumed to be zero, that is useful in explaining the measurement of a z-coordinate representative of touch pressure applied to the touch point of the idealized 5-wire resistive touch screens depicted in  FIGS. 1-4  as a function of y-coordinate parameters. 
         FIG. 7B  is a diagram of an equivalent circuit, in which the wiper resistance is assumed to be zero as in  FIG. 4 , that is useful in explaining the measurement of a z-coordinate representative of touch pressure applied to the touch point of the ideal 5-wire resistive touch screens depicted in  FIGS. 1-4 , as a function of x-coordinate parameters. 
         FIG. 8A  is a diagram of a more simplified equivalent circuit representation of the circuit shown in  FIG. 7A . 
         FIG. 8B  is a diagram of a more simplified equivalent circuit representation of the circuit shown in  FIG. 7B . 
         FIG. 9  is a diagram of an equivalent circuit as in  FIG. 8A  that further includes the effects of wiper resistance as in  FIG. 5  and is useful in explaining the measurement of a z-coordinate representative of touch pressure applied to the touch point of the conventional 5-wire resistive touch screens depicted in  FIGS. 1-5 . 
         FIG. 10  is a block diagram of a touch screen system in which the touch pressure contact area resistance measuring method and touch pressure measuring method of the present invention are implemented. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 7A and 7B  show an equivalent circuit of a touch screen  10  ( FIG. 1 ) on which a touch pressure has been applied on a small area  31  of wiper layer  11 , thereby pressing it against the surface of top layer  12  to thereby form a resistive touch pressure contact area  30  on resistive layer  16 . Touch pressure contact areas  30  and  31  result in a contact resistance R Z  (or a very substantial change in the contact resistance R Z ) between wiper  11  and resistive layer  16 . Dashed line  33  surrounds the touch pressure contact area resistance R Z  as diagrammatically illustrated in  FIGS. 7A and 7B . Resistive layer  16  (also see  FIG. 2 ) is represented as a rectangular grid of discrete resistors with terminals UL, UR, LL, and LR in its upper left, upper right, lower left, and lower right corners corresponding to conductive pads  15 - 1 ,  15 - 2 ,  15 - 4 , and  15 - 3 , respectively, as shown in the exploded view in prior Art  FIG. 1 . 
     Pressure contact area resistance R Z  is connected in series between resistive layer  16  and wiper  11 . The (+) terminal of a reference voltage source  38  produces a voltage V DD  between the contact terminal  35  of wiper  11  and conductor  28  as shown in  FIG. 7A  or between the contact terminal  35  of wiper  11  and conductor  27  as shown in  FIG. 7B . In this case, the wiper resistance (R Wiper  in  FIGS. 5 and 9 ) is assumed to be zero. 
       FIG. 7A  shows that a resistance R Y  of resistive layer  16  between conductors  26  and  28  is equal to the sum of R Y1  and R Y2 , where R Y1  is the resistance in resistive layer  16  between conductor  26  and touch pressure contact area  30  and R Y2  is the resistance between touch pressure contact area  30  and conductor  28 . 
     The simplified equivalent circuit of  FIG. 8A  illustrates more clearly than  FIG. 7A  the coupling of conductor  26  through resistance R Y1  to touch pressure contact area  30 . Touch pressure contact area  30  is coupled by the resistance R Y2  to conductor  28 . The resistance R Z  between contact areas  30  and  31  (which is surrounded by dashed line  33  in  FIGS. 7A and 7B ) is the contact resistance between resistive layer  16  and wiper layer  11 . To measure the touch pressure contact resistance R Z  of 5-wire resistive touch screen  10  (see  FIGS. 1 and 2 ), V DD  is applied between contact terminal  35  of wiper  11  and conductor  28  (see  FIGS. 7A and 8A ). The touch pressure voltage V Z-Y  at the location of touch pressure contact area  30  against resistive layer  16  is the voltage across resistance R Y2 . The voltage on conductor  26  is equal to V Z-Y  because the current through resistance R Y1  is zero, because conductor  26  is electrically “floating”. 
     Consequently, touch resistance R Z  can be determined by measuring the value of touch pressure voltage V Z-Y  measured between conductors  26  and  28 . (Note that by definition, pressure is equal to force per unit area, and that the description of the invention herein is applicable irrespective of whether the intensity of the touch is expressed as a force or as a pressure.) Note that V Z-Y  is the voltage produced by the voltage divider composed of the resistances R Z  and R Y2 , and can be represented by Equation 3: 
                     V     Z   -   Y       =       [       R     Y   ⁢           ⁢   2         (       R   Z     +     R     Y   ⁢           ⁢   2         )       ]     ×       V   DD     .               Eq   .           ⁢   3               
To solve for the touch pressure contact R Z , Equation 3 can be rewritten as Equation 4:
 
