Patent Publication Number: US-2015082890-A1

Title: Biometric sensors for personal devices

Description:
BACKGROUND 
     1. Technical Field 
     Embodiments generally relate to fingerprint sensing. More particularly, embodiments relate to the use of flexible substrates to deploy ultrasonic fingerprint sensors in curved structures and/or pre-existing design elements. 
     2. Discussion 
     Fingerprint sensors may be used to identify and/or authenticate users in a variety of settings. Ultrasonic imaging systems may provide better accuracy and/or quality relative to optical scanners due to an ability to more effectively identify fingerprint ridges (as well as the areas between ridges) in sub-optimal conditions (e.g., environmental contamination). Conventional ultrasonic solutions, however, may be limited to bulky probe designs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various advantages of the embodiments of the present invention will become apparent to one skilled in the art by reading the following specification and appended claims, and by referencing the following drawings, in which: 
         FIG. 1A  is an illustration of an example of an ultrasonic transducer array according to an embodiment; 
         FIG. 1B  is an illustration of an example of an ultrasonic transducer array deployed in a curved structure according to an embodiment; 
         FIGS. 2A-2C  are illustrations of examples of security devices according to embodiments; 
         FIG. 3  is an illustration of an example of a computing platform according to an embodiment; 
         FIGS. 4A and 4B  are illustrations of examples of button according to embodiments; 
         FIG. 5  is a block diagram of an example of a fingerprint identification architecture according to an embodiment; 
         FIG. 6  is a flowchart of an example of a method of fabricating a fingerprint identification system according to an embodiment; 
         FIGS. 7A and 7B  are flowcharts of examples of methods of authenticating users according to embodiments; and 
         FIG. 8  is a block diagram of an example of a system including a fingerprint identification architecture according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  shows an ultrasonic transducer array  10  that may be used to identify fingerprints. In the illustrated example, a bottom electrode  14  (e.g., metallic layer) may be deposited on a flexible substrate  12  such as, for example, a polymer layer. In addition, a set of vacuum cavities  16  may be formed within a layer  18  above the bottom electrode  14 , wherein membranes  20  may rest on sidewalls above the vacuum cavities  16 . In the illustrated example, a top electrode  22  (e.g., metallic layer) is deposited on the membranes  20  of the layer  18 . In one example, the ultrasonic transducer array  10  is a capacitive micromachined ultrasonic transducer (CMUT) array. Other layer configurations and/or manufacturing techniques may be used to fabricate the ultrasonic transducer array  10 . 
     The top electrode  22  may generally function as a read surface for the identification of fingerprints. More particularly, the top electrode  22  and the bottom electrode  14  may form capacitors around the vacuum cavities  16 , wherein application of a direct current (DC) and/or alternating current (AC) voltage across the vacuum cavities  16  may cause the membranes  20  to vibrate. The return echo may be captured and analyzed to identify a fingerprint, a toeprint and/or heartbeat characteristics of a human finger and/or toe. For example, a binary determination may be made for each capacitor as to whether the corresponding membrane  20  is resonating after a given excitation. The binary results may be used to create a highly accurate map of the ridges and valleys of the fingerprint and/or toeprint, as well as the unique heartbeat trace of an individual. 
     The flexible substrate  12  may be initially deposited on a photoresist layer (not shown) that rests on a silicon carrier wafer (not shown). In such a case, the silicon carrier wafer may be removed and the photoresist layer may be etched away as final steps to the fabrication process. As will be discussed in greater detail, the use of the flexible substrate  12  may enable the ultrasonic transducer array  10  to be installed and/or deployed in a wide variety of settings and/or devices. 
     For example, the ultrasonic transducer array  10  may be embedded in an external component  24  such as, for example, the skin of a computing platform, the grip of a security device (e.g., handgun, knife, electroshock weapon, pepper spray, etc.), the handle of a vehicle door, and so forth. The external component  24  may also be a button/key of a keyboard, wherein the button/key has an additional function that is separate from identification of fingerprints. 
       FIG. 1B  demonstrates that the ultrasonic transducer array  10  may be embedded in an external component  26  that has a curved profile. Because the ultrasonic transducer array  10  includes the flexible substrate  12 , the top electrode  22  (e.g., the read surface) of the ultrasonic transducer array  10  conforms to the curved profile, in the example shown. Accordingly, the illustrated ultrasonic transducer array  10  provides relatively high accuracy and quality while being uniquely suitable for a large number of applications. 
