Patent Publication Number: US-2022236816-A1

Title: Combined Capacitive and Piezoelectric Sensing in a Human Machine Interface

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Application No. 63/142,604, filed Jan. 28, 2021, which is hereby fully incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This relates to input devices for electronic systems, and more particularly to touch-sensitive input devices. 
     Mechanical buttons for user actuation of switches or other controls for electronic systems have been commonplace for many years. In newer systems, however, capacitive sensing is rapidly becoming a prevalent human-machine interface (HMI) technology. Actuators using capacitive sensing can be realized in thinner and more elegant forms, of various shapes and sizes, as compared with conventional mechanical buttons and switches. As such, a wide range of HMI applications including appliances, point of sale terminals, security systems, environmental controls, security systems, and other industrial and consumer applications now use capacitive touch sensors. 
     By way of further background, conventional capacitive sensors operate by detecting changes in capacitance due to a user&#39;s finger touching or being in close proximity to the button, slider, wheel, or other actuator. One conventional approach for this measurement is referred to as a “self capacitance” measurement, in which the capacitance at a sensing element relative to earth ground is measured. The user input is detected as increase in this capacitance by the addition of a parallel capacitance from a user&#39;s finger (at earth ground) touching an insulating overlay at the sensing element. Self capacitance measurements are often used to implement buttons (e.g., elevator buttons). Another approach, referred to as a “mutual capacitance” measurement, is based on a capacitive sensing element having one plate as a transmit electrode and a second plate as a receive electrode, between which a potential is maintained. A user input is detected from disruption of electric field propagation between the transmit and receive electrodes caused by a user&#39;s finger (at earth ground) touching an overlay over the electrodes. Mutual capacitance measurements are often used with slider or wheel HMI elements. 
     By way of further background, mixed-signal microcontroller integrated circuits that include measurement capability for capacitive touch sensing are known in the art. One example of such an integrated circuit is the MSP430FR267x microcontroller available from Texas Instruments Incorporated. 
     Certain challenges are presented for capacitive HMI devices deployed in certain environments in which the capacitive effect of user inputs may be attenuated. For example, the user of an outdoor keypad entry system, or such a system in a sterile or clean room environment, may be wearing gloves, which will reduce the capacitive effect of a finger press or movement. Similarly, rain, ice, or other environmental conditions at the keypad may insulate the user&#39;s finger from the capacitive sensor, also reducing the ability of the HMI device to detect the user input. Increasing the amplification gain of the signal from the capacitive element to compensate for the smaller change in capacitance under these conditions can give rise to other problems such as the detection of spurious or false inputs. 
     It is within this context that the embodiments described herein arise. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect, an apparatus is provided that includes capacitive measurement circuitry, coupled to one or more capacitor input terminals at which capacitive touch elements in a human machine interface (HMI) may be connected, and piezoelectric measurement circuit including interface circuitry coupled to one or more piezoelectric terminals at which piezoelectric touch elements in the HMI may be connected. The capacitive measurement circuitry includes a gain stage configured to amplify a signal corresponding to a capacitance at the one or more capacitor input terminals by a gain level for communication to processing circuitry. The apparatus further includes gain control circuitry coupled to the piezoelectric measurement circuitry and the capacitive measurement circuitry, and that is configured to increase the gain level of the gain stage responsive to the piezoelectric measurement circuitry receiving a user input from at least one of the one or more piezoelectric terminals. 
     According to another aspect, a method of detecting user inputs at an HMI is provided. According to one or more example embodiments, the method includes setting a gain level in at least one gain stage in capacitive touch measurement circuitry coupled to a plurality of capacitive touch elements of the HMI to a first gain level, and determining whether one or more piezoelectric touch elements of the HMI is detecting user touch pressure. Responsive to none of the one or more piezoelectric touch elements of the HMI detecting user touch pressure, the capacitive touch measurement circuitry generates measurement signals corresponding to capacitance at one or more of the capacitive touch elements of the HMI using the first gain level. Responsive to one or more piezoelectric touch elements of the HMI detecting user touch pressure, the gain level in at least one gain stage in the capacitive touch measurement circuitry is set to a second gain level greater than the first gain level, and the capacitive touch measurement circuitry generates measurement signals using the second gain level. 
     Technical advantages enabled by one or more of these aspects include a system and method for detecting and sensing of touch inputs at keypads and other HMI input devices deployed outdoors or in other hostile environments, for example in cold environments in which the user may be wearing gloves and in wet environments in which the capacitive sensing of touch inputs is attenuated, even for users using bare fingers. This improved sensing in such environments is enabled without increase vulnerability of the HMI input device to increased noise, thermal drift, and power consumption. The frequency of detecting false inputs, including both false positives and false negatives, can be reduced. 
     Other technical advantages enabled by the disclosed aspects will be apparent to those of ordinary skill in the art having reference to the following specification together with its drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is an electrical diagram, in block form, of a human machine interface (HMI) system according to example embodiments. 
         FIG. 2  is an electrical diagram, in block form, of a capacitive measurement function in the system of  FIG. 1  according to an example embodiment. 
         FIG. 3  is an electrical diagram, in block form, of a capacitive touch measurement block in the function of  FIG. 2  according to an example embodiment. 
         FIG. 4  is an electrical diagram, in block and schematic form, of a piezoelectric measurement function in the system of  FIG. 1  according to an example embodiment. 
         FIG. 5A  is a flow diagram of a method of detecting touch inputs in an HMI system as in  FIG. 1  according to an example embodiment. 
         FIG. 5B  is a flow diagram of a process of detecting changes in capacitance in the method of  FIG. 5A  according to an example embodiment. 
         FIG. 6  is a flow diagram of a method of detecting touch inputs in an HMI system as in  FIG. 1  according to an alternative example embodiment. 
         FIG. 7  is an electrical diagram, in block form, of a human machine interface (HMI) system according to another example embodiment. 
         FIG. 8  is an electrical diagram, in block and schematic form, of piezoelectric driver circuitry in combination with a piezoelectric measurement function in the system of  FIG. 7  according to an example embodiment. 
         FIG. 9  is a flow diagram of a method of detecting touch inputs and producing haptic output in an HMI system as in  FIG. 7  according to an example embodiment. 
     
    
    
     The same reference numbers or other reference designators are used in the drawings to illustrate the same or similar (in function and/or structure) features. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The one or more embodiments described in this specification are implemented into a human machine interface (HMI) device, such as a keypad, as it is contemplated that such implementation is particularly advantageous in that context. However, it is also contemplated that aspects of these embodiments may be beneficially applied in a wide variety of other applications, for example switches, actuators, keyboards, sliders, and other HMI implementations. Accordingly, it is to be understood that the following description is provided by way of example only and is not intended to limit the true scope of this invention as claimed. 
