Abstract:
Systems and methods for determining a user&#39;s touch in a capacitive touch sensor system is provided, including performing a series of potential touch detection tests for a plurality of sensors until a potential touch is detected and measuring a test frequency for one of the sensors, such that a potential touch may be detected when the measured test frequency deviates from a previously measured test frequency for the same sensor. After detecting a potential touch, the method may additionally include performing a series of baseline comparison tests for each of the sensors, for example, measuring a current frequency for one of the sensors, comparing the current frequency to a baseline frequency, and assigning a deviation value based on the comparison the current frequency and the baseline frequency. The method may identify the sensor with the largest deviation value as a touched sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/224,975 filed on Jul. 13, 2009, entitled “CAPACITIVE TOUCH SYSTEM WITH NOISE IMMUNITY”, which is incorporated herein in its entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to capacitive touch sensor systems, and more particularly, to an improved capacitive touch sensor system that uses conducted noise to detect a user&#39;s touch so that the user&#39;s touch can be detected in the presence or absence of conducted noise. 
       BACKGROUND 
       [0003]    Capacitive touch sensors are used as a user interface to electronic equipment, e.g., computers, mobile phones, personal portable media players, calculators, telephones, cash registers, gasoline pumps, etc. In some applications, opaque touch sensors provide soft key functionality. In other applications, transparent touch sensors overlay a display to allow the user to interact, via touch, with objects on the display. Such objects may be in the form of soft keys, menus, and other objects on the display. The capacitive touch sensors are activated (controls a signal indicating activation) by a change in capacitance of the capacitive touch sensor when an object, e.g., a user&#39;s finger tip, causes the capacitance thereof to change. 
         [0004]    One way to detect changes in capacitance on a touch sensor utilizes what is known in the art as a relaxation oscillator. The relaxation oscillator drives an oscillating electric signal onto the conductive elements (e.g., sensors) of the touch sensor while a sensing circuit monitors the frequency of oscillation of the driven elements. When an object contacts the touch screen, the resulting change of capacitance causes the frequency of oscillation of the driven elements to change, indicating a touched condition. 
         [0005]    One problem associated with using a relaxation oscillator-based capacitive touch sensor is that conducted (common mode) noise present on the power supply connections of a capacitive touch sensor can cause interference, false, triggering, and/or out of range values due to the noise overdriving the capacitive touch relaxation oscillator. When this occurs, frequency shifts may be exaggerated, sensitivity, may be significantly increased, and noise at the unpressed frequency may not be detectable as a frequency shift (e.g., blind spots). Current relaxation oscillator-based capacitive touch sensor systems employ measures to either reduce the conducted noise (e.g., filtering) or limit the system&#39;s susceptibility to the conductive noise (e.g., overdriving). However, these approaches have drawbacks. For example, they may require additional or more expensive circuit components. 
       SUMMARY 
       [0006]    In accordance with the teachings of the present disclosure, the disadvantages and problems associated with current approaches to handling conducted noise in a relaxation oscillator-based touch sensor have been substantially reduced or eliminated. More specifically, a system and method are employed wherein the system detects touch by detecting the disruptive actions of conducted noise. Instead of filtering or overdriving to compensate for conducted noise, the system uses the conducted noise to detect touch. 
         [0007]    In accordance with one embodiment of the present disclosure, a method for determining a user&#39;s touch in a capacitive touch sensor system having a plurality of sensors and a relaxation oscillator is provided. The method may include performing a series of potentials touch detection tests for the plurality of sensors until a potential touch is detected. Each potential touch detection test may involve measuring a test frequency for one of the sensors, such that a potential touch is detected by detecting a deviation between the measured test frequency and a previously measured test frequency for the same sensor. In response to detecting a potential touch, the method may additionally include performing a series of baseline comparison tests for each of the sensors. Each baseline comparison test may involve measuring a current frequency for a particular one of the sensors, comparing the current frequency to a baseline frequency for the particular sensor, and assigning to the particular sensor a deviation value based on the comparison of its current frequency with its baseline frequency. The method may further include determining whether any one of the sensors has been assigned a largest deviation value, and if so, identifying the sensor with the largest deviation value as a touched sensor. 
