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`US 20080309625Al
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`(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2008/0309625 A1
`(43) Pub. Date:
`Dec. 18, 2008
`Krah et al.
`
`(54) MULTIPLE SIMULTANEOUS FREQUENCY
`DETECTION
`
`(75)
`
`Inventors:
`
`tK h [.08 All
`ll H
`Christ
`os,
`ors
`op
`ra ,
`.
`CA (US): Steve Porter Hotemng.
`San Jose. CA (US): Sean Erik
`O'Connor, Palo Alto, CA (US);
`Wayne Carl Westrerman, San
`Francisco, CA (US)
`
`Correspondence Address
`APPLE 00 M03350” AND FOERSTER J4“)
`LOS ANCELES
`555 WEST Fll-‘TH STREET SUITE 3500
`LOS ANGELES, CA 90013-1024 (US)
`
`(73) Assignee:
`
`APPIG 13¢-i CUPel1in0- CA (US)
`
`(21) Appl. No.:
`
`1 1/818,345
`
`(22)
`
`Filed:
`
`Jun. 13, 2007
`
`Publication Classification
`
`(51)
`
`Int. (:1.
`6061: 3/041
`(52) US. Cl.
`(57)
`
`(2006.01)
`,
`_
`345/I73, 381/386, 455/422.l
`ABSTRACT
`
`The use of multiple stimulation frequencies and phases to
`generate an image of touch on a touch sensor panel is dis-
`closed. Each ofa plurality ofsense channels can be coupled to
`a column in a touch sensor panel and can have multiple
`mixers. Each mixer in the sense channel can utilize a circuit
`capable generating a demodulation frequency ofa particular
`frequency. At each of multiple steps, various phases of
`selected frequencies can be used to simultaneously stimulate
`the mws ofthe touch sensor panel. and the multiple mixers in
`each sense channel can be configured to demodulate the sig-
`nal received from the column connected to each sense chan-
`nel using the selected tiequencies. After all steps have been
`completed. the demodulated signals from the multiple mixers
`can be used in calculations to detennine an image oftouch for
`the touch sensor panel at each frequency.
`
`Stan
`
`H zms) Update
`LCD
`
`SIBD 0
`
`step 1
`
`
`0 configure sense channels for spectrum analyzer function
`0 no row stimulation
`0 sum all sense channel charge amplifier outputs
`0 demodulate using I 8. Q of multiple frequencies
`
`- complete and store magnitude for each frequency
`
`
`
`
`{ (
`
`~0.2ms)
`
`LCD
`
`Phase
`402
`
`lo_w
`noise
`
`~
`( 0.2ms)
`
`0 demodulate using I & Q of different multiple frequencies
`.
`0 compute and store magnitude for each frequency
`
`(~0-2mS) - pick several low noise frequencies
`step 4-19
`
`- apply +/- phases of low noise frequencies to rows '
`
`
`0 demodulate sense channels using low noise frequencies
`
`- store results from all mixers
`
`step 3
`
`
`
`"3
`‘405
`
`
`
`Panel
`Scan
`408
`
`’I0UCh
`Sensor
`Panel
`phase
`404
`
`Page 1 of 19
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`.
