Patent Publication Number: US-9411462-B2

Title: Capacitive stylus for a touch screen

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/214,048, filed Aug. 19, 2011, which claims the benefit of U.S. Provisional Patent Application 61/385,463, filed Sep. 22, 2010, all of which are incorporated by reference herein in their entirety. This application is related to co-pending U.S. patent application Ser. No. 13/213,895, filed Aug. 19, 2011, and co-pending U.S. patent application Ser. No. 13/213,981, filed Aug. 19, 2011. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to the field of user interface devices and, in particular, to capacitive sensor devices. 
     BACKGROUND 
     The use of a stylus with a touch screen interface is well established. Touch screen designs have incorporated many different technologies including resistive, capacitive, inductive, and radio frequency sensing arrays. Resistive touch screens, for example, are passive devices well suited for use with a passive stylus. The original PalmPilots® devices from the mid-1990s were one of the first successful commercial devices to utilize a resistive touch screen designed for use with a stylus and helped to popularize that technology. Although resistive touch screens can sense the input from nearly any object, multi-touch is generally not supported. An example of a multi-touch application may be applying two or more fingers to the touch screen. Another example may be inputting a signature, which may include simultaneous palm and stylus input signals. Due to these and other numerous disadvantages, capacitive touch screens are increasingly replacing resistive touch screens in the consumer marketplace. 
     Various tethered active stylus approaches have been implemented for use with touch screens and are found in many consumer applications such as point-of-sale terminals (e.g., the signature pad used for credit card transactions in retail stores) and other public uses. However, the need for a tethered cable is a significant drawback for private applications such as personal computers (“PCs”), smart phones, and tablet PCs. 
       FIG. 1A  is a block diagram illustrating a conventional embodiment of a host device  100  for tracking the position of a touch object on an inductive sense array  107 . The host device  100  includes a printed circuit board (“PCB”)  105 , a first matching circuit  110 , a receiver  115 , a host central processing unit (“CPU”)  120 , a personal computer (“PC”)  125 , a transmitter  130 , and a second matching circuit  135 . The PCB  105  is typically placed behind a touch screen (not shown) and includes an inductive sense array  107 . The inductive sense array  107  includes a series of inductive coils. Inductive sense arrays are typically heavy and expensive to manufacture. 
       FIG. 1B  is a block diagram illustrating a conventional embodiment of an active stylus  150  used in a system for tracking the position of a touch object on an inductive sense array  107 . The stylus  150  includes a micro-controller unit (“MCU”)  155 , a driver  160 , and inductor eraser  165 , and inductor tip  170 , a rectifier  175 , a power regulator  180 , a button(s)  185 , a force tip  190 , and a force eraser  195 . The inductor eraser  165  and inductor tip  170  are configured on different stylus edges. 
     In operation, the inductive sense array  107  on PCB  105  generates a magnetic field to provide both stylus power generation and touch position detection. Regarding touch position, the matching circuit  110  provides impedance matching and couples the stylus  150  signal from the inductive sense array  107  to the receiver  115 . The receiver  115  and host CPU  120  receives and process the analog signal, respectively, providing touch position and force data to the PC  125 . Force data is indicative of the amount of pressure provided by the stylus tip to the touch screen. The host CPU  120  calculates the touch position based on the relative inductor signal strength of each coil of the inductive sense array  107 . More specifically, the presence of the stylus  150  changes the individual inductor currents for each coil in the inductive sense array  107  based on their relative proximity to the stylus. The maximum signal strength approximates the stylus  150  touch position on the accompanying touch screen. 
     The host CPU  120  sends a high frequency carrier signal to the stylus  150  via an amplifier (not shown), a transmitter  130 , an impedance matching circuit  135 , and the inductive sense array  107 . The stylus  150  receives and utilizes the high frequency carrier signal for self-powering and data transmission. In operation, the stylus  150  rectifies (rectifier  175 ) and regulates (power regulator  180 ) the carrier signal and feeds the resultant signal to the MCU  155  and driver  160 . The MCU  155  measures force sensors (force tip  190  and force eraser  195 ) and button states (button(s)  185 ) and couples the resultant data signal to the driver  160 . The driver  160  drives the inductor tip  170  and inductor eraser  165 , which inductively couples the stylus  150  to the inductive sense array  107 . 
     Stylus  150  sensing is implemented largely independent of the finger-sensing capability of the touch screen. As described above, stylus tracking requires generating an alternative current (AC) signal by the inductive sense array  107  and inductively coupling the AC signal to the tip of the stylus  150 . The inductive sense array  107 , located behind the touch screen, in turn receives the stylus signal and the Host CPU  120  interpolates the position of the stylus tip (inductor tip  170 ) based on the relative magnitude of the received stylus signals at each of the inductive sensors of the inductive sense array  107 . While inductive sensing may be reliable, inductive stylus tracking solutions exhibit serious commercial disadvantages including high power consumption, high electromagnetic interference (“EMI”), high manufacturing costs, and heavy construction. Furthermore, retro fitting an existing touch sensor (passive touch object sensor) to include independent stylus tracking would require an additional PCB  105  layer to incorporate the inductive sense array  107 . 
       FIG. 2A  is a block diagram illustrating a conventional embodiment of a host device  200  for tracking the position of a touch object on a radio frequency (“RF”) sense array. The host device  200  includes an Indium-Tin-Oxide (“ITO”) panel  205 , a receiver  210 , a data decoder  215 , a host CPU  220 , and a PC  225 . In  FIG. 2B , the stylus  250  includes a force sensor  255 , a measurer  260 , a modulator  265 , an amplifier  270 , a stylus tip  275 , and a reference clock  280 . The stylus  250  is typically battery powered (not illustrated). 
     In operation, the stylus  250  generates, amplifies, and couples an RF carrier signal from the stylus tip  275  to the ITO panel  205  via RF coupling. The ITO panel  205  functions as an antenna and receives the RF carrier signal from the stylus  250  as described below with respect to  FIG. 2B . The selective receiver  210  demodulates the RF carrier signal and couples a touch position signal to the host CPU  220  and a force data signal to the data decoder  215 . The data decoder  215  extracts the force data and couples it to the host CPU  220 . The host CPU  220  calculates the stylus touch position based on the relative maximum amplitude of the RF signal detected on the ITO lines of ITO panel  205 . The host CPU  220  further determines the force applied to the stylus based on the force data. The PC  225  controls the host CPU  220 . 
       FIG. 2B  is a block diagram illustrating a conventional embodiment of an active stylus used in a system for tracking the position of a touch object on an RF sense array. The measurer  260  of stylus  250  measures the force induced on the force sensor  255  and the modulator  265  modulates the resultant force data with a carrier frequency, provided by the reference clock  280 . The amplifier  270  amplifies the modulated signal and transmits the modulated carrier frequency from the stylus tip  275 . As described above, the host  200  decodes the modulated carrier signal and transmits the result to the PC  225 . While an RF sense array solution may offer cost savings and a reduced component count, they require special narrow band receivers on the host  200  and are subject to RF noise and interference. Consequently, conventional touch panel solutions may have significant disadvantages in cost, performance, applicability, and reliability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not of limitation, in the figures of the accompanying drawings in which: 
         FIG. 1A  is a block diagram illustrating a conventional embodiment of a host device for tracking the position of a touch object on an inductive sense array. 