     For example,  FIGS. 2A-2B  show security device configurations in which ultrasonic transducer arrays may be used to scan fingerprints and may be built into the security device so that they may not be easily bypassed. In particular,  FIG. 2A  shows a side view of a handgun  28  having a grip region  30  that may contain an embedded ultrasonic transducer array, a muzzle region  32  that may contain electronics (e.g., processors, microcontrollers, semiconductor chips) to process the signals from the ultrasonic transducer array, and an interconnect region  34  to contain a wired connection between the electronics and the ultrasonic transducer array. Thus, the embedded ultrasonic transducer array may conform to the curved shape of the grip region  30  while maintaining the ability to identify fingerprints at a high level of accuracy. 
     Moreover,  FIG. 2B  shows a side view of a pepper spray container  36  having a head region  38  that may contain an embedded ultrasonic transducer array, a trigger region  40  that may contain electronics to process the signals from the ultrasonic transducer array and an interconnect region  42  to contain a wired connection between the electronics and the ultrasonic transducer array. Accordingly, the embedded ultrasonic transducer array may conform to the curved shape of the head region  38  while maintaining the ability to identify fingerprints at a high level of accuracy. 
     Additionally,  FIG. 2C  shows a side perspective view of an electroshock weapon  44  (e.g., TASER) having a grip region  46  that may contain an embedded ultrasonic transducer array, a muzzle region  48  that may contain electronics to process the signals from the ultrasonic transducer array and an interconnect region  50 . Again, the embedded ultrasonic transducer array may conform to the curved shape of the grip region  30  while maintaining the ability to identify fingerprints at a high level of accuracy. 
     As will be discussed in greater detail, the electronics may selectively activate/deactivate one or more features of the security devices based on the fingerprint identification and/or user authentication results. For example, the ability to fire the handgun  28  ( FIG. 2A ), the pepper spray  36  ( FIG. 2B ) and/or the electroshock weapon  44  ( FIG. 2C ) may all be contingent upon successful identification of the fingerprint of a user of those devices as well as the successful authentication of that user. Other information, such as signals from position sensors and/or radio modules may also be used to identify fingerprints and/or authenticate users. 
       FIG. 3  shows a computing platform  52  having a housing (e.g., enclosure, skin) with a curved profile, wherein an ultrasonic transducer array  56  is embedded in the housing of the computing platform  52  such that the read surface of the ultrasonic transducer array  56  conforms to the curved profile. As discussed above with regard to the security devices, one or more features (e.g., specific applications, read/write capabilities, etc.) of the computing platform  52  may be selectively activated/deactivated based on the fingerprint identification and/or user authentication results. 
       FIG. 4A  shows a button  58  of a keyboard, wherein an ultrasonic transducer array  60  is embedded in the button. Multiple or all buttons of the keyboard may have an ultrasonic transducer array  60  embedded in the button, enabling the ability to perform continuous authentication of the user as they type on the keyboard, wherein detection of a different user subsequently typing on the keyboard might immediately deactivate the user session, or the content that the user created by typing might be annotated as coming from them (for example, online test taking, document authorship, . . . ). 
     The button  58 , which may or may not include a curved profile, may generally have a function that is separate from the identification of fingerprints. In the illustrated example, the separate function is that of a shift key on a keyboard. Thus, the illustrated approach incorporates the fingerprint reader into an existing design element. Moreover, embedding the ultrasonic transducer array  60  into the button  58  may enable existing usage patterns to be leveraged—for example, pushing the button  58  (or any other button on the keyboard) may already be used to wake a device from a power saving mode (e.g., sleep, hibernate, etc.). In such a case, pushing the button  58  may also initiate an authentication of the user based on the fingerprint that is read during the push of the button  58 , wherein the authentication may bypass other authentication methods such as password or PIN (personal identification number) based solutions. 
     The illustrated button  58  also includes a target indicator (e.g., logo, illustration)  62  adjacent to the read surface of the ultrasonic transducer array  60 . The target indicator  62  may visually inform the user as to where on the external profile of the button  58  the user may place his or her finger to conduct a fingerprint scan, and where on the external profile of the button  58  the user may place his or her finger to activate the other function of the button  58  (e.g., the shift function). Other buttons such as, for example, Home buttons on smart phones and/or tablet computers, may also use the illustrated approach. 
