Patent Application
20100180136
This invention relates to fingerprint sensors and more particularly to apparatus and methods for managing power consumption in fingerprint sensing circuits and also to apparatus and method for detecting finger activity with fingerprint sensing circuits.
Power management is increasingly important in today's mobile electronic devices as greater reliance is placed on batteries and other mobile energy sources. This is true for devices such as portable computers, personal data assistants (PDAs), cell phones, gaming devices, navigation devices, information appliances, and the like. Furthermore, with the convergence of computing, communication, entertainment, and other applications in mobile electronic devices, power demands continue to increase at a rapid pace, with batteries struggling to keep pace. At the same time, even where additional features and capability are provided in modern electronic devices, consumers still desire elegant, compact devices that are small enough to be slipped into a pocket or handbag.
While power management continues to increase in importance, access control is also becoming increasingly important as it relates to modern electronic devices. Access control generally refers to methods and techniques for restricting the ability of a user or program to access a system's resources. Access control is gaining importance at least partly because users are storing increasing amounts of private, sensitive, or confidential information on mobile electronic devices. The electronic devices themselves are also valuable. Thus, restricting access to these devices may provide an effective deterrent to theft or misappropriation by reducing the value of the devices for would-be thieves or resellers.
Although reusable passwords are probably the most common technique for authenticating and identifying a user of a device, various other techniques are also being developed to counter the various ways that reusable passwords may be compromised. For example, fingerprint sensors provide one potential method for identifying and authenticating a user. Fingerprints, like various other biometric characteristics, are based on an unalterable personal characteristic and thus are believed to more reliable to identify a user. Nevertheless, like other features, fingerprint and other biometric sensors typically require additional hardware and software for implementation in electronic devices. This hardware and software adds to the already large power demands being placed on these devices.
In view of the foregoing, what are needed are apparatus and methods for efficiently managing and conserving power in fingerprint sensing circuits. For example, apparatus and methods are needed to significantly reduce power consumed by fingerprint sensing circuits when the circuits are idle or waiting for a user to apply a fingerprint. Further needed are methods and techniques to enable fingerprint sensors to quickly “wake up” when finger or non-finger related activity is detected by the circuit. Further needed are apparatus and methods for determining whether a finger is or is not present on a fingerprint sensor when detection begins.
In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific examples illustrated in the appended drawings. Understanding that these drawings depict only typical examples of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which:
The invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been fully solved by currently available fingerprint sensors. Accordingly, the invention has been developed to provide novel apparatus and methods for managing power consumption in fingerprint sensing circuits. The invention has been further developed to provide novel apparatus and methods for detecting finger activity with fingerprint sensing circuits. The features and advantages of the invention will become more fully apparent from the following description and appended claims and their equivalents, and also any subsequent claims or amendments presented, or may be learned by practice of the invention as set forth hereinafter.
Consistent with the foregoing, an apparatus for reducing power consumption in fingerprint-sensing circuits is disclosed in one embodiment of the invention as including a fingerprint sensing area onto which a user can apply a fingerprint. An integrated circuit communicates with the fingerprint sensing area and is configured to detect finger activity over the fingerprint sensing area. The integrated circuit includes a primary logic portion configured to assume control of the integrated circuit when finger activity is detected over the fingerprint sensing area. The integrated circuit also includes a secondary logic portion configured to assume control of the integrated circuit and shut off power to the primary logic portion when finger activity is not detected over the fingerprint sensing area. The secondary logic portion may use significantly less power than the primary logic portion. In selected embodiments, the secondary logic portion is further configured to turn on power to the primary logic portion and pass control back to the primary logic portion when finger activity is detected over the fingerprint sensing area.
In selected embodiments, the fingerprint sensing area is located away from, or off of, the integrated circuit. In other embodiments, however, the fingerprint sensing area is located on the integrated circuit or incorporated into the integrated circuit. To sense finger activity, the fingerprint sensing area may include multiple fingerprint sensing elements. In certain embodiments, the primary and secondary logic portions use the same fingerprint sensing elements to detect finger activity. In other embodiments, the primary and secondary logic portions use different fingerprint sensing elements to detect finger activity.
