(12) United States Patent
`Joharapurkar et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 8,624,870 B2
`Jan. 7, 2014
`
`USOO862487OB2
`
`(54) SYSTEM FOR AND METHOD OF
`TRANSFERRING CHARGE TO CONVERT
`CAPACITANCE TO VOLTAGE FOR
`TOUCHSCREEN CONTROLLERS
`
`(75) Inventors: Ashutosh Ravindra Joharapurkar,
`Karnataka (IN); Karthikeya Kodur,
`Karnataka (IN); Venugopal Reddy,
`Karnataka (IN); Patrick Chan,
`Sunnyvale, CA (US)
`
`(*) Notice:
`
`(73) Assignee: Maxim Integrated Products, Inc., San
`Jose, CA (US)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 305 days.
`(21) Appl. No.: 12/986,841
`
`(22) Filed:
`(65)
`
`Jan. 7, 2011
`O
`O
`Prior Publication Data
`US 2011/0261006A1
`Oct. 27, 2011
`Related U.S. Application D
`elated U.S. Application Data
`(60) Provisional application No. 61/326,830, filed on Apr.
`22, 2010.
`
`(2006.01)
`
`(51) Int. Cl.
`G06F 3/045
`(52) U.S. Cl.
`USPC - - - - - - 345,174. 345/173; 178/1801: 178/18.06
`(58) Field of Classification Search
`USPC . .
`. .
`. . 345/173, 174; 178/ 18.01, 18.06
`See application file for complete search history.
`References Cited
`
`(56)
`
`U.S. PATENT DOCUMENTS
`
`4,527,096 A
`5,313,141 A
`
`7, 1985 Kindlmann
`5, 1994 Kimball
`
`OO
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`
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`5,463,283 A 10/1995 Sanderson
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`8.3. E"
`E is '58 SAEura et a
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`7,176,753 B2
`2/2007 Noda et al.
`7,283,120 B2 10/2007 Grant
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`2/2010 Sadler et al.
`(Continued)
`
`FOREIGN PATENT DOCUMENTS
`
`WO
`
`11, 1997
`9743825
`OTHER PUBLICATIONS
`Publication No. US-2011-0261005-A1, Publication Date: Oct. 27,
`2011, U.S. Appl. No. 12/986,776, filed Jan. 7, 2011, Ashutosh
`Ravindra Joharapurkar.
`
`(Continued)
`
`Primary Examiner — Koosha Sharifi-Tafreshi
`(74) Attorney, Agent, or Firm — Advent, LLP
`
`ABSTRACT
`(57)
`A touchscreen controller system determines the actual loca
`tions of multiple simultaneous touches by eliminating mutual
`capacitance between adjacent rows and columns during self
`capacitance measurements and selectively enabling mutual
`capacitance during mutual capacitance measurements. Dur
`ing the self-capacitance measurements, the controller system
`generates a set of candidate touch locations, which includes
`the locations of real and ghost touches. During the mutual
`capacitance measurements, only the locations in the candi
`date set are measured and, from these measurements, the
`actual touch locations are determined. By limiting the mutual
`capacitive measurements to only a small Subset of the loca
`tions over the entire touch panel, real touch locations are
`determined on a linear order. Also, by using on-chip integra
`tion capacitors, embodiments of the invention are able to
`perform each measurement in a single cycle.
`
`12 Claims, 13 Drawing Sheets
`
`Ref
`
`Ref
`
`DELL EXHIBIT 1029 PAGE 1
`
`