       FIG. 4B  shows a button  59  of a mouse  61 , wherein an ultrasonic transducer array  63  is embedded in the button area of the mouse. The button  59 , which may or may not include a curved profile, may generally have a function that is separate from the identification of fingerprints. In the illustrated example, the separate function is that of a mouse click button. Thus, the illustrated approach incorporates the fingerprint reader into an existing design element. Moreover, embedding the ultrasonic transducer array  63  into the button  59  may enable existing usage patterns to be leveraged—for example, pushing the button  59  may already be used to open an email or make an online purchase. In such a case, pushing the button  59  may also initiate an authentication of the user based on the fingerprint that is read during the push of the button  59 , wherein the authentication might be used to confirm that the user is authorized to perform the intended action of the mouse click, such as viewing an email or authorizing the purchase to proceed. 
     Turning now to  FIG. 5 , a fingerprint identification architecture ( 64   a - 64   e ) is shown. In the illustrated example, one or more ultrasonic transducers  64   a  are coupled to a processor  64   b  (e.g., microcontroller, host processor, input output module/IOM) that is configured to identify fingerprints based on signals from the ultrasonic transducers  64   a . The architecture  64  may also include one or more position sensors  64   c  (e.g., geographic location sensor, inertial sensor, etc.), wherein the processor  64   b  may use signals from the position sensors  64   c  to conduct user authentications. For example, a geographic location sensor such as a Global Positioning System (GPS) sensor might output the coordinates of the system containing the architecture  64 . In such a case, the processor  64   b  may compare those coordinates to known location data to determine if the system is in an approved place (e.g., home, work, particular city/state, etc.). 
     Moreover, an inertial sensor such as an accelerometer or gyroscope may output the orientation of the system containing the architecture  64 . In such a case, the processor  64   b  may compare that orientation to known orientations determine if the system is being held properly (e.g., pointing away from the user, up, down, etc.). If the signals from the position sensor  64   c  indicate that, for example, the system is not located in an approved place or the system not being held properly, the processor  64   b  may deactivate one or more features of the system as a safety measure. Signals from one or more radio modules  64   d  (e.g., WiFi/Wireless Fidelity, e.g., Institute of Electrical and Electronics Engineers/IEEE 802.11-2007, Wireless Local Area Network/LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, Bluetooth, e.g., IEEE 802.15.1-2005, Wireless Personal Area Networks, etc.) may also be used to determine position and other characteristics that may be useful in the fingerprint identification and/or user authentication processes. Moreover, after a predetermined number of unsuccessful attempts to “login”, a signal from the radio module may be used to send a “panic” or “attempted break-in” signal to a central monitoring system. The illustrated architecture  64  also includes a power module  64   e  to manage, balance, supply and/or harvest power for the architecture  64 . 
       FIG. 6  shows a method  66  of fabricating a fingerprint identification system. Portions of the method  66  may be implemented using well documented semiconductor fabrication, hardware manufacturing, programming, surface mount technology (SMT) solder reflow, bonding, assembly, trace layout design, and other techniques, or any combination thereof. In addition, portions of the method  66  may be implemented via a set of logic instructions stored in a machine- or computer-readable storage medium such as random access memory (RAM), read only memory (ROM), programmable ROM (PROM), firmware, flash memory, etc., in configurable logic such as, for example, programmable logic arrays (PLAs), field programmable gate arrays (FPGAs), complex programmable logic devices (CPLDs), in fixed-functionality logic hardware using circuit technology such as, for example, application specific integrated circuit (ASIC), complementary metal oxide semiconductor (CMOS) or transistor-transistor logic (TTL) technology, or any combination thereof. 
     Illustrated block  68  provides a flexible substrate such as, for example a polymer layer deposited on a photoresist layer, which is deposited on a silicon carrier wafer. An ultrasonic transducer array may be coupled to the flexible substrate at block  70 , wherein the silicon carrier wafer and photoresist layer may be removed from the flexible substrate once the ultrasonic transducer array has been coupled to the flexible substrate. Block  72  may embed the ultrasonic transducer array into an external component having a curved profile. In one example, the ultrasonic transducer array includes a read surface that conforms to the curved profile. If the external component is a button having a function that is separate from identification of fingerprints, block  72  might also involve disposing (e.g., printing, stamping, engraving, etc.) a target indicator on the curved profile adjacent to the read surface. In addition, a processor may be configured at block  74  to identify fingerprints based on signals from the ultrasonic transducer array. The processor may be coupled to the ultrasonic transducer array at block  76 . 