         FIG. 1B  is a block diagram illustrating a conventional embodiment of an active stylus used in a system for tracking the position of a stylus on an inductive sense array. 
         FIG. 2A  is a block diagram illustrating a conventional embodiment of a host device for tracking the position of a stylus on a radio frequency sense array. 
         FIG. 2B  is a block diagram illustrating a conventional embodiment of an active stylus used in a system for tracking the position of a stylus on a radio frequency sense array. 
         FIG. 3  is a block diagram illustrating one embodiment of an electronic system having a processing device for detecting a presence of a touch object and a stylus. 
         FIG. 4  is a block diagram illustrating one embodiment of a capacitance sense array including an N×M electrode matrix and a capacitance sensor that converts measured capacitances to touch coordinates. 
         FIG. 5A  is diagram illustrating one embodiment of a method of scanning an all-points-addressable mutual capacitance sense array. 
         FIG. 5B  is diagram illustrating one embodiment of a method of scanning an all-points-addressable mutual capacitance sense array. 
         FIG. 6A  is a block diagram illustrating one embodiment of a system including a capacitive sense array and a touch screen controller that converts measured capacitances to touch coordinates. 
         FIG. 6B  is a block diagram illustrating one embodiment of a system including a capacitive sense array, a stylus, and a touch screen controller that converts measured capacitances to touch coordinates. 
         FIG. 7A  is a block diagram illustrating one embodiment of a stylus including an on/off switch. 
         FIG. 7B  is a block diagram illustrating one embodiment of a stylus including a capacitive sense element to detect when the stylus is in use. 
         FIG. 7C  is a block diagram illustrating one embodiment of a stylus including a stylus tip with multiple function capabilities. 
         FIG. 8  is a block diagram illustrating one embodiment of a stylus. 
         FIG. 9  is a timing diagram for a mutual capacitance touch screen system where no touch object is present on the touch screen, according to an embodiment of the invention. 
         FIG. 10  is a timing diagram for a mutual capacitance touch screen system  950  where a finger is present on the touch screen, according to an embodiment of the invention. 
         FIG. 11  is a timing diagram for a mutual capacitance touch screen system where a stylus is present on the touch screen, according to an embodiment of the invention. 
         FIG. 12  is a timing diagram for a mutual capacitance touch screen system where a finger and a stylus are present on the touch screen, according to an embodiment of the invention. 
         FIG. 13  is a flow chart of one embodiment of a method of detecting a stylus on a capacitive sense array. 
     
    
    
     DETAILED DESCRIPTION 
     Apparatuses and methods of synchronizing a capacitive sense array to a stylus are described. In one embodiment, the stylus is configured to operate as the timing “master,” and a touch screen controller adjusts the timing of the capacitive sense array to match that of the stylus when the stylus is in use. The stylus capacitively couples the stylus transmit (“TX”) signal to the capacitive sense array. The touch screen controller may also be configured to substantially simultaneously track the position of both a passive touch object (e.g., a finger) and the stylus. In one embodiment, the stylus is configured to modulate additional data into the stylus TX signal, including, but not limited to, stylus tip force data, stylus button data, stylus acceleration, and stylus battery data. In an embodiment, the touch screen controller is configured to transmit a TX signal on the row electrodes of the capacitive sense array, and receive the resulting RX signal on the column electrodes of the capacitive sense array to track the position of a passive touch object on the capacitive sense array. In another embodiment, the touch screen controller is configured to receive the stylus TX signal on both the rows and columns of electrodes. It should be noted that the touch screen controller may utilize the row and column electrodes interchangeably to transmit and receive the TX &amp; RX signals for locating and tracking the position of a passive touch object. 
     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques are not shown in detail, but rather in a block diagram in order to avoid unnecessarily obscuring an understanding of this description. 
     Reference in the description to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The phrase “in one embodiment” located in various places in this description does not necessarily refer to the same embodiment. 
       FIG. 3  is a block diagram illustrating one embodiment of an electronic system  300  having a processing device  310  for detecting a presence of a touch object  340  and a stylus  330 . Electronic system  300  includes processing device  310 , touch screen  325 , touch sensor pad  320 , stylus  330 , host processor  350 , embedded controller  360 , and non-capacitance sense elements  370 . In the depicted embodiment, the electronic system  300  includes the touch screen  325  coupled to the processing device  310  via bus  322 . Touch screen  325  may include a multi-dimension capacitive sense array. The multi-dimension sense array includes multiple sense elements, organized as rows and columns. In another embodiment, the touch screen  325  operates as an all-points-addressable (“APA”) mutual capacitance sense array, as described with respect to  FIG. 4 . In another embodiment, the touch screen  325  operates as a coupled-charge receiver, as described with respect to  FIG. 4 . 
     The operations and configurations of the processing device  310  and the touch screen  325  for detecting and tracking the touch object  340  and stylus  330  are described in detail below with respect to  FIGS. 4-6B . In short, the processing device  310  is configured to detect a presence of the stylus  330  on the touch screen  325 , as well as a presence of the touch object  340 . The processing device  310  may detect and track the stylus  330  and the touch object  340  individually on the touch screen  325 . In one embodiment, the processing device  310  can detect and track both the stylus  330  and touch object  340  concurrently on the touch screen  325 . In one embodiment, the stylus  330  is configured to operate as the timing “master,” and the processing device  310  adjusts the timing of the touch screen  325  to match that of the stylus  330  when the stylus  330  is in use. As described herein, the touch screen  325  capacitively couples with the stylus  330 , as opposed to conventional inductive stylus applications. It should also be noted that the same assembly used for the touch screen  325 , which is configured to detect touch objects  340 , is also used to detect and track the stylus  330  without an additional PCB layer for inductively tracking the stylus  330  as done conventionally. 
     In the depicted embodiment, the processing device  310  includes analog and/or digital general purpose input/output (“GPIO”) ports  307 . GPIO ports  307  may be programmable. GPIO ports  307  may be coupled to a Programmable Interconnect and Logic (“PIL”), which acts as an interconnect between GPIO ports  307  and a digital block array of the processing device  310  (not shown). The digital block array may be configured to implement a variety of digital logic circuits (e.g., DACs, digital filters, or digital control systems) using, in one embodiment, configurable user modules (“UMs”). The digital block array may be coupled to a system bus. Processing device  310  may also include memory, such as random access memory (“RAM”)  305  and program flash  304 . RAM  305  may be static RAM (“SRAM”), and program flash  304  may be a non-volatile storage, which may be used to store firmware (e.g., control algorithms executable by processing core  302  to implement operations described herein). Processing device  310  may also include a memory controller unit (“MCU”)  303  coupled to memory and the processing core  302 . 
     The processing device  310  may also include an analog block array (not shown). The analog block array is also coupled to the system bus. Analog block array also may be configured to implement a variety of analog circuits (e.g., ADCs or analog filters) using, in one embodiment, configurable UMs. The analog block array may also be coupled to the GPIO  307 . 