`

`US 8,624,870 B2
`Page 2
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4/2003 Yoshikawa et al.
`2003, OO67449 A1
`9/2003 Poupyrev et al.
`2003/0174121 A1
`2006/01 19586 A1
`6, 2006 Grant et al.
`2008/0055277 A1
`3/2008 Takenaka et al.
`2008.0062145 A1
`3/2008 Shahoian et al.
`6/2010 Paleczny et al.
`2010.0156823 A1
`2010.0245286 A1*
`9/2010 Parker ........................... 345,174
`2011/0025629 A1* 2, 2011 Grivna et al.
`... 345,173
`2011/006 1949 A1* 3, 2011 Krah et al. ..
`178.18.06
`2011/0084936 A1* 4/2011 Chang et al.
`... 345,174
`2011/O156839 A1* 6, 2011 Martin et al. ................. 333,172
`OTHER PUBLICATIONS
`
`
`
`Publication No. US-2011-0261006-A1, Publication Date: Oct. 27,
`2011, U.S. Appl. No. 12/986,841, filed Jan. 7, 2011, Ashutosh
`Ravindra Joharapurkar.
`
`Publication No. US-2011-026.1007-A1, Publication Date: Oct. 27,
`2011, U.S. Appl. No. 12/986,881, filed Jan. 7, 2011, Ashutosh
`Ravindra Joharapurkar.
`Publication No. US-2011-0260990-A1, Publication Date: Oct. 27,
`2011, U.S. Appl. No. 12/986,991, filed Jan. 7, 2011, Yassir Ali.
`Publication No. US-2011-0261008-A1, Publication Date: Oct. 27,
`2011, U.S. Appl. No. 12/987,008, filed Jan. 7, 2011, Ashutosh
`Ravindra Joharapurkar.
`Non-Final Office Action dated Sep. 29, 2011, U.S. Appl. No.
`12/315,690, filed Dec. 5, 2008, Anthony Stephen Doy.
`International Search Report dated Dec. 3, 2009, PCT Patent Appli
`cation Serial No. PCT/US 09/61359, Filed Oct. 20, 2009, Maxim
`Integrated Products, Inc.
`Daniel Carrica et al., “Random Sampling Applied to the Measure
`ment of a DC Signal Immersed in Noise', pp. 1319-1323, IEEE
`Transactions on Instrumentation and Measurement, vol. 50, No. 5,
`Oct. 2001.
`
`* cited by examiner
`
`DELL EXHIBIT 1029 PAGE 2
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`

`

`U.S. Patent
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`DELL EXHIBIT 1029 PAGE 3
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`

`

`U.S. Patent
`
`Jan. 7, 2014
`
`Sheet 2 of 13
`
`US 8,624,870 B2
`
`Integrated
`LCD/touchpad
`
`Display
`Controller
`
`Touchpad
`Circuit
`
`280
`
`Touchpad Circuit 220
`
`
`
`CONTROL
`
`DELL EXHIBIT 1029 PAGE 4
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`

`

`U.S. Patent
`
`Jan. 7, 2014
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`Sheet 3 of 13
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`US 8,624,870 B2
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`DELL EXHIBIT 1029 PAGE 5
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`Jan. 7, 2014
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`Jan. 7, 2014
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`US 8,624,870 B2
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`DELL EXHIBIT 1029 PAGE 8
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`

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`
`Jan. 7, 2014
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`US 8,624,870 B2
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`DELL EXHIBIT 1029 PAGE 9
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`DELL EXHIBIT 1029 PAGE 12
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`

`

`U.S. Patent
`
`Jan. 7, 2014
`
`Sheet 11 of 13
`
`US 8,624,870 B2
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`DELL EXHIBIT 1029 PAGE 13
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`

`

`U.S. Patent
`
`Jan. 7, 2014
`
`Sheet 12 of 13
`
`US 8,624,870 B2
`
`
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`
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`
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`
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`
`DELL EXHIBIT 1029 PAGE 14
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`

`

`U.S. Patent
`
`Jan. 7, 2014
`
`Sheet 13 of 13
`
`US 8,624,870 B2
`
`
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`
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`
`
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`
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`
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`location
`
`Determine difference between readings when mutual
`capacitance enabled and disabled
`
`851
`
`852
`
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`
`853
`
`N
`
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`
`
`
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`candidate
`locations?
`
`855
`
`856
`
`Fig. 12
`
`DELL EXHIBIT 1029 PAGE 15
`
`