                     R   Z     =       [       (       V   DD     -     V     Z   -   Y         )       V     Z   -   Y         ]     ×       R     Y   ⁢           ⁢   2       .               Eq   .           ⁢   4               
Replacing R Y2  in Equation 4 with Equation 2 results in Equation 5A:
 
                     R   Z     =       (       V   Y       V   DD       )     ×     (         V   DD       V     Z   -   Y         -   1     )     ×       R   Y     .                 Eq   .           ⁢   5     ⁢   A               
Thus, the present touch resistance R Z  is a function of the previously known values of V DD  and R Y , and the presently measured values of V Z-Y  and V Y .
 
     Similarly, to measure touch pressure contact resistance R Z  of the 5-wire resistive touch screen  10  (see  FIGS. 1 and 2 ), the voltage V DD  is applied between the contact terminal  35  of wiper  11  and conductor  27  (see  FIGS. 7B and 8B ). The simplified equivalent circuit of  FIG. 8B  illustrates more clearly than  FIG. 7B  the coupling of conductor  29  through resistance R X1  to touch pressure contact area  30 . The touch pressure voltage V Z-X  at the location of touch pressure contact area  30  against resistive layer  16  is the voltage across resistance R X2 . The voltage produced by the voltage divider composed of the resistances R Z  and R X2  and, with equations similar to Equation 4 and Equation 5, touch pressure contact resistance to V Z-X  can also be expressed in Equation 5B: 
     
       
         
           
             
               
                 
                   
                     R 
                     Z 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           V 
                           X 
                         
                         
                           V 
                           DD 
                         
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         
                           
                             V 
                             DD 
                           
                           
                             V 
                             
                               Z 
                               - 
                               X 
                             
                           
                         
                         - 
                         1 
                       
                       ) 
                     
                     × 
                     
                       
                         R 
                         X 
                       
                       . 
                     
                   
                 
               
               
                 
                   
                     Eq 
                     . 
                     
                         
                     
                     ⁢ 
                     5 
                   
                   ⁢ 
                   B 
                 
               
             
           
         
       
     
     To measure R z , users can apply Equation 5A or 5B, or average the results from both of Equations 5A and 5B. To simplify further discussion, only Equation 5A will be used. 
       FIG. 9  is a simplified equivalent circuit that is the same as the one shown in  FIG. 8A  except that  FIG. 9  further includes the resistance R Wiper  of wiper  11 , where R Wiper  includes all resistances of wiper layer  11 , including any other equivalent connection and/or wiring resistances that are coupled between touch pressure contact area  31  and the (+) terminal of voltage source  38 . In many cases, the resistance R Wiper  between R Z  and the (+) terminal of voltage source  38  can be quite significant, due to the resistance of the resistive ITO layer of which wiper layer  11  is composed (see  FIG. 4 ) and any connection/wiring resistances between the contact terminal  35  of wiper  11  and the (+) terminal of voltage source  38 . When the total resistance R Wiper  associated with wiper  11  is considered, Equation 5A becomes Equation 6: 
     
       
         
           
             
               
                 
                   
                     
                       R 
                       Z 
                     
                     + 
                     
                       R 
                       Wiper 
                     
                   
                   = 
                   
                     
                       ( 
                       
                         
                           V 
                           Y 
                         
                         
                           V 
                           DD 
                         
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         
                           
                             V 
                             DD 
                           
                           
                             V 
                             
                               Z 
                               - 
                               Y 
                             
                           
                         
                         - 
                         1 
                       
                       ) 
                     
                     × 
                     
                       
                         R 
                         Y 
                       
                       . 
                     
