Abstract:
System and method for optimizing the consumption of power while maintaining performance in capacitive sensor arrays. A limited sensing area is used to improve the update rate and sensitivity of a row/column array of capacitive sensors. According to one embodiment, a method is provided for scanning a plurality of capacitive sensors by: detecting a stimulus in the field of capacitive sensors, scanning the field of capacitive sensors to determine the position of the stimulus. Once the position of the stimulus is determined, a subsection of the field comprising window corresponding to the position of the stimulus remains activated while the remaining sensors in the field are deactivated.

Description:
RELATED U.S. APPLICATIONS 
       [0001]    This application claims the benefit of and priority to copending provisional patent application Ser. No. 60/947,895, Attorney Docket Number CYPR-CD2007184CSP, entitled “Method For Improving Scan Time and Sensitivity in an Array of Capacitive Sensors,” with filing date Jul. 3, 2007, and hereby incorporated by reference in its entirety. 
     
    
     FIELD OF INVENTION 
       [0002]    The present application relates to sensor user interface systems such as capacitive sensor arrays. More particularly, the present application relates to methods and systems for capturing signals received from sensors and sensor arrays 
       BACKGROUND 
       [0003]    Sensors are devices that respond to a stimulus and produce a signal indicative of the stimulus&#39;s magnitude, relative position, or other characteristic attributable to the stimulus. The stimulus may be any physical quantity or parameter which can affect a sensor. An array of sensors is a collection of individual sensors positioned at discrete locations to form a relative field of perception. 
         [0004]    One type of sensor array is a capacitive sensor array. A capacitive sensor is used to detect the presence and/or absence of a conductive object, although direct contact between the conductive object and sensing element is not necessarily required. Capacitive sensors are typically constructed of a conductive pad, the surrounding ground, and its connection to a controller. In most applications, the conductive pad is a large copper footprint and the surrounding ground is a poured fill. A native (parasitic) capacitance exists between these two objects. When a third conductive object, a stimulus—such as a human finger—is brought into proximity with the sensor, the capacitance of the system is increased by the capacitance of the stimulus. Capacitive sensors are generally resistant to environmental factors, such as water, temperature and humidity, and may be used with a variety of overlay materials and thicknesses. 
         [0005]    A capacitive sensor array typically employs a number of discrete capacitive sensors distributed over a region of the array which may be arranged in a pattern forming a grid. A grid of sensors may comprise a plurality of capacitive sensors which may be individually addressable, addressable in subsections of the grid, or in their entirety. Addressing specific sensors may be accomplished using multiplexers coupled to the sensor array according to data or select signals on multiplexer select lines to determine the individual sensors to be “driven” or “sampled.” 
         [0006]    A sensor is driven by exciting the sensor or energizing the sensor so as to produce a measurement of the stimulus at the sensor. By sampling a sensor, an output signal from the sensor is read to detect the sensor response to the stimulus. Thus, it is possible to selectively measure a signal representative of the sensor capacitance from a given capacitive sensor element located at a particular column and row of the capacitive array. Multiplexers may be used to determine the particular row and column from which a measurement is desired. These grids may be adapted to form interfaces of various sizes and shapes. For example, rectangular touch pads are common in PDAs and mobile handsets. Other embodiments include linear and radial interfaces. 
         [0007]    Applications in which such sensor arrays are useful include touch pads and distributed sensors that provide an indication of the location and magnitude of a force or a pressure applied to a region of interest. These applications may be used in user interfaces of computing devices, such as notebook computers, personal data assistants (PDAs), and mobile handsets. Other applications in which sensor arrays have been incorporated include kitchen appliances, exercise equipment and other consumer electronics. 