`

`US 8,624,870 B2
`
`1.
`SYSTEM FOR AND METHOD OF
`TRANSFERRING CHARGE TO CONVERT
`CAPACITANCE TO VOLTAGE FOR
`TOUCHSCREEN CONTROLLERS
`
`RELATED APPLICATIONS
`
`10
`
`This application claims priority under 35 U.S.C. S 119(e) of
`the co-pending U.S. provisional patent application Ser. No.
`61/326,830, filed Apr. 22, 2010, and titled “Differential
`Capacitive Touchpad Circuit and Method.” which is hereby
`incorporated by reference in its entirety.
`The following co-owned, co-filed, and co-pending appli
`cations, U.S. patent application Ser. No. 12/986,776, titled
`"Method and Apparatus for Improving Dynamic Range of a
`Touchscreen Controller.” U.S. patent application Ser. No.
`12/986,881, titled “Noise Cancellation Technique for Capaci
`tive Touchscreen Controller Using Differential Sensing.”
`U.S. patent application Ser. No. 12/986,991, titled “System
`Integration of Tactile Feedback and Touchscreen Controller
`for Near-Zero Latency Haptics Playout.” U.S. patent applica
`tion Ser. No. 12/987,008, titled “Use of Random Sampling
`Technique to Reduce Finger-Coupled Noise, and U.S. patent
`application Ser. No. 12/986,905, titled “Method and Appara
`tus for Generating Piezoelectric Transducer Excitation Wave
`25
`forms Using a Boost Converter are all also incorporated by
`reference in their entireties.
`
`15
`
`FIELD OF THE INVENTION
`
`30
`
`This invention relates to human-to-machine interfaces.
`More specifically, this invention relates to capacitive touch
`SCCS.
`
`BACKGROUND OF THE INVENTION
`
`35
`
`Touchscreens or touch panels provide an interface for dis
`playing output and receiving input, a structure that makes
`them well Suited for mobile phones, personal digital assis
`tants, digital music players, and other compact devices. Using
`a touchscreen, a user can selector manipulate items displayed
`on the touchscreen, such as buttons, sliders, Scroll wheels, and
`other screen icons.
`Prior art systems use different methods to detect the pres
`ence of an object, such as capacitive charge transfer methods
`and relaxation OSC methods. In a capacitive touchscreen, for
`example, charge is transferred to sensing elements and
`objects adjacent to them. The combined stored charge is then
`read, with charges above a threshold indicating that the object
`is above the sensing element currently being read. By arrang
`ing sensing elements into a grid pattern having rows and
`columns, the particular location of an object on the grid can be
`determined.
`FIG. 1 is a block diagram of prior art capacitive touch
`screen 100. The touchscreen 100 comprises a surface con
`taining multiple sensing elements 101, each configured to
`sense the presence an object, Such as a finger 105, adjacent to
`it. By detecting which ones of the sensing elements the object
`105 is adjacent to, the location of the object along the surface
`of the touchscreen 100 is determined. The capacitive touch
`screen 100 functions by opening and closing Switches A, AB,
`B', C., and C to transfer charge that is measured by measuring
`circuits 110 and 115 and calculated by calculation means 120.
`Those skilled in the art will recognize that the capacitive
`touchscreen 100 requires multiple clock phases and multiple
`charge transfer cycles to determine whether an object is adja
`cent to its surface.
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`The touchscreen 100 has several disadvantages. For
`example, charge must be transferred to external capacitors
`Cs and Cs several times to measure capacitance change.
`This increases the latency and thus reduces the maximum
`frame rate that can be supported. As a result, the touchscreen
`100 has limited sensitivity and a reduced signal-to-noise
`ratio.
`The touchscreen 100, like other prior art capacitive touch
`screens, also Suffers from "ghosting.” during which the actual
`locations of simultaneous touches cannot be resolved. When
`simultaneous touches occurat multiple locations, the system
`is only able to determine that several touches occurred, the
`“real' touches and “ghost' touches. The system is unable to
`easily distinguish between the real and ghost touches. This
`ambiguity, and the processing needed to resolve it, increases
`exponentially with the number of simultaneous touches.
`Some capacitive touchscreens determine simultaneous
`touches using “mutual capacitance, a process that senses
`capacitances at the intersections of row and column lines.
`Systems that Support mutual capacitance measurements
`require much more complex analog hardware, which results
`in higher power dissipation, lower throughput rate, larger die
`size, and more complex signal processing.
`
`SUMMARY OF THE INVENTION
`
`Touchscreen controller systems in accordance with the
`invention Support mutual capacitance cancellation and selec
`tive mutual capacitance measurements between sensors,
`reducing the requirement of offset canceling capacitances.
`These touchscreen controller systems are also capable of
`eliminating ghost touches on a panel, a feature that is critical
`in detecting multi-touches without ambiguity. It does this
`without needing to take mutual capacitance measurements
`between each combination of row and column.
`Touchscreen controller systems in accordance with the
`invention need only a single charge transfer cycle to detect
`capacitance changes that indicate the presence of an object.
`These touchscreen controller systems use on-chip integration
`capacitors, which are much smaller than external integration
`capacitors and can be charged and discharged more quickly.
`These touchscreen controller systems can Support faster
`frame rates, can improve sensitivity, and require no external
`components for the sensors.
`Touchscreen controller systems in accordance with the
`invention convert sensor capacitance of a touch screen panel
`to a directly proportional Voltage. In a reset phase the systems
`are driven to ground and will not pick up noise from external
`Sources. In one embodiment, except for one Switch in the
`feedback path, all node voltages carry either a reference volt
`age or ground. This structure reduces the effect of charge
`injection because most of the charge injection results in a
`constant offset for the system, an offset that can be accom
`modated by calibration.
`In one aspect of the invention, a touchscreen controller
`systems for sensing one or more objects adjacent to and/or in
`contact with a surface of the touchscreen controller includes
`a plurality of sense lines arranged adjacent to the Surface of
`the touch panel. Adjacent sense lines have a mutual capaci
`tance. A plurality of capacitive sensing elements are each
`coupled to one of the senselines. A control logic is configured
`to drive mutual capacitances between selected and unselected
`sense lines to the same potential during a self-capacitance
`measuring stage and to drive mutual capacitances between
`selected and unselected sense lines to different potentials
`during a mutual capacitance measuring stage. In one embodi
`ment, the touchscreen controller systems includes a Voltage
`
`DELL EXHIBIT 1029 PAGE 16
`
`