                   
                 
               
               
                 
                   Eq 
                   . 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     The touch resistance R Z  between the top wiper layer  11  and bottom resistive layer  16  is a function of the touch intensity (e.g., touch pressure or touch force), and therefore can be used to compute the touch intensity P touch . The touch intensity applied against any location on the surface of a 5-wire resistive touch screen is inversely proportional to the touch intensity contact resistance R Z , so a heavier touch reduces R Z  and a lighter touch increases R Z  under the exact same conditions that determine the relationship between R Z  and P touch . As a general matter, the touch intensity P touch  on touch screen  10  is a function of R Z , and can be expressed in the polynomial form:
 
 P   touch   =a 0 +a 1 ×R   Z   +a 2 ×R   Z   2   +a 3 ×R   Z   3 + . . . ,
 
where the coefficients a0, a1, a2, a3, and so on are real values. The coefficients in Equation 7 are different for different touch screens. The resolution or accuracy of measuring the touch pressure contact resistance R Z  resulting from touching a state-of-the-art touch screen is usually quite low, and there is usually no need to use more than about 4 or 5 terms of Equation 7 to be able to calculate an acceptably accurate value of touch intensity P touch . The functional relationship between touch resistance R Z  for any particular touch screen can be determined by a suitable calibration procedure. As a simplified example, Equation 7 may be approximated by the expression
 
                     P   touch     =     1     α   +     β   ×     R   Z                   Eq   .           ⁢   8               
where the coefficients α and β are positive real values, are determined by the touch panel structure and materials, and can be easily obtained by the user by means of a calibration to determine the relationship between P touch  and R Z .
 
     Because the total resistance R Wiper  associated with wiper is a constant at any single touch point, the touch intensity can be derived from Equation 8 by substituting R Z  from Equation 6 and expressed by Equation 9: 
                           P   touch     =       ⁢     1     α   +     β   ×     [         (       V   y       V   DD       )     ×     (         V   DD       V     Z   -   Y         -   1     )     ×     R   Y       -     R   Wiper       ]                       =       ⁢     1     α   -     β   ×     R   Wiper       +     β   ×     R   Y     ×       V   Y       V   DD       ×     (         V   DD       V     Z   -   Y         -   1     )                         =       ⁢       1       α   ′     +     β   ×     R   Y     ×       V   Y       V   DD       ×     (         V   DD       V     Z   .     -   Y           -   1     )           =     1       α   ′     +     β   ×     R   Z               ,                 Eq   .           ⁢   9               
where α′=α−β×R Wiper  is a constant at any fixed point on a 5-wire resistive touch screen. Utilizing this methodology provided by the circuit in  FIG. 9 , the touch resistance at any location on a 5-wire resistive touch screen can be measured in terms of R Z . R Z  can be obtained from Equation 5A and/or 5B, or Equation 6 when considering the screen resistance of wiper layer  11 .
 