`noise frequency
`
`end
`
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`

`

`Panel
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`
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`Panel subsystem 106
`
`
`
`Patent Application Publication
`
`Dec. 18, 2008 Sheet 1 of 9
`
`US 2008/0309625 A1
`
`Computing
`System
`100
`
`\
`
`_
`Peripherals
`104
`
`
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`Stimulation
`Signals
`116
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`Touch sensor panel
`124
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`Fig. 1
`
`Page 2 of 19
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`Patent Application Publication
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`Dec. 18, 2008 Sheet 2 of 9
`
`US 2008/0309625 A1
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`202
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`finger
`
`Fig. 2c
`
`Page 3 of 19
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`Patent Application Publication
`
`Dec. 18, 2008 Sheet 3 of 9
`
`US 2008/0309625 A1
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`1
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`Page 4 of 19
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`Patent Application Publication
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`Dec. 18, 2008 Sheet 4 of 9
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`US 2008/0309625 A1
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`Patent Application Publication
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`Dec. 18, 2008 Sheet 5 of 9
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`US 2008/0309625 A1
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`Patent Application Publication
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`Dec. 18, 2008 Sheet 6 of 9
`
`US 2008/0309625 A1
`
`(~12ms)
`LCD Phase
`
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`touch sensor
`panel phase
`
`
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`Fig. 4a
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`
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`
`step 1
`
`
`
`0 configure sense channels for spectrum analyzer function
`0 no row stimulation
`0 sum all sense channel charge amplifier outputs
`0 demodulate using I & O of multiple frequencies
`
`- complete and store magnitude for each frequency
`
`(~0.2ms)
`
`
`
`Step 2
`(_Q_p_m3) 0 demodulate using I 8. Q of different multiple frequencies
`0 compute and store magnitude for each frequency
`
`(~0-2mS) - pick several low noise frequencies
`
`step 3
`
` 0 apply +/- phases of low noise frequencies to rows ‘
`
`_
`- demodulate sense channels using low noise frequencies
`0 store results from all mixers
`
`step 4-19
`
`low
`
`
`
`
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`freq
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`408
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`t0UCh
`SellS0l
`panel
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`404
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`page 7 0f19
`
`compute image of
`touch for each low
`noise frequency
`
`STEP 20
`
`end
`
`Wintek Exhibit 1024
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`IPR2013—OO567
`|PR2014-00541
`
`

`

`Patent Application Publication
`
`Dec. 18, 2008 Sheet 7 of 9
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`US 2008/0309625 A1
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`Wintek Exhibit 1024
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`IPR2013-00567
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`

`Patent Application Publication
`
`Dec. 18, 2008 Sheet 9 of 9
`
`US 2008/0309625 A1
`
`PSA 534
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`Page 10 of 19
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`

`

`US 2008/0309625 Al
`
`Dec. I8, 2008
`
`MULTIPLE SIMULTANEOUS FREQUENCY
`DETECTION
`
`FIELD OF THE INVENTION
`
`[0001] This relates to touch sensor panels used as input
`devices for computing systems, and more particularly. to the
`use ofmultiple digital mixers to perform spectrum analysis of
`noise and identify low noise stimulation frequencies, and to
`tlte use of multiple stimulation frequencies and phases to
`detect and localize touch events on a touch sensor panel.
`
`BACKGROUND OF THE INVENTION
`
`[0002] Many types of input devices are presently available
`for performing operations in a computing system, such as
`buttons or keys, mice, trackballs, touch sensor panels, joy-
`sticks, touch screens and the like. Touch screens, in particular.
`are becoming increasingly popular because of their case and
`versatility ofoperation as well as their declining price. Touch
`screens can include a touch sensor panel, which can be a clear
`panel with a touch-sensitive surface. and a display device that
`can be positioned behind the panel so that the touch-sensitive
`surface can substantially cover the viewable area of the dis-
`play device. Touch screens can allow a user to perform vari-
`ous functions by touching the touch sensor panel using a
`finger, stylus or other object at a location dictated by a user
`interface (Ul) being displayed by the display device. In gen-
`eral, touch screens can recognize a touch event and the posi-
`tion of the touch event on the touch sensor panel, and the
`computing system can then interpret the touch event in accor-
`dance with the display appearing at the time of the touch
`event, and thereafter can perfonn one or more actions based
`on the touch event.
`[0003] Touch sensor panels can be formed from a matrix of
`row and coltrmn traces, with sensors or pixels present where
`the rows and columns cross over each other while being
`separated by a dielectric material. Each now can be driven by
`a stimulation signal, and touch locations can be identified
`because the charge injected into the columns due to the stimu-
`lation signal is proportional to the amount oftouch. However.
`the high voltage that can be required for the stimulation signal
`can force the sensor panel circuitry to be larger in size, and
`separated into two or more discrete chips. In addition. touch
`screens fonned from capacitance-based touch sensor panels
`and display devices such as liquid crystal displays (LCDs)
`can sufier from noise problems because the voltage switching
`required to operate an LCD can capacitively couple onto the
`columns of the touch sensor panel and cause inaccurate mea-
`surements of touch. Furthermore, alternating current (AC)
`adapters used to power or charge the system can also couple
`noise into the touchscreen. Other sources ofnoise can include
`switching power supplies in the system. backlight inverters.