     As illustrated, capacitance sensor  301  may be integrated into processing device  310 . Capacitance sensor  301  may include analog I/O for coupling to an external component, such as touch-sensor pad  320 , touch screen  325 , touch-sensor slider (not shown), touch-sensor buttons (not shown), and/or other devices. Capacitance sensor  301  and processing device  310  are described in more detail below. 
     In one embodiment, the electronic system  300  includes a touch sensor pad  320  coupled to the processing device  310  via bus  321 . Touch sensor pad  320  may include a multi-dimension capacitive sense array. The multi-dimension sense array includes multiple sense elements, organized as rows and columns. In another embodiment, the touch sensor pad  320  is an APA mutual capacitance sense array, as described with respect to  FIG. 4 . In another embodiment, the touch sensor pad  320  operates as a coupled-charge receiver, as described with respect to  FIG. 4 . 
     In an embodiment, the electronic system  300  may also include non-capacitance sense elements  370  coupled to the processing device  310  via bus  371  and GPIO port  307 . The non-capacitance sense elements  370  may include buttons, light emitting diodes (“LEDs”), and other user interface devices, such as a mouse, a keyboard, or other functional keys that do not require capacitance sensing. In one embodiment, buses  321 ,  322 , and  371  are embodied in a single bus. Alternatively, these buses may be configured into any combination of one or more separate buses. 
     Processing device  310  may include internal oscillator/clocks  306  and communication block (“COM”)  308 . In another embodiment, the processing device  310  includes a spread spectrum clock (not shown). The oscillator/clocks block  306  provides clock signals to one or more of the components of processing device  310 . Communication block  308  may be used to communicate with an external component, such as a host processor  350 , via host interface (“I/F”) line  351 . Alternatively, processing block  310  may also be coupled to embedded controller  360  to communicate with the external components, such as host  350 . In one embodiment, the processing device  310  is configured to communicate with the embedded controller  360  or the host  350  to send and/or receive data. 
     Processing device  310  may reside on a common carrier substrate such as, for example, an integrated circuit (“IC”) die substrate, a multi-chip module substrate, or the like. Alternatively, the components of processing device  310  may be one or more separate integrated circuits and/or discrete components. In one exemplary embodiment, processing device  310  is the Programmable System on a Chip (PSoC®) processing device, developed by Cypress Semiconductor Corporation, San Jose, Calif. Alternatively, processing device  310  may be one or more other processing devices known by those of ordinary skill in the art, such as a microprocessor or central processing unit, a controller, special-purpose processor, digital signal processor (“DSP”), an application specific integrated circuit (“ASIC”), a field programmable gate array (“FPGA”), or the like. 
     It should also be noted that the embodiments described herein are not limited to having a configuration of a processing device coupled to a host, but may include a system that measures the capacitance on the sensing device and sends the raw data to a host computer where it is analyzed by an application. In effect, the processing that is done by processing device  310  may also be done in the host. 
     Capacitance sensor  301  may be integrated into the IC of the processing device  310 , or alternatively, in a separate IC. Alternatively, descriptions of capacitance sensor  301  may be generated and compiled for incorporation into other integrated circuits. For example, behavioral level code describing capacitance sensor  301 , or portions thereof, may be generated using a hardware descriptive language, such as VHDL or Verilog, and stored to a machine-accessible medium (e.g., CD-ROM, hard disk, floppy disk, etc.). Furthermore, the behavioral level code can be compiled into register transfer level (“RTL”) code, a netlist, or even a circuit layout and stored to a machine-accessible medium. The behavioral level code, the RTL code, the netlist, and the circuit layout all represent various levels of abstraction to describe capacitance sensor  301 . 
     It should be noted that the components of electronic system  300  may include all the components described above. Alternatively, electronic system  300  may include only some of the components described above. 
     In one embodiment, the electronic system  300  is used in a tablet computer. Alternatively, the electronic device may be used in other applications, such as a notebook computer, a mobile handset, a personal data assistant (“PDA”), a keyboard, a television, a remote control, a monitor, a handheld multi-media device, a handheld media (audio and/or video) player, a handheld gaming device, a signature input device for point of sale transactions, and eBook reader, global position system (“GPS”) or a control panel. The embodiments described herein are not limited to touch screens or touch-sensor pads for notebook implementations, but can be used in other capacitive sensing implementations, for example, the sensing device may be a touch-sensor slider (not shown) or touch-sensor buttons (e.g., capacitance sensing buttons). In one embodiment, these sensing devices include one or more capacitive sensors. The operations described herein are not limited to notebook pointer operations, but can include other operations, such as lighting control (dimmer), volume control, graphic equalizer control, speed control, or other control operations requiring gradual or discrete adjustments. It should also be noted that these embodiments of capacitive sensing implementations may be used in conjunction with non-capacitive sensing elements, including but not limited to pick buttons, sliders (ex. display brightness and contrast), scroll-wheels, multi-media control (ex. volume, track advance, etc) handwriting recognition, and numeric keypad operation. 
       FIG. 4  is a block diagram illustrating one embodiment of a capacitive sense array  400  sense array including an N×M electrode matrix  425  and the capacitance sensor  301  that converts measured capacitances to touch coordinates. The sense array  400  may be, for example, the touch screen  325  or the touch sensor pad of  FIG. 3 . The N×M electrode matrix  425  includes N×M electrodes (N receive electrodes and M transmit electrodes), which further includes transmit (“TX”) electrode  422  and receive (“RX”) electrode  423 . Each of the electrodes in N×M electrode matrix  425  is connected to the capacitance sensor  301  by one of the conductive traces  450 . In one embodiment, capacitance sensor  301  operates using a charge accumulation circuit, a capacitance modulation circuit, or other capacitance sensing methods known by those skilled in the art. In an embodiment, the capacitance sensor  301  is of the Cypress TMA-3xx family of touch screen controllers. Alternatively, other capacitance sensors may be used. The mutual capacitance sense arrays, or touch screens, as described above, may include a transparent, conductive sense array disposed on, in, or under either a visual display itself (e.g. LCD monitor), or a transparent substrate in front of the display. In an embodiment, the TX and RX electrodes are configured in rows and columns, respectively. In should be noted that the rows and columns of electrodes can be configured as TX or RX electrodes by the capacitance sensor  301  in any chosen combination. In one embodiment, the TX and RX electrodes of the sense array  400  are configured to operate as a TX and RX electrodes of a mutual capacitance sense array in a first mode to detect touch objects, and to operate as electrodes of a coupled-charge receiver in a second mode to detect a stylus on the same electrodes of the sense array. The stylus, which generates a stylus TX signal when activated, is used to couple charge to the capacitive sense array, instead of measuring a mutual capacitance at an intersection of a RX electrode and a TX electrode (a sense element) as done during mutual capacitance sensing. The capacitance sensor  301  does not use mutual capacitance or self-capacitance sensing to measure capacitances of the sense elements when performing a stylus can. Rather, the capacitance sensor  301  measures a charge that is capacitively coupled between the sense array  400  and the stylus as described herein. 
