Abstract:
Active stylus operation when there is no physical connection between the stylus and the touch array requires communication and synchronization. It is possible to use the touchscreen stack-up itself to communicate synchronization signals or other information optically by outfitting the active stylus with an optical receiver and transmitting signals either with additional diodes or by modulating the display clock itself.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to the field of touch-sensors and, in particular, to stylus synchronization and operation with a touch-sensor array. 
       BACKGROUND 
       [0002]    Computing devices, such as notebook computers, personal data assistants (PDAs), kiosks, and mobile handsets, have user interface devices, which are also known as human interface devices (HID). One user interface device that has become more common is a touch-sensor pad (also commonly referred to as a touchpad). A basic notebook computer touch-sensor pad emulates the function of a personal computer (PC) mouse. A touch-sensor pad is typically embedded into a PC notebook for built-in portability. A touch-sensor pad replicates mouse X/Y movement by using two defined axes which contain a collection of sensor elements that detect the position of one or more conductive objects, such as a finger or a stylus pen. Mouse right/left button clicks can be replicated by two mechanical buttons, located in the vicinity of the touchpad, or by tapping commands on the touch-sensor pad itself. The touch-sensor pad provides a user interface device for performing such functions as positioning a pointer, or selecting an item on a display. These touch-sensor pads may include multi-dimensional sensor arrays for detecting movement in multiple axes. The sensor array may include a one-dimensional sensor array, detecting movement in one axis. The sensor array may also be two dimensional, detecting movements in two axes. 
         [0003]    Another user interface device that has become more common is a touch screen. Touch screens, also known as touchscreens, touch windows, touch panels, or touchscreen panels, are transparent display overlays which are typically either pressure-sensitive (resistive or piezoelectric), electrically-sensitive (capacitive), acoustically-sensitive (surface acoustic wave (SAW)) or photo-sensitive (infra-red). The effect of such overlays allows a display to be used as an input device, removing the keyboard and/or the mouse as the primary input device for interacting with the display&#39;s content. Such displays can be attached to computers or, as terminals, to networks. Touch screens have become familiar in retail settings, on point-of-sale systems, on ATMs, on mobile handsets, on kiosks, on game consoles, and on PDAs where a stylus is sometimes used to manipulate the graphical user interface (GUI) and to enter data. A user can touch a touch screen or a touch-sensor pad to manipulate data. For example, a user can apply a single touch, by using a finger to touch the surface of a touch screen, to select an item from a menu. 
         [0004]    A user may also use a stylus to interact with the screen and manipulate data. A stylus may have varying levels of complexity. Basic styli may consist a point by which to contact the surface of a resistive touchscreen. For capacitive systems, styli may have a conductive tip with which to change the capacitance of the sensing electrodes. The change in capacitance may be passive, with only the extra conductive material affecting the capacitance of the sense electrode. The change in capacitance may also be active, whereby a signal is transmitted from the tip of the stylus to the surface of the touchscreen or touch-sensor pad. This signal may be a constant voltage, or in other embodiments, it may be an alternative voltage which may be received by the capacitance sensing circuitry. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The present disclosure is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings. 
           [0006]      FIG. 1  is a block diagram illustrating an embodiment of an electronic system that processes touch sensor data. 
           [0007]      FIG. 2A  illustrates an embodiment of a transmit and receive electrode in a mutual capacitance array. 
           [0008]      FIG. 2B  illustrates a schematic representation of a transmit and receive electrode in a mutual capacitance array. 
           [0009]      FIG. 3  illustrates an embodiment of a capacitive sensor array having a diamond pattern. 
           [0010]      FIG. 4  is a block diagram illustrating an embodiment of an electronic system that processes touch sensor data and is configured to receive on all electrodes. 
           [0011]      FIG. 5  illustrates an embodiment of a synchronized stylus with a touchscreen comprising an inductive synchronization. 