`and light emitting diode (LED) pulse drivers. Each of these
`noise sources has a unique frequency and amplitude of inter-
`ference that can change with respect to time.
`
`SUMMARY OF THE INVENTION
`
`[0004] This relates to the use of multiple digital mixers to
`perform spectrum amlysis of noise and identify low noise
`stimulation frequencies, and to the use ofmultiplestimulation
`frequencies and phases to detect and localize touch events on
`a touch sensor panel. Each ofa plurality ofsense channels can
`be coupled to a column in a touch sensor panel and can have
`multiple mixers. Each mixer in each sense channel can utilize
`
`Page 11 of 19
`
`a circuit capable of being controlled to generate a demodula-
`tion fiequency ofa particular frequency, phase and delay.
`[0005] When performing a spectrum analyzer function, no
`stimulation signal is applied to any of the rows in the touch
`sensor panel. The sum of the output of all sense channels,
`which can represent the total charge being applied to the
`touch sensor panel including all detected noise, can be fed
`back to each of the mixers in each sense channel. The mixers
`
`can be paired up, and each pair of mixers can demodulate the
`stun of all sense channels using the in-phase (I) and quadra-
`ture (Q) signals of a particular frequency. The demodulated
`outputs of each mixer pair can be used to calculate the mag-
`nitude of the noise at that particular frequency, wherein the
`lower the magnitude, the lower the noise at that frequency.
`Several low noise frequencies can be selected for use in a
`subsequent touch sensor panel scan function.
`[0006] When performing the touch sensor panel scan func-
`tion. at each of multiple steps, various phases of the selected
`low noise frequencies can be used to simultaneously stimu-
`late the rows of the touch sensor panel, and the multiple
`mixers in each sense channel can beconfigured to demodulate
`the signal received from the column connected to each sense
`channel using the selected low noise frequencies. The
`demodulated signals from the multiple mixers can then be
`saved. After all steps have been completed, the saved results
`can be used in calculations to determine an imageof touch for
`the touch sensor panel at each frequency.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 illustrates an exemplary computing system
`[0007]
`that can utilize multiple digital mixers to perform spectrum
`analysis of noise and identify low noise stimulation frequen-
`cies, and can utilize multiple stimulation frequencies and
`phases to detect and localize touch events on a touch sensor
`panel according to one embodiment of this invention.
`[0008]
`FIG. 2a illustrates an exemplary mutual capacitance
`touch sensor panel according to one embodiment of this
`invention.
`
`[0009] FIG. 2b is a side view of an exemplary pixel in a
`steady-state (no-touch) condition according to one embodi-
`ment of this invention.
`
`FIG. 2c is a side view of an exemplary pixel in a
`[0010]
`dynamic (touch) condition according to one embodiment of
`this invention.
`
`FIG. 3a illustrates a portion of an exemplary sense
`[0011]
`channel or event detection and demodulation circuit accord-
`ing to one embodiment of this invention.
`[0012]
`FIG. 3b illustrates a simplified block diagram of N
`exemplary sense channel or event detection and demodula-
`tion circuits according to one embodiment of this invention.
`[0013]
`FIG. 30 illustrates an exemplary block diagram of
`IO sense channels that can be configured either as a spectrum
`analyzer or as panel scan logic according to one embodiment
`of this invention.
`
`FIG. 4a illustrates an exemplary timing diagram
`[0014]
`showing an LCD phase and touch sensor panel phase accord-
`ing to one embodiment of this invention.
`[0015]
`FIG. 4/)
`illustrates an exemplary flow diagram
`describing the LCD phase and the touch sensor panel phase
`according to one embodiment of this invention.