         [0008]    As consumer electronic devices continue to reduce in size, so too, do their user interfaces. A smaller capacitive sensor user interface typically means smaller individual capacitive sensors within the user interface. Generally, shrinking the size of a capacitive sensor adversely affects its sensitivity, resulting in a detrimental effect on the user experience (such as slower response time, reduced accuracy). Decreased sensitivity due to shrinking sensor size can be partially compensated by increasing the sampling time of a particular capacitive sensor. However, increasing the sampling time for each capacitive sensor within an array of capacitive sensors reduces the response time of the user interface while simultaneously increasing the rate of power consumption. For mobile devices which have a limited power source (e.g. battery-powered devices), this can contribute to a reduced user experience. 
       SUMMARY 
       [0009]    Conventional implementations of capacitive sensing arrays scan all rows and columns during an active mode to determine the position of stimulus. However, this method wastes power since sensors which are not relevant to the position of the stimulus are scanned as well. Accordingly, scanning these sensors is an inefficient use of time (the time used to scan), and power. Embodiments of the present invention are directed to using a limited sensing area to improve the update rate and sensitivity of an array of capacitive sensors. 
         [0010]    In accordance with one embodiment, scanning includes three modes. The first mode is used when no finger is present and all sensors are scanned at a slower rate, or a subsection of sensors are scanned at the normal rate. This is a coarse scan over the entire row/column array. The second state has all of the sensors scanned at the maximums rate. This is to determine where the finger is on the array and develop the background number of sensors to scan in the third, power saving mode. The third mode scans only those sensors that were determined to be active in the second mode plus some additional sensors in all directions, thus creating a “scan halo.” This third mode is maintained until the finger is released. 
         [0011]    In one embodiment, a method for scanning a plurality of capacitive sensors is provided. The method includes: detecting a stimulus in a field of capacitive sensors, scanning the field of capacitive sensors to determine the position of the stimulus, and scanning a window of sensors corresponding to the position of the stimulus within the field. 
         [0012]    Another embodiment provides a method for scanning a capacitive sensor array. The method includes: detecting the presence of a finger in the capacitive sensor array, determining the location of the finger in said capacitive sensor array during a first mode, forming a halo of capacitive sensors corresponding to the location of the finger, scanning the halo of capacitive sensors during a second mode, and modifying the halo of capacitive sensors according to a displacement of said finger during said second mode. 
         [0013]    Still another embodiment provides a system for scanning for a position of a stimulus in a field of capacitive sensors, the system. The method includes: a controller; and an array of capacitive sensors. When the presence of a stimulus is detected, the capacitive sensors are scanned at a maximum rate to determine the position of the stimulus during a first state, and a subsection of capacitive sensors comprising the position of the stimulus along with a plurality of capacitive sensors corresponding to a possible motion of the stimulus are scanned during a second state. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0014]    The accompanying drawings, which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention: 
           [0015]      FIG. 1  shows block diagram of an exemplary capacitive sensor array, in accordance with one embodiment of the present invention 
           [0016]      FIG. 2  shows an exemplary sensor circuit, in accordance with one embodiment of the present invention. 
           [0017]      FIG. 3  shows the exemplary sensor circuit of  FIG. 2  with an equivalent resistance, in accordance with an embodiment of the present invention. 
           [0018]      FIG. 4  shows another exemplary sensor circuit, in accordance with one embodiment of the present invention. 
           [0019]      FIG. 5  shows the exemplary sensor circuit of  FIG. 4  with an equivalent resistance, in accordance with an embodiment of the present invention. 
           [0020]      FIG. 6  depicts a flowchart of a method for scanning a plurality of capacitive sensors, in accordance with one embodiment. 
           [0021]      FIG. 7  depicts a flowchart of a method for scanning a capacitive sensor array, in accordance with one embodiment. 