`

`3
`measurement circuit that converts a charge on a selected
`capacitive sensing element into a corresponding Voltage that
`indicates whether an object is adjacent thereto.
`In a second aspect of the invention, a method of sensing
`presence of objects simultaneously on or adjacent to a Surface
`of a touch panel includes individually selecting row and col
`umn lines of the touch panel to determine one or more can
`didate locations of the objects along the Surface and reading
`combinations of row and column lines of the touch panel from
`only the candidate locations to determine one or more actual
`locations of the objects along the Surface. In one embodiment,
`the steps of individually selecting row and column lines and
`reading combinations of row and column lines are performed
`on a single integrated circuit. The method also includes can
`celing mutual capacitance between the row and column lines
`when individually reading selected row and column lines and
`selectively enabling mutual capacitance between adjacent
`row and column lines when reading selected combinations of
`row and column lines.
`In one embodiment, determining one or more actual loca
`tions comprises comparing self capacitance measurements of
`the candidate locations with corresponding mutual capaci
`tance measurementS.
`
`10
`
`15
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`25
`
`FIG. 1 is a schematic diagram of a prior art touchscreen.
`FIG. 2 is a block diagram of a mobile telephone having a
`touchscreen, used to explain principles of the invention.
`FIG.3 is a schematic diagram showing several components
`of a touchscreen, used to explain principles of the invention.
`FIG. 4 is a high-level diagram modeling component and
`parasitic capacitive elements of a capacitive touchscreen,
`used to explain principles of the invention.
`FIGS. 5A and 5B show a sensor component during reset
`and measurement phases, respectively, in accordance with
`one embodiment of the invention.
`FIG. 6 is a schematic diagram of an offset capacitor in
`accordance with one embodiment of the invention.
`FIG. 7 shows Voltage diagrams during reset and measure
`ment phases for the sensor components of FIGS.5A and 5B.
`FIGS. 8A-C are schematic diagrams of a sensor component
`that uses integration operations in accordance with one
`embodiment of the invention.
`FIG.9 shows Voltage diagrams of the sensor component in
`FIGS. 8A-C during different phases of operation.
`FIG. 10 shows real and ghost locations of multiple touches
`on a touchscreen, used to explain the principles of the inven
`tion.
`FIG. 11 shows the steps of a method of measuring the
`Voltages on sensing elements using both self capacitance and
`mutual capacitance to eliminate ghosting in accordance with
`the present invention.
`FIG. 12 shows the steps of a method of selectively enabling
`mutual capacitance in combination with self capacitance, as
`illustrated in FIG. 11.
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`DETAILED DESCRIPTION OF THE INVENTION
`
`The present invention encompasses techniques for reduc
`ing or eliminating error in the output of capacitive sensor
`arrays such as touchpads, touchscreens, touch sliders and the
`like, including sensors that detect the presence and position of
`a stylus, as well as those that detect and determine finger
`position. While the illustrative embodiment described herein
`is applied in a mobile telephone, it is understood that capaci
`tive touch sensors are used in a wide variety of devices.
`
`60
`
`65
`
`US 8,624,870 B2
`
`4
`Examples of Such devices are portable devices, such as per
`Sonal digital assistants (PDA5), global positioning systems
`(GPS) receivers, as well as larger devices such as touchscreen