     The TX and RX electrodes in the N×M electrode matrix  425  are arranged orthogonally so that each of the TX electrodes intersects and overlaps each of the RX electrodes at a total of N*M intersections, creating N*M individual sense elements in the array. Thus, each TX electrode is capacitively coupled with each of the RX electrodes. For example, TX electrode  422  is capacitively coupled with RX electrode  423  at the point where TX electrode  422  and RX electrode  423  overlap. The intersections of TX and RX electrodes  422  and  423  each form a capacitive sense element. 
     Because of the capacitive coupling between the TX and RX electrodes, the application of a TX signal at each TX electrode induces a current at each of the RX electrodes. For instance, when a TX signal is applied to TX electrode  422 , the TX signal induces an RX signal on the RX electrode  423  in N×M electrode matrix  425 . The RX signal on each of the RX electrodes can then be measured in sequence by using a multiplexor to connect each of the N RX electrodes to a demodulation circuit in sequence. The 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 RX electrode. 
     When a touch object, such as a finger, approaches the N×M electrode matrix  625 , the object causes a decrease in capacitance affecting only some of the electrodes. For example, if a finger is placed near the intersection of TX electrode  422  and RX electrode  423  (sense element), the presence of the finger decreases the coupling capacitance between the two electrodes  422  and  423 . In another embodiment, the presence of the finger increases the coupling capacitance between the two electrodes  422  and  423 . Thus, the location of the finger on the touchpad can be determined by identifying both the RX electrode having a changed coupling capacitance between the RX electrode and the TX electrode to which the TX signal was applied at the time the changed capacitance was measured on the RX electrode. Therefore, by sequentially determining the capacitances associated with each intersection of electrodes in the N×M electrode matrix  425 , the locations of one or more inputs can be determined. It should be noted that the process can calibrate the sense elements (intersections of RX and TX electrodes by determining baselines for each of the sense elements. It should also be noted that interpolation may be used to detect finger position at better resolutions than the row/column pitch as would be appreciated by one of ordinary skill in the art having the benefit of this disclosure. In addition, various types of centroid algorithms may be used to detect the center of the touch as would be appreciated by one of ordinary skill in the art having the benefit of this disclosure. 
     In other words, sensing is achieved by applying a transmit signal to each row of TX electrodes in turn for a short period of time and during that period, sensing the amount of charge coupled from that row of TX electrodes to each of the columns of RX electrodes. In one embodiment, the charge coupled from TX electrodes to RX electrodes at each intersection is measured one row at a time, (as shown in  FIG. 5A ) until a map of charge measurements has been created for the entire screen. In other embodiments, each row may need to be driven twice and subsequently multiplexed if there are more columns than available sensing channels, as shown in  FIG. 5B . Other variations of scanning patterns may be used as would be appreciated by one of ordinary skill in the art. Furthermore, the conversion of the induced RX signal to touch position coordinates indicating a position of an input on a touch sensor pad would be understood by those of ordinary skill in the art. 
     Although the TX and RX electrodes  422 ,  423 , appear as bars or elongated rectangles in  FIG. 4 , alternative embodiments may use various tessellated shapes such as diamonds, rhomboids, chevrons, and other useable shapes as would be appreciated by one of ordinary skill in the art having the benefit of this disclosure. 
       FIG. 6A  is a block diagram illustrating one embodiment of a system  600  including the sense array  400  and a touch screen controller  605  that converts measured capacitances to touch coordinates. In an embodiment, the touch screen controller  605  is similar to the capacitance sensor  301  described above. In another embodiment, the touch screen controller  605  is the processing device  310 . The sense array  400  includes TX lines  635  and RX lines  640 . In an embodiment, the TX lines  635  and RX lines  640  are the TX electrodes  422  and RX electrodes  423  of  FIG. 4 , respectively. The touch screen controller  605  includes a TX drive circuit  610 , an RX sense circuit  620 , and a multiplexor  630 . 
     In an embodiment, a passive object (e.g., a finger or other conductive object) touches the sense array  400  at contact point  645 . The TX drive circuit  610  drives the TX lines  635  with TX signal  632 . The RX sense circuit  620  measures the RX signal  634  on RX lines  640 . In an embodiment, the touch screen controller  605  determines the location of contact point  645  based on the mapping techniques described above in conjunction with  FIGS. 4-5 . The TX lines  635  and RX lines  640  are multiplexed by multiplexor  630 . The touch screen controller  605  provides the TX signal  632  on the TX lines  635  (rows) and measures the capacitance on the RX lines  640  (columns). In an embodiment, the TX and RX lines  635 ,  640  are orthogonal and may be used interchangeably (e.g., transmitting on columns and receiving on rows). In an embodiment, the TX drive circuit  610  transmits the TX signal  632  through a high impedance ITO panel (TX lines  635 ), thus limiting the upper frequency limit and speed of the system. The total scan time may also dependent upon the number of TX lines  635  and RX lines  640  in the sense array  400 . For example, the TX drive circuit  610  provides a TX signal  632  on each TX line  635  and simultaneously reads the capacitively coupled RX signal  634  on each RX line  640 , according to one embodiment. In another embodiment, the RX lines  640  are multiplexed in two or more scans, as described in conjunction with  FIG. 5B . 
       FIG. 6B  is a block diagram illustrating one embodiment of a system  600  including the sense array  400 , a stylus  680 , and the touch screen controller  605  that converts measured capacitances to touch coordinates. The sense array  400  includes RX lines  640  and  660 . The RX lines  660  are the same as TX lines  635  in  FIG. 6A , but used as a receive sensing channel in system  600  as further described below. In an embodiment, RX lines  640  and  660  are similar to the TX electrodes  422  and RX electrodes  423  of  FIG. 4 . The touch screen controller  605  includes the TX drive circuit  610 , the RX sense circuit  620 , and the multiplexor  630 . The stylus  680  includes a TX drive circuit  685  and a stylus tip  688 . 
     In an embodiment, the stylus TX drive circuit  685  of stylus  680  provides a TX signal  677  directly to contact point  695  on sense array  400 , thus eliminating the need to dedicate the RX  660  lines (previously TX  635  in  FIG. 6A ) to transmitting a TX signal from the TX drive circuit  610 . As such, the RX sense circuit  620  measures the RX signal  634  on both the rows (RX lines  660 ) and columns (RX lines  640 ) of sense array. This results in faster position tracking because the TX signal no longer passes through the high impedance ITO lines, resulting in a faster scan time during stylus sensing than when mutual-capacitance sensing for a conductive object. In one embodiment, the touch screen controller  605  performs a normal scan of the sense array  400  during RX sensing of TX signal from the TX drive circuit  660  (illustrated in  FIG. 6A ), and a stylus scan of the sense array  400  during RX sensing of the stylus TX signal  677  (illustrated in  FIG. 6B ). For the stylus scan, the touch screen controller  605  measures a charge being capacitively coupled to the row and column electrodes of the sense array from the stylus. To further illustrate, a mutual capacitance scan uses both a TX and RX signal  632 ,  634  to locate and track an object. As described above, this is typically done by sensing each of the RX lines  640  for each driven TX line  635  in a successive fashion by the touch screen controller  655 . In an array of N rows (TX signal) and M columns (RX signal), a complete scan would require N×M total scans if one RX line is sensed at a time. For example, transmitting a TX signal (“TX&#39;ing”) on row 1, and receiving a receive signal (“RX&#39;ing”) on columns 1-M, followed by TX&#39;ing on row 2 and RX&#39;ing on columns 1-M, and so on in sequential fashion. 