`[0016]
`FIG. 4c illustrates an exemplary capacitive scanning
`plan according to one embofillwifiték
`1 4
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`|PR2013-00567
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`|PR2014-00541
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`

`US 2008/0309625 A1
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`Dec. 18, 2008
`
`FIG. 4d illustrates exemplary calculations for a par-
`[0017]
`ticular channel M to compute full image results at different
`low noise frequencies according to one embodiment of this
`invention
`[0018]
`FIG. 5a illustrates an exemplary mobile telephone
`that can utilize multiple digital mixers to perform spectrum
`analysis of noise and identify low noise stimulation frequen-
`cies, and can utilive multiple stimulation frequencies and
`phases to detect and localize touch events on a touch sensor
`panel according to one embodiment of this invention.
`[0019] FIG. 5b illustrates an exemplary digital audio player
`that can utilive multiple digital mixers to perform spectrum
`analysis of noise and identify low noise stimulation frequen-
`cies, and can utilize multiple stimulation frequencies and
`phases to detect and localize touch events on a touch sensor
`panel according to one embodiment of this invention.
`
`DETAILED DESCRlI’TlON OF THE PREFERRED
`F.MR()DIMENT
`
`In the following description of preferred embodi-
`[0020]
`ments, reference is made to the accompanying drawings
`which form a part hereof. and in which it is shown by way of
`illustration specific embodiments in which the invention can
`be practiced. It is to be understood that other embodiments
`can be used and structural changes can be made without
`departing from the scope of the embodiments of this inven-
`U011.
`
`[0021] This relates to the use of multiple digital mixers to
`perform spectrum analysis of noise to identify low noise
`stimulation frequencies. and the use of multiple stimulation
`frequencies and phases to detect and localize touch events on
`a touch sensor panel. Each ofa plurality ofsense channels can
`be coupled to a column in a touch sensor panel and can have
`multiple mixers. Each mixer in the sense channel can utilize
`a circuit capable of being controlled to generate a demodula-
`tion frequency of a particular frequency, phase and delay.
`[0022] When performing a spectrum analyzer function, no
`stimulation signal is applied to any of the rows in the touch
`sensor panel. The sum of the output of all sense channels,
`which can represent the total charge being applied to the
`touch sensor panel including all detected noise, can be fed
`back to each of the mixers in each sense channel. The mixers
`
`can be paired up. and each pair of mixers can demodulate the
`sum of all sense channels using the in-phase (I) and quadra-
`ture (Q) signals of a particular frequency. The demodulated
`outputs of each mixer pair can be used to calculate the mag-
`nitude of the noise at that particular frequency. wherein the
`lower the magnitude. the lower the noise at that frequency.
`Several low noise frequencies can be selected for use in a
`subsequent touch sensor panel scan function.
`[0023] When performing the touch sensor panel scan func-
`tion. at each of multiple steps, various phases of the selected
`low noise frequencies can be used to simultaneously stimu-
`late the rows of the touch sensor panel. and the multiple
`mixers in each sense channel can be configured to demodulate
`the signal received from the column connected to each sense
`channel using the selected low noise frequencies. The
`demodulated signals from the multiple mixers can then be
`saved. After all steps have been completed, the saved results
`can be used in calculations to determine an image oftouch for
`the touch sensor panel at each frequency.
`[0024] Although some embodiments of this invention may
`be described herein in terms of mutual capacitance touch
`sensors. it should be understood that embodiments of this
`
`Page 12 of19
`
`invention are not so limited, but are generally applicable to
`other types of touch sensors such as self capacitance touch
`sensors. Funhennore, although the touch sensors in the touch
`sensor panel may be described herein in terms ofan orthogo-
`nal array oftouch sensors having rows and columns, it should
`be tmderstood that embodiments of this invention are not
`limited to orthogonal arrays, but can be generally applicable
`to touch sensors arranged in any number of dimensions and
`orientations, including diagonal. concentric circle, and three-
`dimensional and random orientations. In addition, the touch
`sensor panel described herein can be eitha a single-touch or
`a multi-touch sensor panel, the latter of which is described in
`Applicant‘s co-pending U.S. application Ser. No. 10/842,862
`entitled “Multipoint Touchscreen,“ filed on May 6, 2004 and
`published as U.S. Published Application No. 2006/0097991
`on May I 1, 2006. the contents of which are incorporated by
`reference herein.