       FIG. 1  illustrates the architecture of a touch-sensitive HMI system constructed according to an example embodiment. In this example, keypad  100  includes capacitive array  102 , which includes one or more capacitive touch elements  112 . In this example, capacitive array  102  includes twelve capacitive touch elements  112 , each deployed in keypad  100  at a location corresponding to a button or other HMI actuator. In this example, capacitive touch elements  112  are deployed in the conventional numeric keypad arrangement. Each capacitive touch element  112  may be constructed in the conventional way, for example including two conductive elements or plates in combination with an insulating overlay or other film at which a user&#39;s touch may affect the capacitance of the element to provide a user input. Keypad  100  may be constructed to provide visible indicators of the locations of capacitive touch elements  112 , enabling the user to make meaningful user inputs to the system. Capacitive touch elements  112  may be implemented in either of the self capacitance or mutual capacitance measurement modes in this example embodiment. 
     According to this example embodiment, keypad  100  also includes piezoelectric array  104  including one or more piezoelectric touch elements  114 . In this example, piezoelectric array  104  includes four piezoelectric touch elements  114 A through  114 D, each deployed in keypad  100 . In this example embodiment, piezoelectric touch elements  114 A through  114 D may be embedded within keypad  100  and not visible or otherwise associated with a visible indicator for the user. Each piezoelectric element  114  may be constructed of a conventional piezoelectric material such as lead zirconate titanate (PZT) disposed between electrodes and configured to generate a voltage in response to pressure from a user&#39;s press of the element  114 . 
     In this example, capacitive array  102  and piezoelectric array  104  are arranged in keypad  100  to overlay one another, or in some other arrangement so that the same user touch at certain locations of keypad  100  is detectable by both a capacitive touch element  112  and a piezoelectric touch element  114 . In the example embodiment of  FIG. 1 , the number of piezoelectric touch elements  114  differs from the number of capacitive touch elements  112 , and thus the locations of the two types of touch elements do not necessarily coincide. Alternatively, both a capacitive touch element  112  and a piezoelectric touch element  114  may be deployed at one or more actuator locations of keypad  100 ; in some implementations, both a capacitive touch element  112  and a piezoelectric touch element  114  may be deployed at each actuator location of keypad  100 . As noted above, piezoelectric touch elements  114  may be embedded within keypad  100  so as to not be visible. Alternatively, piezoelectric touch elements  114  may be associated with visible indicators associated separate from those associated with capacitive touch elements  112 . Further in the alternative, one or more piezoelectric touch elements  114  may be associated with the same visible indicator as a capacitive touch element  112 . 
     In the system of  FIG. 1 , capacitive array  102  and piezoelectric array  104  of keypad  100  are coupled to microcontroller  110 . Microcontroller  110  in this example embodiment includes a central processing unit (CPU)  120 , for example arranged as a reduced instruction set computer (RISC) architecture operating on data in a register file. In this example architecture of microcontroller  110 , CPU  120  is coupled to various peripheral functional circuitry modules via address bus ADDR_BUS and data bus DATA BUS. In the example shown in  FIG. 1 , these functional modules include memory resources such as random access memory  124  and read-only memory  126 , one or more input/output interface functions  128 , one or more timers  130 , analog-to-digital converter (ADC) module  132 , and clock system  134 . Other functional circuitry modules may alternatively or additionally be implemented in microcontroller  110  as desired for the particular application. Also as shown in  FIG. 1 , power management function  122  is separately coupled to CPU  120  and is configured for managing power consumption and supply to CPU  120  and the various functional modules. Other support modules such as scan test functionality and the like may also be included. Microcontroller  110  may alternatively be realized with alternative bus architectures and according to other architectural variations from that shown in  FIG. 1 . 
     According to this example embodiment, microcontroller  110  includes capacitive measurement circuitry  140  and piezoelectric measurement circuitry  150  as additional functional modules, each coupled to CPU  120  via address bus ADDR_BUS and data bus DATA_BUS. In this example, capacitive measurement circuitry  140  is coupled to capacitive array  102  in keypad  100  and is configured to acquire measurements of capacitance from the one or more capacitive touch elements  112  in capacitive array  102 . Similarly, piezoelectric measurement circuitry  150  is coupled to piezoelectric array  104  in keypad  100  and is configured to acquire measurements of pressure from the one or more piezoelectric touch elements  114  in piezoelectric array  104 . 
     In this example embodiment, piezoelectric measurement circuitry  150  is constructed or configured to cause a gain control signal to be forwarded to capacitive measurement circuitry  140  in response to measurements acquired from the one or more piezoelectric touch elements  114  in piezoelectric array  104 . For purposes of illustration, microcontroller  110  includes a control line PZO_INT coupled from piezoelectric measurement circuitry  150  to CPU  120  for communicating an interrupt request in response to piezoelectric measurement circuitry  150  detecting a touch event at piezoelectric array  104 . Microcontroller  110  also includes a control line GN_CTRL to communicate a gain control signal from CPU  120  to capacitive measurement circuitry  140 , in response to the interrupt request from piezoelectric measurement circuitry  150  on line PZO_INT. Alternatively, CPU  120  may receive measurements from piezoelectric measurement circuitry  150  and may communicate gain control signals to capacitive measurement circuitry  140  over buses ADDR_BUS, DATA_BUS. Further in the alternative, piezoelectric measurement circuitry  150  may be configured to communicate the gain control signal directly to capacitive measurement circuitry  140 , without involving CPU  120 . 
     Referring now to  FIG. 2 , the construction of capacitive measurement circuitry  140  according to an example embodiment will now be described. In this example embodiment, input/output multiplexer  200  in capacitive measurement circuitry  140  is coupled to one or more terminals of microcontroller  100  that is configured to interface with capacitive touch elements  112 . Input/output multiplexer  200  in this example embodiment operates to couple the one or more selected capacitive touch elements  112  to a corresponding capacitive touch measurement circuit  202 . As suggested in  FIG. 2 , capacitive measurement circuitry  140  may include multiple capacitive touch measurement circuits  202   a ,  202   b ,  202   c  to enables parallel scanning of multiple capacitive touch elements  112 . Each capacitive touch measurement circuit  202  may be configurable to operate in either self capacitance measurement mode or in mutual capacitance measurement mode. As such, as suggested in  FIG. 2 , terminals coupled to input/output multiplexer  200  may include transmit and receive terminals from which capacitive touch measurement circuit  202  detects inputs from a capacitive touch element  112   m  coupled in the mutual capacitance mode, and may include receive terminals from which capacitive touch measurement circuit  202  detects inputs from a capacitive touch element  112   s  coupled in the self capacitance mode. 