     Alternatively, more RX lines can be sensed at a time. In one embodiment, 4 or 8 RX lines are sensed at a time, but in other embodiments, all RX lines may be sense simultaneously or sequentially. With multiple RX channels to sense more than one RX line at the same time, the complete scan would be (N*M)/(# RX channels). In contrast, a stylus scan does not require a TX signal by the TX drive circuit  610  and a complete scan would only require a single RX signal measurement on each row and column, or N+M scans, thus resulting in a significantly reduced stylus scanning time for the entire sense array as compared with mutual capacitance scanning time for the entire sense array. Like above, multiple RX channels can be used to sense multiple RX lines at the same time. In this case, the complete scan would be (N+M)/(# RX channels). 
     As described above, a passive stylus may be used as a touch object to interface with the various touch screens described above. In contrast to passive styluses, an active stylus described herein provides the transmit (“TX”) signal that is typically provided by the touch screen controller  605  in finger sensing modes, as described above in conjunction with  FIGS. 6A and 6B . The stylus is used to couple charge into the capacitive sense array, and the capacitive sense array operates as a coupled-charge receiver to detect the stylus. When operating as the coupled-charge receiver, the capacitive sense array does not use mutual capacitance or self-capacitive sensing. The same capacitive sense array can operate in a different mode when detecting touch objects, for example, using mutual capacitance sensing. 
     The stylus  680  capacitively couples the stylus TX signal  677  to the sense array  400 , as described above in conjunction with  FIGS. 4-6 . In an embodiment, the stylus signal amplitude, frequency, phase, etc., may be the same or similar to that which is utilized for finger sensing by the touch screen controller  605 . Alternatively, the stylus TX signal may be different than the TX signal from the TX drive circuit  610 , in amplitude, frequency, and phase. In another embodiment, the stylus TX signal may have a different sequence for code modulation than a sequence used in the TX signal from the TX drive circuit  610 . In an exemplary embodiment, the stylus TX signal  677  has a greater amplitude than the finger sensing TX signal  632  from the TX drive circuit  610 . For example, in one exemplary embodiment, the stylus TX signal  635  ranges from approximately 20-50V, as compared with the approximately 5-10V typically provided by the touch screen controller  605 . Alternatively, other voltages may be used as would be appreciated by one of ordinary skill in the art. The higher stylus TX voltage couples more charge to the MC array  400  more quickly, thus reducing the amount of time required to sense each row and column of the sense array  400 . Other embodiments may incorporate higher voltages on the MC array TX line  635  to obtain similar time efficiency improvements for finger sensing. 
     In an embodiment, the stylus  680  applies a higher frequency on the stylus TX signal  677  than the TX signal  632  frequency from TX drive circuit  610  to achieve a reduced sensing time. Charge may be capacitively coupled from the stylus  680  to the sense array  400  during the rising and falling edges of the stylus TX signal  677 . Thus, a higher TX frequency provides a greater number of rising and falling edges over a given period of time, resulting in greater charge coupling in a given period than with a low frequency. The practical upper limit of the TX frequency in finger sensing mode (e.g., TX signal on sense array  400  for finger sensing) is dependent upon the resistor-capacitor (“RC”) time constant of the individual sense elements and interconnect (not shown). This is typically due to high impedance materials (e.g. ITO) used in the fabrication of the sense array  400 . A high-impedance sense array  400  may result in a high time constant and resulting signal attentuation of the rows (TX lines  635 ) and columns (RX lines  640 ) of sensors, which may limit the maximum sensing frequency. When using an active stylus to transmit the stylus TX signal  677  directly to a contact point on sense array  400 , the stylus TX signal  677  does not have to pass through all of the high impedance path, and therefore the maximum operating frequency for the stylus TX signal  677  can be increased, as described above in conjunction with  FIGS. 6A and 6B . For example, the time constant of the RX traces (both rows and columns) may be used to determine an upper frequency limit, but this typically is at least double the upper frequency limit used in finger sensing. Typically the impedance is half to the impedance when performing mutual capacitance scanning, since the row&#39;s impedance is eliminated and the column&#39;s impedance remains (or vice versa). It should be noted that both finger sensing and stylus sensing use frequency selection where the operation period should be smaller than the panel&#39;s time constant; so, restrictions for the operation frequency selection are approximately the same for finger and stylus sensing. 
     In an embodiment, the frequency of the stylus TX signal  677  is different than the frequency of the finger sensing TX signal  632 . By using different TX frequencies, the touch screen controller  605  can differentiate between stylus TX signals and finger sensing TX signals. This applies when the controller is performing mutual capacitance sensing. If the controller is not driving any of the TX lines, it can be inferred that any RX signals detected have come from a stylus. Alternatively, the touch screen controller  605  can differentiate the stylus TX signals from the TX drive circuit  610  TX signals  632  using other techniques as would be appreciated by those of ordinary skill in the art with the benefit of this disclosure, such as detecting the difference is signal characteristics (e.g., phase, frequency, amplitude, and code modulation). 
     In an embodiment, the stylus  680  encodes the stylus TX signal  677  to include additional data to be used by the touch screen controller  655 . Such additional information may be useful for the user to obtain additional functionality from the stylus  680 . For example, additional information may include the status of additional buttons, sliders or other operator actuated controls on the stylus  680 . The addition buttons may be electrical (e.g., buttons, potentiometers), inductive, or capacitive. In an embodiment, force sensing, or the pressure with which the stylus tip  688  is pressed against the touch screen or sense array  400 , is detected by the stylus  680  and encoded in the stylus TX signal  677 . A force may be detected by either a passive sensor (e.g., force sensing resistor or capacitor) or active sensor (e.g., capacitive linear position sensor or a moving element in relation to a coil) within the stylus  680 . Furthermore, the orientation or acceleration of the stylus may be detected (e.g., by an accelerometer) and encoded in the stylus TX signal  677  for greater stylus  680  functionality. Such additional information may be transmitted from the stylus  680  to the touch controller  605  in a variety of ways and is not limited to modulating the stylus TX signal  677 . The tip force, for example, may be used to vary the line width in a drawing program, to detect when the stylus is in contact with the screen, or to detect gestures such as a “tap gesture,” or “double-tap” gesture, or the like. 
     In an embodiment, the stylus  680  encodes the stylus TX signal  677  with the additional data using modulation. Such modulation may include frequency modulation (“FM”), frequency-shift keying (“FSK”), amplitude modulation (“AM”), amplitude-shift keying (“ASK”), on-off keying (“OOK”), pulse position modulation, phase modulation (“PM”), Manchester encoding, direct sequence spread spectrum (“DSSS”), and other modulation schemes appreciated by those of ordinary skill in the art. PM modulation may further include binary phase shift keying (“BPS K”) or quadrature phase shift keying (“QPSK”) encoding schemes. 