     Comparing Equation 9 (where R Wiper  is considered, as shown in  FIG. 9 ) with Equation 8 (where R Wiper  is not considered, as in  FIGS. 8A and 8B ), the expression of the relationship between R Z  and P touch  is the same at every touch area on a 5-wire restive touch screen. 
       FIG. 10  shows a touch screen system  40  which includes touch screen  10  coupled to a touch screen controller  41  that can interface with a host processor  66 . Touch screen system  40  provides digital representations of measured values of the V X , V Y , and V Z  (i.e., V Z-Y  or V Z-X ) voltages expressed in the foregoing equations. Or alternatively and often preferably, touch screen system  40  can provide digital x, y, and z “coordinate values” representative of the measured values of the V X , V Y , and V Z  voltages, which completely indicate a three-dimensional touch location on touch screen  10 . 
     There are 5 analog signals coupled between touch screen controller  41  and touch screen  10 . Touch screen controller  41  is connected to contact terminal  35  of wiper  11  of touch screen  10 . Touch screen controller  41  also is connected to terminals UL, UR, LR, and LL of touch screen  10 . Terminals UL, UR, LR, and LL and contact terminal  35  of wiper  11  are connected to touch screen driver circuitry  42  inside touch screen controller  41 , and are further connected to the inputs of a multiplexer  44  of touch screen controller  41 . Multiplexer  44  determines which of these conductors are multiplexed to the input  56  of an ADC (analog to digital converter)  48  which converts V X , V Y , and V Z  to digital touch data. After preprocessing circuit  50  (which, for example, can perform noise filtering), digital touch data is sent to host processor  66  through a conventional digital interface control circuit  54  and a digital bus  64 . 
     Wiper  11  is connected by contact terminal  35  to the drain of a P-channel switching transistor Q 1  having its source connected to V DD . V DD  also is connected to the sources of P-channel switching transistors Q 2 , Q 3 , and Q 5 . The drains of transistors Q 2 , Q 3  and Q 5  are connected to UR terminal  15 - 2 , LR terminal  15 - 3 , and UL terminal  15 - 1 , respectively. The sources of N-channel switching transistors Q 4  and Q 6  are connected to ground. The drain of transistor Q 4  is connected to LR terminal  15 - 3 , and the drain of transistor Q 6  is connected to UL terminal  15 - 1 . The gates of transistors Q 1 ,  2  . . .  6  are connected to a driver controller circuit  68  of touch screen driver  42 , which can be controlled according to either simple logic circuitry or according to appropriate control signals or commands from host processor  66 . 
     Touch screen system  40  can be considered to include touch screen  10 , touch screen controller  41 , and a portion of host processor  66 . A portion  66 A of host processor  66  which can be considered to be part of touch screen system  40  is the portion that communicates with touch screen controller  41  through digital interface control circuit  54  and touch detector  46 . Portion  66 A can be considered to include software that performs the above described calculations associated with touch screen controller  41  and software that is associated with operation of touch screen driver  42 . Portion  66 A of host processor  66  also can be considered to include software and hardware that is associated with storing data associated with touch screen  10  and communicating the data to application software elsewhere in host processor  66 . 
     The switch transistors in driver controller  68  can be easily controlled by various circuitry, such as a simple state machine, that implements subsequently described Table 1. Driver controller  68  can receive a command from host processor  66  via conductor or bus  65  and digital interface control circuit  54 . 
     Multiplexer  44  multiplexes the 5 signals on conductors  35 ,  15 - 1 ,  15 - 2 ,  15 - 3  and  15 - 4  from touch screen  10  to generate reference voltages V REF   +  and V REF   −  and also generate an analog input signal on input conductor  56  of ADC  48 . (PENIRQ is an interrupt output from a touch detector circuit  46  having an input connected to wiper contact terminal  35 , and indicates if a touch on touch screen  10  has been detected.) 
     Table 1 shows the states of the various transistors (or switches) Q 1 - 6  and the connections of the various terminals of resistive layer  16  and wiper  11  (i.e., the analog inputs to multiplexer  44 ) and the voltage reference signals and the analog signal to ADC  48  that are output from multiplexer  44  during operation of touch screen controller  40  to measure V X , V Y , and V Z . 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
               
             
               
               
             
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Measuring 
                 Measuring 
                 Measuring 
               
               
                   
                 V X   
                 V Y   
                 V Z   
               
             
          
           
               
                   
                 ON 
                 OFF 
                 ON 
                 OFF 
                 ON 
                 OFF 
               
               
                   
                   
               
             
          
           
               
                 Input to Multiplexer 44 
               
             
          
           
               
                 Wiper: --&gt; 
                   
                 Q1 
                   
                 Q1 
                 Q1 
                   
               
               
                 UR: --&gt; 
                 Q2 
                   
                 Q2 
                   
                   
                 Q2 
               
               
                 LR: --&gt; 
                 Q3 
                 Q4 
                 Q4 
                 Q3 
                 Q4 
                 Q3 
               
               
                 UL: --&gt; 
                 Q6 
                 Q5 
                 Q5 
                 Q6 
                   