         [0008]    In accordance with another embodiment of the present disclosure, a capacitive touch sensor system may include a touch sensor having a plurality of sensors, a touch controller communicatively coupled to the touch sensor, and a relaxation oscillator circuit as part of the touch controller. The touch controller may be configured to perform a series of potential touch detection tests for the plurality of sensors until a potential touch is detected. Each potential touch detection test may involve measuring a test frequency for one of the sensors, such that a potential touch is detected by detecting a deviation between the measured test frequency and a previously measured test frequency for the same sensor. In response to detecting a potential touch, the touch controller may be further configured to perform a series of baseline comparison tests for each of the sensors. For each baseline comparison test, the touch controller may measure a current frequency for a particular one of the sensors, compare the current frequency to a baseline frequency for the particular sensor, and assign to the particular sensor a deviation value based on the comparison of its current frequency with its baseline frequency. Touch controller may then determine whether any one of the sensors has been assigned a largest deviation value, and if so, identify that sensor as a touched sensor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
           [0010]      FIG. 1  illustrates a block diagram of an example relaxation oscillator-based touch sensor system that uses conducted noise to detect a touch so that the touch can be detected in the presence or absence of conducted noise, in accordance with the present disclosure. 
           [0011]      FIG. 2  illustrates a top view of an example touch sensor in a relaxation oscillator-based touch sensor system, in accordance with the present disclosure. 
           [0012]      FIG. 3  illustrates a partial cross-section, front elevation view of an example touch sensor in a relaxation oscillator-based touch sensor system, in accordance with the present disclosure. 
           [0013]      FIG. 4  illustrates electrical circuits corresponding to an example touch sensor in a relaxation oscillator-based touch sensor system, in accordance with the present disclosure. 
           [0014]      FIG. 5  illustrates an example relaxation oscillator circuit in a relaxation oscillator-based touch sensor system, in accordance with the present disclosure. 
           [0015]      FIG. 6  illustrates an example timing diagram for a relaxation oscillator circuit output in a relaxation oscillator-based touch sensor system, in accordance with the present disclosure. 
           [0016]      FIG. 7  illustrates an example touch controller in a relaxation oscillator-based touch sensor system, in accordance with the present disclosure. 
           [0017]      FIG. 8  illustrates a flow chart of an example method for using conducted noise to detect a touch on a touch sensor in a relaxation oscillator-based sensor system, in accordance with the present disclosure. 
           [0018]      FIG. 9  illustrates an example plot of the percentage change in measured frequency of a conductive element in the presence of conducted noise, in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Preferred embodiments and their advantages over the prior art are best understood by reference to  FIGS. 1-9  below, wherein like numbers are used to indicate like and corresponding parts. 
         [0020]      FIG. 1  illustrates a block diagram of an example relaxation oscillator-based touch sensor system  100  that uses conducted noise to detect a touch so that the touch can be detected in the presence or absence of conducted noise, in accordance with the present disclosure. As depicted in  FIG. 1 , system  100  may comprise touch sensor  200 , touch controller  400 , and host  600 . 
         [0021]    Touch sensor  200  may generally be operable to receive input via contact with a human finger or other hand held object (e.g., stylus, credit card, etc.). In general, touch sensor  200  is configured to recognize a touch event through a change in capacitance that results from the touch event. Touch sensor  200  may include one or more conductive elements that present a natural capacitance to a ground (or virtual ground) plane within touch sensor  200 . Touch sensor  200  may be of a semi-transparent construction, allowing it to be placed in front of or integrated into a graphic (video) display system. Alternatively, touch sensor  200  may be of an opaque construction (e.g., touch pad used in many current laptop computers). A more detailed description of an example touch sensor  200  according to the present disclosure is provided in the discussion of  FIGS. 2-4  below. 
         [0022]    Touch controller  400  may generally be an electronic system operable to detect, measure, and report touch events on touch sensor  200 . Touch controller  400  may comprise a relaxation oscillator circuit  500  in the form of an electronic circuit that produces a voltage signal that oscillates between two voltage levels. Touch controller  400  may be implemented as discrete electrical components, as a part of an integrated circuit, or some combination of both. A more detailed description of an example touch controller  400  according to the present disclosure is provided in the discussion of  FIGS. 5-7  below. 
         [0023]    Host  600  may generally be a system that receives touch reports from touch controller  400 . Host  600  may be configured to initiate some action based on such touch reports. In one embodiment, host  600  may correspond to a computer such as a server, desktop, laptop, or tablet computer. According to another embodiment, host  600  may correspond to, any of a variety of electronic devices including, for example, a mobile phone or a digital media (e.g., music, video, etc.) player. 