     Turning now to  FIG. 7A , a method  78  of authenticating users is shown. The method  78  may be implemented as a set of logic instructions stored in a machine- or computer-readable storage medium such as RAM, ROM, PROM, firmware, flash memory, etc., in configurable logic such as, for example, PLAs, FPGAs, CPLDs, in fixed-functionality logic hardware using circuit technology such as, for example, ASIC, CMOS or TTL technology, or any combination thereof. Illustrated processing block  80  receives a signal from a position sensor and/or a radio module of a mobile device, wherein a determination may be made at block  82  as to whether the mobile device is in a valid position based on the received signal. As already noted, the position might be a geographic position, an orientation, or any combination thereof. If the mobile device is not in a valid position, block  84  may deactivate one or features of the device. 
     If, on the other hand, the device is in a valid position, illustrated block  86  receives a signal from an ultrasonic transducer array embedded in an external component of the mobile device. The external component may include, for example, a grip of a security device, a skin of a computing platform, a button, and so forth. As already noted, the external component may have a curved profile, wherein the ultrasonic transducer array is embedded in the external component and includes a read surface that conforms to the curved profile. 
     The signal from the ultrasonic transducer array may be used at block  88  to identify a fingerprint and conduct an authentication of a user of the device. Thus, the illustrated approach effectively uses the signal from the position sensor and/or radio module to trigger the authentication. The authentication may involve comparing the identified fingerprint to the fingerprints of one or more known individuals. In this regard, the authentication system may be trained. For example, the training may be particularly advantageous for systems in which the ultrasonic transducer array is embedded in a button, as some individuals may not always type on a keyboard or mouse button using the “same” finger every time. If it is determined at block  90  that the authentication was unsuccessful (e.g., no fingerprint match or a match to an unauthorized individual was found), block  84  may deactivate one or more features of the device. If the authentication was successful (e.g., a fingerprint match to an authorized individual was found), illustrated block  92  activates one or more features of the device. 
       FIG. 7B  shows an alternative method  94  of authenticating users. The method  78  may be implemented as a set of logic instructions stored in a machine- or computer-readable storage medium such as RAM, ROM, PROM, firmware, flash memory, etc., in configurable logic such as, for example, PLAs, FPGAs, CPLDs, in fixed-functionality logic hardware using circuit technology such as, for example, ASIC, CMOS or TTL technology, or any combination thereof. Illustrated block  96  receives a signal from an ultrasonic transducer array, wherein the signal may be used at block  98  to identify a fingerprint. Additionally, a signal from a position sensor and/or a radio module may be received at block  100 . Block  102  may use the fingerprint and the signal from the position sensor and/or radio module to authenticate (e.g., verify the identity of) the user. Thus, for example, if the signal from the position sensor indicates that the device is in an unknown geographic location, then the authentication may fail. Additionally, if the fingerprint does not match an authorized individual, the authentication may fail. 
     If it is determined at block  104  that the authentication was unsuccessful, illustrated block  84  deactivates one or more features of the mobile device. If, on the other hand, the authentication was successful, one or more features of the mobile device may be activated at block  92 . 
     Turning now to  FIG. 8 , a computing platform  106  is shown. The platform  106  may be part of a device having computing functionality (e.g., PDA, laptop, tablet computer, desktop computer), communications functionality (e.g., wireless smart phone), imaging functionality, media playing functionality (e.g., smart television/TV), or any combination thereof (e.g., mobile Internet device/MID). In the illustrated example, the platform  106  includes a processor  108 , an integrated memory controller (IMC)  110 , an input output (IO) module  112 , system memory  114 , a radio module  116 , an ultrasonic transducer (UT) array  118 , mass storage  120  (e.g., optical disk, hard disk drive/HDD, flash memory), one or more user interface (UI) devices  122 , one or more position sensors  132  and a power module  124  having a battery  126  to supply power to the platform  106 . The processor  108  may include a core region with one or several processor cores  128 . 
