Method and apparatus for a scaleable touch panel controller

[0002] 1. Field of the Invention

[0003] The invention relates generally to computer architectures and, more particularly, to a method and an apparatus for a scaleable touch panel controller.

[0004] 2. Description of Related Art

[0005] Touch panels are an increasingly popular method of entering information into a computer system. One particular technology of touch panels generally utilizes a screen comprising conductive leads arranged vertically and horizontally terminating at an X-resistance and a Y-resistance element, respectively. When the screen is touched, the conductive leads contact each other, creating an electrical path for current to flow. The position is generally determined by detecting the relative level of voltage via analog-to-digital (A/D) converters. A/D converters, however, generally require a stable power supply and linear circuit techniques, and, therefore, are not practicable for devices that utilize low-voltage linear amplifiers or scaleable-voltage amplifiers, such as some Personal Data Assistants (PDAs).

[0006] Therefore, there is a need for a method and an apparatus for providing a digital interface to a touch panel sensor.

[0007] The present invention provides a method and an apparatus for determining the contact position on a touch panel sensor. The method and apparatus measures the elapsed time that passes for a circuit between two connectors to discharge from its peak value to a predetermined threshold. The contact position is determined by measuring the elapsed time for circuit to discharge between differing points.

[0008] For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

[0009] FIG. 1 is a schematic of a typical touch panel sensor that embodies the present invention;

[0010] FIG. 2 is a data flow diagram illustrating one embodiment of the present invention in which a contact position on a touch panel is determined; and

[0011] FIG. 3 is a table that illustrates connections of a touch panel sensor in accordance with one embodiment of the present invention.

[0012] In the following discussion, numerous specific details are set forth to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known elements have been illustrated in schematic or block diagram form in order not to obscure the present invention in unnecessary detail. Additionally, for the most part, details concerning the operation and the implementation of electrical components, such as resistors, capacitors, and the like have been omitted inasmuch as such details are not considered necessary to obtain a complete understanding of the present invention, and are considered to be within the skills of persons of ordinary skill in the relevant art.

[0013] It is further noted that, unless indicated otherwise, all functions described herein may be performed in either hardware or software, or some combination thereof. In a preferred embodiment, however, the functions are implemented as part of an on-chip circuit design with supporting software.

[0014] Referring to FIG. 1 of the drawings, the reference numeral 100 generally designates a digital touch panel system embodying features of the present invention. The digital touch panel system 100 generally comprises a touch panel sensor 110 that comprises an X-resistance 112 that provides electrical resistance along the X-axis, and a Y-resistance 114 that provides electrical resistance along the Y-axis. Connectors X₁120 and X₂122, and Y₁124 and Y₂126, are electrically coupled to opposing ends of the X-resistance 112 and the Y-resistance 114, respectively.

[0015] Capacitors C_x116 and C_y118 are electrically connected to the X-resistance 112 and the Y-resistance 114, respectively, and electrically connected to an Enable C_x(EnC_x) connector 128 and to an Enable C_y(EnC_y) connector 130, respectively. As one skilled in the art will appreciate, and as will be described in greater detail below, X-resistance 112, the Y-resistance 114, the capacitor C_x116, and the capacitor C_y118 create a Resistor-Capacitor (RC) component in which the X-resistance 112 and the Y-resistance 114 substantially determine the rate at which the capacitors C_x116 and C_y118 discharge. The capacitors C_x116 and C_y118 are preferably selected to provide a discharge slope appropriate to obtain the required accuracy given the X-resistance 112, the Y-resistance 114, and the counter accuracy (discussed below with reference to step 212, FIG. 2).

[0016] Preferably, each connector X₁120, X₂122, Y₁124, Y₂126, EnC_x128, and EnC_y130 comprises a Bi-directional Input/Output (IO) Driver, which is considered well known to a person of ordinary skill in the art. Briefly, each Bi-directional IO Driver provides three connections to the touch panel controller 132: a tri-state enable line, an output data line, and a receiver line. The tri-state enable line enables the data line to be coupled to the touch panel sensor 110. The receiver line provides a sense point for sensing the voltage level of the connection to the touch panel sensor 110.