In certain embodiments, the primary logic portion operates at a significantly different voltage than the secondary logic portion. In certain examples, the primary logic portion operates at a significantly lower voltage than the secondary logic portion. In other examples, the primary logic portion operates at substantially 1.2 volts and the secondary logic portion operates at substantially 3.3 volts.
In other embodiments, the secondary logic portion is also configured to turn on power to the primary logic portion and pass control back to the primary logic portion when “non-finger” activity is detected by the secondary logic portion. Such non-finger activity may include, for example, General Purpose Input/Output (GPIO) activity, USB activity, Serial Peripheral Interface (SPI) activity, parallel port activity, the expiration of one or more timers, or the like.
In another embodiment of the invention, a method for reducing power consumption in fingerprint-sensing circuits includes providing control to a primary logic portion of an integrated fingerprint sensing circuit when finger activity is detected over a fingerprint sensing area. Control is passed to a secondary logic portion of the integrated circuit and power is shut off to the primary logic portion when finger activity is not detected over the fingerprint sensing area. Similarly, power is turned on to the primary logic portion and control is passed back to the primary logic portion when finger activity is detected over the fingerprint sensing area.
In yet another embodiment of the invention, an apparatus for reducing power consumption in fingerprint-sensing circuits includes a fingerprint sensing area onto which a user can apply a fingerprint. An integrated circuit is provided to detect finger activity over the fingerprint sensing area. The integrated circuit includes a primary logic portion configured to assume control of the integrated circuit when finger activity is detected over the fingerprint sensing area. The integrated circuit also includes a secondary logic portion configured to assume control of the integrated circuit and shut off power to the primary logic portion when finger activity is not detected over the fingerprint sensing area. The secondary logic portion further includes a substantially analog portion. The secondary logic portion is configured to periodically turn off power to the analog portion while the secondary logic portion has control of the integrated circuit to further conserve power.
It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of apparatus and methods in accordance with the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of certain examples of presently contemplated embodiments in accordance with the invention. The presently described embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout.
Referring to
In certain embodiments, each transmitting element 14 may successively emit a probing signal, one after the other. As will be explained in more detail hereafter, the probing signal may include a series of probing pulses, such as a series of square waves. A square wave may be used because it is simple waveform that is easy to generate.
In certain embodiments, the probing signal emitted by each transmitting element 14 may be detected on the receiving end by a receiving element 16. In selected embodiments, pairs of receiving elements 16 may be used to cancel out noise. Like the probing signal, the response signal may include a series of response pulses generated in response to the probing pulses. The magnitude of the response signals may depend on factors such as whether a finger is present over the fingerprint sensing area 10 and, more particularly, whether a ridge or valley of a fingerprint is immediately over the transmitting element 14. The magnitude of the signal received at the receiving element 16 may be directly related to the RF impedance of a finger ridge or valley placed near the gap between the transmitting element 14 and the receiving element 16.
Instead of using a separate receiving element 16 for each transmitting element 14, a single receiving element 16 may be used where the transmitting elements 14 transmit the probing signal at different times. The response signals received by the receiving element 16 may then be correlated with each transmitting element 14 on the receiving end. By using a single receiving element 16, the receiver that is coupled to the receiving element 16 may be designed to be very high quality and with a much better dynamic range than would be possible using an array of multiple receiving elements.
The design described above differs from many conventional fingerprint sensors, which may employ a single large transmitting element with a large array of receiving elements and receivers. Nevertheless, many of the power management and finger detection features described herein are not limited to the illustrated transmitter and receiver design. Indeed, the apparatus and methods disclosed herein may be used with fingerprints sensors using a small number of transmitting elements and a relatively large number of receiving elements, a large number of transmitting elements and a relatively small number of receiving element, or a roughly equal number of transmitting elements and receiving elements.
In selected embodiments, the fingerprint sensing area 10 (including the transmitting and receiving elements 14, 16) may be physically decoupled from the fingerprint sensing integrated circuit, as will be described in more detail in
In certain embodiments, the fingerprint sensing area 10 may include one or more transmitting and receiving elements 18, 20 to “wake up” the fingerprint sensing circuit when a user swipes a finger over the fingerprint sensing area 10. These elements may stay active when other elements (e.g., elements 14, 16) have been turned off, disabled, or have been put into a sleep or hibernation mode. Although only two elements 18, 20 are shown, the fingerprint sensing area 10 may include additional transmitting and/or receiving elements placed at various locations on the fingerprint sensing area 10. Like the transmitting and receiving elements 14, 16, the elements 18, 20 may be used to wake up the fingerprint sensing circuit by detecting changes in impedance when a finger is placed or swiped over the fingerprint sensing area 10.