           [0022]      FIG. 8  depicts an illustration of an exemplary capacitive sensor array exhibiting behavior of sensors during a power saving mode corresponding to the presence of a detected stimulus, in accordance with one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Reference will now be made in detail to several embodiments. While the subject matter will be described in conjunction with the alternative embodiments, it will be understood that they are not intended to limit the claimed subject matter to these embodiments. On the contrary, the claimed subject matter is intended to cover alternative, modifications, and equivalents, which may be included within the spirit and scope of the claimed subject matter as defined by the appended claims. 
         [0024]    Furthermore, in the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the claimed subject matter. However, it will be recognized by one skilled in the art that embodiments may be practiced without these specific details or with equivalents thereof. In other instances, well-known methods, procedures, and components, have not been described in detail as not to unnecessarily obscure aspects and features of the subject matter. 
         [0025]    Portions of the detailed description that follows are presented and discussed in terms of a method. Although steps and sequencing thereof are disclosed in a figure herein (e.g.,  FIG. 5 ) describing the operations of this method, such steps and sequencing are exemplary. Embodiments are well suited to performing various other steps or variations of the steps recited in the flowchart of the figure herein, and in a sequence other than that depicted and described herein. 
         [0026]      FIG. 1  shows block diagram of an exemplary capacitive sensor array, in accordance with one embodiment of the present invention. Capacitive sensor array  100  may provide a user interface (e.g., touchpad, track pad, touch screen, and the like) for a variety of devices including, but not limited to, servers, desktop computers, laptops, tablet PCs, mobile devices, music devices, video devices, cellular telephones, and smartphones. Capacitive sensor array  100  may include a plurality of sensor devices in a row and column configuration. Sensor circuit  102  illustrates an exemplary sensor device in capacitive sensor array  100 . Capacitive sensor array  100  may be configured in a variety of ways including, but not limited to, a square, a rectangle, a circle, or a ring. Connections  104  facilitates coupling of capacitive sensor array  100  to a reporting or data processing resources for reporting to a coupled device (e.g., computing device). Another possible structure for a capacitive sensing array is described in US Patent Application 2007/0229470. 
         [0027]    In one embodiment, capacitive sensor array  100  is made of a material having an impedance which impacts signals received from sensor circuits remote from connections  104 . For example, signals from sensor circuits in the upper left of capacitive sensor array  100  may be reduced or impacted by serial impedance as the signal travels to connections  104 . This is especially true for high impedance capacitive sensors. It is appreciated that as the distance between connections  104  and a sensor circuit of capacitive sensor circuit array  100  increases the impact of the impedance of capacitive sensor circuit array  100  increases. 
       Exemplary Circuits 
       [0028]      FIGS. 2-5  illustrate example components used by various embodiments of the present invention. Although specific components are disclosed in circuits  200 ,  300 ,  400 , and  500  it should be appreciated that such components are examples. That is, embodiments of the present invention are well suited to having various other components or variations of the components recited in systems  200 ,  300 ,  400 , and  500 . It is appreciated that the components in systems  200 ,  300 ,  400 , and  500  may operate with other components than those presented, and that not all of the components of systems  200 ,  300 ,  400 , and  500  may be required to achieve the goals of systems  200 ,  300 ,  400 , and  500 . 
         [0029]      FIG. 2  shows an exemplary sensor circuit  200 , in accordance with one embodiment of the present invention. Sensor circuit  200  includes Vdd signal  202 , current source  204 , comparator  208 , timer  210 , data processing module  212 , oscillator  214 , reference voltage  226 , external modification capacitor  216 , ground signal  224 , sensor capacitor  222 , switch  220  and switch  218 . Current source  204  may be a current DAC (Digital to Analog converter). Circuits of the type shown in  FIG. 2  are described in more detail in U.S. Pat. Nos. 7,307,485 and 7,375,535. 
         [0030]    In one embodiment, circuit  200  may operate in three phases. In a first phase, switch  218  alternatively couples current source  204  to sensor capacitor  222  and current source  204  charges or settles external modification capacitor  216  to a start voltage, Vstart. In one embodiment, the start voltage may be governed the voltage current function expressed by: 
         [0000]    
       