[0017] The touch panel controller 132 is preferably configured to control the touch panel sensor 110, and to detect the presence and location of contact on the touch panel sensor 110 by altering the electrical connections of connectors X₁120, X₂122, Y₁124, Y₂126, EnC_x128, and EnC_y130, and by measuring the transition time required for given voltage to discharge as discussed in greater detail below. The touch panel controller 132 is preferably configured to comprise of one or more internal registers 134 and electrically connected to a peripheral bus 136, which, among other things, provides an electrical connection to other electrical components (not shown), such as processors, I/O devices, and the like.

[0018] Preferably, a weak keeping element 138 is electrically connected to connector X₁to provide a mechanism to detect contact, as described below with reference to FIG. 2. Briefly, the weak keeping element 138, which is preferably implemented as a pull-up resistor, allows the touch panel controller 132 to hold X₁in a high state until contact is detected, at which time the X₁becomes grounded without substantially affecting the reading being taken.

[0019] It should be noted that other components, such as power supplies, processors, peripheral bus controller, and the like, necessary for the operation of the present invention are not shown in order to not obscure the present invention in unnecessary detail that is considered to be obvious to one of ordinary skill in the art upon a reading of the present disclosure.

[0020] It should also be noted that the touch panel controller 132 is preferably implemented as part of an on-chip design that provides for direct connection to the touch panel sensor 110.

[0021] FIG. 2 is a flow chart depicting steps that may be performed by the digital touch panel system 100 in accordance with one embodiment of the present invention that digitally determines the position of a contact on a touch panel sensor. Processing begins in step 205, wherein the amount of resistance is measured in the X-resistance 112 and the Y-resistance 114, namely, the resistance measurements R(X₁,X₂) and R(Y₁,Y₂), where R(a,b) represents the resistive value between connector a and connector b, respectively, are made. Preferably, the amount of resistance is measured by coupling the connectors X₁120, X₂122, Y₁124, Y₂126, EnC_x128, and EnC_y130 and measuring the transition time for a given voltage level to discharge as described below with reference to R(X₁,X₂) and R(Y₁,Y₂) of FIG. 3.

[0022] Preferably, the measurement of R(X₁,X₂) and R(Y₁,Y₂) is performed when contact is not detected on the touch panel sensor 110. Contact is detected in the preferred embodiment by setting X₂122 and Y₂126 in tri-state, connecting Y₁124 to ground and setting X₁120 high via the weak keeping element 138. Contact is then determined when X₁120 returns low as a result of the charge of X₁120 being connected to ground at connector Y₁124.

[0023] In step 210, a measurement is selected and the connectors X₁120, X₂122, Y₁124, Y₂126, EnC_x128, and EnC_y130 are configured accordingly. As will be discussed in greater detail below with reference to FIG. 3, a total of four measurements are taken when contact is detected. Specifically, the preferred calculation of the location of a contact on the touch panel 110 requires the resistive values of R(X₁,Y₁), R(X₁,Y₂), R(Y₁,X₁), and R(Y₁,X₂) when contact is detected.

[0024] After selecting a measurement and configuring the connectors accordingly in step 210, processing proceeds to step 212, wherein a counter is initialized to zero. In step 214, the circuit comprising the X-resistance 112, the Y-resistance 114, the C_xcapacitor 116, and the C_ycapacitor 118, is charged to its peak value, preferably the supply voltage for the bi-directional IO driver. In step 216 the circuit is allowed discharge via the appropriate GND connection via the touch panel sensor 110 while the counter is incremented.

[0025] In step 220, a determination is made whether the voltage at the appropriate sense connector (as indicated in FIG. 3) is less than a predefined threshold. Preferably, the predefined threshold is set as a fraction, such as one-half, of the maximum supply voltage of the receiver of the tri-state, bi-directional driver/receiver buffers, commonly referred to as (V_DD/2). If, in step 220, a determination is made that the voltage at the sense connector is not less than the predefined threshold, then processing returns to step 216, wherein the circuit is allowed to discharge further and the counter incremented.