                 Q5, Q6 
               
             
          
           
               
                 LL: --&gt; 
                 always connected to GND 
               
               
                   
               
             
          
           
               
                 Output from Multiplexer 44 
               
             
          
           
               
                 ADC 
                   
                   
                   
               
               
                 input 56: --&gt; 
                 Wiper contact 35 
                 wiper contact 35 
                 UL/UR 
               
               
                   
               
               
                 V REF   + : --&gt; 
                 UR/LR 
                 UL/UR 
                 Wiper contact 35 
               
               
                 V REF   − : --&gt; 
                 UL/LL 
                 LR/LL 
                 LR/LL 
               
               
                   
               
             
          
         
       
     
     ADC (analog to digital converter)  48  converts the measured analog voltages V X , V Y , and V Z  on conductor  56  to a digital value on digital bus  60  in accordance with the various conditions indicated in Table 1. 
     As previously indicated, wiper contact  35  is selectively coupled to V DD  through Q 1 , and UR is selectively coupled to V DD  through Q 2 . LR is selectively coupled to V DD  through Q 3  and to ground through Q 4 . UL is selectively coupled to V DD  through Q 5  and to ground through Q 6 . 
     Referring to Table 1, to measure V X , Q 1  is off, so wiper contact  35  is coupled through multiplexer  44  to ADC input  56 . Q 2  is on, so UR is coupled to V DD . Q 3  is on, so LR is coupled to V DD . Q 3  and Q 4  can not both be on at the same time, and Q 3  is on, so Q 4  is off. Q 5  is off and Q 6  is on, which means UL and LL both are coupled to ground while UR and LR both are coupled to V DD . Q 2  and Q 3  both are on UR and LR both are at V DD . Wiper contact  35  is electrically floating because Q 1  is off. UL is at ground because Q 6  is on, and LL is always at ground. The V REF   +  reference voltage input of ADC  48  is connected to UL and LR, which results in a voltage nearly equal to V DD  being coupled to the V REF   +  input of ADC  48 . The V REF   −  reference voltage input of ADC  48  is connected to UL and LL, which results in a voltage nearly equal to ground being connected to the V REF   −  reference voltage input of ADC  48 . See  FIG. 6B . 
     To measure V Y , Q 1  is off, so wiper contact  35  is coupled through multiplexer  44  to ADC input  56 . Q 2  is on, so UR is coupled to V DD . Q 4  is on, so LR is coupled to GND. Q 3  and Q 4  can not both be on at the same time, and Q 4  is on, so Q 3  is off. Q 5  is on and Q 6  is off, which means LR and LL both are coupled to ground while UR and UL both are coupled to V DD . Since Q 2  and Q 4  are on, UR and UL both are at V DD . Wiper contact  35  is electrically floating because Q 1  is off. LR is at ground because Q 5  is on, and LL is always at ground. The V REF   +  reference voltage input of ADC  48  is connected to UL and UR, which results in a voltage nearly equal to V DD  being coupled to the V REF   +  input of ADC  48 . The V REF   −  reference voltage input of ADC  48  is connected to the LR and LL, which results in a voltage nearly equal to ground being connected to the V REF   −  reference voltage input of ADC  48 . See  FIG. 6A . 
     To measure V Z , Q 1  is on so wiper contact  35  is connected to V DD . Q 2  is off so UR is electrically floating. Q 3  is off and Q 4  is on, so LR is at ground. Q 5  and Q 6  both are off so UL is electrically floating. LL is at ground. The input of the analog to digital conversion circuitry is connected to UL and UR. The V REF   +  reference voltage input of ADC  48  is connected to wiper contact  35 . The V REF   −  reference voltage input of ADC  48  is connected to LR and LL. See  FIGS. 7A and 8A . 
     The digital output generated on digital bus  60  by ADC  48  is provided as an input to a pre-processing circuit  50 , which can function as a digital averaging filter to reduce noise before sending the measured quantity to host processor  66 . Pre-processing circuit  50  also can perform various other functions, such as data validation. The output of pre-processing circuit  50  is coupled by digital bus  62  to a digital interface control circuit  54 , which is coupled by means of bidirectional digital bus  64  to host processor  66 . 