     The measurements made by each capacitive touch measurement circuit  202  in the example of  FIG. 2  are controlled by various support circuitry  205  in capacitive measurement circuitry  140 . As shown in  FIG. 2 , this support circuitry  205  for making the relevant measurements include low drop-out (LDO) voltage regulator  210 , reference voltage generator circuitry  212 , and reference capacitors  214 . In this example embodiment, support circuitry  205  includes frequency hopping and spread spectrum oscillator circuitry  216  to enable performing the capacitance measurements at various frequencies, so that capacitance measurements at frequencies corrupted by common-mode noise may be rejected. Control and reporting of results of the capacitance measurements made by capacitive touch measurement circuits  202   a  through  202   c  is managed by conversion and control logic  218 , which interfaces with data bus DATA_BUS and, in this example, is capable of communicating interrupt requests to CPU  120  on separate control line INT in response to the capacitance measurements or other events. Capacitive measurement circuitry  140  in this example also includes event timer  220  and other timer circuitry, operating based on a clock signal received on line CLK and generating interrupt requests on line INT as appropriate. 
       FIG. 3  illustrates the construction of capacitive touch measurement circuit  202   a  in capacitive measurement circuitry  140  according to an example embodiment in which the capacitive measurement is made using a charge transfer measurement technique. An example of a charge transfer measurement technique as may be applied by capacitive touch measurement circuit  202   a  according to this example embodiment is described in U.S. Patent Application Publication No. US2021/0050852 A1, commonly assigned herewith and incorporated herein by this reference. As shown in  FIG. 3 , input/output multiplexer  200  couples a capacitive touch element  112  at its terminals, configured in the self capacitance or mutual capacitance mode, to charge transfer engine  300  in capacitive touch measurement circuit  202   a . According to the charge transfer measurement technique applied in this example, charge transfer engine  300  is configured to measure the capacitance at the capacitive touch element  112  by alternately charging capacitive touch element  112  to a selected voltage and transferring the charge from capacitive touch element  112  to capacitor  312  in integrator/trip detector  310  (via signal conditioning circuitry  304  in this example). Integrator/trip detector  310  also includes comparator  314 , which compares the voltage at capacitor  312  with a reference voltage VREF. In this example, the charge/transfer cycles applied to capacitive touch element  112  by charge transfer engine are clocked by a conversion clock from frequency hopping and spread spectrum oscillator circuitry  216  of  FIG. 2 . In this example, the conversion clock frequency is varied by frequency hopping and spread spectrum oscillator circuitry  216  so that measurements may be obtained at multiple frequencies, enabling measurements made at noisy frequencies to be omitted from the capacitance measurement. 
     As noted above, capacitive touch measurement circuit  202  in this implementation includes signal conditioning function  304 . In this example, signal conditioning function  304  includes gain stage  306  for amplifying the charge transferred from capacitive touch element  112  by charge transfer engine  300 , and may also include offset compensation  308  to compensate for offset. According to this example implementation, the gain applied by gain stage  306  is at least in part determined by a gain control signal communicated on line GN_CTRL from CPU  120  in response to measurements acquired by piezoelectric measurement circuitry  150  from piezoelectric array  104 . Other signal conditioning including filtering may also be applied in signal conditioning function  304 . 
     According to the charge transfer measurement technique, transfer counter  302  counts the number of charge/transfer cycles performed until comparator  314  detects that the voltage at capacitor  312  reaches reference voltage VREF, in response to which comparator  314  issues an end of conversion signal, for example as an interrupt request to CPU  120 . In response to the end of conversion, CPU  120  can then interrogate transfer counter  302  to obtain the number of charge/discharge cycles counted as the conversion result, which CPU  120  can process to determine whether a touch event has occurred. For example, CPU  120  may compare the conversion result (e.g., as obtained at the less noisy conversion clock frequencies) relative to a long term average corresponding to a filtered version of conversion counts previously obtained from that capacitive touch element  112 . For example, a difference in the measured capacitance at a capacitive touch element  112 , as determined from the conversion result, as compared to the long term average indicates a change in capacitance that may be due to a user touch at that element  112 . Accordingly, in response to the obtained conversion result exceeding a touch threshold value (e.g., a count corresponding to a proportional offset from the long term average), CPU  120  can identify a touch event at capacitive touch element  112  and process the input accordingly. 
     According to this example embodiment using a charge transfer measurement technique, the gain applied by gain stage  306  to the transferred charge signal determines the sensitivity of the capacitance measurement, in that a higher gain enables smaller differences in capacitance (e.g., as may occur from a touch of capacitive touch element  112  by a user wearing gloves) to be reliably detected as a touch event. In alternative implementations, it is contemplated capacitive measurement circuitry  140  may be implemented so as to measure capacitance at capacitive touch elements  112  (including changes in capacitance) according to other approaches for generating a measurement signal that may be amplified by a selected gain that at least in part determines the sensitivity of the measurement. It is contemplated that the aspects described in this specification may similarly be applied in such alternative implementations. 
       FIG. 4  illustrates the architecture and construction of piezoelectric measurement circuitry  150  according to an example embodiment. In this example and as shown in  FIG. 1 , piezoelectric measurement circuitry  150  includes an instance of receiver circuitry coupled to each of four external terminals  401 A through  401 D of microcontroller  110 , each of which one of piezoelectric touch elements  114 A through  114 D is respectively coupled. In this example, referring to piezoelectric touch element  114 A, this receiver circuitry includes analog interface  400 A and amplifier circuitry  402 A. Analog interface  400 A is coupled to terminal  401 A and includes the appropriate circuit components to forward voltage signals from piezoelectric touch element  114 A via diode  405 A and to an input of amplifier circuitry  402 A. For example, analog interface  400 A may include a resistor and capacitor network as shown in  FIG. 4  to level-shift the input signal and apply a low-pass filter characteristic. Diode  405 A has its anode coupled to an output node of analog interface  400 A, and its cathode coupled to a wired-OR node PZO_OR. 
     In this example embodiment, amplifier circuitry  402 A has an input coupled to receive the signal from piezoelectric touch element  114 A after conditioning by analog interface  400 A. Amplifier circuitry  402 A may be constructed in any one of a number of configurations.  FIG. 4  illustrates one example of such construction as including operational amplifier  415  with its positive input coupled to an output node of analog interface  400 A, and its negative input receiving feedback from its output via a voltage divider arrangement in the conventional manner. Amplifier circuitry  402 A in this example also includes switch  412  coupled in series with resistor  413  between the positive input of amplifier  415  and ground. Switch  412  may be controlled by a signal (not shown) from CPU  120  or other control circuitry to reset the voltage at the input of amplifier  415  between measurement instances. As noted above, amplifier circuitry  402 A may be constructed and configured in other arrangements as desired for a particular application. In this example embodiment, the output of amplifier  415  is coupled to one input of multiplexer  430 . 
     Piezoelectric measurement circuitry  150  includes similarly constructed analog interfaces  400 B,  400 C,  400 C and amplifier circuitry  402 B,  402 C,  402 D coupled to terminals  401 B,  410 C,  401 D, respectively. In similar fashion as amplifier circuitry  402 A described above, the output of each instance of amplifier circuitry  402 B,  402 C,  402 D is coupled to a corresponding input of multiplexer  430 . 