     Alternative embodiments may be implemented to transfer additional data from the stylus  680  to the touch screen controller  605  in lieu of modulating the stylus TX signal  677 . In one embodiment, the stylus  680  transmits the additional data separately from the stylus TX signal  677 , but using the same capacitive coupling to the sense array  400 . For example, in time division multiplexing (“TDM”) the stylus  680  transmits the stylus TX signal  677  in one time slot, and transmits the additional data (e.g., force data, acceleration data) in another time slot. In one embodiment, the stylus TX signal  677  and additional data utilize the same or different frequencies. In another embodiment, the stylus TX signal  677  and additional data utilize one or more of the modulation methods previously described. In further embodiments, optical, ultrasonic, inductive, or RF signal transmissions may be utilized to transfer the additional data from the stylus  680  to the touch screen controller  655 . It should be noted that additional hardware, such as antennas and amplifiers may be needed to transmit the additional data on the separate communication link. 
     In one embodiment, in order to minimize the power consumption of the stylus  680  and maximize battery life, the stylus  680  does not generate or transmit stylus TX signals  677  unless it is touching or in proximity to the sense array  400 . In one embodiment, the user operates an on/off switch to begin generating the stylus TX signal  677 .  FIG. 7A  is a block diagram illustrating one embodiment of a stylus  700  including an on/off switch  705 . In an embodiment, the on/off switch  705  may be an actuator configured to be enabled (i.e., switched from the “off” position to the “on” position and vice versa) by depressing, rotating, or sliding the actuator. The on/off switch  705  may be mechanical, electrical, or other type of manual switch known by those of ordinary skill in the art. 
       FIG. 7B  is a block diagram illustrating one embodiment of a stylus  720  including a capacitive sense element  730  to detect when the stylus is in use. In one embodiment, the stylus  720  includes an on/off switch  725  similar to that which is described above in conjunction with  FIG. 7A . The on/off switch  725  is combined with a sleep mode. The sleep mode is a low power mode where the stylus  720  uses a small amount of power to detect whether it is currently in use by a user. For example, a user turns on the stylus  720  by depressing the on/off switch  725  and the stylus  720  begins transmitting a stylus TX signal  677 . After a predetermined period of non-use, the stylus  720  enters a low power state (“sleep mode”) and stops transmitting the stylus TX signal  677 . The stylus  720  remains in the low power state until a touch object (e.g., a finger, hand, etc.) is detected on the capacitive sense element  730 . In an embodiment, the stylus  720  periodically measures the capacitance on the capacitance sense element  730  to detect use (e.g., every 250 ms); however, the detect period or duty cycle may be configured as necessary. Upon detection of a touch object, the stylus  720  begins retransmitting the stylus TX signal  677  until either the predetermined period lapses or the user turns the stylus off via the on/off switch  725 . In another embodiment, the stylus  720  turns off the stylus after a predetermined period has lapsed in the low power state. 
       FIG. 7C  is a block diagram illustrating one embodiment of a stylus  740  including a stylus tip  745  with multiple function capabilities. In an embodiment, the stylus tip  745  is configured to sense mechanical pressure (e.g., force data) when the stylus tip  745  makes contact with the sense array  400  or touch panel (not shown). The stylus  740  may be configured to employ a “wake on force sensing” feature. For example, the stylus  740  utilizes the force data to detect use of the stylus  740  to change the mode of operation from a low power sleep mode to an active stylus TX transmit mode, as similarly described in conjunction with  FIG. 7B . In other embodiments, the stylus  740  transmits stylus TX signals  677  while a force signal is present and remains active for an additional time out period afterwards (e.g., 15 seconds). In an embodiment, the stylus tip  745  may also detect use by detecting the touch screen TX signal (TX signal  632 ), detecting liquid crystal display (“LCD”) noise, and/or detecting LCD optical signals. The features descried in  FIGS. 7A-7C  may be used alone or in any combination thereof. 
       FIG. 8  is a block diagram illustrating one embodiment of a stylus  800 . The stylus  800  includes a waveform generator  810 , a wave shaper  830 , and an enable block  850 . In one embodiment, the waveform generator  810  is an oscillator. The oscillator may be a crystal, a ceramic resonator, ring oscillator, CPU internal oscillator, relaxation oscillator, or other waveform generating circuits known by those skilled in the art. In an embodiment, the waveform generator  810  generates a stylus TX signal  820  and couples it to the wave shaper  830  for further processing. 
     The wave shaper  830  may be configured to shape the frequency, duty cycle, phase, or amplitude of the TX signal  820  as required by the receiving touch screen controller (not shown). It should be noted that the square wave shape of  820 ,  840 , and  860  is not intended to be indicative of the actual signal waveform, but merely generically indicative of the presence of a signal. The wave shaper  830  is configured to modulate or encode the TX signal  820  as described above in conjunction with  FIG. 6B . In an embodiment, the wave shaper  830  couples the TX signal  840  to the enable block  850 . For some applications, the wave shaper  830  may not be an integral part of TX signal generation. For example, the TX signal  820  from the waveform generator  810  may not require additional wave shaping. As such, some embodiments may not require the wave shaper  830  in the stylus  800  design. 
     In an embodiment, the enable block  850  is configured to enable low power or sleep mode operation, as described above in conjunction with  FIG. 7C , for example. The enable block  850  may function as a separate block, as shown in  FIG. 8 , or may be integrated in the waveform generator  810  or wave shaper  830  blocks. In an embodiment, the enable block  850  provides a feedback path to the waveform generator  810  and/or wave shaper  830  blocks to control their activity during active and sleep modes. For example, if the enable block  850  determines that the stylus  800  should be switched from active to sleep mode due to user inactivity, the enable block  850  provides a feedback signal  870  to stop the waveform generator  810  and wave shaper  830  from generating a TX signal  820 ,  840 . In one embodiment, the enable block  850  couples the TX signal  860  to the stylus tip (not shown) in the stylus active mode. 
     In an embodiment, the stylus  800  may be powered from a local primary battery, such as one or more AAA cells, button cells, hearing aid batteries, etc. Alternatively, rechargeable cells may be used including, but not limited to, nickel and lithium based batteries, flexible polymer cells, fuel cells, as well as super or ultra-capacitors. The stylus  740  may also be housed within the touch screen as in a charging station configuration, or receive power through resistive, capacitive or inductive means, as would be appreciated by one having ordinary skill in the art. 
     In an embodiment, the stylus  800  is configured to transmit a TX signal  860  that appears electrically similar to the TX signal  632  provided by a touch screen TX drive circuit  610  in  FIG. 6A . In one embodiment, the touch screen controller (not shown) is the Cypress CY8CTMA300 processor. Alternatively, other touch screen controllers may be used. The touch screen controller typically produces a square wave TX signal  820 ,  840 ,  860  in the range of 100 KHz to 300 KHz, although other TX waveforms (e.g., sine, triangle, spread spectrum) and operating frequencies may be used as known by those skilled in the art. 