     The illustrated IO module  112 , sometimes referred to as a Southbridge or South Complex of a chipset, functions as a host controller and communicates with the radio module  116 , which could provide off-platform communication functionality for a wide variety of purposes such as, for example, cellular telephone (e.g., Wideband Code Division Multiple Access/W-CDMA (Universal Mobile Telecommunications System/UMTS), CDMA2000 (IS-856/IS-2000), etc.), WiFi, 4G LTE (Fourth Generation Long Term Evolution), Bluetooth, WiMax (e.g., IEEE 802.16-2004, LAN/MAN Broadband Wireless LANS), Global Positioning System (GPS), spread spectrum (e.g., 900 MHz), and other radio frequency (RF) telephony purposes. Other standards and/or technologies may also be implemented in the radio module  116 . The IO module  112  may also include one or more wireless hardware circuit blocks to support such functionality. Although the processor  108  and IO module  112  are illustrated as separate blocks, the processor  108  and IO module  112  may be implemented as a system on chip (SoC) on the same semiconductor die. 
     The system memory  114  may include, for example, double data rate (DDR) synchronous dynamic random access memory (SDRAM, e.g., DDR3 SDRAM JEDEC Standard JESD79-3C, April 2008) modules. The modules of the system memory  114  may be incorporated into a single inline memory module (SIMM), dual inline memory module (DIMM), small outline DIMM (SODIMM), and so forth. 
     The illustrated IO module  112  includes logic  130  to identify fingerprints based on signals from the UT array  118 , which may have a wired connection  119  to the IO module  112  that includes leads, connectors, contacts, and so forth. The logic  130  may also use the identified fingerprints and signals from the one or more position sensors  132  and/or the radio module  116  to conduct authentications of users of the platform  106 . In one example, the logic  130  deactivates one or more features of the platform  106  if a given authentication is unsuccessful and activates one or more features of the platform  106  if the authentication is successful. 
     Additional Notes and Examples 
     Example 1 may include a fingerprint identification system comprising a flexible substrate, an ultrasonic transducer array coupled to the flexible substrate and a processor coupled to the ultrasonic transducer array, the processor to identify a fingerprint based on a signal from the ultrasonic transducer array. The system may also include an external component having a curved profile, wherein the ultrasonic transducer array is embedded in the external component and includes a read surface that conforms to the curved profile. 
     Example 2 may include the system of Example 1, wherein the external component includes one of a grip of a security device or a skin of a computing platform. 
     Example 3 may include the system of Example 1, wherein the external component includes a button having a function that is separate from identification of the fingerprint. 
     Example 4 may include the system of Example 3, wherein the button includes a target indicator adjacent to the read surface. 
     Example 5 may include the system of any one of Examples 1 to 4, further including a position sensor coupled to the processor, the processor to use the fingerprint and a signal from the position sensor to conduct an authentication of a user. 
     Example 6 may include the system of Example 5, wherein the processor is to deactivate one or more features of a system containing the apparatus if the authentication is unsuccessful. 
     Example 7 may include the system of Example 5, wherein the signal from the position sensor is to trigger the authentication. 
     Example 8 may include the system of Example 5, wherein the processor is to use the signal from the position sensor to verify an identity of the user. 
     Example 9 may include the system of Example 5, wherein the position sensor includes one or more of a geographic location sensor and an inertial sensor. 
     Example 10 may include the system of Example 1, wherein the ultrasonic transducer array is a capacitive micromachined ultrasonic transducer (CMUT) array. 
     Example 11 may include the system of Example 1, further including a semiconductor chip that contains the processor, and a wired connection coupled to the semiconductor chip and the ultrasonic transducer array. 
     Example 12 may include the system of Example 1, further including a power module to manage power delivered to the apparatus, and a radio module coupled to the processor. 
     Example 13 may include a method of fabricating a fingerprint identification system, comprising providing a flexible substrate, coupling an ultrasonic transducer array to the flexible substrate, configuring a processor to identify a fingerprint based on a signal from the ultrasonic transducer array and coupling the processor to the ultrasonic transducer array. 
     Example 14 may include the method of Example 13, further including embedding the ultrasonic transducer array into an external component having a curved profile, wherein the ultrasonic transducer array includes a read surface that conforms to the curved profile. 
     Example 15 may include the method of Example 14, wherein the ultrasonic transducer array is embedded into one of a button having a function that is separate from identification of the fingerprint, a grip of a security device or a skin of a computing platform. 
     Example 16 may include the method of Example 15, further including disposing a target indicator on the curved profile adjacent to the read surface. 
     Example 17 may include a fingerprint identification apparatus comprising a flexible substrate, an ultrasonic transducer array coupled to the flexible substrate and a processor coupled to the ultrasonic transducer array, the processor to identify a fingerprint based on a signal from the ultrasonic transducer array. 