     According to this example embodiment and similarly as described above relative to analog interface  400 A, each of analog interfaces  400 B,  400 C,  400 C has an output node coupled to an anode of a corresponding diode  405 B,  405 C,  405 D. The cathodes of diodes  405 A,  405 B,  405 C,  405 D are all coupled together at wired-OR node PZO_OR, which is coupled via bias network  420  to a positive input of comparator  422 . Comparator  422  has a negative, or reference, input coupled to an output of digital-to-analog converter (DAC)  424  via line DET_LVL. Comparator  422  and DAC  424  may be realized as a functional module in microcontroller  110 , residing on address bus ADDR_BUS or data bus DATA_BUS as shown in  FIG. 1 , or alternatively may be deployed within piezoelectric measurement circuitry  150  itself. In any event, DAC  424  has an input coupled to receive a digital signal indicating a reference level on digital lines DREF, for example from CPU  120  over data bus DATA_BUS. DAC  424  operates to convert this digital reference level DREF into an analog level communicated to the negative input of comparator  422  on line DET_LVL. The output of comparator  422  presents a logic signal on line PZO_INT as an interrupt request to CPU  120  in response to a comparison of the signal at wired-OR node PZO_OR to the analog level on line DET_LVL. 
     According to this example embodiment, comparator  422  asserts an interrupt request on line PZO_INT in response to a user touch at one or more of piezoelectric elements  114 A through  114 D. For example, a user touch at piezoelectric element  414 A causes a voltage at the corresponding terminal  401 A that is coupled via analog interface  400 A to the anode of diode  405 A. If the user touch is of sufficient pressure to produce a voltage that forward biases diode  405 A, that voltage will appear at wired-OR node PZO_OR (less a diode voltage drop). Similarly, user touches at any one or more of piezoelectric elements  414 B through  414 D will also source current into wired-OR node PZO_OR, while diodes  405 B through  405 D for those piezoelectric elements  414 B through  414 D not experiencing a touch will remain reverse-biased. Upon the voltage at wired-OR node PZO_OR resulting from a touch input exceeding the reference level from DAC  424  on line DET_LVL, comparator  422  asserts (e.g., drives a logic “1” level) at its output, which is communicated to CPU  120  as an interrupt request indicating from a user touch somewhere at piezoelectric array  104  of keypad  100 . 
     As will be described in further detail below in connection with one or more example embodiments, CPU  120  responds to the piezoelectric interrupt request generated by piezoelectric measurement circuitry  150  to increase the gain applied by gain stage  306  in one or more capacitive touch measurement circuits  202 , and thus increase the sensitivity of microcontroller  110  to capacitive touch user inputs. 
     As mentioned above in connection with  FIG. 4 , multiplexer  430  has inputs coupled to the outputs of amplifier circuits  402 A,  402 B,  402 C,  402 D. Multiplexer  430  also has a select input coupled to receive a select signal on line SEL, for example from CPU  120 , to select one of its inputs for forwarding to an input of ADC  432 . ADC  432  in  FIG. 4  may correspond to ADC module  132  shown in  FIG. 1  as residing on address bus ADDR_BUS and data bus DATA_BUS, or alternatively may be realized within piezoelectric measurement circuitry  150  itself. ADC  432  receives a reference voltage on line VREF, for example from a reference voltage generator elsewhere in microcontroller  110  and converts the analog voltage at its input to a digital signal DOUT for presentation to CPU  120 , for example via data bus DATA_BUS. 
     In operation, multiplexer  430  and ADC  432  of piezoelectric measurement circuitry  150  can operate to interrogate the receiver circuitry associated with each of its terminals  401 A through  401 C to determine which one or more of piezoelectric elements  114 A through  114 D is receiving a touch input, and a measure of the pressure of that touch input. For example, this interrogation of individual piezoelectric elements may be performed in response to comparator  422  indicating the presence of a touch input somewhere at piezoelectric array  104 . In this case, multiplexer  430  may individually forward the output from each amplifier circuit  402 A through  402 D to ADC  432  for determining which one is presenting the highest amplitude output, and thus determine which piezoelectric touch element  114  received the input. In addition, to reduce the power consumption of piezoelectric measurement circuitry  150 , amplifier circuitry  402 A through  402 D may be disabled until such time as comparator  422  indicates that a touch input was received at one or more of piezoelectric elements  114 A through  114 D; in response to this indication, amplifier circuitry  402 A through  402 D may then be powered up, for interrogation via multiplexer  430  and ADC  432  as described above. 
     In some implementations in which it is not required to identify which individual piezoelectric element  114 A through  114 D is receiving a touch input, piezoelectric measurement circuitry  150  may omit amplifier circuitry  402 A through  402 D, multiplexer  430 , and ADC  432  altogether. In the alternative, comparator  422  would remain to indicate the presence of a touch input somewhere at piezoelectric array  104 , by issuing the interrupt request on line PZO_INT as noted above. 
       FIG. 5A  and  FIG. 5B  illustrate the generalized operation of the system of  FIG. 1 , including the response of microcontroller  110  to touch inputs received at keypad  100  according to one or more example embodiments. In this example, it is contemplated that these operations will be carried out by and under the direction and control of CPU  120  in combination with other functions in microcontroller  110 . For example, CPU  120  may carry out and control these operations by executing program instructions stored in machine-readable form in the memory resources of the system, such as ROM  126  and in some implementations RAM  124 . Alternatively or in addition, some or all of the operations described herein may be executed by special-purpose or dedicated logic circuitry. 
     As shown in  FIG. 5A , operation of the system begins with process  500  in which the gain applied by gain stages  306  of capacitive measurement circuitry  140  capacitance signals as sensed at capacitive touch elements  112  in keypad  100  is set to a nominal level. In this example embodiment, this nominal level corresponds to a relatively low gain level suitable for detection of changes in capacitance at capacitive touch elements  112  in response to a normal touch input by the bare finger of a user in good environmental conditions (e.g., dry conditions). Various inaccuracies and other problems in the touch input system can occur if the gain applied by gain stages  306  is too high, including vulnerability to “false positives” due to noise in capacitive measurement circuitry  140 , vulnerability to thermal drift, and higher than optimal power consumption, especially in battery-powered systems. The setting of a low nominal gain in process  500  avoids these issues for those situations in which the low gain adequately detects user touch inputs at keypad  100 . 
     Decision  501  determines whether piezoelectric measurement circuitry  150  has detected the presence of a touch input at one or more piezoelectric touch elements  114 A through  114 D in this example. As described above relative to  FIG. 4 , this determination may be made by comparator  422  comparing the voltage at wired-OR node PZO_OR with a reference level on line DET_LVL from DAC  424 . In this example, if a touch input of sufficient pressure is present at one or more of piezoelectric elements  114 A through  114 D, the resulting voltage generated by that piezoelectric element will forward-bias the corresponding diode  405 A through  405 D and appear at wired-OR node PZO_OR at a voltage exceeding reference level DET_LVL. In response, comparator  422  issues an interrupt request in the form of a logic “1” level at its output, which is communicated on line PZO_INT to CPU  120  in this example implementation. Decision  501  thus returns a “yes” result. 