`enabled displays and computer systems, as well as appli
`aCCS,
`Referring now to FIG. 2, a mobile telephone 200 in accor
`dance with one embodiment of the present invention is
`shown. The mobile telephone 200 includes a microprocessor
`(uP) 214 coupled to a memory 215 that stores program
`instructions for execution by the microprocessor 214, and
`generally includes non-volatile storage for Such program
`instructions, as well as temporary storage for use by the
`microprocessor 214. The program instructions stored in a
`memory 215 include program instructions forming computer
`program products in accordance with embodiments of the
`present invention that determine the position of one or more
`fingers and/or styli at the Surface of a touch sensor included in
`an integrated liquid crystal display (LCD)/touchpad 212. The
`LCD/touchpad 212 is coupled to a touchpad circuit 220 in
`accordance with one embodiment of the present invention
`that includes the capability of simultaneously measuring the
`capacitance of two or more elements of the touchpad within
`the LCD/touchpad 212. Alternatively, as will be described in
`further detail below, the present invention may integrate a
`Voltage present on the two or more elements which is gener
`ally produced by providing a reference potential to another
`layer. Integrator circuit(s) may be provided to perform the
`integration, or the present invention may include program
`instructions within the memory 215 that measure the voltages
`present on the two or more elements, from simultaneous
`samples of the Voltages, and integrate the Voltages. The
`mobile telephone 200 also includes a display controller 216
`for coupling the microprocessor 214 to the LCD within the
`integrated LCD/touchpad 212 and radio circuits 218 for pro
`viding wireless telephone communication connections. The
`mobile telephone 200 also includes an audio codec 217
`coupled to a microphone 213 and a speaker element 211 that
`provide voice communications with a user.
`In accordance with the present invention, the capacitance
`of two or more rows/columns of a two-dimensional touchpad
`array or two or more elements of a one-dimensional touchpad
`array, Such as a slider, are measured simultaneously, either
`directly, or by measuring a Voltage present on the elements.
`The measurements are subtracted to yield a measurement free
`of common-mode error that is primarily due to incident noise,
`such as that produced by operation of the LCD, any backlight
`or other power supply under integrated LCD/touchpad 212
`and other external environmental sources of noise.
`Referring now to FIG.3, details of the touchpad circuit 220
`in the mobile telephone 200 of FIG. 2 are shown in accor
`dance with one embodiment of the present invention. As in all
`the figures, the same label refers to the same or similar ele
`ment. The touchpad circuit 220 includes a pair of multiplexers
`260A and 260B coupled to each of eight rows of touchpad
`elements 280. Single touchpad elements 280 are illustrated,
`but it is understood that more or fewer rows and columns of
`equal or unequal numbers and both one and two-dimensional
`touchpads can utilize the techniques of the present invention.
`The multiplexer 260A selects a first row/element from among
`the touchpad elements 280. The multiplexer 260B selects
`another second row/element from among the touchpad ele
`ments 280. A pair of capacitance measuring circuits 270A and
`270B each measures the capacitance at the output of the
`multiplexers 260A and 260B, respectively, which will change
`due to the presence of a finger/stylus near the selected ele
`ment, and converts the capacitances to corresponding Voltage
`signals.
`
`DELL EXHIBIT 1029 PAGE 17
`
`