     Example 18 may include the apparatus of Example 17, further including a position sensor coupled to the processor, the processor to use the fingerprint and a signal from the position sensor to conduct an authentication of a user. 
     Example 19 may include the apparatus of Example 18, wherein the processor is to deactivate one or more features of a system containing the apparatus if the authentication is unsuccessful. 
     Example 20 may include the apparatus of Example 18, wherein the signal from the position sensor is to trigger the authentication. 
     Example 21 may include the apparatus of Example 18, wherein the processor is to use the signal from the position sensor to verify an identity of the user. 
     Example 22 may include the apparatus of Example 18, wherein the position sensor includes one or more of a geographic location sensor and an inertial sensor. 
     Example 23 may include the apparatus of Example 17, wherein the ultrasonic transducer array is a capacitive micromachined ultrasonic transducer (CMUT) array. 
     Example 24 may include the apparatus of Example 17, further including a semiconductor chip that contains the processor, and a wired connection coupled to the semiconductor chip and the ultrasonic transducer array. 
     Example 25 may include the apparatus of any one of Examples 17 to 24, further including a power module to manage power delivered to the apparatus, and a radio module coupled to the processor. 
     Example 26 may include an apparatus to fabricate a fingerprint identification system, comprising means for performing the method of any one of Examples 13 to 16. 
     Thus, techniques described herein may enable secure personal identification in a low cost, low power solution that scales across multiple applications and devices. Additionally, any need for users to remember multiple passwords or experience delayed access to devices may be obviated. The instantaneous and secure access may be particularly advantageous in personal security scenarios involving weaponry, self defense sprays, etc. For example, “smart” personal handheld security devices may automatically limit their usage to their rightful owners or against specifically designated targets. Moreover, embedding ultrasonic transducer arrays in buttons having functions separate from fingerprint identification may facilitate enhanced security with no impact on the overall form factor design (e.g., physical size of the button is unaltered from standard buttons). 
     Embodiments of the present invention are applicable for use with all types of semiconductor integrated circuit (“IC”) chips. Examples of these IC chips include but are not limited to processors, controllers, chipset components, programmable logic arrays (PLAs), memory chips, network chips, systems on chip (SoCs), SSD/NAND controller ASICs, and the like. In addition, in some of the drawings, signal conductor lines are represented with lines. Some may be different, to indicate more constituent signal paths, have a number label, to indicate a number of constituent signal paths, and/or have arrows at one or more ends, to indicate primary information flow direction. This, however, should not be construed in a limiting manner. Rather, such added detail may be used in connection with one or more exemplary embodiments to facilitate easier understanding of a circuit. Any represented signal lines, whether or not having additional information, may actually comprise one or more signals that may travel in multiple directions and may be implemented with any suitable type of signal scheme, e.g., digital or analog lines implemented with differential pairs, optical fiber lines, and/or single-ended lines. 
     Example sizes/models/values/ranges may have been given, although embodiments of the present invention are not limited to the same. As manufacturing techniques (e.g., photolithography) evolve over time, it is expected that devices of smaller size could be manufactured. In addition, well known power/ground connections to IC chips and other components may or may not be shown within the figures, for simplicity of illustration and discussion, and so as not to obscure certain aspects of the embodiments of the invention. Further, arrangements may be shown in block diagram form in order to avoid obscuring embodiments of the invention, and also in view of the fact that specifics with respect to implementation of such block diagram arrangements are highly dependent upon the platform within which the embodiment is to be implemented, i.e., such specifics should be well within purview of one skilled in the art. Where specific details (e.g., circuits) are set forth in order to describe example embodiments of the invention, it should be apparent to one skilled in the art that embodiments of the invention can be practiced without, or with variation of, these specific details. The description is thus to be regarded as illustrative instead of limiting. 
     The term “coupled” may be used herein to refer to any type of relationship, direct or indirect, between the components in question, and may apply to electrical, mechanical, fluid, optical, electromagnetic, electromechanical or other connections. In addition, the terms “first”, “second”, etc. may be used herein only to facilitate discussion, and carry no particular temporal or chronological significance unless otherwise indicated. 
     Those skilled in the art will appreciate from the foregoing description that the broad techniques of the embodiments of the present invention can be implemented in a variety of forms. Therefore, while the embodiments of this invention have been described in connection with particular examples thereof, the true scope of the embodiments of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and following claims.