         
           
             
               V 
               Start 
             
             = 
             
               
                 1 
                 
                   f 
                   · 
                   
                     C 
                     x 
                   
                 
               
               · 
               iDAC 
             
           
         
       
     
         [0000]    Where f is frequency of the switching of switch  218 , Cx is the capacitance of the sensor capacitor  222 , and iDAC is the current of current source  204 . 
         [0031]    It is appreciated that the capacitance of sensor capacitor  222  varies with the presence of objects (e.g., a finger). For example, the presence of a finger may increase the capacitance and thereby result in a lower starting voltage. Conversely, a higher starting voltage may result from no finger being present. 
         [0032]    In a second phase, the sensor capacitor  222  is decoupled from current source  204  by switch  218  and sensor capacitor  222  is discharged by coupling to ground signal  224  via switch  220 . External modification capacitor  216  may be charged by current source  204  until it increases to reference voltage  226  and comparator  208  is tripped which disables timer  210 . In one embodiment, voltage on the external modification capacitor  216  is reduced through a low pass filter in series with external modification capacitor  216  to the input of comparator  208 . Scan time is measured during this second phase. 
         [0033]    In one embodiment, timer  210  is a counter (e.g., 16-bit). The raw number of counts on timer  210  may be used to determine if a finger is present on sensor capacitor  222 . The raw counts are taken after each measurement sequence (e.g., after each charge of sensor capacitor  222  and tripping of comparator  208 ) and then compared to a stored baseline number of counts with no finger present on sensor capacitor  222 . If the difference between the raw counts and the baseline counts exceeds a threshold, then sensor activation is detected. The value of the counter may measure the elapsed time to get to the threshold or reference voltage  226  and can then be used to determine what the start voltage was, and therefore the capacitance value. 
         [0034]    For example, when no finger present 100 cycles may be required to bring the voltage across external modification capacitor  216  to reference voltage  226 . When a finger is present, 105 cycles may be required to bring the voltage across external modification capacitor  216  to reference voltage  226 . If there is a threshold of three cycles to indicate the presence of an object, as long as the number of the change in cycles is greater than three, the sensor may be determined to be active. It is appreciated that a difference threshold of larger than zero prevents noise or other interference from appearing as an active sensor. 
         [0035]    The time (or count) measured by timer  210  may be used by data processing module  212  to process the data and make decisions based on the capacitive inputs (e.g., sensors triggered by presence of a finger). Lower starting voltages (e.g., when a finger is present) leads to longer charge times as the current from the current source  204  flows into the external modification capacitor  216  and increases the voltage at the same rate. If the start voltage is low, the time or count measured by timer  210  will be relatively large because current source  204  will have to provide more charge to external modification capacitor  216  to reach reference voltage  226 . If the start voltage is relatively high (e.g., no finger present), the time or count measured by time  210  is low as current source  204  provides less current to external modification capacitor  216  to reach reference voltage  226 . 
         [0036]    In a third phase, the sensor scan is completed and current source  204  is turned off. During the third phase, the time or count from timer  210  may be processed and stored. Voltage on the external modification capacitor  216  decreases as charge dissipates by leakage currents until the next scan begins. In one embodiment, the amount of time that the voltage decreases is strictly dependent upon the firmware between each scan and the CPU (Central Processing Unit) clock speed. It is appreciated that the next scan may then start with the first phase on the same sensor circuit or another sensor circuit (e.g., an adjacent or active sensor circuit). 
         [0037]    In another embodiment, comparator  208  may be replaced with an analog-to-digital converter (ADC). Charge from the sensor capacitor  222  is transferred to external modification capacitor  216  acting as a filter capacitor for a prescribed number of cycles. After the prescribed number of cycles is complete, the voltage on external modification capacitor  216  is measured and the output of the ADC is proportional to the size of sensor capacitor  222 . The measured value of the ADC may then be used to determine the presence of an object. 
         [0038]      FIG. 3  shows the exemplary sensor circuit of  FIG. 2  with an equivalent resistance. Sensor circuit  200  includes Vdd signal  202 , current source  204 , comparator  208 , timer  210 , data processing  212 , oscillator  214 , reference voltage  226 , external modification capacitor  216 , ground signal  224 , sensor capacitor  222 , and equivalent resistance  330 . 
         [0039]    In the first phase, a current value for current source  204  may be determined which results in a start voltage across equivalent resistance  330  that is below reference voltage  326 . 
         [0040]    The equivalent resistance  330  may be governed the voltage current function expressed by: 
         [0000]    
       
         
           
             
               R 
               Equivalent 
             
             = 
             
               1 
               
                 
                   f 
                   s 
                 
                 · 
                 
                   C 
                   x 
                 
               
             
           
         
       
     
         [0000]    Where fs is the switching frequency of phases 1 and 2 as described herein, and Cx is the capacitance of sensor capacitor  222 . 
         [0041]    Equivalent resistance  330  is inversely proportional to the capacitance of sensor capacitor  222 . The presence of an object (e.g., finger) on a sensor increases the capacitance of the sensor, which decreases equivalent resistance  330  formed by the switching phases 1 and 2. A decreased equivalent resistance results in a lower starting voltage. In one embodiment, the start voltage may be governed the voltage current function expressed by: 
         [0000]    
       
