Integrated technique for filtering and measuring signals of four and five wire digitizers

This invention describes a method and apparatus for filtering noise from a measurement of the X and Y coordinates of a resistive digitizer. The method applies to both four and five wire resistive digitizers biased with a DC voltage. The same filtering and measurement apparatus can be used on both types of digitizers with inclusion of an extra signal pin to accommodate the sense lead of the five wire digitizer. This approach involves connecting the signal to be read to a filter, reading the filtered voltage, and disconnecting the signal from the filter before disconnecting bias voltage from the planes of the digitizer. A separate filter is used for the X and Y coordinate signals and each filter voltage can be read at any time before the next measurement. A reset voltage is available to an established reference on the filter capacitor when a "pen up" status is detected.

BACKGROUND OF THE INVENTION
 1. Field of Invention
 This invention relates to resistive digitizers and in particular noise
 filtration and coordinate voltage measurement for four and five wire
 digitizers.
 2. Description of Related Art
 Resistive digitizers when placed on an LCD or CRT screen pick up noise that
 is superimposed on the coordinate voltage that is to be read. This type of
 digitizer has two resistively coated surfaces that come into contact when
 pressed. The noise cannot be easily filtered because the different
 terminals are multi-function and see different voltages at different
 times. For example, in a four wire digitizer a terminal can be used to
 connect power to the digitizer panels or to read the coordinate voltage of
 the other panel. A filter one might use on a power terminal would have
 time constant implications when switching between functional use of the
 terminal, and a filter one might use in a signal voltage would have a
 voltage drop implication when used as a power terminal
 In U.S. Pat. No. 4,306,110 (Nelson et al.) is described an apparatus for
 determining planar coordinates. A four wire digitizer is discussed with
 the un-powered plane providing the location voltage through a high
 impedance conditioning circuit to a sample and hold circuit. As power is
 alternated between digitizer planes, un-powered plane is changed to read
 X-plane coordinate voltages and then Y-plane coordinate voltages. These
 voltages are connected to different inputs of the high impedance
 conditioning circuitry and further connected to sample and hold circuits
 at the output of the high impedance and conditioning circuits.
 In U.S. Pat. No. 4,484,026 (Thornburg) a touch tablet is shown for entering
 data into a computer. The touch tablet is made of two resistive sheets in
 parallel and oriented orthogonal to each other. Conductive strips on two
 ends of each resistive sheet and having orthogonal orientation between
 sheets are electrically connected to the resistive material of the sheet.
 A voltage is applied between the strips on one resistive sheet, and a
 coordinate location is read from the un-powered sheet. By connecting the
 output amplifier to both strip on the un-powered resistive sheet, the
 sensitivity to noise is reduced and system performance is improved.
 In U.S. Pat. No. 5,041,701 (Wolfe et al.) is described an edge
 linearization device for producing orthogonal electric fields in a
 resistive surface to be used in a contact input system. The system is
 capable of locating an object in contact with the resistive surface. In
 U.S. Pat. No. 5,083,118 (Kazama) is described a coordinate measuring
 apparatus to be mounted to a CRT or an LCD screen in which a fist layer is
 used as an electromagnetic screen and resistors of value approximately one
 hundred ohms are connected to one end of the resistive layers to help
 reduce noise. Shown in U.S. Pat. No. 5,191,175 (Protheroe et al.) is shown
 a self tuning digitizer control circuit in which a narrow bandpass filter
 is used to filter out extraneous noise picked up by a coordinate sensing
 stylus. In U.S. Pat. No. 5,365,253 (Cheng et al.) is shown a digitizer
 device with anti-noise capability. The digitizer operates at several
 hundred thousand Hertz and uses a hardware circuit controlled by a
 software program to eliminate outside noises.
 Resistive digitizers have two resistively coated surfaces that come into
 contact when pressed. A standard four wire technique involves applying a
 voltage across one of the surfaces while reading the voltage at the point
 of contact through the other layer. A controller switches between the two
 resistive surfaces and measures the voltage corresponding to the X and Y
 coordinates. The noise induced into the resistive surfaces when the
 digitizer is placed on a CRT or an LCD screen can produce errors in the
 coordinate readings.
