Abstract:
An integrated circuit device inductive touch analog front end excites selected ones of a plurality of inductive touch sensors and provides analog output signals representative of voltages across the coils of the plurality of inductive touch sensors. Various characteristics of the inductive touch analog front end are programmable. A digital processor controls selection of each one of the plurality of inductive touch sensors and receives the respective analog output voltage signal from the inductive touch AFE. The digital processor may program the characteristics of the inductive touch analog front end. When a sufficient change in the coil voltage is determined by the digital processor, that inductive touch sensor is assumed to have been actuated and the digital processor takes action based upon which one of the plurality of inductive touch sensors was actuated (touched).

Description:
RELATED PATENT APPLICATIONS 
       [0001]    This application claims priority to commonly owned U.S. Provisional Patent Application Ser. No. 61/104,012; filed Oct. 9, 2008; entitled “Integrated Circuit Device to Support Inductive Sensing,” by Sean Steedman, Keith E. Curtis, Radu Ruscu and Petru Cristian Pop; and is related to commonly owned U.S. patent application Ser. No. 12/560,855; filed Sep. 16, 2009; entitled “Integrated Circuit Device to Support Inductive Sensing,” by Sean Steedman, Keith E. Curtis, Radu Ruscu and Petru Cristian Pop; and both are hereby incorporated by reference herein for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to integrated circuits, and more particularly to, an integrated circuit device that supports inductive sensing. 
       BACKGROUND 
       [0003]    Capacitive sensors, e.g., touch sensors, are widely used as user interfaces for a wide variety of consumer, commercial, industrial and military applications. However, capacitive touch sensors suffer from several shortcomings such as sensitivity to spilled liquids and unreliable operation when a user is wearing heavy gloves. Inductive touch sensors solve the shortcomings of capacitive touch sensors and have started to replace them in certain specialized applications not completely suited for the capacitive touch sensors. Inductive touch sensors require appropriate specialized interface circuits when used in an electronic system. Present technology inductive sensor interface circuits require a significant number of external discrete components to operate. These external discrete components are expensive and require a large amount of circuit board area for use in a system application. 
       SUMMARY 
       [0004]    Therefore there is a need for an integrated circuit device that supports detection of the actuation of inductive sensors and provides useful outputs therefrom. The inductive sensor may be used to sense a touch causing an inductive change in the sensor. The integrated circuit interface that supports the operation of the inductive sensor will have control mechanisms that substantially eliminate the need for external components and adjustments by having user programmable characteristics, thus reducing physical size and costs of manufacture for systems using inductive sensors. 
         [0005]    The integrated circuit device adapted for operation with the inductive sensor may be programmable so as to allow further reductions in manufacturing cost and board space by eliminating additional external components. Performance of the inductive sensor and integrated circuit interface circuits may be used to tailor the performance of the inductive sensor system to specific applications. Performance parameters may be easily changed before, during and/or after design and implementation of an inductive sensor system. 
         [0006]    A control interface(s) may be used to control device parameters and may be comprised of a serial interface, e.g., I 2 C, SPI, UNI/O, UART, etc., a parallel interface, and/or direct input-output (I/O) level control. Parameter control may be, for example but not limited to, gain selection of a operational amplifier, corner frequency of the operational amplifier, output drive strength of a current driver of the operational amplifier, virtual ground voltage level, quiescent operating current of the operational amplifier, offset control of the operational amplifier, etc. 
         [0007]    According to a specific example embodiment of this disclosure, an integrated circuit device configured as a programmable analog front end for supporting inductive touch sensing comprises: a voltage reference; a synchronous detector having a first input coupled to an inductive reference coil external connection, wherein the inductive reference coil external connection is adapted for coupling to an inductive reference coil; a second input coupled to an inductive touch sensor coil external connection, wherein the inductive touch sensor coil external connection is adapted for coupling to at least one inductive touch sensor coil; a third input coupled to a reference select external connection, wherein the reference select external connection is adapted for coupling to a reference select signal; a fourth input coupled to a clock external connection, wherein the clock external connection is adapted for coupling to a clock signal, and a fifth input coupled to the voltage reference; a coil driver having an output coupled to a coil drive output external connection, an input coupled to a coil drive input external connection; an amplifier configured