`
`FIG. 1 illustrates exemplary computing system 100
`[0025]
`that can utilize multiple digital mixers to perform spectrum
`analysis of noise and identify low noise stimulation frequen-
`cies. and can utilize multiple stimulation frequencies and
`phases to detect and localize touch events on a touch sensor
`panel according to embodiments ofthe invention. Computing
`system 100 can include one or more panel processors 102 and
`peripherals 104. and panel subsystem 106. One ormorc panel
`processors 102 can include, for example. ARM968 proces-
`sors or other processors with similar functionality and capa-
`bilities. I lowever, in other embodiments. the panel processor
`functionality can be implemented instead by dedicated logic.
`such as a state machine. Peripherals 104 can include. but are
`not limited to. random access memory (RAM) or other types
`of memory or storage, watchdog timers and the like. Panel
`subsystem 106 can include. but is not limited to, one or more
`sense channels 108. channel scan logic 110 and driver logic
`ii-6. (Iharmei scan logic 110 can access RAM 112, autono-
`mously read data from the sense channels and provide control
`for the sense channels. Inaddition, channel scan logic 1 1 0 can
`control driver logic 114 to generate stimulation signals 116 at
`various frequencies and phases that can be selectively applied
`to rows of touch sensor panel 124. In some embodiments.
`panel subsystem 106. panel processor 102 and peripherals
`104 can be integrated into a single application specific inte-
`grated circuit (ASIC).
`[0026] Touch sensor panel 124 can include a capacitive
`sensing medium having a plurality of row traces or driving
`lines and a plurality of column traces or sensing lines,
`althougi other sensing media can also be used. The row and
`column traces can be formed from a transparent conductive
`medium such as Indium Tin Oxide (ITO) or Antimony Tin
`Oxide (ATO). although other transparent and non-transparent
`materials such as copper can also be used. In some embodi-
`ments. the row and column traces can be perpendicular to
`each other, although in other embtxliments other non-Carte-
`sian orientations are possible. For example. in a polar coor-
`dinate system, the sensing lines can beconcentric circles and
`the driving lines can be radially extending lines (or vice
`versa). It should be understood, therefore. that the terms
`“row" and "column." “firs! dimension" and "second dimen-
`sion," or “first axis" and “second axis" as used herein are
`intended to encompass not only orthogonal grids, but the
`intersecting traces of other geometric configurations having
`first and second dimensions (e.g. the concentric and radial
`lines of a poIar—coordinate arrangement). The rows and col-
`“n"s°m°f°""ed°"“i"%'ewin‘tek Exhibit 1024
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`parent substrate separated by a substantially transparent
`dielectric material, on opposite sides of the substrate. or on
`two separate substrates separated by the dielectric material.
`[0027] At the “intersections“ of the traces, where the traces
`pass above and below (cross) each other (but do not make
`direct electrical contact with each other), the traces can esch-
`tially form two electrodes (although more than two traces
`could intersect as well). Each intersection of row and column
`traces can represent a capacitive sensing node and can be
`viewed as picture element (pixel) 126, which can be particu-
`larly useful when touch sensor panel 124 is viewed as cap-
`turing an “image" of touch. (In other words, after panel sub-
`system 106 has determined whether a touch event has been
`detected at each touch sensor in the touch sensor panel, the
`pattern of touch sensors in the multi-touch panel at which a
`touch event occuned can be viewed as an “image" of touch
`(e.g. a pattern offingers touching the panel). ) The capacitance
`between row and column electrodes appears as a stray capaci-
`tance when the given row is held at direct current (DC) volt-
`age levels and as a mutual signal capacitance Csig when the
`given mw is stimulated with an alternating current (AC) sig-
`nal. The presence of a finger or other object near or on the
`touch sensor panel can be detected by measuring changes to
`a signal charge Qsig present at the pixels being touched.
`which is a function of Csig. Each column of touch sensor
`panel 124 can drive sense channel 108 (also referred to herein
`as an event detection and demodulation circuit) in panel sub-
`system 106.
`[0028] Computing system 100 can also include host pro-
`cessor 128 for receiving outputs from panel processor 102
`and performing actions based on the outputs that can include.