         
           
             
               V 
               Start 
             
             = 
             
               
                 1 
                 
                   f 
                   · 
                   
                     C 
                     x 
                   
                 
               
               · 
               iDAC 
             
           
         
       
     
         [0042]    Where fs is the switching frequency of phases 1 and 2 as described herein, and Cx is the capacitance of sensor capacitor  222  and iDAC is current of current source  204 . 
         [0043]    A lower starting voltage corresponds to an increased time for current source  204  to charge up external modification capacitor  216 , thereby resulting in a larger time that timer  210  will operate. Data processing module  212  may thus process the increased value from timer  210  to indicate the presence of an object relative to the equivalent resistance  330 . 
         [0044]      FIG. 4  shows another exemplary sensor circuit, in accordance with one embodiment of the present invention. Sensor circuit  400  includes Vdd  402 , pseudo random generator  404 , oscillator  406 , frequency modifier  408 , pulse-width modulator  410 , counter  412 , data processing module  414 , and gate  416 , latch  418 , comparator  420 , reference voltage  422 , discharge resistor  426 , ground signal  424 , external modification capacitor  428 , sensor capacitor  430 , switch  432 , switch  434 , and switch  436 . 
         [0045]    Switches  432  and  434  are controlled by pseudo random generator  404 , which modulates the voltage across external modification capacitor  428  about reference voltage  422  in charge up and charge down steps. Pseudo random generator  404  reduces the electromagnetic inference susceptibility and radiated emissions of capacitive sensing circuits. In one embodiment, external modification capacitor  428  is larger than sensor capacitor  430 . 
         [0046]    In one embodiment, switch  434  is used to charge sensor capacitor  430 . The capacitance of sensor capacitor  430  varies with the presence of an object (e.g., a finger). After the charging of sensor capacitor  430 , switch  434  is decoupled and switch  432  is coupled thereby allowing the charge of sensor capacitor  430  to flow to external modification capacitor  428 . 
         [0047]    As the charge in external modification capacitor  428  increases, so does the voltage across external modification capacitor  428 . The voltage across external modification capacitor  428  may be an input to comparator  420 . When the input to comparator  420  reaches the threshold voltage or reference voltage  422 , discharge resistor  426  is connected and charge is bled off of external modification capacitor  428  at a rate determined by the starting voltage across the external modification capacitor  428  and the value of discharge resistor  426 . As the voltage across external modification capacitor  428  decreases and the voltage passes reference voltage  422 , discharge resistor  426  is disconnected from ground  424  via switch  436 . 
         [0048]    The charge/discharge cycle of the external modification capacitor  428  is manifested as a bit stream on the output of comparator  420 . The bitstream of comparator  420  is ‘ANDed’ with pulse-width modulator  410  via and gate  416  to enable timer  412 . Pulse width modulator  410  sets the timeframe or measurement window in which the bit-stream enables and disables timer  412 . The capacitance measurement and therefore the presence of an object is a matter of comparing the bit-stream of the comparator to the known, baseline value. 
         [0049]    The value of reference voltage  422  affects the baseline level of counts or time measured by timer  412  from a sensor when no finger is on the sensor. This voltage on an external modification capacitor  428  may reach the reference voltage before comparator  420  trips, so the value of reference voltage  422  affects the amount of time that it takes external modification capacitor  428  to charge to reference voltage  422 , decreasing the density of the bit-stream during a scan. 
         [0050]    The output of timer  412  is used for processing the level of capacitance change and determining the sensor activation state. The duration of these steps is compared relative to each other by looking at the comparator bit-stream density. If the density of the bit-stream is relatively high, the sensor is read as “on”. The bit-stream output of comparator  420  is synchronized with system clock via latch  418 . 
         [0051]      FIG. 5  shows the exemplary sensor circuit of  FIG. 4  with an equivalent resistance. Sensor circuit  500  includes Vdd  402 , oscillator  406 , frequency modifier  408 , pulse-width modulator  410 , counter  412 , data processing module  414 , and gate  416 , latch  418 , comparator  420 , reference voltage  422 , discharge resistor  426 , ground signal  424 , external modification capacitor  428 , switch  436 , and equivalent resistance  540 . 