In selected embodiments, certain elements 18, 20 may be dedicated to waking up the fingerprint sensor while other elements 14, 16 may be dedicated to scanning fingerprints. In other embodiments, one or more of the elements 14, 16, 18, 20 may double as “wake up” elements and fingerprint scanning elements. For example, a small subset of the transmitting and receiving elements 14, 16 may be kept active even as other elements 14, 16 are turned off or put into sleep mode. When finger activity is detected by the small subset, the remainder of the transmitting and receiving elements may be woken up to begin scanning a fingerprint. In selected embodiments, the fingerprint sensing circuit may be programmable to allow different fingerprint sensing elements 14, 16 to be used as “wake up” elements.
Referring to
Digital data output by the ADC 32 may then be stored in a FIFO buffer 38. The FIFO 38 may be coupled to a bus 40, which may communicate with various components, such as a CPU 42, memory 44, direct memory access controller 46, and the like. The bus 40 may also communicate with one or more interfaces, such as a USB interface 48, Serial Peripheral Interface (SPI) interface 50, parallel port (PP) interface 52, or the like. The FIFO 38 may provide a data storage medium to compensate for differences in timing and transfer rate between the components 30, 32, 38 and the components 42, 44, 46, 48, 50, 52.
In selected embodiments, some or all of the components illustrated in
Referring to
In selected embodiments in accordance with the invention, the integrated circuit 56 described in
As will be explained in more detail hereafter, the power domains 60, 62, 64 may, in certain embodiments, operate at different voltages. For example, components within the “always on” and “low” power domains 60, 62 may operate at 3.3V (the voltage supplied to the integrated circuit 56), whereas components within the “core” power domain may operate at 1.2V. Level shifters 65 may be placed between the power domains 60, 62, 64 operating at different voltages to change the voltage of signals transmitted therebetween. Although the higher operating voltages of the “always on” and “low” power domains 60, 62 may require use of larger components (e.g., transistors, resistors, etc.), which may increase power consumption, the increased power consumption may be offset by eliminating voltage regulators needed to reduce the operating voltage. That is, by eliminating the voltage regulators for the “always on” and “low” power domains 60, 62, the net power consumption may be reduced even while using larger, less efficient components (assuming that the number of larger components is relatively small).
In certain embodiments, the “always on” power domain 60 may include very low power control logic 66 which may remain operational even when other portions of the integrated circuit 56 are turned off or disabled. In certain embodiments, the “low” power domain 62 may include a “wake-on-event” module 68 which may listen for finger or non-finger activity and wake up other parts of the circuit 56 (e.g., core components 70) when such activity is detected. The “wake-on-event” module 68 may include both low power digital components 69 and analog components 71, as will be explained in more detail hereafter. The “core” power domain 64 may include core components 70, which may include core digital components 72 and core analog components 74. Core digital components 72, for example, may include the CPU 42, memory 44, DMAC 46, and other digital components. The core components 70 may comprise the largest portion of the circuit 56 and may provide most of the processing power for the circuit 56.
In certain embodiments, the very low power control logic 66 may control the power supply to each of the power domains 60, 62, 64. For example, the very low power control logic 66 may turn power on and off to components in the low power domain 62. The very low power control logic 66 may also control the power supply to voltage regulators 76, 78, which may reduce the operating voltage from 3.3V to 1.2V to power the core components 70. These regulators 76, 78, in selected embodiments, may include a core digital and core analog voltage regulator 76, 78, each of which may be turned on and off independently. By controlling power supplied to the regulators 76, 78, the core analog and core digital components may be turned on or off as needed.
In certain embodiments, output latches 80 may be provided to latch or gate output values to fixed states when the components within particular power domains are powered off. That is, when components within a particular power domain are powered off, the outputs from these components may “float” or assume uncertain states that may confuse downstream components, which may be unaware that the upstream components are powered off. To correct this problem, the outputs may be latched or gated to fixed states to avoid confusion or erratic behavior that may result when various power domains or components are powered off.