`but are not limited to. moving an object such as a cursor or
`pointer. scmlling or panning. adjusting control settings, open-
`ing a file or document, viewing a menu, making a selection.
`executing instructions, operating a peripheral device con-
`nected to the host device. answering a telephone call, placing
`a telephone call, terminating a telephone call, changing the
`volume or audio settings. storing information related to tele-
`phone communications such as addrcsses, frequently dialed
`numbers, received calls, missed calls, logging onto a com-
`puter ora computer network, permitting authorized individu-
`als access to restricted areas of the computer or computer
`network, loading a user profile associated with a user’s pre-
`ferred arrangement of the computer desktop, permitting
`access to web content.
`launching a particular program,
`encrypting or decoding a message, and/or the like. Host pro-
`cessor 128 can also pqform additional functions that may not
`be related to panel processing, and can be coupled to program
`storage 132 and display device 130 such as an LCD display
`for providing a U] to a user of the device.
`[0029]
`In some systems, sensor panel 124 can be driven by
`high-voltage driver logic. The high voltages that can be
`required by the high-voltagedriver logic (e.g. 18V) can force
`the high-voltage driver logic to be formed separate from panel
`subsystem 106, which can operate at much lower digital logic
`voltage levels (e.g. 1 .7 to 3.3V). However. in embodiments of
`the invention, on-chip driver logic 1 14 can replace the off-
`chip high voltage driver logic. Although panel subsystem 106
`can have low. digital logic level supply voltages. on-chip
`driver logic 114 can generate a supply voltage greater that the
`digital logic level supply voltages by cascoding two transis-
`tors together to form charge pump 115. Charge pump 1 15 can
`be used to generate stimulation sigtals 116 (Vstim) that can
`have amplitudes of about twice the digital logic level supply
`
`Page 13 of19
`
`voltages (e.g. 3.4 to 6.6V). Although FIG. 1 shows charge
`pump 115 separate from driver logic 114. the charge pump
`can be part of the driver logic.
`[0030]
`FIG. 2a illustrates exemplary mutual capacitance
`touch sensor panel 200 according to embodiments of the
`invention. FIG. 2a indicates the presence of a stray capaci-
`tance C stray at each pixel 202 located at the intersection ofa
`row 204 and a column 206 trace (although Cstray foronly one
`colunm is illustrated in FIG. 2a for purposes of simplifying
`the figure). In the example ofI’lG. 2a, AC stimuli Vstim 214,
`Vstim 215 and Vstim 217 can be applied to several rows.
`while other rows can be connected to DC. Vstim 214, Vstim
`215 andVstim 217 can be at different frequencies and phases,
`as will he explained later. Each stimulation signal on a row
`can cause a charge Qsig=CsigxVstim to be injected into the
`colmnns through the mutual capacitance present at
`the
`aflected pixels. A change in the injected charge (Qsig_sense)
`can be detected when a finger. palm or other object is present
`at one or more of the affected pixels. Vstim signals 214, 215
`and 217 can include one or more bursts of sine waves. Note
`
`that although FIG. 2a illustrates rows 204 and columns 206 as
`being substantially perpendicular,
`they need not be so
`aligned. as described above. As described above. each column
`206 can be connected to a sense channel (see sense channels
`108 in FIG. 1).
`[0031]
`FIG. 2b is a side view of exemplary pixel 202 in a
`steady-state (no~touch) condition according to embodiments
`of the invention. In FIG. 2b. an electric field of electric field
`lines 208 ofthc mutual capacitance between column 206 and
`row 204 traces or electrodes separated by dielectric 210 is
`shown.