         [0052]    Sensor capacitor  430  is replaced with equivalent resistance  540 . Equivalent resistance  540  is inversely proportional to the capacitance of sensor capacitor  430 . As such, the presence of an object (e.g., a finger) will result in an increase in capacitance and a corresponding reduction in the resistance of equivalent resistance  540 . The reduction of equivalent resistance  540  thereby allows more current to charge external modification capacitor  428  and thereby allowing the voltage across external modification capacitor  428  to reach reference voltage  422  relatively faster. 
       Scanning a Plurality of Capacitive Sensors 
       [0053]    With reference now to  FIG. 6 , a flowchart  600  of a method for scanning a plurality of capacitive sensors is depicted, in accordance with one embodiment. Steps  601 - 605  describe exemplary steps comprising the process  600  in accordance with the various embodiments herein described. 
         [0054]    At step  601 , a stimulus is detected in a field of capacitive sensors. The stimulus may be any physical stimulant. In many contemporary applications, the stimulus is typically a physical object, such as a stylus or human finger. The field of capacitive sensors may be implemented according to various shapes and applications. For example, a field of capacitive sensors implemented as a touch pad comprising an interface of a computerized device may be implemented as a grid of capacitive sensors, arranged in intersecting columns and rows spanning the area encompassed by the touch pad. In one embodiment, the capacitive sensors comprising the field of capacitive sensors are implemented as circuits as herein described. In one embodiment, at  601  all sensors are scanned at a slow rate or a subsection of sensors are scanned at a normal rate. This is a coarse scan. 
         [0055]    At step  603 , the field of capacitive sensors is scanned (e.g., activated and sampled) to determine the position of the stimulus within the field. The position of the stimulus within the field may be determined according to a centroid positioning algorithm. In one embodiment, the entire field of capacitive sensors is scanned at a high rate or its maximum rate during  603 , to achieve a high resolution (accuracy). In a further embodiment, a window of sensors to be scanned in step  605  is also developed during step  603  by determining the precise position of the stimulus. 
         [0056]    At step  605 , once the position of the stimulus within the field of capacitive sensors is determined (in step  603 ), a window of sensors is scanned while the other sensors in the field outside the window are not scanned. Step  605  persists for as long as the stimulus is detected in its current (original) position. In one embodiment, the window of sensors includes the sensors directly corresponding to the position of the stimulus (e.g., the sensors positioned for sensing the area in the field of sensors currently occupied by the stimulus). These are the active sensors whose output is used to determine the position of the stimulus 
         [0057]    In one embodiment, the window of sensors also comprises one or more sensors proximate to the position of the stimulus in the field of capacitive sensors but which may not be deemed active in  603 . While being scanned, these sensors provide the window the ability to detect a motion of the stimulus within the corresponding proximate sensors. For example, in one embodiment, the window of sensors comprises an additional, adjacent sensor in each of four directions (above, below, to the right and to the left) relative to the position of the active sensors. 
         [0058]    If the stimulus were displaced from its original position to a position corresponding to one or more of the additional scanned sensors in the window, the already-scanned sensor(s) would become active and would be able to detect the presence (and thus, the motion) of the stimulus from its original position. In a further embodiment, the window would be adjusted to account for the new position (and subsequent positions) of the stimulus. Adjustment may comprise, for example, scanning additional sensors proximate to the new position of the stimulus and not scanning the sensors corresponding to the original position of the stimulus but no longer corresponding to the new position of the stimulus. 
         [0059]    In a further embodiment, an entire arrangement of sensors (e.g., a row or column) in one or more directions relative to the position of the stimulus is also scanned. In alternate embodiments, the window may include an additional sensor (or arrangement of sensors) in other denominations. In still further embodiments, the additional sensors comprise sensors which may not be immediately adjacent to the detected position of the stimulus. Instead, the additional sensors may comprise sensors at some other discrete location calculated (or designed) to provoke or entice a motion. 
         [0060]    In one embodiment, a window of scanned sensors may comprise only the sensors directly corresponding to the position of the stimulus. When the sensors comprising the perimeter of the stimulus position detect a change (e.g., the stimulus achieves a motion), the window may anticipate the motion of the stimulus by referencing the detected change in position. For instance, a stimulus which observes a motion upwards would detect a change in the one or more sensors comprising the lower perimeter of the original position (e.g., the presence is no longer detected). According to this embodiment, the window may adjust appropriately by scanning the sensors directly opposite to the sensors comprising the lower perimeter of the original position (e.g., sensors “above” the upper perimeter of the original position) and not scanning the sensors comprising the (former) lower perimeter of the original position. 