           [0012]      FIG. 6  illustrates am embodiment of an optical synchronization stylus. 
           [0013]      FIG. 7  illustrates an embodiment of a synchronized stylus with a touchscreen comprising an optical synchronization. 
           [0014]      FIG. 8  illustrates an embodiment of an optically synchronized stylus in cross-section. 
           [0015]      FIGS. 9A  and B illustrate embodiments of an optically synchronized stylus in cross-section. 
           [0016]      FIG. 10  illustrates embodiment of an optically synchronized stylus in cross-section. 
           [0017]      FIGS. 11A-D  illustrate embodiments of synchronization transmitters and receivers. 
           [0018]      FIG. 12A-C  illustrate example synchronization waveform and signal strength at various supply voltages. 
           [0019]      FIG. 12D  illustrates an example LCD drive signal waveform. 
           [0020]      FIG. 13  illustrates an embodiment of a method optically synchronizing a stylus in a capacitive sensing system. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The following description sets forth numerous specific details such as examples of specific systems, components, methods, and so forth, in order to provide a good understanding of several embodiments of the present invention. It will be apparent to one skilled in the art, however, that at least some embodiments of the present invention may be practiced without these specific details. In other instances, well-known components or methods are not described in detail or are presented in a simple block diagram format in order to avoid unnecessarily obscuring the present invention. Thus, the specific details set forth are merely exemplary. Particular implementations may vary from these exemplary details and still be contemplated to be within the spirit and scope of the present invention. 
         [0022]    An embodiment of a capacitive sensor array may include sensor elements arranged such that each unit cell corresponding to an intersection between sensor elements may include a main trace and one or more primary subtraces branching away from the main trace. In one embodiment, a sensor element may also include one or more secondary subtraces branching from a primary subtrace, or one or more tertiary subtraces branching from a secondary subtrace. In one embodiment, a sensor array having such a pattern may have decreased signal disparity and reduced manufacturability problems as compared to other patterns, such as a diamond pattern. Specifically, a capacitive sensor array with sensor elements having main traces and subtraces branching from the main trace, such as a totem pole pattern, may be manufactured with decreased cost and increased yield rate, as well as improved optical quality. 
         [0023]    An embodiment of such a capacitive sensor array may include a first and a second plurality of sensor elements each intersecting each of the first plurality of sensor elements. Each intersection between one of the first plurality of sensor elements and one of the second plurality of sensor elements may be associated with a corresponding unit cell. A unit cell may be a single node or pixel of capacitance measurement on the capacitive sensor array. In one embodiment, a unit cell corresponding to an intersection may be understood as an area including all locations on the surface of the sensor array that are nearer to the corresponding intersection than to any other intersection between sensor elements. 
         [0024]    In one embodiment of a capacitive sensor array, each of the second plurality of sensor elements includes a main trace that crosses at least one of the plurality of unit cells, and further includes, within each unit cell, a primary subtrace that branches away from the main trace. In one embodiment, the primary subtrace may be one of two or more primary subtraces branching symmetrically from opposite sides of the main trace, resembling a “totem pole”. Alternatively, the primary subtraces may branch asymmetrically from the main trace. 
         [0025]      FIG. 1  is a block diagram illustrating one embodiment of a capacitive touch sensor array  121  and a capacitance sensor  101  that converts measured capacitances to coordinates. The coordinates are calculated based on measured capacitances. In one embodiment, touch sensor array  121  and capacitance sensor  101  are implemented in a system such as electronic system  100 . Touch sensor array  121  includes a matrix  110  of N×M electrodes (N receive electrodes and M transmit electrodes), which further includes transmit (TX) electrode  122  and receive (RX) electrode  123 . Each of the electrodes in matrix  110  may be connected with capacitance sensor  101  through demultiplexer  112  and multiplexer  113 . 