     If a touch input of sufficient pressure is not present at keypad  100 , the signal level at wired-OR node PZO_OR does not cause comparator  422  to assert the interrupt request. In this case (decision  501  is “no”), microcontroller  110  continues to sense capacitance at capacitive touch elements  112  of keypad  100 , amplifying the corresponding measurement signals using the nominal gain value set in process  500  for communication to CPU  120  over data bus DATA_BUS to indicate which capacitive touch elements  112  of keypad  100  received the user input. Operation in this manner continues until detection of a touch input of sufficient force by piezoelectric measurement circuitry  150 . 
     If a piezoelectric touch input is detected (decision  501  is “yes”), CPU  120  receives the interrupt request issued by piezoelectric measurement circuitry  150  on line PZO_INT. CPU  120  processes this requested interrupt in process  504  by increasing the gain applied by gain stages  306  in capacitive measurement circuitry  140  to a selected increased level. For example, as suggested in  FIG. 1 , CPU  120  may issue a signal to capacitive measurement circuitry  140  (e.g., to gain stages  306 ) on control line GN_CTRL; alternatively, CPU  120  may issue the gain increase signal over data bus DATA_BUS. Further in the alternative, piezoelectric measurement circuitry  150  may itself issue the gain control signal directly to capacitive measurement circuitry  140 . In any case, this increased gain level applied in process  504  increases the sensitivity of the touch input detection at capacitive touch elements  112 , for example to be sensitive enough to reliably detect a touch input by a gloved finger, or to detect a touch input by a bare finger in wet (e.g., rainy) conditions or through ice overlaying keypad  100 , etc. The capacitive measurement signals are thus amplified using the increased gain value set in process  504  for communication to CPU  120 , from which CPU  120  can determine which capacitive touch elements  112  received the user input. 
     According to this example embodiment, the sensing of changes in capacitance at capacitive touch elements  112  is to be performed using the increased gain level for at most a particular duration, after which the gain applied by gain stages  306  returns to its nominal or other lower gain level. As such, also in process  504 , CPU  120  starts a timer operation, for example as may be monitored by timer  130  in microcontroller  110 . 
     Once the gain level is increased in process  504 , capacitive measurement circuitry  150  then operates to sense capacitance at one or more of capacitive touch elements  112  of keypad  100  in process  506 . In this example embodiment, capacitive measurement circuitry  150  may repeatedly scan capacitive touch measurement blocks  202  to interrogate each capacitive touch element  112  (e.g., each numeric key in keypad  100 ). Inputs based on the user touch inputs at keypad  100  are then detected by capacitive measurement circuitry  150  using this increased gain level and forwarded to CPU  120  for decoding and other processing to carry out the desired operations of microcontroller  110  in response. 
     It is conceivable that a user may make a touch input with a bare finger, in good environmental conditions, that is sufficient to not only provide an adequate input for detection and measurement by capacitive measurement circuitry  140  using nominal gain, but that is also of sufficient pressure to be detected by piezoelectric measurement circuitry  150  (decision  501  is “yes”), causing the gain applied by capacitive measurement circuitry  140  to be increased in process  504 . But for bare finger inputs in good conditions, this increased gain level may be too high for accurate operation and can cause excessive power consumption.  FIG. 5B  illustrates an optional method of executing process  506  to adjust the gain of capacitive measurement circuitry  140  for this situation. 
     In this alternative approach, process  506  is performed by capacitive measurement circuitry  140  first detecting and measuring capacitance at one or more capacitive touch elements  112  in process  520 . For this first pass through process  520 , the gain applied by gain stages  306  is the increased gain applied in process  504 . The measured capacitance signals are forwarded by capacitive measurement circuitry  140  to CPU  120  for its comparison with a maximum threshold level T_MAX in decision  525 . For example, this maximum threshold level T_MAX may be selected to detect an amplitude of capacitance change corresponding to a bare finger at a capacitive touch element  112  under ideal conditions. If CPU  120  determines that the amplitude of the capacitance change detected in process  520  is below this maximum threshold level T_MAX (decision  525  is “no”), for example as in the case of a touch input at keypad  100  by a gloved finger or a bare finger in wet or icy conditions, the increased gain level will be maintained and process  540  will be performed at this increased gain level to acquire the user inputs from keypad  100  for processing and response by CPU  120 . If, however, the amplitude of the capacitance change detected in process  520  is above this maximum threshold level T_MAX (decision  525  is “yes”), for example as in the case of a touch input at keypad  100  by a bare finger in dry conditions, the gain level applied by gain stages  306  will be reduced to a lower gain level in process  530 , for example reduced to the nominal level originally set in process  500 . Process  540  will then be performed at this nominal gain level to acquire the user inputs from keypad  100  for processing and response by CPU  120 . 
     As noted above in connection with process  504 , the increased gain at gain stages  306  is to be applied by gain stages  306  to the capacitance measurements from keypad  100  for a certain duration, after which the gain is to return to the nominal or other lower level. It is contemplated that this duration may be preselected and stored in a register or other memory location in advance, for example as a fixed value set at manufacture or as a user-programmable value. For example, this duration may be set to a value of a few seconds to allow sufficient time for a user touch input at keypad  100 . Referring back to  FIG. 5A , while sensing capacitance at capacitive touch elements  112  in process  506  using the increased gain level applied in process  504 , microcontroller  110  periodically interrogates the timer (e.g., timer  130 ) set in process  504  to determine whether this duration has elapsed. If the duration of increased gain has not yet elapsed (decision  507  is “no”), capacitance at capacitive touch elements  112  in keypad  100  will continue to be measured by capacitive touch measurement blocks  202  using the increased gain level set in process  504 . 
     If the duration of increased gain has elapsed (decision  507  is “yes”), microcontroller  110  resets the gain applied by gain stages  306  in capacitive touch measurement blocks  202  to its nominal or other lower value. Sensing of touch inputs at keypad  100  then continues from process  500  as described above. 
     The combination of piezoelectric and capacitive touch input detection in the system and methods described above in this and other example embodiments, provides important technical advantages in the operation of HMI systems utilizing touch inputs. In a general sense, these example embodiments provide a system and method for detecting and sensing of touch inputs at keypads and other HMI input devices deployed outdoors or in other hostile environments, for example in cold environments in which the user may be wearing gloves and in wet environments in which the capacitive sensing of touch inputs is attenuated, even for users using bare fingers. Increased sensitivity of capacitive touch inputs is efficiently provided, according to these example embodiments, by invoking the increased gain in response to significant touch pressure as sensed at piezoelectric touch elements, as well as by limiting the duration of the increased gain. Vulnerability to increased noise, thermal drift, and power consumption as would otherwise result from unconditionally increasing the gain for capacitive touch measurement are avoided and limited. The frequency of detecting false inputs, including both false positives and false negatives, can be reduced accordingly. 