Referring to
Sleep mode 80 may refer to the circuit's lowest level of hibernation or inactivity and may provide the lowest power consumption. In this mode 80, the “low” power domain 62 and “core” power domain 64 (both the analog portion 74 and digital portion 72) may be turned off. Furthermore, the 1.2V and 3.3V level shifters 65 may be disabled and the outputs may be latched or gated to fixed states. In sleep mode 80, the fingerprint sensing elements (i.e., the transmitting and receiving elements 14, 16, 18, 20) may be shut down such that they are neither transmitting nor receiving. As a result, finger activity over the fingerprint sensing area 10 will go undetected and thus have no effect on the fingerprint sensor.
“Wake-on-event” mode 82 may correspond to a slightly greater level of activity and power consumption compared to sleep mode 80, while still leaving most of the circuit 56 (i.e., the core components 70) turned off. Thus, “wake-on-event” mode 82 may be similar to sleep mode 80 in that it consumes very little power, while providing some additional features not provided by sleep mode 80. In “wake-on-event” mode 82, the “low” power domain 62 may be turned on while the “core” power domain 64 (both the analog portion 74 and digital portion 72) may be turned off. The 1.2V and 3.3V level shifters may be disabled and the outputs may be latched or gated to fixed states. In general, the “wake-on-event” mode 82 may differ from sleep mode 80 in that finger activity over the fingerprint sensing area 10 may be detected and used to wake up the rest of the circuit 56.
As will be explained in more detail in association with
When operating in the “sensing” sub-mode, a relatively small number of fingerprint sensing elements (i.e., the transmitting and receiving elements 14, 16, 18, 20) may be kept active to sense activity over the fingerprint sensing area 10 and wake up the circuit 56 and the remaining fingerprint sensing elements. When operating in the “wait for event” sub-mode, the fingerprint sensing elements may be turned off while the “wake-on-event” module 68 may continue to listen for non-finger activity such as General Purpose Input/Output (GPIO) activity, USB activity, SPI activity, parallel port activity, the expiration of one or more timers, or the like. Non-finger activity, like finger activity, may be used to wake up the circuit (e.g., the core components 70) and the remaining fingerprint sensing elements in order to read a fingerprint. In selected embodiments, the integrated circuit 56 may be programmable with respect to the type of activity that will wake up the core components 70.
Although the lowest power may be achieved when all non-essential components (including the CPU 42) are turned off, in selected embodiments the CPU 42 may be kept active in wake-on-event mode 82. If desired, other core components 70, such as the memory 44 or DMAC 46 may be disabled or turned off completely to conserve power. By keeping the CPU 42 active, the CPU 42 may be available for quick response and/or calibration. This may allow the CPU 42 to respond more rapidly to events by eliminating the need to power up and go through initialization routines. As will be explained in more detail in association with
In active mode 84, various core components 70, most notably the CPU 42, may be turned on to resume normal, full power operation. This may significantly raise power consumption while also significantly increasing the functionality of the circuit 56. Unlike the other two modes 80, 82, the 1.2V and 3.3V level shifters may be enabled and the outputs may be opened (i.e., not latched or gated to fixed states). Like the “wake-on-event” mode 82, the active mode 84 may include various sub-modes. These sub-modes may include (1) a lower power “idle” sub-mode where the core digital components 72 (e.g., the CPU 42, the memory 44, etc.) are turned on but the core analog components 74 (e.g., the analog front end 30, ADC 32, etc.) are turned off; and (2) a higher power “scanning” sub-mode where both the core digital components 72 and core analog components 74 are turned on. The “scanning” sub-mode may be used to scan a fingerprint and the “idle” sub-mode may be used during periods where scanning is not performed, will imminently be performed, has just ended, or the like, such that the analog components 74 are not needed but the CPU 42 is still turned on or active.
Various events may cause the circuit 56 to transition between each of the modes 80, 82, 84. For example, a “wake up” event may cause the circuit 56 to transition from sleep mode 80 directly to active mode 84. For example, if a fingerprint sensor is implemented in a “flip type” cell phone, opening up the cell phone may expose the fingerprint sensor and may be considered a “wake up” event 86 to wake up the fingerprint sensor and put it into active mode 84. This will cause the fingerprint sensor to begin scanning the fingerprint sensing area 10 for a fingerprint. Similarly, closing the cell phone may be considered a “go to sleep” event 88 which may cause the circuit 56 to transition back to sleep mode 80. Such a transition 88 may occur, for example from either the active mode 84 or the wake-on-event mode 82.
