Drive current calibration for an analog resistive touch screen
First Claim
1. A method for controlling power supplied to a load resistance, the method comprising the steps of:
- converting a digital control value to a current supplied to said load resistance, to thereby produce a voltage across said load resistance;
comparing said voltage across said load resistance to a desired voltage to produce a comparison signal; and
adjusting said digital control value in response to said comparison signal so that said voltage across said load resistance becomes substantially equal to said desired voltage, wherein said digital control value is adjusted in response to said comparison signal by successively adjusting individual bits of said digital control value beginning with a more significant bit of said digital control value and ending with a less significant bit of said digital control value.
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Accused Products
Abstract
An analog resistive touch screen is powered by a current source responsive to a stored digital control value. The current source is calibrated by comparing the excitation voltage across the touch screen to a desired voltage to produce a comparison signal, and adjusting the digital control value in response to the comparison signal so that the excitation voltage becomes substantially equal to the desired voltage. The power supply can therefore be easily constructed in a low-voltage CMOS integrated circuit having a minimal power consumption without sacrificing touch screen resolution. Preferably the comparison of the excitation voltage to the reference voltage is performed by an analog-to-digital converter that is later used in a normal conversion mode for digitizing the touch screen coordinates. Moreover, because the touch screen presents a different load resistance depending on whether a first resistive sheet is energized for measuring an X coordinate or a second resistive sheet is energized for measuring a Y coordinate, a separate calibration is performed for each of the two resistive sheets, and the digital control value from each calibration is stored in a separate register for selective retrieval to measure either an X coordinate or a Y coordinate.
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Citations
32 Claims
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1. A method for controlling power supplied to a load resistance, the method comprising the steps of:
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converting a digital control value to a current supplied to said load resistance, to thereby produce a voltage across said load resistance; comparing said voltage across said load resistance to a desired voltage to produce a comparison signal; and adjusting said digital control value in response to said comparison signal so that said voltage across said load resistance becomes substantially equal to said desired voltage, wherein said digital control value is adjusted in response to said comparison signal by successively adjusting individual bits of said digital control value beginning with a more significant bit of said digital control value and ending with a less significant bit of said digital control value. - View Dependent Claims (2, 3, 4, 5, 6)
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7. A method for controlling power supplied to a load resistance, the method comprising the steps of:
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(a) converting a digital control value to a selected value of substantially constant current supplied by an adjustable substantially-constant current source to said load resistance, to thereby produce an excitation voltage across said load resistance; (b) applying said excitation voltage to an analog-to-digital converter to produce a digitized value responsive to said excitation voltage; and (c) adjusting said digital control value in response to said digitized value so that said excitation voltage becomes substantially equal to a desired voltage. - View Dependent Claims (8, 9, 10)
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11. A method of operating an analog resistive touch screen having a screen resistance and producing a screen voltage indicating a selected location on the analog resistive touch screen, said method including the steps of:
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(a) converting a digital control value to a selected value of substantially-constant current supplied by an adjustable substantially-constant current source to said screen resistance, to thereby produce an excitation voltage across said screen resistance; (b) applying said excitation voltage to an analog-to-digital converter to produce a digitized value responsive to said excitation voltage; (c) adjusting said digital control value in response to said digitized value so that said voltage across said screen resistance becomes substantially equal to a desired voltage; and
then(d) applying said screen voltage to said analog-to-digital converter to produce a digital output indicating said selected location on the analog resistive touch screen. - View Dependent Claims (12, 13)
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14. A method of operating an analog resistive touch screen having a first resistive sheet and a second resistive sheet, said first resistive sheet having a first screen resistance and producing a first screen voltage indicating an X coordinate of a selected location on the analog resistive touch screen, said second resistive sheet having a second screen resistance and producing a second screen voltage indicating a Y coordinate of the selected location on the analog resistive touch screen, said method including:
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(a) performing respective calibrations for each of said first resistive sheet and said second resistive sheet, said calibration for said first resistive sheet being performed by; (i) converting a first digital control value to a first current supplied to said first screen resistance, to thereby produce a first excitation voltage across said first screen resistance; (ii) applying said first excitation voltage to an analog-to-digital converter to produce a first digitized value responsive to said first excitation voltage; (iii) adjusting said first digital control value in response to said first digitized value to obtain an adjusted value for said first digital control value; and (iv) storing said adjusted value for said first digital control value in a first register; and said calibration for said second resistive sheet being performed by; (i) converting a second digital control value to a second current supplied to said second screen resistance, to thereby produce a second excitation voltage across said second screen resistance; (ii) applying said second excitation voltage to said analog-to-digital converter to produce a second digitized value responsive to said second excitation voltage; (iii) adjusting said second digital control value in response to said second digitized value to produce an adjusted value for said second digital control value; and (iv) storing said adjusted value for said second digital control value in a second register; and
then(b) obtaining digital values of the X and Y coordinates of said selected location on said analog resistive touch screen, wherein the digital value of the X coordinate is obtained by; (v) retrieving said adjusted value for said first digital control value from said first register; (vi) converting said adjusted value for said first digital control value to said first current supplied to said first screen resistance, to thereby produce said first excitation voltage across said first screen resistance; (vii) applying said first screen voltage to said analog-to-digital converter to produce said digital value of said X coordinate; and wherein the digital value of the Y coordinate is obtained by; (v) retrieving said adjusted value for said second digital control value from said second register; (vi) converting said adjusted value for said second digital control value to said second current supplied to said second screen resistance, to thereby produce said second excitation voltage across said second screen resistance; and (vii) applying said second screen voltage to said analog-to-digital converter to produce said digital value of said Y coordinate.