       FIG. 6  illustrates the operation of the system of  FIG. 1  according to another example embodiment in which keypad  100  includes more than one piezoelectric touch element  114 , for example four such piezoelectric touch elements  114 A through  114 D physically deployed at quadrants of keypad  100  as suggested in  FIG. 1 . As will now be described, the multiple piezoelectric touch elements  114  are used to advantage to adjust the sensitivity of the system more precisely to user touch inputs. 
     In the example embodiment of  FIG. 6 , the operation of the system begins with process  600  in which the gain applied by gain stages  306  of capacitive measurement circuitry  140  is set to a nominal level. In decision  601 , microcontroller  110  determines whether piezoelectric measurement circuitry  150  has detected the presence of a touch input at one or more piezoelectric touch elements  114 A through  114 D, for example by comparator  422  comparing the voltage at wired-OR node PZO_OR with a reference level on line DET_LVL from DAC  424  as described above. If not (decision  601  is “no”), microcontroller  110  continues to sense capacitance changes at capacitive touch elements  112  of keypad  100  using the nominal gain value set in process  600 . 
     According to this example embodiment, if a touch input of sufficient pressure is present at one or more of piezoelectric elements  114 A through  114 D (decision  601  is “yes”), comparator  422  will issue an interrupt request in the form of a logic “1” level at its output, which is communicated on line PZO_INT to CPU  120  in this example implementation. In this example, CPU  120  will handle this interrupt resulting from detection of a piezoelectric input by determining which of piezoelectric touch elements  114 A through  114 D received the detected touch input. Referring to  FIG. 4 , process  602  may be executed by CPU  120  issuing select control signals on line SEL to multiplexer  430  in piezoelectric measurement circuitry  150  to select each of amplifiers  402 A through  402 D in turn. As described above, the one of amplifiers  402 A through  402 D selected by multiplexer  430  will apply its output to ADC  432  for conversion of the analog amplifier output signal to a digital value on lines DOUT. The digital values on lines DOUT are communicated to CPU  120 , for example as a sequence of values communicated over data bus DATA_BUS. In process  602 , CPU  120  determines, from this sequence of digital values, which of the amplifiers  402 A through  402 D produced the highest amplitude signal, and thus which one of the piezoelectric touch elements  114 A through  114 D was nearest to the user&#39;s touch input. This determines the region of keypad  100  that was touched by the user. For the example of piezoelectric touch elements  114 A through  114 D physically deployed in quadrants of keypad  100 , this determination of process  602  identifies which quadrant of keypad  100  was pressed by the user. 
     After identification of the touched region of keypad  100  in this example embodiment, CPU  120  operates in process  604  to increase the gain applied by gain stages  306  in capacitance touch measurement blocks  202  associated with capacitive touch elements  112  that are located in the identified region. As described above, CPU  120  may increase the gain of the identified capacitive touch measurement blocks  202  by issuing a signal to capacitive measurement circuitry  140  (e.g., to gain stages  306 ) on control line GN_CTRL, or alternatively by forwarding a gain increase signal over data bus DATA_BUS. Further in the alternative, in some implementations piezoelectric measurement circuitry  150  may itself issue the gain control signal to the capacitive measurement circuitry  140  for the identified region. In any case, this increased gain level applied in process  604  increases the sensitivity of the touch input detection at capacitive touch elements  112  in the identified region, for example to a sensitivity sufficient to detect a touch input by a gloved finger, or by a bare finger in wet (e.g., rainy) conditions or through ice overlaying keypad  100 , etc. In this example, nominal gain will continue to be applied by gain stages  306  for those capacitance touch measurement blocks  202  associated with capacitive touch elements  112  that are not located in the identified region. A timer (e.g., timer  130 ) is also set in process  604 . 
     Following the increasing of gain for the selected gain stages  306  in process  604 , sensing of changes in capacitance at capacitive touch elements  112  is then performed in process  606  using the increased gain level in the region of keypad  100  identified in process  602 , and using the nominal gain level for elements in other regions of keypad  100 . As described above, capacitive measurement circuitry  150  may repeatedly scan capacitive touch measurement blocks  202  in process  606  to interrogate each capacitive touch element  112  (e.g., each numeric key in keypad  100 ), including both those in the region with enhanced gain and those in other regions of keypad  100 . Detected user touch inputs are then forwarded by capacitive measurement circuitry  150  to CPU  120  for processing to carry out the desired operations of microcontroller  110  in response. 
     Sensing at the increased gain level for the identified region continues for a particular duration. In this regard, decision  607  is performed by CPU  120  to interrogate the timer set in process  604  to determine whether the selected duration of increased gain has elapsed. If not (decision  607  is “no”), process  606  continues with the increased gain at the region identified in process  602 . 
     In this example embodiment, process  606  may be performed according to the approach described above relative to  FIG. 5B , in which the increased gain for the identified region of keypad  100  is decreased to the nominal or another lower gain level in response to the amplitude of the capacitive touch input exceeding a threshold (e.g., threshold T_MAX). Alternatively, the increased gain for the identified region of keypad  100  may be used for the entire duration regardless of amplitude. 
     If the duration of increased gain has elapsed (decision  607  is “yes”), microcontroller  110  resets the gain applied by gain stages  306  in capacitive touch measurement blocks  202  to its nominal or other lower value. Sensing of touch inputs at keypad  100  then continues from process  600  in the manner described above. 
     According to this alternative example embodiment of  FIG. 6 , additional efficiency is attained in HMI systems, specifically by limiting the application of increased gain in sensing capacitive touch elements of a keypad or other HMI device to regions receiving greater physical pressure, as sensed by piezoelectric touch elements. 
     According to another example embodiment, haptic output functionality is provided in the HMI system to provide user feedback and other functions. In this example embodiment, the HMI system includes driver circuitry to actuate the piezoelectric elements in the keypad or other HMI device to provide haptic output. This haptic output can be used to provide positive feedback to the user in response to a user input, and can additionally be used to clear ice, water droplets or other forms of moisture, as well as dirt and debris from the touch elements. 
     Referring now to  FIGS. 7 through 9 , the construction and operation of a touch-sensitive HMI system according to alternative example embodiments will be described. In these  FIGS. 7 through 9 , the same reference numbers are used for the same or similar (in function and/or structure) features in the previously described examples. 
       FIG. 7  illustrates the architecture of a touch-sensitive HMI system constructed according to an example embodiment. In this example, keypad  700  includes capacitive array  702 , which includes one or more capacitive touch elements  112  as in the example of  FIG. 1 , arranged in the conventional numeric keypad arrangement. Each capacitive touch element  112  may be constructed as described above, and may be implemented in either of the self capacitance or mutual capacitance measurement modes. Visible indicators may additionally be provided for each of capacitive touch elements  112  to facilitate user actuation. As in the example of  FIG. 1 , keypad  700  also includes piezoelectric array  704  with four piezoelectric touch elements  114 A through  114 D, for example as embedded within keypad  700  and not visible or otherwise associated with a visible indicator. 