         [0026]    Capacitance sensor  101  may include multiplexer control  111 , demultiplexer  112  and multiplexer  113 , clock generator  114 , signal generator  115 , demodulation circuit  116 , and analog-to-digital converter (ADC)  117 . ADC  117  is further coupled with touch coordinate converter  118 . Touch coordinate converter  118  outputs a signal to the processing logic  102 . 
         [0027]    The transmit and receive electrodes in the electrode matrix  110  may be arranged so that each of the transmit electrodes overlap and cross each of the receive electrodes such as to form an array of intersections, while maintaining galvanic isolation from each other. Thus, each transmit electrode may be capacitively coupled with each of the receive electrodes. For example, transmit electrode  122  is capacitively coupled with receive electrode  123  at the point where transmit electrode  122  and receive electrode  123  overlap. 
         [0028]    Clock generator  114  supplies a clock signal to signal generator  115 , which produces a TX signal  124  to be supplied to the transmit electrodes of touch sensor array  121 . In one embodiment, the signal generator  115  includes a set of switches that operate according to the clock signal from clock generator  114 . The switches may generate a TX signal  124  by periodically connecting the output of signal generator  115  to a first voltage and then to a second voltage, wherein said first and second voltages are different. 
         [0029]    The output of signal generator  115  is connected with demultiplexer  112 , which allows the TX signal  124  to be applied to any of the M transmit electrodes of touch sensor array  121 . In one embodiment, multiplexer control  111  controls demultiplexer  112  so that the TX signal  124  is applied to each transmit electrode  122  in a controlled sequence. Demultiplexer  112  may also be used to ground, float, or connect an alternate signal to the other transmit electrodes to which the TX signal  124  is not currently being applied. 
         [0030]    Because of the capacitive coupling between the transmit and receive electrodes, the TX signal  124  applied to each transmit electrode induces a current within each of the receive electrodes. For instance, when the TX signal  124  is applied to transmit electrode  122  through demultiplexer  112 , the TX signal  124  induces an RX signal  127  on the receive electrodes in matrix  110 . The RX signal  127  on each of the receive electrodes can then be measured in sequence by using multiplexer  113  to connect each of the N receive electrodes to demodulation circuit  116  in sequence. In one embodiment, multiple multiplexers may allow RX signals to be received in parallel by multiple demodulation circuits. 
         [0031]    The mutual capacitance associated with each intersection between a TX electrode and an RX electrode can be sensed by selecting every available combination of TX electrode and an RX electrode using demultiplexer  112  and multiplexer  113 . To improve performance, multiplexer  113  may also be segmented to allow more than one of the receive electrodes in matrix  110  to be routed to additional demodulation circuits  116 . In an optimized configuration, wherein there is a 1-to-1 correspondence of instances of demodulation circuit  116  with receive electrodes, multiplexer  113  may not be present in the system. 
         [0032]    When an object, such as a finger or stylus, approaches the electrode matrix  110 , the object causes a decrease in the mutual capacitance between only some of the electrodes. For example, if a finger or stylus is placed near the intersection of transmit electrode  122  and receive electrode  123 , the presence of the finger will decrease the mutual capacitance between electrodes  122  and  123 . Thus, the location of the finger on the touchpad can be determined by identifying the one or more receive electrodes having a decreased mutual capacitance in addition to identifying the transmit electrode to which the TX signal  124  was applied at the time the decreased mutual capacitance was measured on the one or more receive electrodes. 
         [0033]    By determining the mutual capacitances associated with each intersection of electrodes in the matrix  110 , the locations of one or more touch contacts may be determined. The determination may be sequential, in parallel, or may occur more frequently at commonly used electrodes. 
         [0034]    In alternative embodiments, other methods for detecting the presence of a finger or conductive object may be used where the finger or conductive object causes an increase in capacitance at one or more electrodes, which may be arranged in a grid or other pattern. For example, a finger placed near an electrode of a capacitive sensor may introduce an additional capacitance to ground that increases the total capacitance between the electrode and ground. The location of the finger can be determined from the locations of one or more electrodes at which an increased capacitance is detected. 