         [0061]    By performing the scan in this fashion, embodiments of the present invention provide a fast scan time with greater resolution and sensitivity. The scan also provides for lower power consumption due to the faster scan time and more sleep operation. 
       Scanning a Capacitive Sensor Array 
       [0062]    With reference now to  FIG. 7 , a flowchart  700  of a method for scanning a capacitive sensor array is depicted, in accordance with one embodiment. Steps  701 - 709  describe exemplary steps comprising the process  700  in accordance with the various embodiments herein described. 
         [0063]    At step  701 , the presence of a finger (e.g., a physical stimulant such as a human finger) is detected in the capacitive sensor array. The capacitive sensor array may be implemented according to various shapes and applications. For example, a capacitive sensor array may be employed as a two dimensional array of intersecting columns and rows forming a rectangular grid. Other embodiments include a linear capacitive sensor array (that is, a two dimensional array with but one row or column), or a radial capacitive sensor array. In one embodiment, each capacitive sensor comprising the array of capacitive sensors is implemented as a circuit as described herein. 
         [0064]    In one embodiment, the presence of a finger is detected at  701  in the capacitive sensor array during a preliminary, power-saving state. During this state, the sensors comprising the capacitive sensor array are all scanned at a relatively slow rate, or a subsection of the sensors are scanned at a standard rate. The reduction in rate and/or coverage adversely affects the sensitivity of the corresponding sensors (and thus, the sensor array as a whole). Accordingly, the presence of a finger is detected with coarse position accuracy, although further information (e.g., specific location, magnitude) may be unavailable during this state. 
         [0065]    At step  703 , the location of the finger in the capacitive sensor array is determined during a first mode. The location of the finger within the field may be determined according to a centroid positioning algorithm based on the active sensors. During the first mode, the entire capacitive sensor array is scanned at its maximum rate, to achieve the highest available resolution, and to determine the location of the finger with the greatest degree of accuracy. An active sensor means a sensor whose output signal contributes to a position detecting computation, e.g., that contributes to the centroid computation. 
         [0066]    At step  705 , a halo of sensors encompassing the location of the finger (as determined in step  703 ) is formed by activating the sensors corresponding to the location of the finger. In one embodiment, the halo includes an additional perimeter of one or more capacitive sensors surrounding the active sensors corresponding to the location of the finger. In a further embodiment, where available, the perimeter comprises a ring of sensors of at least one sensor in width, encapsulating the finger&#39;s location. Accordingly, the ring of sensors may be abridged by, for example, the boundaries comprising the capacitive sensor array (e.g., the edge of a touchpad). In alternate embodiments, the ring of sensors may encapsulate other sensors in addition to those corresponding to the location of the finger 
         [0067]    At step  707 , the capacitive sensors in the halo of sensors (formed in step  705 ) are scanned during a second mode. All other sensors not within the halo of sensors are no longer being scanned. Step  707  persists for as long as the finger is detected in this current (original) location. The halo of sensors includes the active sensors directly corresponding to the location determined of the finger and other nearby sensors (e.g., the sensors located in, or directed to sensing the area in the field of sensors currently occupied by the finger). While scanned, these sensors in the halo provide the halo the ability detect a contiguous motion of the finger within the corresponding proximate sensors. 
         [0068]    By limiting the scanning of sensors to a subsection of the array (e.g., the halo), the rate at which power is consumed while the finger&#39;s presence is detected is greatly reduced. In one embodiment, the halo of sensors is scanned at a normal or maximum rate. In this embodiment, the sensitivity and resolution of the sensor array is largely maintained (equivalent for sensors within the halo), while the rate of power consumption is reduced by not scanning the sensors outside of the halo. In an alternate embodiment, the halo of sensors may be scanned at a rate less than that of normal (or maximum). According to this embodiment, the sensitivity (and resolution) of the capacitive sensor array may be reduced (even within the halo), but further gains may be achieved in reducing the rate of power consumption. 