`
`FIG. 2c is a side View of exemplary pixel 202 in a
`[0032]
`dynamic (touch) condition. In FIG. 2c, finger 212 has been
`placed near pixel 202. Finger 212 is a low-impedance object
`at signal frequencies, and has an AC capacitance Cfinger from
`the column trace 204 to the body. The body has a self-capaci-
`tance to ground Cbody ofabout 200pF, where Cbody is much
`larger than Cfingcr. If finger 212 blocks some electric field
`lines 208 between the row and column electrodes (those
`fringing fields that exit the dielectric and pass through the air
`above the row electrode), those electric field lines are shunted
`to ground through the capacitance path inherent in the finger
`and the body, and as a result. the steady state signal capaci-
`tance Csig is reduced by ACsig. In other words, the combined
`body and finger capacitance act to reduce Csig by an amount
`ACsig (which can also be referred to herein as Csig_sense),
`and can act as a shunt or dynamic return path to ground,
`blocking some of the electric fields as resulting in a reduced
`net signal capacitance. The signal capacitance at the pixel
`becomes Csig—ACsig, where Csig represents the static (no
`touch) component and ACsig represents the dynamic (touch)
`component. Note that Csig—ACsig may always be nonzero
`due to the inability of a finger, palm orothcr object to block all
`electric fields, especially those electric fields that remain
`entirely within the dielectric material. In addition, it should be
`understood that as a finger is pushed harder or more com-
`pletely onto the multi-touch panel. the finger can tend to
`flatten, blocking more and moreofthe electric fields, and thus
`ACsig can be variable and representative of how completely
`the finger is pushing down on the panel (i.e. a range from
`"no-touch” to “full-touch").
`[0033]
`FIG. 3a illustrates a portion of exemplary sense
`channel or event detection and demodulation circuit 300
`
`according to embodiments of t\/Vintek
`
`IIPRZO13-00567
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`US 2008/0309625 A1
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`Dec. 18, 2008
`
`scnsc channels 300 can be present in the panel subsystem.
`Each column from a touch sensor panel can be connected to
`sense channel 300. Each sense channel 300 can include vir-
`
`tual-ground amplifier 302, amplifier output circuit 309 (to be
`explained in greater detail below), signal mixer 304, and
`accumulator 308. Note that amplifier output circuit 309 can
`also be connected to other signal mixers and associated cir-
`cuitry not shown in FIG. 3a to simplify the figure.
`[0034] Vurtual-ground amplifier 302. which can also be
`referred to as a DC amplifier orachargearnplifier, can include
`feedback capacitor Cfb and feedback resistor Rib. In some
`etnbodiments, because of the much smaller amount ofcharge
`that can be injected into a row due to lowerVstim amplitudes,
`Cfb can be made much smallerthan in some previous designs.
`However, in other embodiments. because as many as all rows
`can be simultaneously stimulated at the same time, which
`tends to add charge, Ctb is not reduced in size.
`[0035]
`FIG. 3a shows, in dashed lines, the total steady-state
`signal capacitance Csig_tot that can be contributed by a touch
`sensor panel column connected to sense channel 300 when
`one or more input stimuli Vstim are applied to one or more
`rows in the touch sensor panel and no finger. palm or other
`object is present. In a steady-state, no-touch condition, the
`total signal charge Qsig_tot injected into the column is the
`sum of all charge injected into the column by each stimulated
`row. In other words. Qsig_tot=2(Csig‘Vstim for all stimu-
`lated rows). Each sense channel coupled to a column can
`detect any change in the total signal charge due to the pres-
`ence of a finger. palm or other body pan or object at one or
`more pixels in thatcolumn. In other words, Qsig_tot_sense=2
`(((‘sig-Csig_sense)*Vstim for all stimulated rows).
`|0036] As noted above.
`there can be an inherent stray
`capacitance Cstray at each pixel on the touch sensor panel. In
`virtual ground charge amplifier 302. with the +(noninverting)
`input tied to reference voltage ‘Vref, the —(invening) input can
`also be driven to Vref, and a DC operating point can be
`established. Therefore, regardless of how much Csig is
`present at the input to virtual ground charge amplifier302. the
`-input can always be driven to Vref. Because of the charac-
`teristics of virtual ground charge amplifier 302, any charge
`Qstray that is stored in Cstray is constant, because the voltage
`across Cstray is kept constant by the charge amplifier. There-
`fore. no matter how much stray capacitance Cstray is added to
`the -input, the net charge into Cstray will always be 7cm. The
`input charge is accordingly zero when the corresponding row
`is kept at DC and is purely a function ofCsig and