Abstract:
An integrated circuit device inductive touch analog front end (AFE) excites selected ones of a plurality of inductive touch sensors, measures voltages across the coils of the plurality of inductive touch sensors, and provides analog output signals representative of these coil voltages. A physical displacement (touch) to the inductive sensor causes the inductance value of the inductive touch sensor to change with a corresponding change in a voltage across the coil of the inductive touch sensor. 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. 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 
     This application claims priority to commonly owned U.S. Provisional Patent Application Ser. No. 61/104,012; filed Oct. 9, 2008, and is related to commonly owned U.S. patent application Ser. No. 12/560,906; filed Sep. 16, 2009; and both are hereby incorporated by reference herein for all purposes. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to integrated circuits, and more particularly to, an integrated circuit device that supports inductive sensing. 
     BACKGROUND 
     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 
     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, thus reducing physical size and costs of manufacture for systems using inductive sensors. 
     According to a specific example embodiment of this disclosure, an integrated circuit device configured as an 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; and 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; 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 the low-pass filter substantially attenuates the sum mixing product and passes the difference mixing product to the output of the low-pass filter. 
     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 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; and 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; 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. 
     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. 
     The synchronous detector may also comprise: a frequency mixer; a decoder having a first input coupled to the clock and a second input coupled to the reference; and a plurality of multiplexers for selectively coupling the voltage reference, the inductive touch sensor, and the inductive reference coil external connection to the frequency mixer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a schematic block diagram of an electronic system having an inductive touch keypad, an inductive touch analog front end and a digital processor, according to a specific example embodiment of this disclosure; 
         FIG. 2  is a more detailed schematic block diagram of the inductive touch analog front end shown in  FIG. 1 ; 
         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; 
         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; 
         FIG. 5  is a more detailed schematic block diagram of the amplifier and low-pass filter shown in  FIG. 3 , according to a specific example embodiment of this disclosure; and 
         FIG. 6  is a more detailed schematic block diagram of the amplifier and low-pass filter shown in  FIG. 4 , according to another specific example embodiment of this disclosure. 
     
    
    
     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 foil is disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
     DETAILED DESCRIPTION 
     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. 
     Referring to  FIG. 1 , depicted is a schematic block diagram of an electronic system having an inductive touch keypad, an 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 (DSP), application specific integrated circuit (ASIC), programmable logic array (PLA), etC., is coupled to an 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. 
     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 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. 
     The digital processor  106  supplies clock and control functions to the inductive touch AFE  104 , reads the analog voltage detector output of the 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 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. 
     Referring to  FIG. 2 , depicted is a more detailed schematic block diagram of the inductive touch analog front end shown in  FIG. 1 . The inductive touch AFE  104  comprises a synchronous detector  212 , a control module  214 , a coil driver  210 , a voltage reference  220 , and an amplifier/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 . 
     The amplifier/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 amplifier/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 amplifier/low-pass filter  216  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 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. 
     The voltage reference  220  may be set to have a voltage output of approximately one-half the supply voltage, and may be an operational amplifier configured with unity gain and a non-inverting input to a resister ladder voltage divider. Other types of voltage references may be used effectively so long as there is adequate voltage stability and sufficient low impedance drive capability. 
     The 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 comprising capacitor  252  and resistor  250  attenuate the higher frequency components of the clock square wave signal so as to produce approximately a sinusoidal waveform that is input to the coil driver  210 , amplified, and then made available to 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 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 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 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. 
     Referring to  FIG. 3 , depicted 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. The synchronous detector  212   a  comprises a decoder  360  and a plurality of analog pass-gates  362 . The 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 decoder  360  controls the on and off states of the plurality of analog pass-gates  362 , as more fully described hereinafter. 
     The 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   a  through external connections  224  and  226 , respectively. 
     The plurality of analog pass-gates  362  may operate according to the following table: 
     
       
         
               
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 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 
                 LBTN 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                 selected 
               
               
                   
               
             
          
         
       
     
     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 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 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 differential amplifier/low-pass filter  216   a.    
     The differential amplifier/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 . The amplifier/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. 
     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 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 amplifier/low-pass filter  216   a . The voltage reference  220  DC biases the circuits of the synchronous detector  212   a  at about one-half the operating voltage for optimal operation of the amplifier/low-pass filter  216   a . The amplifier/low-pass filter  216   a  converts the differential output from the synchronous detector  212   a  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 amplifier/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. 
     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 multiplexers  440 ,  442  and  444 , and a frequency mixer  446 . 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. 
     Referring to  FIG. 5 , depicted is a more detailed schematic block diagram of the amplifier/low-pass filter shown in  FIG. 3 , according to a specific example embodiment of this disclosure. The amplifier/low-pass filter  216  may comprise an amplifier  550  having differential inputs, e.g., differential input operational amplifier; feedback resistors  258  and  260 , and capacitors  262  and  264  arranged in a low-pass filter configuration. It is contemplated and within the scope of this disclosure that other amplifier and low pass filter configurations may be used for the amplifier/low-pass filter  216 , as would be know to one of ordinary skill in analog circuit design and having the benefit of this disclosure. 
     Referring to  FIG. 6 , depicted is a more detailed schematic block diagram of the amplifier and low pass filter shown in  FIG. 4 , according to a specific example embodiment of this disclosure. The amplifier/low-pass filter  216   b  may comprise an amplifier  550  having differential inputs, e.g., differential input operational amplifier; feedback resistor  260 , and capacitor  264  arranged in a low-pass filter configuration. It is contemplated and within the scope of this disclosure that other amplifier and low pass filter configurations may be used for the amplifier/low-pass filter  216   b , as would be know to one of ordinary skill in analog circuit design and having the benefit of this disclosure. 
     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.