     Touch screen  10  and touch driver  42  produce values of analog voltages V X , V Y , and V Z  to the input  56  of ADC  48 . V X , V Y , and V Z  represent three-dimensional touch position coordinates on touch screen  10 , namely V X , V Y , and V Z  as expressed by Equations 1 to 3, respectively. Typically, the digitized values of V X , V Y , and V Z  produced by ADC  48  actually are digital x, y, and z coordinate location numbers, e.g.  2046 ,  4096  or the like corresponding to each of the analog values of V X , V Y , and V Z  produced by multiplexer  44  on the input  56  of ADC  48 . ADC  48  performs the conversions of the analog values of V X , V Y , and V Z  to the digital x, y, and z coordinate location numbers and provides them to host processor  66  via preprocessing circuit  50  and digital interface control  54 . Host processor  66  presents, applies, and/or interprets the data for specific user applications. 
     Thus, the digital representations of V X , V Y , and V Z  (i.e., V Z-Y  or V Z-X ) for the current touch on touch screen  10 , for example, digital x, y, and z touch screen coordinate location number representations of the analog voltages V X , V Y , and V Z , are provided by the controller  41  to host processor  66 . If directly digitized representations of the measured analog voltages V X , V Y , and V Z  are provided by driver  42 , then host processor  66  then can use that information to locate the touch position corresponding to V X  and V Y , compute values of R Z , and further compute the value of P touch . Host processor  66  then can use those values for the present user application or purpose. 
     Host processor  66  might then use the value of R Z  to eliminate system noise and/or improve the accuracy of the touch point information in other ways. For example, the z coordinate information may help determine whether what appears to be a very light pressure touch point is actually just due to vibration. 
     The relationship between touch point resistance R Z  and touch pressure or intensity P touch  may be complex, and various users may use host processor  66  to execute various algorithms to compute the touch pressure or intensity P touch  on the basis of the digital representations of V X , V Y , and V Z  generated by touch screen system  40  shown in  FIG. 10 . Host processor  66  can be used to establish/calibrate the relationship between P touch  and R Z  for any particular touch screen  10  and compute the touch pressure or intensity P touch  being applied to wiper layer  11  on the basis of values of V X , V Y , and V Z  generated by touch screen system  40 . 
     It should be appreciated that the present invention is believed to provide the first 5-wire touch screen system that generates measurements from which a third dimensional coordinate value R Z  can be obtained at any point on a 5-wire touch screen and touch point pressure can be computed, and thereby enables the host processor to perform more functions with more accuracy than previously has been possible using 5-wire touch screen systems. This can be very useful in some applications, such as graphic drawing, determining line or dot size, signature verification, in which the touch intensity applied to provide a valid signature can be very significant. With the intensity/z-coordinate technique of the present invention, the touch screen system can generate 3-dimensional coordinates and therefore supports “3-dimensional” or “real-world” applications. In general, information regarding how much touch intensity is applied to the touch screen can help improve the overall performance of the touch screen system. 
     While the invention has been described with reference to several particular embodiments thereof, those skilled in the art will be able to make various modifications to the described embodiments of the invention without departing from its true spirit and scope. It is intended that all elements or steps which are insubstantially different from those recited in the claims but perform substantially the same functions, respectively, in substantially the same way to achieve the same result as what is claimed are within the scope of the invention. For example, in addition to using the above mentioned state machine to implement driver controller  68 , there are other ways of controlling touch screen driver  42 . Driver controller  68  could be implemented by means of logic circuitry other than a state machine. Host processor  66  may initiate operation of driver controller  68  so as to cause operation of touch screen driver  42  in accordance with Table 1. Driver controller  68  itself could be programmable so as to cause touch screen driver  42  to automatically operate as desired to measure V X , V Y  and V Z  if a touching on the touch screen surface is detected. Alternatively, the preprocessing circuitry  50  could be configured to control driver controller  68  in response to a valid touch on the surface of touch screen  10 .