         [0024]    As illustrated in  FIG. 1 , touch sensor  200 , touch controller  400 , and host  600  may be communicatively coupled via connections  101  and  102  to form system  100 . Connections  101  and  102  may be any type of structure suitable for facilitating the communication of electronic signals, data, and/or messages (generally referred to as data). In addition, touch sensor  200 , touch controller  400 , and host  600  may communicate via connections  101  and  102  using any suitable communication protocol. In one embodiment, communication over connections  101  and  102  may be in the form of a custom communication protocol. According to another embodiment, communication over connections  101  and  102  may be according to any of a variety of known protocols/bus architectures. For example, such protocols/architectures may include, but are not limited to, Micro Channel Architecture (MCA) bus, Industry Standard Architecture (ISA) bus, Enhanced ISA (EISA) bus, Peripheral Component Interconnect (PCI) bus, PCI-Express bus, HyperTransport (HT) bus, Universal Serial Bus (USB), Video Electronics Standards Association (VESA) local bus, Internet protocol (IP), other packet-based protocol, small computer system interface (SCSI), Internet SCSI (iSCSI), Serial Attached SCSI (SAS) or any other transport that operates with the SCSI protocol, advanced technology attachment (ATA), serial ATA (SATA), advanced technology attachment packet interface (ATAPI), serial storage architecture (SSA), integrated drive electronics (IDE), and/or any combination thereof. 
         [0025]    While touch sensor  200 , touch controller  400 , and host  600  are depicted as separate blocks in  FIG. 1 , any physical configuration may be provided. For example, in one embodiment touch controller  400  and host  600  may be implemented as a single integrated circuit. In another embodiment, touch controller  400  and touch sensor  200  may be implemented as a standalone device separate from host  600 . In yet another embodiment, touch sensor  200 , touch controller  400 , and host  600  may be implemented as one physical device with connections  101  and  102  as internal connections within the device. For embodiments including more than one physical device corresponding to touch sensor  200 , touch controller  400 , and host  600 , the physical devices may be physically located at the same location or at remote locations. For example, connection  101  may be the internet and host  600  may be a server computer located many miles away from touch sensor  200  and touch controller  400 . 
         [0026]    In operation, touch controller  400  may use relaxation oscillator circuit  500  and other circuitry to continually measure, via connection  102 , the capacitance value of one or more conductive elements within touch sensor  200 . When a user touches touch sensor  200  with a finger or other object, the touch changes the capacitance value at conductive element(s) near the touch location. Touch controller  400  may recognize the changed capacitance and determine that the touch sensor  200  has been touched. In embodiments where touch sensor  200  has more than one conductive element, touch controller  400  may determine the location of the touch or the specific conductive element that was touched. Touch controller  400  may then report the touch touched location to host  600 . Host  600  may initiate some action based in whole or in part on the location of the touch. 
         [0027]      FIG. 2  illustrates a top view of an example touch sensor  200  in a relaxation oscillator-based touch sensor system  100 , in accordance with the present disclosure. According to the depicted embodiment, touch sensor  200  may include dielectrically separated conductive elements X 1 -X 7  and Y 1 -Y 7  arranged in a grid pattern and forming a Cartesian coordinate system (x and y) in which each conductive element represents a different x or y coordinate. According to another embodiment, touch sensor  200  may include conductive elements arranged according to a Polar coordinate system or some other coordinate system. In an embodiment having only one conductive element (e.g., a soft button), no coordinate system is required. 
         [0028]    Each of conductive elements X 1 -X 7  and Y 1 -Y 7  may be electrically connected via traces  202  and  204  to ports  252  and  254 . In the embodiment shown, each conductive element is separately and directly connected to a respective one of ports  252  and  254 . According to another embodiment, traces  202  and  204  may be connected directly or indirectly (e.g., with intervening logic) to more than one of conductive elements X 1 -X 7  and Y 1 -Y 7 . 
         [0029]    Conductive elements X 1 -X 7  and Y 1 -Y 7  may be formed with any suitable conductive medium. In a semi-transparent touch sensor configuration, capacitive elements X 1 -X 7  and Y 1 -Y 7  may be formed with, for example, indium tin oxide (ITO). In an opaque touch sensor configuration, capacitive elements X 1 -X 7  and Y 1 -Y 7  may be formed with, for example, copper. 
         [0030]    Ports  252  and  254  may provide an interface to which the touch controller  400  of  FIG. 1  may be coupled (via connection  102 ). While the disclosed embodiment includes one port  252  corresponding to conductive elements Y 1 -Y 7  and a separate port  254  corresponding to conductive elements X 1 -X 7 , other embodiments may comprise a single port or more than two ports. In these cases, traces  202  and  204  are routed to the desired port(s). 