 SUMMARY OF THE INVENTION
 In this invention an R-C series filter is connected by a switch to the
 measurement terminal of a four or five wire resistive digitizer. Once the
 capacitor of the filter has been charged to the voltage value of the
 coordinate being measured, an ADC (analog to digital converter) measures
 the voltage across the capacitor and the switch disconnects the filter
 from the digitizer. The voltage is maintained on the capacitor until the
 next reading or until a reference voltage resets the capacitor voltage.
 Since the coordinate voltage is maintained on the filter capacitor after
 the switch disconnects the filter from the digitizer, the ADC could
 measure the voltage at anytime until the voltage is changed by a new
 measurement or a reset signal.
 In a four wire digitizer a bias voltage is applied to first one resistive
 plane with the second plane floating, and then to a second plane with the
 first plane floating. When the bias voltage is applied to the X plane of
 the digitizer, the X coordinate location of the digitizer pen is measured
 by connecting the "Y-" terminal, or the "Y+" terminal, of the floating Y
 plane to an R-C filter. Either "Y" terminal can be used to measure the X
 coordinate voltage since the Y plane is floating when the X plane is
 powered. The X coordinate voltage is connected to the floating Y plane at
 the location of the digitizer pen. An ADC measures the X coordinate
 voltage across the capacitor with respect to the "X-" terminal of the X
 plane, and then the R-C filter is disconnected from the "Y-" terminal, or
 the "Y+" terminal. Since the X coordinate voltage is maintained on the R-C
 filter, the measurement of the X coordinate voltage by the ADC can be done
 after the R-C filter is disconnected from the terminal of the Y plane.
 Next a voltage is applied to the Y plane of the digitizer, and the Y
 coordinate location of the digitizer pen is measured by connecting the
 "X-" terminal of the floating X plane to an R-C filter. Either "X"
 terminal can be used to measure the Y coordinate voltage since the X plane
 is floating when the Y plane is powered. The Y coordinate voltage is
 connected to the floating X plane at the location of the digitizer pen. An
 ADC measures the Y coordinate voltage across the capacitor of the filter
 with respect to the "Y-" terminal of the Y plane, and then the R-C filter
 is disconnected from the "X-" terminal. Since the Y coordinate voltage is
 maintained on the R-C filter, the measurement of the Y coordinate voltage
 by the ADC can be done after the R-C filter is disconnected from the
 terminal of the X plane.
 The X and the Y filter voltages can be reset to a reference voltage to
 allow the capacitors of the filters to be charged from a known value. This
 is done at reset and initial powering of the digitizer controller, and
 when a pen up condition is detected. If there is adequate drive
 capability, the reference voltage can be connected directly to the
 capacitor of the filter; otherwise, the reference voltage is connected to
 the signal input of the filter.
 In a five wire digitizer there is an X-Y voltage coordinate plane and a
 sense plane. Reference voltages are connected to the corners of the X-Y
 coordinate plane and are designated; UL upper left, LL lower left, UR
 upper right and LR lower right. To measure an X coordinate a first
 reference voltage is connected to UR and LR and a second reference voltage
 is connected to UL and LL. When a Y coordinate voltage is measured, the
 first reference voltage is connected to LL and LR and the second reference
 voltage is connected to UL and UR.
 The sense plane of a five wire digitizer is floating with respect to the
 X-Y coordinate plane except when the digitizing pen forces contact between
 sense plane and the coordinate plane. At this point of contact the X or Y
 coordinate voltage is connected to the sense plane. A sense terminal of
 the sense plane is connected by means of a first switch to an R-C filter
 for X coordinate voltage values and by means of a second switch to an R-C
 filter for Y coordinate voltage values. The appropriate R-C filter is
 connected to the sense terminal by means of switches as the coordinate
 voltage plane is connected to voltages to measure the X coordinate and
 then the Y coordinate. The voltages to which each of the capacitors in the
 two filters were charged are maintained on the capacitors until the next
 coordinate measurement or until being reset by a reference voltage. The
 coordinate voltages can be read form the filters at any time after the
 filter is properly charged until the next coordinate measurement and a
 reset voltage is applied.