     As in the example of  FIG. 1 , capacitive array  702  and piezoelectric array  704  in the example embodiment of  FIG. 7  may overlay one another in keypad  700  so that the same user touch at keypad  700  can be detected by both one or more of capacitive touch elements  112  and one or more of piezoelectric touch elements  114 . As noted above, the particular association of the position of piezoelectric touch elements  114  relative to capacitive touch elements  112  in keypad  700  may vary from application to application. 
     In the system of  FIG. 7 , capacitive array  702  and piezoelectric array  704  of keypad  700  are coupled to microcontroller  710 . Microcontroller  710  in this example embodiment is constructed similarly as microcontroller  110  described above in connection with  FIG. 1 , with multiple functional circuit modules coupled to CPU  120  via address bus ADDR_BUS and data bus DATA_BUS. Microcontroller  710  in this example embodiment also includes one or more modules in addition to those included in microcontroller  110  as described above, including piezoelectric driver circuitry  740 . 
     According to this example embodiment, piezoelectric driver circuitry  740  has one or more outputs coupled to piezoelectric touch elements  114  in piezoelectric array  104  of keypad  700 . As known in the art, conventional piezoelectric elements function both to produce a voltage in response to an applied mechanical pressure and also to produce a mechanical deformation in response to an applied voltage. As described above, piezoelectric measurement circuitry  150  described above receives the voltages produced by piezoelectric touch elements  114  in response to pressure from a user input. Conversely, in this example embodiment, piezoelectric driver circuitry  740  is adapted to provide drive signals to cause a deformation by one or more of piezoelectric touch elements  114 , thus producing a haptic output at keypad  700 . Piezoelectric driver circuitry  740  may be constructed in the conventional manner, an example of which is the DRV2667 piezoelectric haptic driver available from Texas Instruments Incorporated. In the example embodiment of  FIG. 7 , piezoelectric driver circuitry  740  operates to provide drive signals to piezoelectric touch elements  114  under the control of CPU  120 , for example as communicated to piezoelectric driver circuitry  740  via data bus DATA_BUS or via dedicated control lines (not shown). 
     Alternatively, piezoelectric driver circuitry  740  may be implemented externally to microcontroller  710 . In this alternative implementation, digital or analog control signals may be provided from microcontroller  710  by input/output interface function  128  or another function of microcontroller  710 . 
     Keypad  700  optionally includes, according to this example embodiment, proximity sensor  705  arranged as a rectangular ring encircling the array of capacitive touch elements  112  at keypad  700 . Proximity sensor  705  may be realized as a capacitive, inductive, or magnetic element arranged to detect the presence of a user&#39;s finger or other actuating element in the proximity of keypad  700 . 
     In this example in which keypad  700  includes optional proximity sensor  705 , microcontroller  710  also includes the module of proximity detection circuitry  750 . Proximity detection circuitry  750  has an input coupled to receive signals from proximity sensor  705  in keypad  700 , and is constructed and operates to process those signals from proximity sensor  705  and communicate signals to CPU  120  (e.g., via data bus DATA_BUS) indicating whether an actuating element such as a user&#39;s finger is in the proximity of keypad  700 . The particular construction and operation of proximity detection circuitry  750  will depend upon the technology used to implement proximity sensor  705  (e.g., capacitive, inductive, magnetic, etc.). 
     Referring now to  FIG. 8 , the implementation of piezoelectric driver circuitry  740  in combination with piezoelectric measurement circuitry  150  according to an example embodiment will be described. As described above in connection with  FIG. 4 , piezoelectric measurement circuitry  150  has inputs coupled to terminals  401 A through  401 D, at which piezoelectric touch elements  114 A through  114 D, respectively, are connected. Each of terminals  401 A through  401 D is coupled to a corresponding analog interface  400 A through  400 D. Each analog interface  400  has an output coupled to the anode of a corresponding diode  405 , and an output coupled to a corresponding instance of amplifier circuitry  402 . The cathodes of diodes  405 A through  405 D are connected in common to a wired-OR node PZO_OR, which is coupled to an input of comparator  422  as described above. 
       FIG. 8  shows piezoelectric driver circuitry  740  coupled to data bus DATA_BUS, over which control and data signals may be communicated to and from CPU  120 . For purposes of haptic output in this example in which multiple piezoelectric touch elements  114  are implemented in keypad  700  with a twelve-key arrangement of capacitive touch elements  112 , it is contemplated that a user would generally be unable to distinguish a haptic output at one piezoelectric touch element  114  from a haptic output at another. As such, in this example, piezoelectric driver circuitry  740  has an output coupled in common to all of terminals  401 A through  401 D, and thus coupled in common to piezoelectric touch elements  114 A through  114 D. This results in any output drive signal produced by piezoelectric driver circuitry  740  to be simultaneously applied in common to all of piezoelectric touch elements  114 A through  114 D to produce the haptic output at keypad  700 . 
     Alternatively, piezoelectric driver circuitry  740  may have multiple outputs, each coupled to a single one of terminals  401 A through  401 D, in order to drive individual ones of piezoelectric touch elements  114 A through  114 D in response to data communicated by CPU  120  over data bus DATA_BUS. 
       FIG. 9  illustrates the generalized operation of the system of  FIG. 7  according to one or more example embodiments. In this example, it is contemplated that these operations will be carried out by and under the direction and control of CPU  120  in combination with other functions in microcontroller  710 , including piezoelectric driver circuitry  740 . For example, CPU  120  may carry out and control these operations by executing program instructions stored in machine-readable form in the memory resources of the system, such as ROM  126  and in some implementations RAM  124 . Alternatively or in addition, some or all of the operations described herein may be executed by special-purpose or dedicated logic circuitry. 
     As described above, the system of  FIG. 7  includes proximity sensor  705  and proximity detect circuitry  750 , both of which are optional. When these functions are included in the HMI system as in this example embodiment, additional power savings can be attained by enabling microcontroller  710  to be placed in a sleep mode as shown by state  900  of  FIG. 9 . In this sleep mode, many functions of microcontroller  710  (e.g., CPU  120 , capacitive measurement circuitry  140 ) may be powered down, with proximity detect circuitry  750  remaining powered up to the extent necessary to receive and process signals from proximity sensor  705 . In this sleep mode, the polling of capacitive touch elements  114  by capacitive measurement circuitry  140  is paused, avoiding the consumption of the significant power required by that operation. If a user or other actuating element is not in the proximity of keypad  700  (decision  901  is “no”), microcontroller  710  remains in sleep mode in state  900 . If a user approaches keypad  700  and is about to make an input at keypad  700  (e.g., the user&#39;s finger is in proximity of keypad  700 ), proximity sensor  705  communicates a corresponding signal to proximity detect circuitry  750  of microcontroller  710 . In this event (decision  901  is “yes), proximity detect circuitry  750  issues the appropriate signals to CPU  120  and other functions of microcontroller  710  to “wake” the device from the sleep state in process  902 , placing microcontroller  710  in an active condition and initiating the polling of capacitive touch elements  114  by capacitive measurement circuitry  140 . 