         [0035]    The induced current signal (RX signal  127 ) is rectified by demodulation circuit  116 . The rectified current output by demodulation circuit  116  can then be filtered and converted to a digital code by ADC  117 . 
         [0036]    The digital code is converted to touch coordinates indicating a position of an input on touch sensor array  121  by touch coordinate converter  118 . The touch coordinates are transmitted as an input signal to the processing logic  102 . In one embodiment, the input signal is received at an input to the processing logic  102 . In one embodiment, the input may be configured to receive capacitance measurements indicating a plurality of row coordinates and a plurality of column coordinates. Alternatively, the input may be configured to receive row coordinates and column coordinates. 
         [0037]    In one embodiment, touch sensor array  121  can be configured to detect multiple touches. One technique for multi-touch detection uses a two-axis implementation: one axis to support rows and another axis to support columns. Additional axes, such as a diagonal axis, implemented on the surface using additional layers, can allow resolution of additional touches. 
         [0038]      FIG. 2A  illustrates an embodiment of a simplified representation  200  of an embodiment of a single intersection, or node  210 , of a transmit electrode  122  and a receive electrode  123 . Transmit electrode  122  is coupled to 1→N demultiplexer  112  and receive electrode  123  is coupled to N→1 demultiplexer  113  as shown in  FIG. 1 . Node  210  is characterized by mutual capacitance, Cm, between transmit electrode  122  and receive electrode  123 . A mutual capacitance, Cm, exists for every intersection between every transmit electrode and every receive electrode in N×M electrode matrix  125  (shown in  FIG. 1 ). 
         [0039]      FIG. 2B  illustrates a circuit representation  201  of a single intersection, or node  210 , from  FIG. 2A . Transmit electrode  122  comprises a resistance, R 1 , and a parasitic capacitance, C 1 . Receive electrode  123  comprises a resistance, R 2 , and a parasitic capacitance, C 2 . Resistances R 1  and R 2  are a function of the impedance of transmit electrode  122  and receive electrode  123 , respectively. High impedance materials, such as indium tin oxide, limit the rate of charge and discharge on transmit and receive electrodes. Capacitors C 1  and C 2  represent the parasitic capacitance each electrode has to the rest of the array and the rest of the system for each transmit electrode  122  and each receive electrode  123 , respectively. Parasitic capacitances C 1  and C 2  are the capacitance of each electrode to everything in the system except the transmit or receive electrode that comprises the other side of node  210 . Voltage source  220  provides may provide an alternating voltage source (signal) on transmit electrode  122 , thus generating a current, Iin, through resistor R 1 , and building a voltage potential on mutual capacitor Cm. This voltage potential is then converted to a current, Tout, through resistor R 2 . Current Tout is representative of the mutual capacitance, Cm, between transmit electrode  122  and receive electrode  123 . 
         [0040]      FIG. 3  illustrates an embodiment of a capacitive touch sensing system  300  that includes a capacitive sensor array  320 . Capacitive sensor array  320  includes a plurality of row sensor elements  331 - 345  and a plurality of column sensor elements  350 - 359 . The row and column sensor elements  331 - 445  and  350 - 359  are connected to a processing device  305 , which may include the functionality of capacitance sensor  101  of  FIG. 1 . In one embodiment, the processing device  305  may perform TX-RX scans of the capacitive sensor array  320  to measure a mutual capacitance value associated with each of the intersections, or nodes,  310 , between a row sensor element and a column sensor element in the sensor array  320 . The measured capacitances may be further processed to determine centroid, or center of mass, locations of one or more contacts at the capacitive sensor array  320 . 
         [0041]    In one embodiment, the processing device  305  is connected to a host  360  which may receive the measured capacitances or calculated centroid locations from the processing device  305 . 