 Filters other than R-C filters can be used with the four or five wire
 digitizers. The only requirements is that the filters sufficiently reduce
 the noise picked up by the digitizer and that resulting values the X and Y
 coordinate voltages be maintained long enough so the measurements can be
 made.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 In FIG. 1 is shown a schematic of the connections of a four wire digitizer
 in which a resistive X plane Dx 10 is powered and the coordinate position
 is read from the floating Y plane of the digitizer by an ADC 11. Connected
 to the "X+" terminal 12 is a voltage V 14 through a switch SAx 13.
 Connected to the "X-" terminal 15 is circuit ground 17 through a switch
 SCx 16. The "Y-" terminal 18 of the floating Y plane is connected to a
 filter input Fx 20 through a switch SBx 19. Since the Y plane is floating,
 the "Y+" terminal could also be used to connect the signal on the Y plane
 to the filter input Fx 20 through switch SBx 19 or both the "Y+" and the
 "Y-" terminals could be connected the filter input Fx 20 through switch
 SBx 19. The filter consists of a resistor Rx 21 connected in series with a
 capacitor Cx 22. The filtered voltage on the capacitor Cx 22 is connected
 to the ADC 11 through a filtered voltage input terminal INx 23 and an
 analog selector 24. A reference voltage Vref 25 is connected to the filter
 Fx 20 through a voltage follower 26 and a switch SDx 27.
 Continuing to refer to FIG. 1, power is connected to the X plane of the
 digitizer Dx 10 by closing switches SAx 13 and SCx 16. A voltage V 14 is
 connected by means of switch SAx 13 to the "X+" digitizer terminal 12 and
 the Ref. HI terminal of the ADC 11 through an analog selector 28. A ground
 voltage 17 is connected by means of switch SCx 16 to the "X-" digitizer
 terminal 15 and the Ref. LO terminal of the ADC 11 through an analog
 selector switch 29. A voltage representing the location of a pen
 contacting the planes of the digitizer is induced into the floating Y
 plane and is connected to the measurement circuitry through the "Y-"
 terminal 18. This voltage could also be connected through the "Y+"
 terminal. A switch SBx 19 is closed connecting the voltage on "Y-" 18 to
 the filter input Fx 20. After the filter capacitor Cx 22 is charged, the
 voltage across the capacitor is read by the ADC 11 and the switch SBx 19
 is opened.
 Continuing to refer to FIG. 1, the voltage on the filter capacitor Cx 22 is
 maintained until the next measurement of the X plane coordinate voltage.
 This can allow the voltage on the capacitor Cx 22 to be read any time
 after the capacitor Cx 22 is filly charged and until the next X plane
 measurement is initiated or a reset is initiated using a reference voltage
 Vref 25. A reference voltage Vref 25 can be connected to the filter input
 Fx 20 by momentarily closing switch SDx 27 to reset the capacitor Cx 22 to
 the reference voltage Vref 25 in preparation for the next reading of an X
 coordinate. If the voltage follower 26 has enough drive capability, the
 reference voltage could be momentarily connected to the capacitor Cx 22 of
 the X filter to reset the capacitor voltage.
 In FIG. 2 is shown a schematic of the connections of a four wire digitizer
 in which a resistive Y plane Dy 30 is powered and the coordinate position
 is read from a floating X plane of the digitizer by an ADC 11. Connected
 to the "Y+" terminal 32 is a voltage V 14 through a switch SAy 33.