with a low-pass filter and having inputs coupled to the synchronous detector and an output having voltage values representative of inductance values of the at least one inductive touch sensor coil and the inductive reference coil, the output of the amplifier is coupled to a voltage detector output external connection; and an interface and control module having a communications port, wherein the interface and control module controls at least one characteristic of the programmable analog front end; wherein the synchronous detector mixes the clock signal with a signal from the inductive reference coil or the at least one inductive touch sensor coil, as selected by the reference select signal, to produce sum and difference mixing products, whereby the amplifier amplifies the sum and difference mixing products and substantially attenuates the sum mixing product and passes the difference mixing product at the output of the amplifier. 
         [0008]    According to another specific example embodiment of this disclosure, an electronic system having an inductive touch interface comprises: an inductive touch interface comprising a plurality of inductive touch sensor coils and an inductive reference coil; a first integrated circuit digital processor; a second integrated circuit programmable inductive touch analog front end comprising: a voltage reference; a synchronous detector having a first input coupled to the inductive reference coil; a second input coupled to the plurality of inductive touch sensor coils; a third input coupled to a reference select signal from the digital processor; a fourth input coupled to a clock signal from the digital processor, and a fifth input coupled to the voltage reference; a coil driver having an output coupled in series with the inductive reference coil and selected ones of the plurality of inductive touch sensor coils, an input coupled through an external low-pass filter to a clock output from the digital processor; an amplifier configured with a low-pass filter and having inputs coupled to the synchronous detector and an output having voltage values representative of inductance values of the plurality of inductive touch sensor coils and the inductive reference coil, the output of the amplifier is coupled to an analog input of the digital processor; and an interface and control module having a communications port, wherein the interface and control module controls at least one characteristic of the programmable inductive touch analog front end; wherein the synchronous detector mixes the clock signal with a signal from the inductive reference coil or the at least one inductive touch sensor coil, as selected by the reference select signal, to produce sum and difference mixing products, whereby the amplifier amplifies the sum and difference mixing products and passes the difference mixing product at the output of the amplifier. 
         [0009]    The synchronous detector may comprise: a decoder having a first input coupled to the clock external connection and a second input coupled to the reference select external connection; and six analog pass-gates controlled by the decoder, wherein inputs of first and fourth analog pass-gates are coupled to the voltage reference, inputs of second and fifth analog pass-gates are coupled to the plurality of inductive touch sensor coils, inputs of third and sixth analog pass-gates are coupled to the inductive reference coil; whereby the third and fourth analog pass-gates are closed when the reference select and clock signals are at first logic levels, the first and sixth analog pass-gates are closed when the reference select signal is at the first logic level and the clock signal is at a second logic level, the second and fourth analog pass-gates are closed when the reference select signal is at the second logic level and the clock signal is at the first logic level, and the first and fifth analog pass-gates are closed when the reference select and clock signals are at the second logic level. 
         [0010]    The synchronous detector may also comprise: a frequency mixer having a first input coupled the voltage reference, and an output coupled to the amplifier; a frequency divider having a first input coupled to the clock external connection, and an output coupled to a second input of the frequency mixer; and a multiplexer for selectively coupling the inductive touch sensor external connection and the inductive reference coil external connection to a third input of the frequency mixer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    A more complete understanding of the present disclosure thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
           [0012]      FIG. 1  is a schematic block diagram of an electronic system having an inductive touch keypad, a programmable inductive touch analog front end and a digital processor, according to a specific example embodiment of this disclosure; 
           [0013]      FIG. 2  is a more detailed schematic block diagram of the programmable inductive touch analog front end shown in  FIG. 1 ; 
           [0014]      FIG. 3  is a more detailed schematic block diagram of a synchronous detector and the circuit functions shown in  FIG. 2 , according to a specific example embodiment of this disclosure; and 
           [0015]      FIG. 4  is a more detailed schematic block diagram of a synchronous detector and the circuit functions shown in  FIG. 2 , according to another specific example embodiment of this disclosure. 
       