`

`5
`A control logic circuit 225 provides a reset signal Rst for
`resetting capacitance measuring circuits 270A and 270B,
`which prepares capacitance measuring circuits 270A and
`270B to make capacitance measurements. The control logic
`can be integrally formed with the microprocessor 214. In such
`a case the signal Rst will be provided by the microprocessor
`214. Selection values Sel1 and Sel2 are set at address inputs of
`multiplexers 260A and 260B for selecting the measurement
`elements prior to performing the measurement. The reset
`signal Rst is de-asserted, thereby permitting the capacitance
`measuring circuits 270A and 270B to measure the capaci
`tance of the respective selected touchpad elements 280. The
`outputs of the capacitance measuring circuits 270A and 270B
`are applied differentially to the inputs of an analog-to-digital
`converter (ADC) 240, which provides digital values corre
`sponding to the difference between the capacitances mea
`sured at the outputs of the multiplexers 270A and 270B, after
`a sample control signal, Sample, signal is asserted for Sam
`pling the outputs of the capacitance measuring circuits 270A
`and 270B. Therefore any common-mode noise and offset
`present at the outputs of multiplexers 260A and 260B will be
`substantially canceled in the output of ADC 240.
`FIG. 4 is a schematic diagram 400 modeling the touchpad
`circuit 220 of FIG. 3., used to explain principles of the inven
`tion. The schematic 400 shows that each intersecting row and
`column lines (e.g., R1 and C1) that form the touchpad 212
`contains a mutual capacitance C. Each row line contains a
`sensor capacitance (C) and a sensor offset capacitance
`(C), both coupled to ground. For modeling purposes, it
`will be appreciated that C
`can be combined into its cor
`responding C.
`Referring now to FIG. 5A, details of capacitance measur
`ing circuits 270A and 270B (collectively, 270) of FIG. 3 are
`shown. A capacitance measuring circuit, as provided in one
`embodiment of the present invention, includes a Switching
`35
`network formed by the switches S1A-S1D, the switches S2A
`S2C, and the switch S3. A feedback or integration capacitor
`C is selectively coupled to an operational amplifier 420 by
`the switches S1C and S2B. As shown in FIG. 5A, when the
`reset signal Rst is asserted, the switch S3 couples the input of
`40
`mutual capacitor C to ground and the Switch S1B couples
`the inverting input of the amplifier 420 to a reference voltage
`V. Also during assertion of the reset signal RSt, the Switch
`S2B is open and the switch S1C is closed, causing the inte
`gration capacitor C to be charged to the reference Voltage
`V. Also during the assertion of the reset signal Rist, the
`switch S1A is closed, coupling the output of the multiplexer
`260 to ground, which discharges a capacitance C of the
`selected element, and any stray offset capacitance along the
`path to the selected element C. An offset cancellation
`capacitor C is coupled to the inverting input of the opera
`tional amplifier 420 and charged to the reference voltage
`V, when the reset signal RSt is asserted by the activation of
`the switch S1D that grounds the second terminal of the offset
`cancellation capacitor C. Therefore, after a settling time,
`the condition of the capacitances in the capacitance measure
`ment circuit 270 are Such that the integration capacitor Cy
`and the offset cancellation capacitor C are charged to the
`reference Voltage V, and the element capacitance C and
`any stray offset capacitance C
`are Substantially dis
`charged.
`As shown in FIG. 5B, when a measurement control signal
`Meas is asserted after the reset signal Rst is de-asserted,
`which is generally performed by ensuring non-overlapping
`control signals generated by the control logic 225, the
`switches S1C and S1B are open due to the de-assertion of the
`reset signal RSt, and the Switch S3 couples the mutual capaci
`
`6
`tance C to the reference Voltage V. The Switch S2E is
`closed by assertion of the measurement control signal Meas,
`causing the integration capacitor Cry to be coupled between