 Connected to the "Y-" terminal 35 is circuit ground 17 through a switch
 SCy 36. The "X-" terminal 38 of the floating X plane is connected to a
 filter input Fy 40 through a switch SBy 39. Since the X plane is floating,
 the "X+" terminal could also be used to connect the signal on the X plane
 to the filter input Fy 40 through switch SBy 39 or both the "X+" and the
 "X-" could be connected the filter input Fy 40 through switch SBy 39. The
 filter consists of a resistor Ry 41 connected in series with a capacitor
 Cy 42. The filtered voltage on the capacitor Cy 42 is connected to the ADC
 11 through a filtered voltage input terminal INy 43 and an analog selector
 24. A reference voltage Vref 25 is connected to the filter Fy 20 through a
 voltage follower 26 and a switch SDy 37.
 Continuing to refer to FIG. 2, power is connected to the Y plane of the
 digitizer Dy 30 by closing switches SAy 33 and SCy 36. A voltage V 14 is
 connected by means of switch SAy 33 to the "Y+" digitizer terminal 32 and
 the Ref. HI terminal of the ADC 11 through an analog selector 28. A ground
 voltage 17 is connected by means of switch SCy 36 to the "Y-" digitizer
 terminal 35 and the Ref. LO terminal of the ADC 11 through an analog
 selector switch 29. A voltage representing the location of a pen
 contacting the planes of the digitizer is induced into the floating X
 plane and is connected to the measurement circuitry through the "X-"
 terminal 38. This voltage could also be connected through the "X+"
 terminal since the X plane is floating. A switch SBy 39 is closed
 connecting the voltage on "X-" terminal 38 to the filter input Fy 40.
 After the filter capacitor Cy 42 is charged, the voltage across the
 capacitor is read by the ADC 11 and the switch SBy 39 is opened.
 Continuing to refer to FIG. 2, the voltage on the filter capacitor Cy 42 is
 maintained until the next measurement of the Y plane coordinate voltage.
 This can allow the voltage on the capacitor Cy 42 to be read any time
 after the capacitor Cy 42 is fully charged and until the next Y plane
 measurement is initiated or a reset is initiated using a reference voltage
 Vref 25. A reference voltage Vref 25 can be connected to the filter input
 Fy 40 by momentarily closing switch SDy 37 to reset the capacitor Cy 42 to
 the reference voltage Vref 25 in preparation for the next reading of an Y
 coordinate. If the voltage follower 26 has enough drive capability, the
 reference voltage could be momentarily connected to the capacitor Cy 42 of
 the Y filter to reset the capacitor voltage.
 Referring to FIG. 3a, connections to filter and read coordinate voltages
 for a five wire digitizer is shown The circuitry to filter and read the
 five wire digitizer is similar to that used with the four wire digitizer
 with the addition of a sense terminal 60, an analog selector 61 for
 delivering power the digitizer panel, and the renaming of the terminals
 connecting power to the five wire digitizer to lower right LR 62, lower
 left LL 63, upper right UR 64 and upper left UL 65. In a five wire
 digitizer there is a sense plane and a resistive coordinate plane. The
 resistive coordinate plane is powered from the corners with the upper left
 being the "0,0" coordinate. A sense terminal 60 is connected to the sense
 plane of the five wire digitizer which in turn is connected to a filter
 input Fx 20 through a switch SBx 19 and to a filter input Fy 40 through
 switch SBy 39. The filter for filtering X coordinate voltages consists of
 a resistor Rx 21 connected in series with a capacitor Cx 22 to circuit
 ground The filtered voltage on the capacitor Cx 22 is connected to the ADC
 11 through INx 23 and an analog selector 24. The filter for filtering Y
 coordinate voltages consists of a resistor Ry 41 connected in series with
 a capacitor Cy 42 to circuit ground. The filtered voltage on the capacitor
 Cy 42 is connected to the ADC 11 through INy 43 and an analog selector 24.
 A reference voltage Vref 25 is connected to the input of the X filter Fx
 20 through a voltage follower 26 and a switch SDx 27, and to the input of
 the Y filter Fy 40 through a voltage follower 26 and a switch SDy 37.