On the other hand, if the cell phone is left open (with the fingerprint sensor exposed), but the sensor has not detected finger activity for some time (t), the circuit 56 may transition 90 from active mode 84 to “wake-on-event” mode 82. This will cause the circuit 56 to transition to a much lower power state while still retaining the ability to listen for finger or non-finger activity. More specifically, the wake-on-event module 68 may take control of the circuit 56 and shut off power to the core components 70, including the CPU 42, to eliminate or reduce both switching and leakage current in the core components 70.
On the other hand, if the wake-on-event module 68 detects finger or non-finger activity (i.e., a finger or non-finger event 92), the circuit 56 may transition 92 back to active mode 84. More specifically, the wake-on-event module 68 may turn on power to the core components 70 and pass control back to the CPU 42. By shutting down the primary logic portion (e.g., the core components 70) of the integrated circuit 56 and passing control to a secondary logic portion (e.g., the wake-on-event module 68) during periods of inactivity, very low power consumption may be achieved. In fact, an integrated circuit 56 operating in the manner described herein has been shown to consume less then 10 μA (usually 5-6 μA) when operating in wake-on-event mode 82.
The illustrated modes 80, 82, 84, including the sub-modes discussed herein, and the events 86, 88, 90, 92 used to transition between the modes 80, 82, 84, simply provide one example of various modes and events that may be used with an integrated circuit 56 in accordance with the invention. Thus, the illustrated example is not intended to be limiting. Indeed, the modes, names of the modes, components and power domains effected by the modes, and events used to transition between the modes, may be modified, added to, or subtracted from, as needed, without departing from the principles and characteristics of the invention as described herein.
Referring to
In selected embodiments, before the CPU 42 is turned off, the CPU 42 may be configured to send various types of data (e.g., settings, calibration values, security values, etc.) to the state machine 100 for retrieval at a later time. This is because the CPU 42 and memory 44 may be powered down and thus may be unable to retain any data when in wake-on-event mode 82. This data may be saved in one or more persistent registers 104 in the wake-on-event module 68 when the CPU 42 is powered down. The CPU 42 may retrieve this data from the registers 104 when power is turned back on in order to start up and initialize correctly.
The wake-on-event module 68 may also communicate with sensor I/O drivers 106 that interface with the transmitting and receiving elements 14, 16, 18, 20. These sensor I/O drivers 106 may be shared by the wake-on-event module 68 and the CPU 42 and may be programmable to function as either transmitters or receivers. A sensor I/O driver 106 may be provided for each transmitting and receiving element 14, 16, 18, 20 in the fingerprinting sensing area 10. The wake-on-event module 68 and CPU 42 may use decoders 108 to select which sensor I/O drivers 106 are utilized by the wake-on-event module 68 and CPU 42 respectively.
When in wake-on-event mode 82, the wake-on-event module 68 may take control of the sensor I/O drivers 106. Similarly, when in active mode 84, the CPU 42 may take control of the sensor I/O drivers 106. A multiplexer 110, controlled by the state machine 100, may be used to select whether the state machine 100 or the CPU 42 controls the sensor I/O drivers 106. One or more sensor I/O select registers 112, coupled to a multiplexer 114 (also controlled by the state machine 100) may store data that selects which sensor I/O drivers 106 are configured to transmit and receive when in wake-on-event mode 82.
In selected embodiments, the CPU 42 may communicate with the wake-on-event module 68 through a number of I/O lines. These I/O lines may pass through level shifters 65 due to the difference in operating voltages between the CPU and wake-on-event module power domains. For example, the I/O lines may include “reset,” “control,” “data,” and “sensor I/O control” lines. In selected embodiments, the reset and control I/O lines may communicate with a demultiplexer 116 to select which registers 104, 112 are written to by the CPU 42. The sensor I/O control line(s) may communicate with the decoders 108 by way of the multiplexer 110.