         [0042]    The capacitive sensor array  320  illustrated in  FIG. 3  may include sensor elements arranged in a diamond pattern. Specifically, the sensor elements  331 - 348  of capacitive sensor array  320  may be arranged in a single solid diamond (SSD) pattern as shown in  FIG. 3A . In other embodiments, the sensors elements  331 - 345  and  350 - 359  may be hollow diamonds (“single hollow diamonds”) or may be pairs of diamonds coupled at one or both ends (“dual solid diamonds”). In another embodiment, pairs of hollow diamonds may be coupled at one or both ends (“dual hollow diamonds”). 
         [0043]      FIG. 4  is a block diagram of an embodiment  400  of touch sensor array  121  and capacitance sensor  101 . The embodiment  400  is similar to the electronic system  100  of  FIG. 1 , however transmit electrodes  122  may be coupled to the demodulation circuit  116  through demultiplexer  112 , multiplexer  401 . In this embodiment, all of the electrodes are configured to receive RX signal  127 . Multiplexer  401  may be controlled by multiplexer control  111  and enables the capacitance sensor  101  to function as a transmit-and-receive sensor in one configuration and a receive-only sensor in another configuration. A receive-only sensor may require an external signal. This external signal may be provided by an active stylus  401 , which may be configured to provide a signal which is attenuated to the expected RX single  127 . 
         [0044]    The measured mutual capacitance of embodiment  400  when all the electrodes are configured to receive is between the tip of a stylus and the receive electrodes. Because of the capacitive coupling between the stylus tip and receive electrodes, the TX signal  124  applied to each transmit electrode induces a current within each of the receive electrodes. For instance, when the TX signal  124  is applied to transmit electrode  122  through demultiplexer  112 , the TX signal  124  induces an RX signal  127  on the electrodes in matrix  110 . The RX signal  127  on each of the electrodes can then be measured in sequence by using demultiplexer  112  and multiplexer  113  to connect each of the N receive electrodes to demodulation circuit  116  in sequence. 
         [0045]    The mutual capacitance associated with each intersection between stylus  401  and an RX electrode can be sensed by selecting every available RX electrode using demultiplexer  112  and multiplexer  113 . To improve performance, multiplexer  113  may also be segmented to allow more than one of the receive electrodes in matrix  110  to be routed to additional demodulation circuits  116 . In an optimized configuration, wherein there is a 1-to-1 correspondence of instances of demodulation circuit  116  with electrodes, multiplexers  112  and  113  may not be present in the system. 
         [0046]      FIG. 5  illustrates an embodiment  500  of capacitive sensing array  510 , sensing device, and stylus  550 . Capacitive sensing array  510  may be analogous to matrix  110  of  FIGS. 1 and 4  and is coupled to sensing device  501  through multiplexers  512  and  513 . RX signal  127  may be received the electrodes ( 122  and  123  of  FIGS. 1 and 4 ) and routed to the demodulation circuit ( 116  of  FIGS. 1 and 4 ). Sensing device  501  may be coupled to transmitter  520 . Sensing device  501  may pass TX signal  124  to transmitter  520  so that transmitter  520  may transmit the TX signal to receiver  530 . Receiver  530  may be coupled to tip driver  555  which is part of stylus  550 . In one embodiment, receiver  530  may be integrated into stylus  550 . TX signal  124  may be driven out the stylus tip  552  to capacitance sensing array  510 , where it may be received by the electrodes and passed to sensing device  501  as RX signal  127  through multiplexers  512  and  513 . In one embodiment, battery  540  boost circuit may be coupled to receiver  530  and tip driver  555 . 