         [0031]      FIG. 3  illustrates a partial cross-section, front elevation view of an example touch sensor  200  in a relaxation oscillator-based touch sensor system  100 , in accordance with the present disclosure. As depicted, touch sensor  200  may comprise substrate layer  306  onto which conductive elements X 1 -X 3  are formed. Insulating layer  308  may dielectrically separate conductive elements X 1 -X 3  from conductive element Y 1 . Surface layer  310  may be formed on top of conductive element Y 1  and provide the input surface of touch screen  200  (i.e., the surface that the user touches with a finger or other object). In a semi-transparent touch sensor configuration, substrate  306  and surface layer  310  may be formed with, for example, glass or clear plastic (e.g., Plexiglas); and insulating layer  308  may be formed with, for example, a clear adhesive or other semi-transparent materials having good insulating characteristics. In an opaque touch sensor configuration, substrate  306  may be formed with, for example, a fiberglass (FR-4) printed circuit board (PCB) material; insulating layer may be formed with, for example, any suitable adhesive or other material having good insulating characteristics; and surface layer  310  may be formed with, for example, glass or plastic. 
         [0032]    In operation, the touch sensor  200  illustrated in  FIGS. 2 and 3  provide a physical interface through which a user may provide input to touch sensor system  100 . Each conductive element X 1 -X 7  and Y 1 -Y 7  has a natural resistance. Each conductive element X 1 -X 7  and Y 1 -Y 7  also has a natural capacitance to a ground (or virtual ground) plane within touch sensor  200 . Thus, each conductive element X 1 -X 7  and Y 1 -Y 7  may be used to form an RC circuit such as those depicted in  FIG. 4 . For example, circuit  412  of  FIG. 4  may represent an RC circuit corresponding to an untouched, individual conductive element having a natural resistance depicted as resistor  413  and a natural capacitance Cp. 
         [0033]    When a user touches touch sensor  200  with a finger or other object, a second capacitance may be added in parallel to the natural capacitance of the conductive element(s) near the location of touch. This second capacitance is illustrated as capacitance Cf in circuit  414  of  FIG. 4 . Again, resistor  415  of circuit  414  may correspond to the natural resistance of the conductive element, and capacitance Cp may correspond to the natural capacitance of the conductive element. Parallel capacitances Cp and Cf in circuit  414  may be added together to form a total sensor capacitance (Cs), as depicted in circuit  416 . Thus, circuit  416  illustrates an RC circuit that may be formed in the presence of a touch. 
         [0034]    As described more fully below, touch controller  400  of  FIG. 1 , via relaxation oscillator circuit  500 , may repeatedly measure the sensor capacitance Cs of each conductive element X 1 -X 7  and Y 1 -Y 7  to determine if a user has touched touch sensor  200 . In other words, by repeatedly measuring Cs, touch controller  400  may determine that a user has touched touch screen  200  when the value of Cs increases. 
         [0035]      FIG. 5  illustrates an example relaxation oscillator circuit  500  in a relaxation oscillator-based touch sensor system  100 , in accordance with the present disclosure. According to this embodiment, capacitor  432  having a capacitance Cs and resistor  534  correspond to an RC circuit (e.g., circuit  416 ) of an individual conductive element X 1 -X 7  or Y 1 -Y 7 . This RC circuit may be connected to comparators  520  and  522  and SR latch  524 . As depicted, voltage Vcs at node  530  may correspond to the voltage across sensor capacitor  532 . Voltage Vcs at node  530  may be used as the inverting input to both comparators  520  and  522 . The non-inverting input of comparator  520  may be connected to voltage V 2 , and the non-inverting input of comparator  522  may be connected to voltage V 1 . In this embodiment, voltage V 2  is greater than voltage V 1 . 
         [0036]    The output of comparator  520  may be inverted and connected to the S input of SR latch  524 . The output of comparator  522  may be connected to the R input of SR latch  524 . The inverted output of SR latch  524  (i.e., Q-bar output) may be connected to the RC circuit formed by one of conductive elements X 1 -X 7  or Y 1 -Y 7 . 
         [0037]    In operation, relaxation oscillator circuit  500  may be used to create a window of operation in which the voltage Vcs at node  530  is cyclically charged to voltage level V 2  and discharged to voltage level V 1 . Relaxation oscillator circuit  500  may achieve this function in the following manner. First, if the voltage at node  530  (i.e., the voltage across capacitor  532 ) drops below voltage V 1 , the output of comparator  522  will go HIGH. Similarly, if the voltage at node  530  rises above voltage V 2 , the output of comparator  520  will go LOW (because of the inverted output). Next, comparator outputs are connected to SR latch  524 , which behaves according to the truth table in TABLE 1. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 S 
                 R 
                 Q 
                 Q-bar 
                 Operation 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0 
                 0 
                 HOLD (output holds last known value) 
               
             
          
           
               
                 0 
                 1 
                 0 
                 1 
                 RESET 
               
               
                 1 
                 0 
                 1 
                 0 
                 SET 
               
               
                 1 
                 1 
                 0 
                 1 
                 RESET 
               
               
                   
               
             
          
         
       
     
         [0038]    Thus, if the SET (S) input of SR latch  524  is driven HIGH, the Q-bar output of the latch will be driven LOW. If the RESET (R) input of latch  524  is driven HIGH, the Q-bar output of the latch will be driven HIGH. SR latch  524  may be a reset-dominant latch so that when both the S and R inputs are driven HIGH, SR latch  524  will be in RESET mode (i.e., Q-bar output will be driven HIGH). Finally, where both S and R inputs are driven LOW, the outputs of SR latch  524  will hold the last known output value. 