     A mutual capacitance touch screen system supporting an active stylus  680  is described herein, where the stylus is configured to operate as the timing “master,” and the touch screen controller  605  adjusts its timing to match that of the stylus  680  when the stylus is in use. In other words, touch screen controller  605  is configured to synchronize the sense array  400  to the stylus  680 . Various embodiments described herein are applicable to any mutual capacitance touch screen system using an untethered, or wireless active stylus configured to be capacitively coupled to the mutual capacitance array, where the stylus does not receive synchronization or timing data from the touch screen controller. Furthermore, the timing of the various different sequenced steps and process relating to synchronization are derived from TX signals transmitted by the stylus, as contrasted with systems that perform synchronization by the host. 
     To sufficiently describe the operation of an embodiment of the system  600 , and the timing transitions that occur between the different operating modes of the system  600 , a sample sequence of events is provided below beginning with the system  600  in an off state. At startup, the user turns of the power on and the initialization process of system  600  begins. The display (not shown) turns on and the touch screen subsystem (system  600 ) initializes. At this point, no finger, touch object, or stylus in proximity or contact with the sense array  400 . 
       FIG. 9  is a timing diagram  900  for a mutual capacitance touch screen system  600  where no touch object is present on the touch screen, according to an embodiment of the invention. The timing diagram  900  illustrates a controller TX signal  910 , a controller RX signal  920 , and a stylus TX signal  930 . The controller TX signal  910  includes controller TX pulses  912 . The controller RX signal  920  includes wait-for-touch (“WFT”) interval  922  and wait-for-stylus (“WFS”) interval  923 . In an embodiment, the touch screen includes the sense array  400 . 
     In operation, a user turns the power on and the system initializes. In an embodiment, the system  600  begins operation by transmitting a TX signal  632  by TX drive circuit  610  at a predetermined duty cycle. In an embodiment, the duty cycle for TX signal  632  is lower than the duty cycle for active mode where a finger or stylus is detected for improved power dissipation of the system  600 . During each WFT TX sensing period  912  the controller generates signal  632 , the touch screen controller  605  listens for a corresponding RX signal during WFT RX sensing period  922  to detect if a finger is present on the sense array  400 ; in one implementation the WFT TX sensing period  912  may occur at substantially the same time as the WFT RX sensing period  922 . After each WFT interval  912 / 922 , the touch screen controller  605  performs a WFS sensing operation during WFS sensing period  923  to sense for stylus use. The WFS interval  923  is performed similarly to the mutual capacitance WFT  922 , except that the touch screen controller  605  does not transmit the TX signal  632 . Instead, the touch screen controller  605  performs RX sensing, and any received signal occurring after the WFT  923  interval (ie during the WFS period  923 ) that is above a predetermined noise threshold is determined to be a stylus  680  signal. The WFT/WFS process continues as a background process for as long as no finger, touch object, or stylus is detected; the system may be in a low power mode outside of the WFT/WFS sensing periods, resulting in low overall average power consumption when no finger or stylus is proximate to the screen. 
     In an alternative embodiment, the WFT (mutual capacitance) and WFS (coupled-charge) are utilized, as described above in conjunction with  FIG. 6B . In another embodiment, there is no reduced duty cycle WFT/WFS mode, and after initialization, the touch screen controller  605  begins finger tracking as described below even with no finger or stylus present on the sense array  400 . 
       FIG. 10  is a timing diagram  1000  for a mutual capacitance touch screen system  600  where a finger is present on the touch screen, according to an embodiment of the invention. The timing diagram  1000  illustrates a controller TX signal  1010 , a controller RX signal  1020 , and a stylus TX signal  1030 . The controller TX signal  1010  includes controller TX pulse  1012 . The controller RX signal  1020  includes WFT interval  1022 , and WFS interval  1023 . 
     In operation, a user places a finger on the touch screen. The touch screen controller  605  detects the finger during the WFT interval  912 / 922  and begins finger-tracking sensing. Finger tracking may include sending TX signals on some or all TX lines (rows)  635  of sense array  400  in sequence ( 1012 ), while RX sensing ( 1022 ) on some or all of RX lines (columns)  640 , as previously described. In an embodiment, the scanning may be done periodically (e.g., at 10 ms intervals) or continuously (not shown). Once a finger is detected, the controller TX signal  1010  may increase the duty cycle of the controller TX pulses  1012 , for example to improve the signal to noise ratio of the sensing operation. After the Mutual Capacitance Sensing (MCS) period  1022 , the touch screen controller  605  performs a WFS operation during period  1023 . In an embodiment, some or all of the columns and/or rows during the WFS interval  1023  are “ganged” or shorted together by the pin multiplexor  630  and the touch screen controller  605  scans the entire sense array  400  as a single sensor to detect for the presence (but not location) of stylus TX signals  677 . Once the presence of the stylus  680  is detected, then the touch screen controller  605  locates and/or tracks the stylus as previously described. Alternatively, the rows and/or columns are not “ganged” during the WFS interval  1023 . The WFS interval  1023  is typically shorter in duration (e.g. 100-200 μs) than MCS periods  1022 . In an embodiment, the touch screen controller  605  performs the WFS sensing periodically, rather than after every WFT interval. For example, WFS may occur at every 100 ms or after every ten MCS scans. 
       FIG. 11  is a timing diagram  1100  for a mutual capacitance touch screen system  600  where a stylus  680  is present on the touch screen, according to an embodiment of the invention. It should be noted that the stylus appears at around 1132, but is not present initially. The timing diagram  1100  illustrates a controller TX signal  1110 , a controller RX signal  1120 , and a stylus TX signal  1130 . The controller TX signal  1110  includes controller TX signaling periods  1112 ,  1114 ,  1116 , and  1118 . The controller RX signal  1120  includes WFT intervals  1122 ,  1124 , and  1129 . The controller RX signal  1120  further includes WFS intervals  1123  and  1125 . The stylus TX signal  1130  includes stylus TX pulses  1132 ,  1134 , and  1136 . 
     In one embodiment (not shown), the stylus  680  transmits stylus TX signals  1130  continuously when transmitting. In another embodiment, the stylus  680  transmits a stylus TX signal  1130  at a certain duty cycle. For example, the stylus  680  transmits a stylus TX signal  1130  for 2 ms per every 10 ms. In another embodiment, the stylus  680  may transmit bursts on a duty cycle. For example, the stylus  680  transmits a stylus TX signal  1130  for 100 μs bursts with 50 μs gaps for a total of 4 ms, and then ceases transmission for six ms. Other duty cycles may be implemented as would be appreciated by one of ordinary skill in the art. In an embodiment, the duty cycle may change when the force sensor in the stylus tip  688  detects first contact with the touch screen. In another embodiment, the stylus  680  may not begin transmitting the TX signal  1130  until contact is detected. In each case, however, once the stylus  680  is in contact with the touch screen, it will begin transmitting—either at a predetermined duty cycle or burst pattern or continuously. 