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15. A digitally-calibrated power supply comprising, in combination:
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an adjustable substantially-constant current source for supplying current to a load resistance, said current causing a voltage to be produced across said load resistance, said adjustable substantially-constant current source having a digital-to-analog converter for adjusting said current in response to a digital control value; a register for storing said digital control value, said register being connected to said digital-to-analog converter for providing the stored digital control value to said digital-to-analog converter; comparator circuitry for comparing said voltage across said load resistance to a desired voltage to produce a comparison signal; and control logic connected to said register and to said comparator circuitry for adjusting said digital control value stored in said register in response to said comparison signal so that said voltage across said load resistance is adjusted toward said desired voltage. - View Dependent Claims (16, 17, 18, 19, 20, 21)
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22. A digitally-calibrated power supply comprising, in combination:
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an adjustable substantially-constant current source for supplying current to a load resistance, said current causing a voltage to be produced across said load resistance, said adjustable substantially-constant current source including a digital-to-analog converter for adjusting said current in response to a digital control value, and an amplifier having an output regulating the current supplied to said load resistance; a register for storing said digital control value, said register being connected to said digital-to-analog converter for providing the stored digital control value to said digital-to-analog converter; an analog-to-digital converter having an analog voltage input, a reference voltage input, and a digital output;
one of said analog voltage input and said reference voltage input being connected for receiving said voltage across said load resistance and the other of said analog voltage input and said reference voltage input being connected for receiving a reference voltage;control logic connected to said register and to said digital output of said analog-to-digital converter for adjusting said digital control value stored in said register in response to said digital output of said analog-to-digital converter so that said voltage across said load resistance is adjusted toward said desired voltage. - View Dependent Claims (23, 24, 25)
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26. A power supply and electronic interface for an analog resistive touch screen having a screen resistance and producing a screen voltage indicating a selected location on the analog resistive touch screen, said power supply and electronic interface comprising, in combination:
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an adjustable substantially-constant current source for supplying current to said screen resistance, said current causing an excitation voltage to be produced across said screen resistance, said adjustable substantially-constant current source including a digital-to-analog converter for adjusting said current in response to a digital control value, a register for storing said digital control value, said register being connected to said digital-to-analog converter for providing the stored digital control value to said digital-to-analog converter; an analog-to-digital converter having an analog voltage input, a reference voltage input, and a digital output; an analog multiplexer connected to said analog voltage input of said analog-to-digital converter and responsive to a calibration mode signal for providing said screen voltage to said analog voltage input during a normal mode of operation and for providing one of a reference voltage and the excitation voltage to said analog voltage input during a calibration mode of operation, and for providing the other one of said reference voltage and excitation voltage to said reference voltage input of said analog-to-digital converter during said calibration mode, and control logic connected to said register and to said digital output of said analog-to-digital converter for adjusting said digital control value in said register in response to said digital output of said analog-to-digital converter during said calibration mode so that said voltage across said screen resistance is adjusted toward said desired voltage during said calibration mode. - View Dependent Claims (27, 28, 29, 30, 31, 32)
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Specification