`the output and the inverting input of the operational amplifier
`420. Since the integration capacitor C is charged to a
`Voltage of V with respect to the input terminal, and since
`the non-inverting input of the operational amplifier 420 is also
`referenced to the reference Voltage V, the initial output of
`the operational amplifier 420 will be substantially Zero. Dur
`ing assertion of the measurement control signal Meas, the
`switch S2A is closed, and the switch S1A was previously
`opened at de-assertion of the reset signal RSt. Also, the offset
`cancellation capacitor C is de-coupled from ground when
`the reset signal Rst causes the switch S1D to open, and is
`coupled to the reference Voltage V at the previously
`grounded terminal when the measurement control signal
`Meas causes the switch S2C to close. As a result of the closure
`of the Switches S2A and S2C, a positive charge amount equal
`to Co.V., where Co. is the capacitance of the offset
`cancellation capacitor C, is transferred from the offset
`cancellation capacitor C to the integration capacitor Cy
`and a negative charge amount equal to -CV is trans
`ferred from the output terminal of the multiplexer 260 to the
`integration capacitor C, where C is the Sum of the capaci
`tances of the element capacitance C and any stray offset
`capacitance Co. (Based on a particular context, those
`skilled in the art will recognize when a label. Such as C,
`refers to an element, a value of an element, a charge on an
`element, etc.)
`The output voltage V of the operational amplifier 420 is
`equal to the output-referred Voltage across the integration
`capacitor C. plus V, which can be computed from
`Vo-Q/C+V where Q is the charge on the integration
`capacitor C. The total charge transferred to the integration
`capacitor C, when the measurement control signal Meas is
`asserted is CV-C.V-AQ. The initial charge
`on the integration capacitor Civis-CMV, so the final
`value of charge Q is given by Equation (1):
`
`Equation (1)
`-CNT*VRE-i-Cx*VRE-CoFF'VREF
`and the output Voltage Vo is given by Equation (2):
`
`Vo (-CNT*VREF--Cx VREF-Corf'VREF)CNT+VREF
`Equation (2)
`(C/CNT-Coffy'CNT) REF
`The same result can be obtained by applying charge conser
`Vation at the inverting input terminal of the operational ampli
`fier 420. Therefore, the output voltage V is proportional to
`the capacitance of the element plus any stray offset capaci
`tance. The offset cancellation capacitor C provides bias
`ing of the measurement away from an always negative result,
`and the relative capacitance of the integration capacitor C.
`sets the dynamic range of the measurement. In one embodi
`ment of the present invention, the integration capacitor Cy
`can have a much higher capacitance relative to the capaci
`tances C and C, and the Switches S1A and S2A are
`operated by separate clock signals having a higher frequency
`than the measurement control signal Meas and the reset signal
`Rst. The operational amplifier 420 and the integration capaci
`tor Cy will then act as an integrator, integrating the charge
`transferred from the capacitance at the output of the multi
`plexer 260, providing additional filtering of noise.
`The feedback factor for the operational amplifier 420 is
`determined by the ratio of C and C to C. C is an
`external capacitor that the designers have no control over. It
`can change from a very Small value to a large value. C is
`used to cancel the effect of parasitic capacitance, Co. In
`one embodiment, shown in FIG. 6, C is a capacitive digi
`
`US 8,624,870 B2
`
`5
`
`10
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`15
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`25
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`30
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`45
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`50
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`55
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`60
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`65
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`DELL EXHIBIT 1029 PAGE 18
`
`