     As CPU  120  and capacitive measurement circuitry  140  are awakened in process  902 , a nominal gain level is set at gain stages  306  of capacitive measurement circuitry  140 , as described above relative to  FIG. 5A  (e.g., process  500 ). Detection of a user input at capacitive touch elements  112 , piezoelectric touch elements  114 , or both is then enabled. According to this example embodiment, if either or both a piezoelectric or a capacitive touch input is received (decision  903  is “yes”), the corresponding inputs are processed in process  904 , in the manner described above in connection with  FIG. 5A  or  FIG. 6 . As described above, this operation of the HMI system in which a gain level in capacitive measurement circuitry  140  is increased in response to piezoelectric measurement circuitry  150  detecting significant pressure from the user input, enables improved receipt of user inputs in challenging environmental conditions while reducing vulnerability of the HMI system to increased noise, thermal drift, and excess power consumption as could result from excessively high gain. 
     In addition to the enabling of these benefits, this example embodiment also provides haptic output in response to the receipt of the input in decision  903 . In this example embodiment, piezoelectric driver circuitry  740  is enabled to drive signals at terminals  401 A through  401 D in process  906 . If desired, piezoelectric measurement circuitry  150  may be disabled from receiving inputs from terminals  401 A through  401 D at this time, to avoid responding to voltages driven by piezoelectric driver circuitry  740  at terminals  401 A through  401 D. Alternatively, piezoelectric measurement circuitry  150  and piezoelectric driver circuitry  740  may be operated in a “half-duplex” fashion to avoid interference between the signals driven and received at piezoelectric touch elements  114 A through  114 D. 
     Once piezoelectric driver circuitry  740  is enabled in process  906 , process  908  may optionally be performed to clear ice, water and moisture, or dirt and debris from the surface of keypad  700 . Process  906  in this example may be performed by piezoelectric driver circuitry  740  driving voltages at terminals  401 A through  401 D, for example at a selected amplitude and at sonic or ultrasonic frequencies for a selected duration, to cause vibrations at keypad  700  for clearing foreign substances from its surface. Alternatively, process  908  may be performed unconditionally as microcontroller  710  wakes from the sleep state in process  902 , to prepare keypad  700  for receiving user inputs. 
     In process  910 , after piezoelectric driver circuitry  740  is activated in process  906 , CPU  120  communicates signals over data bus DATA_BUS to piezoelectric driver circuitry  740  in response to user inputs received and processed in process  904 . The signals communicated by CPU  120  in process  910  in this example embodiment cause piezoelectric driver circuitry  740  to drive voltages at one or more of terminals  401 A through  401 D in response to the receipt by CPU  120  of the touch inputs at keypad  700 , to provide haptic feedback to the user. This haptic feedback provides the user with positive confirmation of the receipt of each user input, improving the user experience with the HMI system. System operation continues with processes  904 ,  910  during such time as user inputs are received, until a timeout or other events places the system back into sleep mode  900 . 
     In addition to the advantages enabled by the example embodiment of  FIGS. 1 through 6 , this example embodiment further enables other technical advantages. Haptic feedback to the user in response to user inputs provides an improved user experience, especially in challenging conditions in which the user may not be confident that the HMI system is sensing a touch input. Examples of such challenging conditions include cold weather, in which the user may be wearing gloves, and wet or icy conditions, in which the user may be unsure whether his input is sensed. And as described above, the HMI system according to this example embodiment enables the driving of piezoelectric elements to clear ice, water, and other debris from the surface of the keypad, further increasing the reliability of detection and processing of user inputs. The inclusion of the optional proximity sensor at the keypad of the HMI system according to this example embodiment can provide additional power consumption savings, as the microcontroller can be largely powered down, and the power-intensive polling of capacitive touch elements paused, until such time as the proximity sensor detects a user in the proximity of the keypad. Overall system power consumption over system life can thus also be significantly reduced. 
     As used herein, the terms “terminal”, “node”, “interconnection” and “pin” are used interchangeably. Unless specifically stated to the contrary, these terms are generally used to mean an interconnection between or a terminus of a device element, a circuit element, an integrated circuit, a device, or other electronics or semiconductor component. 
     Unless otherwise stated, “about,” “approximately,” or “substantially” preceding a value means+/−10 percent of the stated value. Modifications are possible in the described examples, and other examples are possible within the scope of the claims. 
     A device that is “configured to” perform a task or function may be configured (e.g., programmed and/or hardwired) at a time of manufacturing by a manufacturer to perform the function and/or may be configurable (or re-configurable) by a user after manufacturing to perform the function and/or other additional or alternative functions. The configuring may be through firmware and/or software programming of the device, through a construction and/or layout of hardware components and interconnections of the device, or a combination thereof. 
     A circuit or device that is described herein as including certain components may instead be adapted to be coupled to those components to form the described circuitry or device. For example, a structure described as including one or more semiconductor elements (such as transistors), one or more passive elements (such as resistors, capacitors, and/or inductors), and/or one or more sources (such as voltage and/or current sources) may instead include only the semiconductor elements within a single physical device (e.g., a semiconductor die and/or integrated circuit (IC) package) and may be adapted to be coupled to at least some of the passive elements and/or the sources to form the described structure either at a time of manufacture or after a time of manufacture, for example, by an end-user and/or a third-party. 
     Circuits described herein are reconfigurable to include the replaced components to provide functionality at least partially similar to functionality available prior to the component replacement. Components shown as resistors, unless otherwise stated, are generally representative of any one or more elements coupled in series and/or parallel to provide an amount of impedance represented by the shown resistor. For example, a resistor or capacitor shown and described herein as a single component may instead be multiple resistors or capacitors, respectively, coupled in parallel between the same nodes. For example, a resistor or capacitor shown and described herein as a single component may instead be multiple resistors or capacitors, respectively, coupled in series between the same two nodes as the single resistor or capacitor. 
     Uses of the phrase “ground” in the foregoing description include a chassis ground, an Earth ground, a floating ground, a virtual ground, a digital ground, a common ground, and/or any other form of ground connection applicable to, or suitable for, the teachings of this description. 
     While one or more embodiments have been described in this specification, it is of course contemplated that modifications of, and alternatives to, these embodiments, such modifications and alternatives capable of obtaining one or more of the technical effects of these embodiments, will be apparent to those of ordinary skill in the art having reference to this specification and its drawings. It is contemplated that such modifications and alternatives are within the scope of the claims presented herein.