         [0047]      FIG. 6  illustrates an embodiment of stylus  650  comprising a tip driver, and optical RX circuit, a photo diode  658 , a light tube  659  and a stylus tip  552 . Tip driver  555  and optical RX circuit  657  may be coupled to a power source and boost circuit (not shown). Light may enter stylus  650  through light tube  659  and be received by photodiode  658 . The light received by photodiode may be converted to a drive signal by optical RX circuit  657  and sent to tip driver  555 . Tip driver then takes the converted drive signal and transmit that signal through stylus tip  552 . In one embodiment, the light entering light tube  659  may be in the infrared (IR) range such that it is not visible to a user. The IR light may contain a signal that can be received and demodulated by optical RX circuit  657 . 
         [0048]      FIG. 7  illustrates an embodiment  700  of a capacitive sensing array  510 , a sensing device  501 , an optical transmitter  720  and a stylus  650 . The operation of embodiment  700  is similar to that of embodiment  500  of  FIG. 5 , however the synchronization of sensing device  501  and the tip driver  555  is achieved through an optical signal and not inductive (through transmitter  520  and  530 ). Sensing device  501  may pass TX signal  124  to optical transmitter  720 , which drives LED  725 . IR signal  727  from LED  725  may then be received by the optical RX circuit ( 657  of  FIG. 6 ) through the light tube and photodiode ( 659  and  658  of  FIG. 6 ). 
         [0049]    In one embodiment, optical transmitter may be configured to produce a plurality of IR signals. In this embodiment, multiple styli may be synchronized by the same optical synchronization circuit. Multiple styli may be configured to provide different TX signals which may be demodulated by sensing device  501  configured with corresponding clock dividers. In this embodiment, different users may be able to input on the display simultaneously and have their movements and inputs easily tracked. In another embodiment, different TX signals may correspond to different types of input by the same stylus or multiple styli, allowing for different interactions depending on received TX signal, if within a predefined range. 
         [0050]      FIG. 8  illustrates an embodiment  800  of a capacitance sensing array stack-up. The sensing array may comprise several layers including a display  820  configured to display graphical information to a user. Display  820  may be a liquid crystal display, AMOLED display, OLED display, or vacuum fluorescence displays. Display  820  may be backlit by an LED  820 , the light  815  of which may be channeled toward the user through light guide  810 . Light guide  810  may channel the backlight from LED  812  and light that is not channeled up to the user may be reflected off mirror  815  to the user. Electrodes  223  and  224  from  FIG. 2  may be disposed between the display  820  and an overlay  860 . Overlay  860  may be a glass layer or a polyethylene teraphthalate (PET). Overlay  860  may be constructed of other transparent materials. Electrodes  223  and  224  may be deposited on overlay  860  or on display  820 , or both. Electrodes  223  and  224  may also be deposited on a third substrate disposed between overlay  860  and display  820 . In one embodiment, the layers of the stackup may be adhered to each other through an optically clear adhesive. The optically clear adhesive may be configured to have an index of refraction matching that of the electrode material, or any of the substrates. 
         [0051]    Display driver  811  may be configured to control the graphical display on display  820  and to drive the backlight LED  812 . In another embodiment, the LED backlight drive and the display drive may be on separate controllers. 
         [0052]    Sensing device  401  may be configured to receive the RX signal  127  from electrodes  223  and  224 . To synchronize the sensing device receive channel (demodulation circuit  116 ,  FIG. 1 ), sensing device  401  may send TX signal  524  to optical transmitter  720 . Optical transmitter may be configured to then drive IR LED  822  with TX signal  524 . TX signal  524  may enter light guide  810  and be channeled through the stackup to stylus  650 . As described with regard to figures  6  and  7 , stylus  650  may be configured to receive TX signal  527  optically and convert that signal to an electromagnetic signal that can be received on electrodes  223  and  224 . 
         [0053]      FIG. 9A  illustrates an embodiment of stackup  900 , wherein IR LED  822  passes light into overlay  860 , rather than light guide  810 . In one embodiment IR LEDs may be positioned on both sides of overlay  860 . Light from IR LEDs  822  may travel through overlay  860  and be channeled up to stylus  650  by means of a polarizing treatment. 