         [0039]      FIG. 6  illustrates an example timing diagram for a relaxation oscillator circuit  500  output in a relaxation oscillator-based touch sensor system  100 , in accordance with the present disclosure.  FIG. 6 , along with  FIG. 5  and TABLE 1, further describes the function of relaxation oscillator circuit  500 . 
         [0040]    Starting with the very beginning of operation (i.e., device power-up), at time t 0  in  FIG. 6 , the voltage Vcs across the sensor capacitor  532  is 0. Therefore, comparator  522  output goes HIGH while the inverted output of comparator  520  goes LOW since both inverting inputs are less than the non-inverting input voltages V 2  and V 1 , respectively. This places SR latch  524  into RESET, driving the Q-bar output to 1, which in turn charges the sensor capacitor  532 . 
         [0041]    At time t 1  depicted in  FIG. 6 , the voltage Vcs across sensor capacitor  532  increases until it surpasses voltage threshold V 1  present on the non-inverting input of comparator  522 . This causes the output of comparator  522  to go to 0. Thus, at time t 1 , both comparator outputs are LOW and the SR latch  524  output holds the last known value, which means that the Q-bar output remains at 1 and continues to charge sensor capacitor  532  (between time t 1  and t 2 ). 
         [0042]    At time t 2 , the voltage Vcs across sensor capacitor  532  exceeds voltage threshold V 2  present on the non-inverting input of comparator  520 . This causes the inverted output of comparator  520  to transition to 1. Thus, at time t 2 , the S input of SR latch  524  is HIGH, and the R input of SR latch  524  is LOW. This causes the Q-bar output of SR latch  524  to transition to 0. At this time (t 2 ), sensor capacitor  532  begins to discharge (between time t 2  and t 3 ). When voltage Vcs drops below voltage threshold V 2  (between time t 2  and t 3 ), the output of comparator  520  again goes LOW, and SR latch  524  holds the last known value (i.e., 0) and allows capacitor  532  to continue to discharge. 
         [0043]    At time t 3 , the voltage Vcs across sensor capacitor  532  drops below voltage threshold V 1 . This causes comparator  522  output to go to 1, driving the Q-bar output of SR latch  524  HIGH and again charging sensor, capacitor  532 . This charging and discharging process repeats as long as there is power to the system. 
         [0044]    The timing of the above-described function of relaxation oscillator circuit  500  may be affected by the electrical properties of the RC circuit formed by each conductive element X 1 -X 7  and Y 1 -Y 7  of touch sensor  200  ( FIG. 2 ). For example, RC circuit  416  of  FIG. 4  (corresponding to capacitor  532  and resistor  534  in  FIG. 5 ), like all other RC circuits, may have an RC time constant corresponding to the amount of time necessary to charge capacitor Cs. The RC time constant is typically represented by the Greek letter Tau, and satisfies the following equation: 
         [0000]      τ= R*Cs  
 
         [0045]    According to this equation, □ represents the time it takes to charge capacitor Cs to about 63% of the supply voltage, and 5□ represents the time it takes to charge capacitor Cs to within 1% of the supply voltage. According to the equation, charging time is directly proportional to capacitance Cs. As a result, the sensor capacitance Cs with no touch will charge and discharge more quickly than it does when a touch occurs. In other words, because a touch may increase the capacitance Cs of the RC circuit, the RC time constant may also be increased, and may result in longer charging and discharging periods. Longer charging and discharging periods, in, turn, may result in a reduced frequency of relaxation oscillation circuit  500 . 
         [0046]    Given these properties of relaxation oscillator circuit  500 , touch controller  400  may determine a touched condition by measuring the frequency of relaxation oscillator circuit  500 .  FIG. 7  illustrates an example touch controller  700  (corresponding to touch controller  400  of  FIG. 1 ) and provides further details regarding how the frequency of relaxation oscillator circuit  500  may be measured. For example, touch controller  700  may implement counter circuit  702  that is connected to output  701  of relaxation oscillator circuit  500  (i.e., the Q-bar output of SR latch  524  in  FIG. 5 ). Counter circuit  702  may be operable to increment a value stored in counter register  704  on every positive edge of the output  701 . 