 Continuing to refer to FIG. 3a, when an X coordinate voltage is being
 measured, the analog selector 61 connects a voltage V 14 to the LR
 terminal 62 and the UR terminal 64, and ground to the LL terminal 63 and
 the UL terminal 65. The sense terminal 60 connected to the sense plane has
 an X coordinate voltage that is connected to the X filter input Fx 20 by
 means of a switch SBx 19. The X filter consists of a resistor Rx 21 and a
 capacitor Cx 22. After the capacitor Cx 22 has been charged with the X
 coordinate voltage, the voltage on the capacitor Cx 22 is read by the ADC
 11 through terminal INx 23 and an analog selector 24. After the ADC 11
 reads the voltage on capacitor Cx 22, the switch SBx 19 is opened. The
 voltage on capacitor Cx 22 can be reset to the reference voltage Vref 25
 by momentarily closing switch SDx 27. If the capacitor voltage is not
 reset, then the X coordinate voltage is maintained on the capacitor Cx 22
 until the next X coordinate measurement; therefore an X coordinate
 measurement by the ADC 11 could be made anytime before the next X
 coordinate measurement or a reset of the capacitor voltage by the
 reference voltage Vref 25 through the voltage follower 26 and switch SDx
 27.
 Continuing to refer to FIG. 3a, when a Y coordinate voltage is being
 measured, the analog selector 61 connects a voltage V 14 to the LR
 terminal 62 and the LL terminal 63, and ground to the UR terminal 64 and
 the UL terminal 65. The sense terminal 60 connected to the sense plane has
 a Y coordinate voltage that is connected to the Y filter input Fy 40 by
 means of a switch SBy 39. The X filter consists of a resistor Ry 41 and a
 capacitor Cy 42. After the capacitor Cy 42 has been charged with the Y
 coordinate voltage, the voltage on the capacitor Cy 42 is read by the ADC
 11 through terminal INy 43 and an analog selector 24. After the ADC 11
 reads the voltage on capacitor Cy 42, the switch SBy 39 is opened, and the
 voltage on capacitor Cy 42 can be reset to the reference voltage Vref 25
 by momentarily closing switch SDy 37. If the capacitor voltage is not
 reset, then the Y coordinate voltage is maintained on the capacitor Cy 42
 until the next Y coordinate measurement; therefore a Y coordinate
 measurement by the ADC 11 could be made anytime before the next Y
 coordinate measurement or a reset of the capacitor voltage by the
 reference voltage Vref 25 through the voltage follower 26 and switch SDy
 37. When a Y coordinate voltage is on the sense terminal 60, switch SBy 62
 is closed and SBx 61 is open. The Y coordinate voltage is connected to the
 input of the Y filter Fy 67. After the capacitor Cy 69 has been charged
 with the Y coordinate voltage, the voltage on the capacitor Cy 69 is read
 by the ADC 72 through terminal INy 70 and the analog selector 71. The
 voltage on capacitor Cy 69 can be reset to the reference voltage Vref 73
 by momentarily closing switch SDy 76.
 In FIG. 3b a table is shown that has the various voltages to be connected
 to the resistive coordinate plane of a five wire digitizer. The voltages
 are connected to the four corners of the five wire digitizer as noted by
 LR (upper left), LL (lower left), UR (upper right) and UL (upper left).
 When measuring an X coordinate, the LR and UR corners are connected to a
 voltage V, and the LL and UL corners are connected to ground G. When
 measuring a Y coordinate, the LR and LL corners are connected to a voltage
 V, and the UR and UL corners are connected to ground G. For touch
 detection, TD, all corners are connected to a voltage V.
 Shown in FIG. 4 is a method for filtering coordinate voltages using a four
 terminal digitizer. When a pen is detected in the down position 80, a
 voltage is applied across the "X+" and "X-" terminals of the digitizer 81.
 The the word "down" as used in the previous sentence implies contact of
 the pen with the digitizer regardless of the orientation of the digitizer.