Referring to
The response signal may be received by the detector 120 of the wake-on-event module 68. In certain embodiments, the response signal may be passed through various analog components, such as a high pass filter 128 to remove noise, and an amplifier 130. The amplifier 130, and more particularly the high pass filter 128, may generate a response signal 132 that includes a series of response pulses which may resemble a series of sharp peaks. This response signal 132 may be input to a comparator 134a, which may compare the response signal 132 to an upper reference signal, and a comparator 134b, which may compare the response signal 132 to a lower upper reference signal. In selected embodiments, the upper and lower reference signals may be digitally programmed to a desired level using a pair of resistive digital-to-analog converters, or RDACs 136a, 136b.
As will be explained in more detail in
Referring to
As shown, an upper reference signal 150a may be established above the peaks of the response signal 132 and a lower reference signal 150b may be established below the peaks of the response signal 132. As explained in association with
For example, when a user places a finger over the fingerprint sensing area 10, the magnitude of the response signal 132 may increase significantly due to the reduced impedance between the transmitting element and the receiving element. This will cause the magnitude of the response pulses 152 to increase above the upper reference signal 152. When the response pulses 152 increase above the upper reference signal 150a, the comparator 134a may output a high logic value, as shown by the comparator output signal 154a. Similarly, when the magnitude of the response pulses 152 is greater than the lower reference signal 150b, the comparator 134b may also output a high logic value, as shown by the comparator output signal 154b. In this example, the peaks of the response signal 132 always exceed the lower reference signal 150b and thus each response pulse will generate a pulse on the comparator output signal 154b.
As explained previously, the pulses of the comparator output signals 154a, 154b may be captured and held by one or more latches 138. Thus, a first latch output 156a may generate a high value each time a high value is encountered in the comparator output signal 154a. A second latch output 156b may generate a high value each time a high value is encountered in the comparator output signal 154b. These latches 138 may be reset after each pulse so they may be counted by the state machine 100 and be ready to capture and hold the next pulse on the comparator output signals 154a, 154b. After the number of pulses of the first and second latch output signals 156a, 156b have been counted, the number may be processed by the wake-on-event logic 142 in the state machine 100 to take action. Based on the number of pulses that are counted above or below the reference signals 150a, 150b, the wake-on-event module 68 may determine that finger activity has been detected and wake up the circuit 56.
Referring to
Referring to
However, the wake-on-event module 68 may still be configured to detect non-finger activity, such as GPIO activity, USB activity, SPI activity, parallel port activity, or the like, even during the idle time. In selected embodiments, the state machine 100 may include a programmable timer to automatically wake up the core components 70 after a specified amount of time has passed, regardless of whether finger or non-finger activity was detected. By disabling or turning off the analog components 71 during idle time, the power that is consumed by the wake-on-event module 68 during idle time may be significantly reduced.
Conversely, the wake-on-event module 68 may be put in the higher power “sensing” sub-mode when actively transmitting and receiving from the fingerprint sensing area 10. In this sub-mode, the low power analog components 71 may be enabled and the power domain 62b for the switched outputs may be turned on.
Each time the series of probing pulses is transmitted to the fingerprint sensing area 10, the response signal 132 may be compared to the upper and lower references signals 150a, 150b, as described in
If no event occurs (i.e., a finger is neither placed on nor removed from the fingerprint sensing area 10), the counter 140 should count zero responses above the upper reference signal 150a and sixteen responses above the lower reference signal 150b. On the other hand, if a finger is placed over the sensor, the counter 140 should count sixteen responses above the upper reference signal 150a and sixteen responses above the lower reference signal 150b. Similarly, if a finger was already placed on the fingerprint sensor when the sensor enters wake-on-event mode 82 but was then removed, the counter 140 should count zero responses above the upper reference signal 150a and zero responses above the lower reference signal 150b.
These scenarios are the ideal cases where the counter 140 counts either zero or sixteen responses for all cases. In practice, the counter 140 will likely arrive at a number between zero and sixteen due to the effect of noise or other variations in the response signal 132. In selected embodiments, the wake-on-event logic 142 may be programmed to trigger a wakeup for different numbers of response pulses that cross the reference signals 150a, 150b. For example, the wake-on-event logic 142 may be programmed to trigger a wake-up if ten of the sixteen responses are above the upper reference signal 150a. Similarly, the wake-on-event logic 142 may be programmed to trigger a wake-up if four of the sixteen responses are below the lower reference signal 150b. These numbers may be adjusted as needed to account for noise and other signal fluctuations.
As shown in
The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described examples are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.