         [0047]    According to one embodiment, which is referred to herein as a “frequency measurement method,” touch controller  700  may read the counter register  704  at regular, pre-defined time intervals, for example, according to a pre-defined timer interrupt. Touch controller  700  may compare successive reads of counter register  704  to determine the number of times relaxation oscillator circuit  500  has oscillated during the pre-defined time interval. Accordingly, this number provides a measurement related to the frequency of relaxation oscillator circuit  500 . Touch controller  700  may compare successive measurements to determine whether a touch event has occurred. 
         [0048]    As described above, a touch may increase the capacitance Cs at capacitor  706 , resulting in a reduced frequency of relaxation oscillator circuit  500 . Thus, according to the frequency measurement method, if the value of counter register  704  decreases from one pre-defined time interval to the next, touch controller  700  may determine that a touch event has occurred. In some embodiments, touch controller  700  may not determine that a touch event has occurred unless the value of counter register  704  has decreased more than a pre-determined threshold. In such embodiments, touch controller  700  may be less prone to falsely reporting a touch event as a result of minor changes in the frequency of relaxation oscillator  500  due to conditions other than a touch event (e.g., noise, drift, etc.). 
         [0049]    In another embodiment, which is referred to herein as a “period measurement method,” touch controller  700  may count the time (or period) needed to fill up counter register  704 . According to this period measurement embodiment, touch controller  700  may include a system time register  708 . Touch controller  700  may reset the value of counter register  704  to ZERO and, at the same or substantially same time, may store the current value of system time register  708  into storage register  710 . Here again, counter register  704  may increment on every positive edge of output  701  of relaxation oscillator circuit  500 . At some point, this may cause an overflow condition of counter register  704 . Touch controller  700  may be configured to respond to an overflow condition of counter register  704  (e.g., via an interrupt) by reading the value of system time register  708  and comparing that value to the time value stored in storage register  710 . This comparison provides the number of system time units needed to overflow the counter, and is an indication of the frequency of relaxation oscillator circuit  500 . 
         [0050]    As described above, a touch may increase the capacitance Cs at capacitor  706 , resulting in a reduced frequency of relaxation oscillator circuit  500 . Thus, according to the period measurement method, if the number of system time units needed to overflow counter register  704  increases between successive measurements, touch controller  700  may determine that a touch event has occurred. In some embodiments, touch controller  700  may not determine that a touch event has occurred unless the number of system time units taken to overflow counter register  704  has increased more than a pre-determined threshold. In such embodiments, touch controller  700  may be less prone to falsely reporting a touch event as a result of minor changes in the frequency of relaxation oscillator circuit  500  due to conditions other than a touch event (e.g., noise, drift, etc.) 
         [0051]    According to the frequency measurement method, the sampling window may be adjusted by modifying the length of the pre-defined timer interrupt. According to the period measurement method, the sampling window may be adjusted by changes in the maximum value of the counter register. For example, a small maximum value will result in a shorter sampling window and more frequent frequency measurements. The ratio between the speed of the scanning mechanism and the resolution of system  100  must always be considered when adjusting the sampling window. 
         [0052]    As discussed above with respect to  FIGS. 1-7 , the frequency of relaxation oscillator circuit  500  may be disturbed when a user touches touch screen  200  with a finger or other object. In addition, the frequency of relaxation oscillator circuit  500  may be disturbed by conducted noise that is present in system  100 . In either case, there is a deviation in the measured frequency of relaxation oscillator circuit  500  (e.g., the measured frequency changes between successive sampling windows). Accordingly, touch controller  400  must be able to distinguish between three different scenarios. 
         [0053]    First, scenario A may correspond to the condition where only a touch event affects the frequency of relaxation oscillator circuit  500 . In scenario A, no conducted noise is present, and as described above with respect to  FIG. 7 , the frequency deviation may tend to be constant and relatively easy to detect. Second, scenario B may correspond to the condition where only conducted noise affects the frequency of relaxation oscillator circuit  500 . In scenario B, there is no touch event. Third, scenario C may correspond to the condition where both a touch event and conducted noise affect the frequency of relaxation oscillator circuit  500 . 
         [0054]    In both scenarios B and C, the frequency of relaxation oscillator  500  may be easily overpowered by the frequency of the conducted noise. As a result, the frequency of relaxation oscillator  500  may be close or equal to the frequency of the conducted noise. This new frequency presents a deviation from the natural frequency of relaxation oscillator circuit  500 . Compared to the natural frequency, this deviation may be multiple orders of magnitude or zero. Thus, touch controller  400  may be configured to accurately report a touch event in the presence or absence of such deviations. Likewise, touch controller  400  may be configured to not report a touch event when a frequency deviation is caused by conducted noise alone (scenario B). 