     In an embodiment, the touch screen controller  605  senses the stylus TX signal  1130  and synchronizes RX sense timing to the stylus TX signal  1130  before RX sensing of the stylus signal begins. Typically, the stylus TX signal  1130  is easy to detect and synchronize with the touch screen controller  605  with a high-impedance RX sensing circuit (ITO is typically a high impedance material). In an embodiment with a relatively low impedance RX sensing circuit, it may be necessary to electrically disconnect the RX sensing circuit from the individual sensors on the sense array  400  in order for the stylus TX signal  1120  to be a high enough amplitude to be detected. 
     In an embodiment, a phase locked loop (“PLL”) (not shown) may be used to detect and lock the RX sensing circuits of the touch screen controller  605  to the stylus TX signal  1120 . Typically, the touch screen controller  605  performs the phase locking process at the beginning of each RX sensing operation in stylus mode (as shown in timing diagram  1100 ), and not just during WFS sensing. The PLL may be analog or digitally based. In an embodiment, the digital PLL is implemented in firmware. 
     In operation, the system  600  will initially be in WFT/WFS mode before the first touch from a touch object or stylus on the touch screen (e.g., WFT  1122  and WFS  1123 ). The active and enabled stylus  680  makes contact with the touch screen and transmits a TX signal during period  1132 ). In an embodiment, the touch screen controller  605  detects the stylus TX signal  1132  during WFS interval  1125 . The controller RX signal  1120  then remains active (controller RX period  1126 ) to detect the next Stylus TX pulse  1134  to obtain sufficient timing information to synchronize the touch screen controller  605  to the stylus  680 . By the end of controller RX period  1126 , touch screen controller  605  establishes synchronization with the stylus TX signal  1130 . Typically, synchronization will occur within two complete signal cycles. As shown, the controller RX signal  1128  begins at the start of the next stylus TX pulse  1136 . In addition, the touch screen controller  605  synchronizes the controller TX WFT period  1118  to the stylus TX pulse  1136 , which may change the WFT sensing rate. In another embodiment, the WFT signals (no finger detected) may be configured to occur before the synchronized stylus TX pulses. In either case, the WFT sensing is performed during a period when the stylus is known not to be sending stylus TX signals. 
     Although not shown in the timing diagrams, the stylus  680  may be configured to modulate the stylus TX signal  1130  to include additional data (e.g., force data, acceleration data, button data, power levels, etc.). Alternatively, the stylus  680  may be configured to send the additional data over a communication link as described herein. 
     In one embodiment, the touch screen controller  605  only senses the rows and columns in the vicinity of the last detected point of contact of the stylus to the touch screen. If the stylus is detected within that area, then the touch screen controller does not perform any further sensing outside of that vicinity. For example, the touch screen controller  605  may scan 12 rows and 12 columns centered on the row and column of the last detected location of the stylus, and continue to scan the 12 rows/columns around the last detected location of the stylus for as long as the stylus continues to be detected in that area; note that this area will move as the stylus moves, unless the stylus moves so fast as to move outside the area being sensing in a single scan rate interval. In another embodiment, the touch screen controller  605  determines the velocity and direction of the stylus based on the previous two or more scans to predict the current location of the stylus and centers the area of detection at the predicted location. In an embodiment, if both a stylus and finger are detected, as illustrated in  FIG. 12 , the entire screen may need to be scanned or separate location prediction algorithms may need to be applied to each input source to determine a reduced scanning area. 
     The presence of fingers on the touch screen may affect the stylus tracking accuracy. For example, fingers (touch objects) may couple TX signals from one row to another, or between rows and columns. Therefore, even if some embodiments do not support simultaneous tracking of both a stylus and a finger, it may advantageous to perform WFT sensing during stylus tracking in order to adjust for electrical changes induced by the presence of a finger. 
       FIG. 12  is a timing diagram  1200  for a mutual capacitance touch screen system  600  where a finger and a stylus  680  are simultaneously present on the touch screen, according to an embodiment of the invention. The timing diagram  1200  illustrates a controller TX signal  1210 , a controller RX signal  1220 , and a stylus TX signal  1230 . The controller TX signal  1210  includes controller TX pulses  1212  and  1214 . The controller RX signal  1220  includes MCS periods  1223  and  1225 , and Stylus sensing (SS) periods  1222 / 1232  and  1224 / 1234 . The stylus TX signal  1230  includes stylus TX signaling periods  1232  and  1234 . 
     Timing diagram  1200  depicts substantially simultaneous tracking the position of a finger and stylus, according to an embodiment of the invention. As described above, the touch screen controller  605  synchronizes the timing of the controller TX signal  1210  to the stylus TX signal  1230  such that controller TX signal is not being transmitted when the stylus TX signal is active. For example, the MCS period  1222  is synchronized to the stylus TX period  1232 . Furthermore, touch screen controller adjusts the controller TX period  1212  and MCS period  1223  to occur after the stylus TX pulse  1232  (and before the anticipated start of the following Stylus TX period  1234 ). As described above, stylus TX sensing may be faster than finger sensing. For example, a stylus TX pulse may occur every 10 ms for a duration of 3 ms. The remaining 7 ms are available to the touch screen controller to perform finger sensing/tracking. 
     When the stylus is removed from the touch screen, if the finger(s) remain in proximity with the touch screen, then the system  600  will continue finger tracking as described above in conjunction with  FIG. 13 . In an embodiment, the stylus may cease sending stylus TX signals when the stylus tip sensors no longer detect contact with the touch screen, after a predetermined timeout period, after a hand is no longer detected on the stylus, or when the stylus is turned off, as previously described. When neither finger nor stylus remains in contact with the touch screen, the system  600  begins executing WFT and WFS sensing at a reduced duty cycle, as previously described. 
       FIG. 13  is a flow chart  1300  of one embodiment of a method of detecting a stylus  680  on a sense array  400 . At block  1310 , a stylus locally generates a stylus TX signal  677 . In an embodiment, the stylus TX signal is generated by a waveform generator in the TX drive circuit  610 . The stylus TX signal  677  may be modulated with additional data including force data, button data, power supply data, etc., as previously described. 
     The stylus  680  capacitively couples the stylus TX signal  677  to the sense array  400  (block  1320 ). A touch screen controller  605  synchronizes the sense array  400  to the stylus TX signal  677  (block  1330 ). The touch screen controller  605  tracks the position of the stylus  680  on the sense array  400  (block  1340 ). In an embodiment, the touch screen controller  605  is configured to substantially simultaneously track the position of the stylus  680  and another touch object (e.g., finger, palm, etc.). 
     Embodiments of the present invention, described herein, include various operations. These operations may be performed by hardware components, software, firmware, or a combination thereof. As used herein, the term “coupled to” may mean coupled directly or indirectly through one or more intervening components. Any of the signals provided over various buses described herein may be time multiplexed with other signals and provided over one or more common buses. Additionally, the interconnection between circuit components or blocks may be shown as buses or as single signal lines. Each of the buses may alternatively be one or more single signal lines and each of the single signal lines may alternatively be buses. 
     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. The computer-readable transmission medium includes, but is not limited to, electrical, optical, acoustical, or other form of propagated signal (e.g., carrier waves, infrared signals, digital signals, or the like), or another type of medium suitable for transmitting electronic instructions. 
     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. 
     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. 
     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.