    
    
       [0016]    While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0017]    Referring now to the drawings, the details of an example embodiment is schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
         [0018]    Referring to  FIG. 1 , depicted is a schematic block diagram of an electronic system having an inductive touch keypad, a programmable inductive touch analog front end and a digital processor, according to a specific example embodiment of this disclosure. A digital processor  106 , e.g., a microprocessor, microcomputer, digital signal processor, application specific integrated circuit (ASIC), programmable logic array, etc., is coupled to a programmable inductive touch analog front end (AFE)  104  and a matrix of inductive touch sensors  102 . Preferred inductive touch sensors  102  are Microchip inductive mTouch™ sensors more fully described at www.microchip.com. 
         [0019]    The inductive touch AFE  104  facilitates, with a single low-cost integrated circuit device, all active functions used in determining when there is actuation of inductive sensors, e.g., by pressing and deflecting a target key that changes the inductance value of an associated inductive sensor. The programmable inductive touch AFE  104  measures the inductance value of each key of the matrix of inductive touch sensors  102  and converts the inductance values into respective analog direct current (dc) voltages that are read and converted into digital values by the digital processor  106 . A reference inductor (e.g., coil) ( FIGS. 2 and 3 ) may also be included in the matrix of inductive touch sensors  102  for use as a comparison reference between an un-activated inductive sensor (coil) and an activated inductive sensor (coil), as more fully described hereinafter. 
         [0020]    The digital processor  106  supplies clock and control functions to the inductive touch AFE  104 , reads the analog voltage detector output of the programmable inductive touch AFE  104 , and selects each key of the matrix of inductive touch sensors  102  and the reference inductive sensor for processing by the programmable inductive touch AFE  104 , as more fully described hereinafter. When actuation of a key of the matrix of inductive touch sensors  102  is determined, the digital processor  106  will take an appropriate action. The digital processor  106  and/or an another source may be used to program attributes and characteristics of the various functions of the programmable inductive touch AFE  104 , as more fully described hereinafter. 
         [0021]    Referring to  FIG. 2 , depicted is a more detailed schematic block diagram of the inductive touch analog front end shown in  FIG. 1 . The programmable inductive touch AFE  104  may comprise a synchronous detector  212 , an interface and control module  214 , a programmable coil driver  210 , a programmable voltage reference  220 , and a programmable gain amplifier (PGA)/programmable low-pass filter  216 . The synchronous detector  212  is used to extract signals from excitation of each touch sensor coil. Use of a synchronous detector (e.g., mixer) improves the signal-to-noise ratio of the detection process so as to produce useful or desired signals (utile signals) for use by the digital processor  106 . As explained more fully hereinafter, the alternating current (AC) voltage amplitude from each touch sensor coil is mixed with a clock signal to produce sum and difference frequencies of the two AC signals. Since the AC voltage amplitude from each inductive touch sensor coil is at the same frequency as the clock signal, there will be a direct current (DC) voltage component (difference frequency) and twice the clock signal frequency (sum frequency) signals as mixing products from the synchronous detector  212 . 
         [0022]    The PGA/low-pass filter  216  is used as a buffer-amplifier/low pass filter between the synchronous detector  212  and the V DETOUT  node  236 . The PGA/low-pass filter  216  functions as an integrator and passes the DC voltage (difference frequency mixing result) while effectively suppressing the twice clock frequency (sum frequency mixing result). At the output of the functions a DC voltage is available to the digital processor  106  that is proportional to the inductance value of a selected one of the plurality of inductive sensors  242  or the reference inductor  240 , as more fully described hereinafter. The digital processor  106  converts the analog DC voltage from the programmable inductive touch AFE  104  to a digital voltage representation thereof and associates that digital voltage representation with the selected inductive sensor  242  or reference inductor  240 . Since the digital processor  106  selects the inductive sensor  242  or the reference inductor  240 , matching of the DC voltage values to associated ones of the plurality of inductive sensors  242  and reference inductor  240  are easily made. 