         [0055]    Accordingly, touch controller  400  may be configured to exploit one or more properties of the conducted noise in order to accurately detect a touch event. For example, conducted noise in touch sensor system  100  will generally affect each conductive element X 1 -X 7  and Y 1 -Y 7  of touch sensor  200  causing, for each conductive element, a frequency deviation that is similar in magnitude to that experienced by all the other conductive elements. Thus, while all conductive elements may experience a similar frequency deviation in the presence of conducted noise, a touched conductive element will show a deviation that is higher in magnitude, compared to the untouched conductive elements. In both scenarios A and C, the touched conductive element will likely be the one showing the greatest deviation. As a result, touch controller may detect a touched conductive element in all scenarios by searching for a “most pressed button,” i.e., the conductive element showing the greatest frequency deviation relative to all other conductive elements. 
         [0056]    Notwithstanding the above, scenarios B and C may present a situation in which the conducted noise creates dead frequencies (i.e., blind spots). This situation may be presented, for example, when the frequency of the conducted noise has a value near or equal to the natural frequency of relaxation oscillator circuit  500 . When this occurs, touch controller  400  may fail to detect a touch event because the measured frequency does not show a deviation. In order to detect a touch event under these conditions, relaxation oscillator circuit  500  may be configurable to operate according to more than one operating range. According to this aspect of the disclosure, relaxation oscillator circuit  500  may produce an output signal at more than one drive current, where a higher drive current results in a natural frequency that is greater than the natural frequency resulting from a lower drive current. 
         [0057]    Thus, for each conductive element X 1 -X 7  and Y 1 -Y 7 , touch controller  400  may measure the frequency of relaxation oscillator circuit  500  at two different drive currents (operating ranges). Deviations caused by conducted noise in scenarios B and C may be detected if a deviation at either operating range is detected. According to this embodiment of the present disclosure, touch controller  400  may detect a touch event based on a measurement at one operating range while a measurement at the other operating range shows no deviation as a result of a blind spot. 
         [0058]      FIG. 8  illustrates a flow chart of an example method  800  for using conducted noise to detect a touch on a touch sensor  200  in a relaxation oscillator-based sensor system  100 , in accordance with the present disclosure. 
         [0059]    According to one embodiment, method  800  preferably begins at step  802 . As noted above, teachings of the present disclosure may be implemented in a variety of configurations of system  100 . As such, the preferred initialization point for method  800  and the order of the steps  802 - 820  comprising method  800  may depend on the implementation chosen. 
         [0060]    At step  802 , touch controller  400  may select a conductive element of touch sensor  200  to be measured. For example, touch controller may set a control signal that electrically connects a selected element X 1 -X 7  or Y 1 -Y 7  to relaxation oscillator circuit  500 . At step  804 , touch controller  400  may measure the frequency of relaxation oscillator circuit  500  according to the methods described above. For example, touch controller  400  may use the frequency measurement method or the period measurement method to measure the frequency of relaxation oscillator circuit  500 . 
         [0061]    At step  806 , touch controller  400  determines whether the measured frequency indicates a deviation from the natural frequency of relaxation oscillator circuit  500 . In order to do this, touch controller may store a running baseline average representing the average frequency of a given conductive element. This running baseline average may be used to eliminate noise created by changes in temperature, voltage, and environment. Accordingly, touch sensor  400  may compare the measured frequency with the baseline average. In one embodiment, touch controller  400  determines that a deviation is present if the frequency measurement is different than the baseline average. In another embodiment, touch controller  400  determines that a deviation is present if the frequency measurement differs from the baseline average more than a pre-defined threshold value. 
         [0062]    If no frequency deviation is detected at step  806  (i.e., at either operating range of relaxation oscillator circuit  500 ), touch controller  400  determines that no touch is present and proceeds to step  808 . At step  808 , touch controller  400  may add the measured frequency to the running baseline average. Thus, the baseline averaging system may be a “gated” system, so that the system keeps track of the presence/absence of noise in any operating range, and disables the baseline averaging whenever noise affects even a single sensor. This ensures that the baseline average reflects the natural frequency of relaxation oscillator circuit  500  rather than the frequency as affected by conducted noise. Further, a fairly slow averaging method (e.g., step  808  is not performed for every scanning cycle) may be implemented if avoiding averaging deviations with a slow slope is desired (e.g., as would be present when an object approaches touch screen  200  very slowly). 
         [0063]    After step  808  is complete, touch controller  400  may proceed to step  810  where it may determine if the currently selected conductive element is the last element to be measured. If the currently selected conductive element is the last element to be measured, method  800  may exit. If the currently selected conductive element is not the last element to be measured, method  800  may loop back to step  802  where the next conductive element is selected, and the previous steps repeated for the newly selected conductive element. 