         [0054]      FIG. 9B  illustrates an embodiment of stackup  901 , wherein a signal IR LED is positioned on one side of overlay  860  and configured to pass light into that side of overlay  860 . Light from IR LED  822  that is not channeled to stylus  650  may be reflected off mirror  823 . Mirror  823  ensures that the light emitted from IR LED  822  is channeled to mirror  823  and does not exit overlay  860  without serving its purpose. 
         [0055]      FIG. 10  illustrates an embodiment of stackup  1000  wherein display driver  1010  is configured to drive both the backlight and the synchronization signal. In this embodiment, an optical TX circuit may be disposed between the sensing device  401  and display driver  1010 . The optical TX circuit may be configured to receive a signal from sensing device  401  and to transmit the signal to an optical receiver through the display driver. In this embodiment, sensing device  401  may pass the synchronization signal to display driver. The synchronization signal my then be passed into the light guide through the same LED that is used for backlight control. In another embodiment, the display driver may be configured to drive separated LEDs for the LCD backlight and the synchronization IR LED. In another embodiment, the display driver may modulate the pixel clock per signals from the sensing device  401  and optical TX circuit  1020  to provide the optical signal to the receiver in stylus  650 . 
         [0056]      FIGS. 11A-D  illustrate alternative representations of the optical synchronization circuit shown in  FIG. 7 .  FIG. 11A  illustrates an embodiment  1101  comprising capacitance sensor  101  which may be coupled to LED driver  1120 . LED driver  1120  may be constructed using inductance and switches. In this embodiment, a clock may be required for operation. LED driver  1120  may have an clock signal synchronization input which is coupled to capacitance sensor  101 . The clock frequency for the synchronization of LED driver  1120  may be higher than the TX drive frequency to be passed to receiver  1130 . The LED driver synchronization signal and the TX drive frequency may be derived from the same clock signal using a clock divider  1111  coupled between the clock  1110  and a panel scan engine  1112 . The panel scan engine may comprise the capacitance sensing circuits described in  FIGS. 1 and 4 . LED driver  1120  may be coupled to IR LED  1125  and configured to transmit the touchscreen clock information to the stylus  1150  through the overlay  1105 , which is analogous to overlay  860  from  FIGS. 8 ,  9 A,  9 B and  10 . Touchscreen clock information or synchronization information is received by photodiode  1135  and demodulated by receiver  1130 . Clock divider  1131  may be coupled between receiver  1130  and stylus tip driver  1140 . In one embodiment clock divider  1131  may have the same dividing coefficient as clock divider  1111 , which may enable an efficient translation of the clock signal or synchronization signal from LED driver  1120 . 
         [0057]      FIG. 11B  illustrates an embodiment  1102  wherein the synchronization signal is generated by LED driver  1120  and sent to capacitance sensor  101 . The synchronization signal may be divided down by clock divider  1111  and passed to panel scan engine  1112  to produce the TX drive signal. LED driver  1120  may be coupled to IR LED  1125  and configured to transmit the touchscreen clock information to the stylus  1150  through the overlay  1105 , which is analogous to overlay  860  from  FIGS. 8 ,  9 A,  9 B and  10 . Touchscreen clock information or synchronization information is received by photodiode  1135  and demodulated by receiver  1130 . Clock divider  1131  may be coupled between receiver  1130  and stylus tip driver  1140 . In one embodiment clock divider  1131  may have the same dividing coefficient as clock divider  1111 , which may enable an efficient translation of the clock signal or synchronization signal from LED driver  1120 . 