         [0023]    The programmable voltage reference  220  may be set to have a voltage output, for example but is not limited to, of approximately one-half the supply voltage, and may be an operational amplifier having programmable output voltage and/or output impedance. Other types of programmable voltage references having programmable output voltage and/or output impedance may be used effectively so long as there is adequate voltage stability and sufficient drive capability. 
         [0024]    The programmable coil driver  210  receives a signal derived from the clock supplied by the digital processor  106  or from any other clock source available. A low-pass filter may comprise, for example but is not limited to, a capacitor  252  and a resistor  250  configured as a low-pass filter for attenuating the higher frequency components of the clock square wave signal. The low-pass filter may also be an active filter (not shown). From the output of the low-pass filter, a substantially sinusoidal waveform is applied to the input of the coil driver  210 , amplified by the programmable coil driver  210 , and then made available to excite selected ones of the plurality of inductive sensors  242  and the reference inductor  240  through a series connected resistor  244 . Each of the plurality of inductive sensors  242  is selected by the digital processor  106  by connecting one end of the selected one of the plurality of inductive sensors  242  to a supply common, thereby completing the circuit from the programmable coil driver  210  and producing a voltage across the selected one of the plurality of inductive sensors  242  from the AC current supplied by the programmable coil driver  210 . The synchronous detector  212  detects the voltage from each one of the plurality of inductive sensors  242  and the reference inductor  240  for subsequent processing by the digital processor  106 . The programmable coil driver  210  supplies current at the clock frequency to the reference coil  240  and the selected one of the plurality of inductive sensors  242  connected in series. When the approximately sinusoidal current is flowing through the reference coil  240  and the selected one of the plurality of inductive sensors  242 , voltages proportional to the inductances are thereby generated. 
         [0025]    Referring to  FIG. 3 , depicted is a more detailed schematic block diagram of the synchronous detector and circuit functions shown in  FIG. 2 , according to a specific example embodiment of this disclosure. The synchronous detector  212  comprises a programmable decoder  360  and a plurality of analog pass-gates  362 . The programmable decoder  360  receives a clock (C LK ) signal at external connection  230  and an input selection (R EFSEL ) at external connection  228 , both from the digital processor  106 . The programmable decoder  360  controls the on and off states of the plurality of analog pass-gates  362 , as more fully described hereinafter. 
         [0026]    The programmable coil driver  210  generates AC voltages across the reference inductor  240  and the selected one of the plurality of inductive sensors  242  proportional to the inductances thereof. The selected one of the plurality of inductive sensor voltages (L BTN ) and the reference inductor voltage (L REF ) are coupled through DC blocking capacitors  246  and  248  to inputs of the synchronous detector  212  through external connections  224  and  226 , respectively. 
         [0027]    The plurality of analog pass-gates  362  may operate according to the following table: 
         [0000]    
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 REFSEL 
                 CLK 
                 Tx1 
                 Tx2 
                 Tx3 
                 Tx4 
                 Tx5 
                 Tx6 
                 Remarks 
               