`

`7
`tal-to-analog control element made up of unit-sized capaci
`tors. Instead of Switching unit capacitors in and out, depend
`ing on the value of C, the number of fingers at the bottom
`plate of Col driven to V or ground during the measure
`ment phase can be changed. If the bottom plate of some of the
`fingers of C is driven to ground in the measurement phase,
`there is no net charge transfer from these fingers as the Voltage
`across them stays constant in the two cycles. The feedback
`factor will be set by all the fingers of the C capacitor and
`the external capacitor C. This will significantly reduce the
`variation of the feedback factor of the operational amplifier.
`The embodiment of C shown in FIG. 6 allows for the
`calibration of the capacitance measurement circuit 270 in
`FIG. 3. In the embodiment of FIG. 6, C. can be made
`programmable. Additional fingers of a capacitor having a
`common plate are selected by a plurality of Switches S.
`through Sy in response to a set of digital signals trim, effec
`tively providing for a set of selectable capacitive elements
`C1-CN that can be used to adjust C. on a static per-element
`basis after calibration. In one embodiment, C is Zeroed for
`offset cancellation.
`Referring to FIG. 5A, in one embodiment, the integration
`capacitor C, and the operational amplifier 420 are formed
`on a single integrated circuit. As one example, all the ele
`ments to the right of the dotted line are on a single integrated
`circuit. It will be appreciated that because the integration
`capacitor C is on-chip, it is Smaller than similarly func
`tioning capacitors in the prior art, reducing the time needed to
`charge and discharge it. Such structure decreases the latency
`of the touchscreen processing.
`Referring now to FIG. 7, a signal waveform diagram
`depicting the operation of the capacitance measurement cir
`cuit 400 of FIG. 5A is illustrated. The voltage V, is the
`Voltage across the feedback capacitor Cry and the Voltage
`Vis the voltage at the output of the multiplexer 260. During
`the first assertion of the reset signal Rist, at a time T, the
`Voltage V is set to V and the Voltage V is set to Zero.
`The depicted condition for the first measurement is
`C-0.50C and C-0. At a time T, the reset signal Rst is
`de-asserted and the measurement control signal Meas is
`asserted. The Voltage V, falls to 0.5V, and the Voltage V
`rises to 0.5* V, which is consistent with Equation (2),
`above. Also at the time T, the sample control signal Sample
`is asserted and the value of the difference between the outputs
`of the capacitance measurement circuits 270A and 270B is
`45
`sampled and then captured at a time T on the falling edge of
`sample control signal Sample. Since the operation of both of
`the capacitance measurement circuits 270A and 270B is iden
`tical except for the capacitance present at their inputs, only
`one set of waveforms