         [0058]      FIG. 11C  illustrates an embodiment  1103  wherein the synchronization signal is generated by LED driver  1120  and sent to capacitance sensor  101 . The synchronization signal may be divided down by clock divider  1111  and passed to panel scan engine  1112  to produce the TX drive signal. LED driver  1120  may be coupled to IR LED  1125  and configured to transmit the touchscreen clock information to the stylus  1150  through the overlay  1105 , which is analogous to overlay  860  from  FIGS. 8 ,  9 A,  9 B and  10 . Touchscreen clock information or synchronization information is received by photodiode  1135  and demodulated by receiver  1130 . Frequency modifier may be coupled to the output of receiver  1130  and the input of a phase lock loop (PLL)  1138 . PLL may comprise a detector configured to receive the input from receiver  1130  and measure the phase of the input and a frequency relative to a feedback signal. The measurement signals received on PLL  1138  may be sent to a voltage controlled oscillator or charge pump (not shown) and may change the output frequency which may be routed back to the input. The feedback circuit of the PLL may be repeated until a desired output frequency from PLL is achieved nd passed to the stylus tip driver. Frequency modifier  1137  may comprise a single DFF and a chained DFF pair. In this embodiment, the frequency output may be divided by two and four as well as pass the native frequency. Frequency modifier  1137  may allow for higher synchronization signal processing without altering the voltage controlled oscillator or charge pump. In another embodiment, frequency modifier may comprise a inverter and buffer which are applied to the signal from receiver  1130 . The output of the inverter and buffer may be received by PLL  1138  and recognized as a different frequency. In another embodiment, frequency modifier may comprise a series of chained buffers. In this embodiment, the buffers may be selected by the stylus tip driver of other circuit to create multiple frequencies depending on their combination. 
         [0059]      FIG. 11D  illustrates an embodiment where in frequency modifier  1137  is integrated into PLL  1138 . 
         [0060]      FIGS. 12A-D  illustrate representations of the synchronization light signals from the IR LED and the LCD backlight LED.  FIGS. 12A-C  illustrate the synchronization signals  1221 ,  1222  and  1223  based on clock frequency  1232 . While some signal is absorbed by the stackup, enough signal survives to be received by the IR photodiode.  FIG. 12D  illustrates a representation  1214  of the LCD drive signal  1224 . The LCD drive frequency may be much lower than the synchronization frequency and easily filtered by a simple high-pass filter. The amplitude of the LCD drive signal is also fairly low and incapable of saturating the photodiode. 
         [0061]      FIG. 13  illustrates an embodiment of a method  1300  for synchronizing the stylus drive signal to the receive demodulation circuit shown in  FIG. 7 . The TX frequency is first set in block  1310 . The TX frequency may then be transmitted by the LED driver in block  1320 . The TX frequency may be received and demodulated by the receiver in block  1330 . The TX driver may then be configured according to the synchronization clock frequency in block  1340 . TX driver may transmit the TX signal to the capacitance sensing array in block  1350 . Capacitance sensing array  1360  may receive the TX signal from the stylus on the receive electrodes (illustrated in  FIG. 1-4 ). Capacitance sensor ( 101  from  FIG. 1 ) may demodulate the signal from the capacitance sensing array in block  1370  and calculate the position of the stylus on the capacitance sensing array based on the demodulated signal in block  1380 . 
         [0062]    Certain embodiments may be implemented as a computer program product that may include instructions stored on a computer-readable medium. These instructions may be used to program a general-purpose or special-purpose processor to perform the described operations. A computer-readable medium includes any mechanism for storing or transmitting information in a form (e.g., software, processing application) readable by a machine (e.g., a computer). The computer-readable storage medium may include, but is not limited to, magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto-optical storage medium; read-only memory (ROM); random-access memory (RAM); erasable programmable memory (e.g., EPROM and EEPROM); flash memory, or another type of medium suitable for storing electronic instructions. 
         [0063]    Additionally, some embodiments may be practiced in distributed computing environments where the computer-readable medium is stored on and/or executed by more than one computer system. In addition, the information transferred between computer systems may either be pulled or pushed across the transmission medium connecting the computer systems. 
         [0064]    Although the operations of the method(s) herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operation may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be in an intermittent and/or alternating manner. 
         [0065]    In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.