[0026] The method and apparatus for charging/discharging the circuit and using the counter discussed above is considered to be within the skills of a person of ordinary skill in the art upon a reading of the present disclosure. In the preferred embodiment, however, a counter is implemented in hardware that is enabled upon the circuit reaching its peak value. The counter is allowed to increment while the circuit is discharging. Upon the circuit discharging to a predefined threshold, the value of the counter is stored in the registers 134, and an interrupt signal is generated. Application software receives the interrupt and evaluates the contents of the registers 134 to determine the contact position as discussed below with reference to step 226.

[0027] If, in step 220, a determination is made that the voltage at the sense connector is less than the predefined threshold, then processing proceeds to step 222, wherein the value of the counter is captured and stored, preferably in the registers 134 (FIG. 1).

[0028] In step 224, a determination is made whether all resistive measurements have been made. As discussed above, preferably four measurements are made for use in calculating the contact position on the touch panel sensor 110. If, in step 224, a determination is made that all four measurements have not been made, then processing returns to step 210, wherein a measurement is selected and processed as described above with reference to steps 210-220.

[0029] If, however, in step 224, a determination that all four measurements have been made, then processing proceeds to step 226, wherein the measurements, i.e., the counter values, are used to determine the contact position upon the touch panel sensor 110. Preferably, the contact position is determined according to the following equation:

[0030] R(X_a)=[R(Y₁,X₁)−R(Y₁,X₂)+R(X₁,X₂)]/2

[0031] R(Y_a)=[R(X₁,Y₁)−R(X₁,Y₂)+R(Y₁,Y₂)]/2

[0032] where:

[0033] R(X_a) represents the position of the contact along the X-axis relative to X₁120; and

[0034] R(Y_a) represents the position of the contact along the Y-axis relative to Y₁124.

[0035] Therefore, the coordinate of the contact is given by (R(X_a), R(Y_a)) relative to the origin (X₁,Y₁). It should be noted that the contact position is a relative position and is unitless, i.e., a percentage of the touch panel sensor 110 from the origin.

[0036] FIG. 3 is a table illustrating the connections and the measurements used in making the six measurements discussed above with reference to FIG. 2 in accordance with the preferred embodiment of the present invention. Specifically, FIG. 3 depicts how each of the connectors X₁120, X₂122, Y₁124, Y₂126, EnC_x128, and EnC_y130 are connected to measure each of R(X₁,Y₁), R(X₁,Y₂), R(Y₁,X₁), R(Y₁,X₂), R(X₁,X₂), and R(Y₁,Y₂). As noted above, it is preferred that R(X₁,X₂) and R(Y₁,Y₂) are measured when no contact is detected, and R(X₁,Y₁), R(X₁,Y₂), R(Y₁,X₁), and R(Y₁,X₂) are measured when contact is detected.

[0037] For example, to make the measurement of the resistance from connector X₁120 to connector Y₁124, specified as R(X₁Y₁), it is preferred that X₁120 is connected as a sense point, X₂122 is in tri-state (not connected or dangling), Y₁124 is connected to ground, Y₂126 is in tri-state, EnC_x128 is connected to ground, and EnC_y130 is in tri-state. Upon making the connections specified, the circuit is charged and the voltage of X₁120 is connected as a sense point to measure the voltage as discussed above with reference to steps 214-220 (FIG. 2).

[0038] As will be appreciated by one skilled in the art, the contact position may be determined by connecting the touch panel sensor 110 differently and obtaining different measurements. The connections and measurements presented herein is the preferred embodiment and are provided for illustrative purposes only, and, therefore, should not be interpreted as limiting the present invention to the specific connections and/or measurements specified herein in any manner.

[0039] It is understood that the present invention can take many forms and embodiments. Accordingly, several variations may be made in the foregoing without departing from the spirit or the scope of the invention. Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Many such variations and modifications may be considered obvious and desirable by those skilled in the art based upon a review of the foregoing description of preferred embodiments. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.