 Next the "Y-" terminal is connected to the X filter input 82. The Y panel
 of the digitizer is not powered and is floating with respect to voltage
 bias. The voltage representing the position of a digitizing pen is
 superimposed on the Y plane at the point of contact with the pen. The
 filtered voltage across the filter capacitor is read by an ADC with
 respect to the "X-" terminal 83 which is ground after the capacitor is
 fully charged. After the filtered voltage is read, the "Y-" terminal is
 disconnected from the X filter input 84. Then the bias voltage is
 disconnected from the "X+" and the "X-" terminals 85 and a voltage bias is
 applied across the "Y+" and "Y-" terminals of the digitizer 86. The "X-"
 terminal, which is now floating with respect to applied power, is now
 connected to the Y filter input 87, and the voltage representing the Y
 location of the pen is superimposed onto the floating X plane. When the
 capacitor of the Y filter is charged up with the Y coordinate voltage from
 the floating X plane, the voltage on the capacitor is read with respect to
 the "Y-" terminal of the digitizer 88. The "X-" terminal is disconnected
 for the Y filter 89, and the bias voltage is disconnected from the "Y+"
 and "Y-" terminals of the digitizer 90. When the pen is detected in the up
 position (up meaning not in contact with the digitizer panels) 91, Vref is
 momentarily connected to the X filter 92 to establish the same starting
 voltage on the X filter capacitor for the next measurement. Next Vref is
 momentarily connected to the Y filter 93 to establish a starting voltage
 on the Y filter capacitor, and the process returns to the beginning step
 waiting for the pen to be down in contact with the digitizer 80. If the
 pen is not up 94, the process continues with a return to the beginning
 step, and since the pen is down 80 continues on to applying a voltage
 across the X terminals of the digitizer 81. It should be noted that all
 steps in the process may take varying time delays for voltages and signals
 to stabilize. Adequate time delays are assumed to exist in each of the
 steps of the process.
 Shown in FIG. 5 is a method for filtering coordinate voltages using a five
 terminal digitizer. When a pen is detected in the down position 100, a
 voltage is connected to the UR (upper right) and LR (lower left), and
 ground is connected to UL (upper left) and LL (lower left) terminals of
 the digitizer 101. The the word "down" as used here implies contact of the
 pen with the digitizer regardless of the orientation of the digitizer.
 Next a sense terminal is connected to the X filter input 102. The filtered
 voltage across the filter capacitor is read by an ADC with respect ground
 103. After the filtered voltage is read, the sense terminal is
 disconnected from the X filter input 104. Then the bias voltage and ground
 is disconnected from the Up, LR, UL and LL terminals 105 and a voltage
 bias is connected to LR and LL and ground is connected to UR and UL
 terminals of the digitizer 106. Next a sense terminal is connected to the
 Y filter input 107. When the capacitor of the Y filter is charged up with
 the Y coordinate voltage, the voltage on the capacitor is read with
 respect to ground 108. The sense terminal is then disconnected from the Y
 filter 109, and the bias voltage and ground is disconnected from LR, LL,
 UR and UL terminals of the digitizer 110. When the pen is detected in the
 up position (up meaning not in contact with the digitizer panels) 111,
 Vref is momentarily connected to the X filter 112 to establish the same
 starting voltage on the X filter capacitor for the next measurement. Next
 Vref is momentarily connected to the Y filter 113 to establish a starting
 voltage on the Y filter capacitor, and the process returns to the
 beginning step waiting for the pen to be down in contact with the
 digitizer 100. If the pen is not up 114, the process continues with a
 return to the beginning step, and since the pen is down 100, the process
 continues by applying a voltage across the X terminals of the digitizer
 81. It should be noted that all steps in the process may take varying time
 delays for voltages and signals to stabilize. Adequate time delays are
 assumed to exist in each of the steps of the process.
 While the invention has been particularly shown and described with
 reference to preferred embodiments thereof, it will be understood by those
 skilled in the art that various changes in form and details may be made
 without departing from the spirit and scope of the invention.