         [0064]    Turning back to step  806 , if a frequency deviation is detected in this step, touch controller  400  proceeds to step  812  where it measures and stores the frequency deviations for all conductive elements X 1 -X 7  and Y 1 -Y 7 . For example, touch controller  400  may loop through steps similar to steps  802  and  804  in order to measure these frequency deviations. According to one embodiment, touch controller  400  may measure the frequency of each conductive element once. According to another embodiment, step  812  may correspond to an acquisition window during which touch controller measures and stores frequency deviations based on multiple measurements, or an average of multiple measurements, for each conductive element X 1 -X 7  and Y 1 -Y 7 . According to this latter embodiment, touch controller  400  may employ slope detection during the acquisition window of step  812  to determine when the user is finished touching the touch sensor. For example, as describe more fully below with respect to  FIG. 9 , touch controller may wait for the frequency to settle (i.e., the slope flattens), before detecting a completion of a touch. 
         [0065]      FIG. 9  illustrates an example plot  900  of the percentage change in measured frequency of a conductive element in the presence of conducted noise, in accordance with the present disclosure. As depicted, plot  900  represents the injected noise frequency in megahertz (MHz). The frequency measurements according to two operating ranges (High Power and Medium Power) are depicted. The plotted curve having a peak that occurs earlier in frequency corresponds to the High Power operating range. As illustrated by the frequency response in plot  900 , if noise is being injected on the system at 418 KHz (0.4180 on the x-axis), a medium power scan of the sensor may result in the same value (e.g., approximately 0% absolute percentage change) whether or not the user is pressing the sensor. A high power scan of the sensor at 418 KHz (0.4180 on the x-axis), may result in a 110% absolute percentage change if a user is pressing on the sensor. Because current operating ranges may have frequencies that result in little or no change when a user presses the sensor, method  800  may be performed using at least two operating ranges that do not share 0% shift frequencies. According to one embodiment, the percentage change is calculated in absolute value, as the frequency of the conducted noise may take a value higher or lower than the natural frequency of relaxation oscillator circuit  500 . In an alternative embodiment, the raw percentage change may be calculated and/or measured. 
         [0066]    After touch controller  400  has measured and stored frequency deviations for all conductive elements of touch sensor  200 , touch controller may proceed to step  812 . At step  812 , touch controller  400  may determine whether a most pressed button exists. For example, touch controller  400  may employ a sorting procedure such that a conductive element having the highest deviation compared to all other conductive elements is determined to be the most pressed button. According to one embodiment of the sorting procedure, the most pressed button must reveal a deviation larger than the other conductive elements by at least a predefined threshold. Accordingly, if touch controller  400  does not determine a most pressed button at step  812 , method  800  may exit. This would likely be the case in scenario B where the measured deviations were due to noise only and not a touch event. Alternatively, if touch controller  400  determines a most pressed button at step  812 , touch controller  400  may proceed to step  816 . 
         [0067]    At step  816 , touch controller  400  may debounce the most pressed button for a number of times in an effort to check for consistency and to avoid false triggers. Touch controller  400  may then determine at step  818  whether the most pressed button has been successfully debounced. If not, method  800  may exit. If so, touch controller  400  may proceed to step  820  where it may report to host  600  (or other application) the most pressed button and exit. 
         [0068]    As described above, touch controller  400  may measure the frequency of relaxation oscillator circuit  500  at two different drive currents (operating ranges). Accordingly, all conductive elements X 1 -X 7  and Y 1 -Y 7  of touch sensor  200  may be scanned alternatively according to one operating range and then the other. As a result of this embodiment of the present disclosure, a mirror-like system and method may be implemented where there are similar functions and variables belonging to each of the two operating range modes. For example, method  800  may be performed once at one operating range and then again at another operating range. 
         [0069]    Although  FIG. 8  discloses a particular number of steps to be taken with respect to method  800 , method  800  may be executed with greater or lesser steps than those depicted in  FIG. 8 . In addition, although  FIG. 8  discloses a certain order of steps to be taken with respect to method  800 , the steps comprising method  800  may be completed in any suitable order. For example method  800  may be used in conjunction with standard capacitive touch detection methods currently used in the industry. According to this aspect of this disclosure, the method may determine whether conducted noise is present in the touch system (e.g., step  806  of method  800 ). If conducted noise is present, the method may proceed according to the disclosed steps of method  800 . If conducted noise is not present, the method may determine the user&#39;s touch using standard capacitive touch detection methods currently used in the industry. 
         [0070]    While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of disclosure.