               
                   
               
             
             
               
                 0 
                 0 
                 open 
                 open 
                 close 
                 close 
                 open 
                 open 
                 LREF 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 selected 
               
               
                 0 
                 1 
                 close 
                 open 
                 open 
                 open 
                 open 
                 close 
                 LREF 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 selected 
               
               
                 1 
                 0 
                 open 
                 close 
                 open 
                 close 
                 open 
                 open 
                 LBTN 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 selected 
               
               
                 1 
                 1 
                 close 
                 open 
                 open 
                 open 
                 close 
                 open 
                 LBT 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 selected 
               
               
                   
               
             
          
         
       
     
         [0028]    The synchronous detector  212   a  has three signal inputs used for measurement of the inductance of the inductive touch sensors, the reference inductor voltage (L REF ), the selected one of the plurality of inductive sensor voltage (L BTN ) and the reference voltage (V REF ) from the programmable voltage reference  220 . The synchronous detector  212   a  (mixer) can mix between two of the these three inputs at any given time at the frequency provided at the clock connection  230  (C LK ). For example, if R EFSEL  is at a logic zero, then the synchronous detector  212   a  mixes the reference inductor voltage and reference voltage signals. If the R EFSEL  is at a logic one, then the synchronous detector  212   a  mixes the selected one of the plurality of inductive sensor voltage (L BTN ) and reference voltage (L REF ) signals. By alternately mixing the reference inductor voltage or the selected one of the plurality of inductive sensor voltages (L BTN ) with the reference voltage (L REF ) at the same frequency as the approximately sinusoidal voltage being produced by the programmable coil driver  210 , a DC signal and an AC signal are generated at the output of the synchronous detector  212   a  (mixer) that is applied to the differential inputs of the PGA/low-pass filter  216   a.    
         [0029]    The PGA/low-pass filter  216   a  is used as a buffer-amplifier/low pass filter between the synchronous detector  212   a  and the V DETOUT  node  236 . PGA/low-pass filter  216   a  functions as an integrator and passes the DC voltage (difference frequency mixing result) while effectively suppressing the twice clock frequency (sum frequency mixing result). This DC voltage represents the inductance of the measured reference or selected touch sensor inductor, as discussed more fully above. The DC voltage may be fed to an analog-to-digital converter (ADC) (not shown) that is part of the digital processor  106 , whereby the digital processor  106  samples and performs inductive touch calculations in determining when a touch sensor is actuated. 
         [0030]    Also by alternating the polarity of connecting the touch inductor or reference inductor signals frequency mixing occurs that produces the sum and the difference frequencies between the clock input frequency and the frequency of the coil voltage. Since both frequencies are the same (the output of the programmable coil driver  210  is derived from the clock signal input) the mixing product sum of the frequencies will be twice the clock frequency and the difference of the frequencies will be at zero frequency, a DC voltage that is proportional to the inductance value of the measured coil. The differential outputs from the closed ones of the plurality of analog pass-gates  362  are applied to the differential inputs of the PGA/low-pass filter  216   a.  The programmable voltage reference  220  DC biases the circuits of the synchronous detector  212   a  at, for example but not limited to, about one-half the operating voltage for optimal operation of the differential input PGA/low-pass filter  216   a.  The PGA/low-pass filter  216   a  converts the differential output from the synchronous detector  212  to a single-ended voltage output, whereby DC utile (useful, desired) signals are made available to an analog input of the digital processor  106 . An isolate signal may be applied at node  250  to turn off all of the plurality of analog pass-gates  362  so as to isolate the PGA/low-pass filter  216   a  from the plurality of inductive sensors  242  and the inductive sensor  240  during a Vref measurement at node  252 , otherwise during normal operation the synchronous detector  212   a  functions as described hereinabove. 
         [0031]    Any or all of the functions (modules) associated with the programmable inductive touch AFE  104  may be programmable or fixed. An advantage of being able to programmable one or more of the functions during fabrication and/or by the user is that the programmable inductive touch AFE  104  may be tailored to a specific application and thereby further reduce the number of external components necessary to implement an electronic system having an inductive touch interface. 
         [0032]    The interface and control module  214  may communicate with the digital processor  106 , or any other appropriate communications and programming device, through a communications port coupled to external connection  234 . The communications port may be a serial interface, e.g., I 2 C, SPI, UNI/O, UART, etc., a parallel interface, and/or direct input-output (I/O) level control. The interface and control module  214  upon receiving programming parameters may control the aforementioned circuits and functions, for example but not limited to, as follows. 
         [0033]    The configuration and performance of the programmable coil driver  210  may be controlled by adjusting the current drive strength from the output thereof. The output signal waveform shaping characteristics may also be programmed for a desired output wave shape, thus reducing or eliminating the requirement for a low-pass filter between the drive-in external connection  232  (D RVIN ) and the clock external connection  230  (C LK ). The characteristics of the programmable coil driver  210  may be controlled by the interface and control module  214  through a serial or parallel (e.g., n-bit) bus  270 . 
         [0034]    The mixing frequency of the synchronous detector  212  may be controlled by the interface and control module  214  through a serial or parallel (e.g., y-bit) bus  276 . A programmable frequency divider may be included in the decoder &amp; frequency divider  360  ( FIG. 3 ). 
         [0035]    The configuration and performance of the PGA/low-pass filter  216   a  may be controlled by adjusting the voltage gain, output impedance, offset, gain-bandwidth product (GBWP) and/or corner roll-off frequency thereof by the interface and control module  214  through a serial or parallel (e.g., x-bit) bus  274 . 
         [0036]    The configuration of the programmable voltage reference  220  may be controlled by adjusting the output reference voltage value and/or the output impedance thereof by the interface and control module  214  through a serial or parallel (e.g., m-bit) bus  272 . 
         [0037]    Referring to  FIG. 4 , depicted is a more detailed schematic block diagram of a synchronous detector and the circuit functions shown in  FIG. 2 , according to another specific example embodiment of this disclosure. The synchronous detector  212   b  comprises a multiplexer  440  and a frequency mixer  446 . A programmable frequency divider  460  may also be used to divide the clock frequency, and the frequency division thereof may be controlled by the interface and control  214  over control line  276 . Operation of this embodiment of the synchronous detector  212   b  is similar to the operation of the synchronous detector  212   a  shown in  FIG. 3  and described hereinabove. The frequency mixer  446  may be, for example but is not limited to, a Gilbert Cell mixer. 
         [0038]    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 the disclosure.