         [0069]    Finally, at step  709 , the halo of capacitive sensors is modified according to a displacement of the finger during the second mode. In one embodiment, the halo of sensors comprises an additional, adjacent sensor in each of four directions (above, below, to the right and to the left) relative to the location of the finger. If the finger were displaced from its original location to a location corresponding to one or more of the additional sensors in the halo, the already-scanned sensor would be able to detect the presence (and thus, the motion) of the finger from its original location. In a further embodiment, the halo would be adjusted to account for the new location (and subsequent locations) of the finger. Adjustment of the halo may comprise, for example, scanning additional sensors proximate to the new location of the finger and not scanning the sensors corresponding to the original location of the finger but no longer corresponding to the new location of the finger. 
         [0070]    In a further embodiment, an entire arrangement of sensors (e.g., a row or column) in one or more directions relative to the location of the finger is also scanned. In still further embodiments, the additional sensors comprise sensors which may not be immediately adjacent to the detected location of the finger. Instead, the additional sensors may comprise sensors at some other discrete location calculated (or designed) to provoke or entice a motion. Alternate embodiments may employ a halo comprising only the sensors directly corresponding to the location of the finger, wherein the halo may anticipate the motion of the finger by referencing the detected change in the finger&#39;s presence at one or more sensors in the halo. 
       Sensor Behavior Corresponding to Stimulus in Power Saving Mode 
       [0071]    With reference now to  FIG. 8 , an illustration of an exemplary capacitive sensor array  800  exhibiting behavior of sensors during a power saving mode corresponding to the presence of a detected stimulus is depicted, in accordance with one embodiment. As depicted,  FIG. 8  includes a stimulus  801 , activated sensor arrangements (e.g., activated rows  803 , activated columns  805 ), and halo portions (e.g., halo sensor arrangements  807 ,  809 ,  811 ,  813 ). 
         [0072]    As presented in  FIG. 8 , capacitive sensor array  800  is a grid of capacitive sensors (depicted as diamond tiles). The position of stimulus  801  is located in roughly the center of the lower right quadrant of capacitive sensor array  800 , and occupies, in whole or in part, a circular area of roughly sixteen sensors, comprising four rows and four columns of sensors. The position of stimulus  801  may correspond, for example, to the position of a fingertip proximate to the capacitive sensor array  800 . 
         [0073]    As depicted, capacitive sensor array  800  also includes activated sensor arrangements, specifically, a plurality of activated rows of sensors  803  (depicted as black tiles) and a plurality of activated columns of sensors  805  (presented as grey tiles) corresponding to the presence of the detected stimulus  801 . As shown, the presence of a detected stimulus  801  activates an entire arrangement of sensors (e.g., every sensor in a row or column) associated with the sensors corresponding to the position of the detected stimulus. 
         [0074]    In alternate embodiments, the presence of a detected stimulus may activate only one or more immediate sensors, or a subsection of an arrangement of sensors. As presented, the activated sensors comprising activated rows  803  would detect a motion (e.g., displacement) of stimulus  801  along the horizontal axis relative to the current position of the stimulus. Likewise, the activated sensors comprising activated columns  805  would detect a motion of stimulus  801  along the vertical axis relative to the current position of the stimulus. 
         [0075]    During a power saving mode, not every sensor will be activated. According to various embodiments, a window or halo of sensors may be scanned in addition to the activated sensor arrangements (e.g., activated rows  803  and activated columns  805 ). As depicted, capacitive sensor array  800  employs a window (or halo) of a full two sensors in width. For example, halo sensor arrangement  807  includes the next two rows immediately above the current detected position of stimulus  801 . Halo sensor arrangement  809  includes the next two rows immediately below the current detected position of stimulus  801 . Meanwhile, halo sensor arrangement  811  includes the next two columns immediately to the left of the current detected position of stimulus  801 . Halo sensor arrangement  813  includes the next two columns immediately to the right of the current detected position of stimulus  801 . According to this configuration, a displacement of stimulus  801  in the additional sensor arrangements (e.g., halo sensor arrangement  807 ,  809 ,  811  and  813 ) would still be detected, even if outside the original sensor arrangements activated by the presence of the stimulus. 
         [0076]    Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.