U.S. patent application number 14/331760 was filed with the patent office on 2014-11-06 for apparatus and method for reducing noise in fingerprint sensing circuits.
The applicant listed for this patent is SYNAPTICS INCORPORATED. Invention is credited to Gregory Lewis DEAN, Richard Alexander ERHART, Jaswinder S. JANDU, Erik Jonathon THOMPSON.
Application Number | 20140328522 14/331760 |
Document ID | / |
Family ID | 41133328 |
Filed Date | 2014-11-06 |
United States Patent
Application |
20140328522 |
Kind Code |
A1 |
DEAN; Gregory Lewis ; et
al. |
November 6, 2014 |
APPARATUS AND METHOD FOR REDUCING NOISE IN FINGERPRINT SENSING
CIRCUITS
Abstract
An apparatus for reducing noise 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 analog front end is coupled to the fingerprint sensing area and
is configured to generate an analog response signal. An
analog-to-digital converter (ADC) samples the analog response
signal and converts the sample to a digital value, which may be
received by a digital device such as a processor or CPU. To reduce
the amount of the noise that is present in the analog response
signal and therefore reflected in the digital value, the digital
device may be shut down while the ADC is sampling the analog
response signal.
Inventors: |
DEAN; Gregory Lewis;
(Standish, ME) ; ERHART; Richard Alexander;
(Tempe, AZ) ; JANDU; Jaswinder S.; (Chandler,
AZ) ; THOMPSON; Erik Jonathon; (Phoenix, AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SYNAPTICS INCORPORATED |
San Jose |
CA |
US |
|
|
Family ID: |
41133328 |
Appl. No.: |
14/331760 |
Filed: |
July 15, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13965978 |
Aug 13, 2013 |
8787632 |
|
|
14331760 |
|
|
|
|
13372153 |
Feb 13, 2012 |
8520913 |
|
|
13965978 |
|
|
|
|
12098367 |
Apr 4, 2008 |
8116540 |
|
|
13372153 |
|
|
|
|
Current U.S.
Class: |
382/124 |
Current CPC
Class: |
G06K 9/0002
20130101 |
Class at
Publication: |
382/124 |
International
Class: |
G06K 9/00 20060101
G06K009/00 |
Claims
1. An apparatus for reducing noise in fingerprint sensing circuits,
the apparatus comprising: a fingerprint sensing area onto which a
user can apply a fingerprint; an analog front end coupled to the
fingerprint sensing area to generate an analog response signal; an
analog-to-digital converter (ADC) to sample the analog response
signal and convert the sample to a digital value; and a digital
device to receive the digital value from the ADC, the digital
device further configured to shut down while the ADC is sampling
the analog response signal.
Description
CROSS-REFERENCE
[0001] This application is a continuation of Ser. No. 13/965,978
filed Aug. 13, 2013, which is a continuation of Ser. No.
13/372,153, filed Feb. 13, 2012, now U.S. Pat. No. 8,520,913,
issued Aug. 27, 2013, which is a continuation of Ser. No.
12/098,367, filed Apr. 4, 2008, now U.S. Pat. No. 8,116,540, issued
Feb. 14, 2012, both entitled "Apparatus and Method for Reducing
Noise in Fingerprint Sensing Circuits," which are incorporated
herein by reference in its entirety and to which application we
claim priority under 35 USC .sctn.120.
BACKGROUND
[0002] This invention relates to fingerprint sensing technology,
and more particularly to apparatus and methods for reducing the
effects of noise in fingerprint sensing circuits.
[0003] Fingerprint sensing technology is increasingly recognized as
a reliable, non-intrusive way to verify individual identity.
Fingerprints, like various other biometric characteristics, are
based on unalterable personal characteristics and thus are believed
to be more reliable when identifying individuals. The potential
applications for fingerprints sensors are myriad. For example,
electronic fingerprint sensors may be used to provide access
control in stationary applications, such as security checkpoints.
Electronic fingerprint sensors may also be used to provide access
control in portable applications, such as portable computers,
personal data assistants (PDAs), cell phones, gaming devices,
navigation devices, information appliances, data storage devices,
and the like. Accordingly, some applications, particularly portable
applications, may require electronic fingerprint sensing systems
that are compact, highly reliable, and inexpensive.
[0004] Various electronic fingerprint sensing methods, techniques,
and devices have been proposed or are currently under development.
For example, optical and capacitive fingerprint sensing devices are
currently on the market or under development. Like a digital
camera, optical technology utilizes visible light to capture a
digital image. In particular, optical technology may use a light
source to illuminate an individual's finger while a charge-coupled
device (CCD) captures an analog image. This analog image may then
be converted to a digital image.
[0005] There are generally two types of capacitive fingerprint
sensing technologies: passive and active. Both types of capacitive
technologies utilize the same principles of capacitance to generate
fingerprint images. Passive capacitive technology typically
utilizes an array of plates to apply an electrical current to the
finger. The voltage discharge is then measured through the finger.
Fingerprint ridges will typically have a substantially greater
discharge potential than valleys, which may have little or no
discharge.
[0006] Active capacitive technology is similar to passive
technology, but may require initial excitation of the epidermal
skin layer of the finger by applying a voltage. Active capacitive
sensors, however, may be adversely affected by dry or worn minutia,
which may fail to drive the sensor's output amplifier. By contrast,
passive sensors are typically capable of producing images
regardless of contact resistance and require significantly less
power.
[0007] Although each of the fingerprint sensing technologies
described above may generate satisfactory fingerprint images, each
may be adversely affected by noise, interference, and other
effects. For example, capacitive sensors may be particularly
susceptible to noise and parasitic capacitive coupling, which may
degrade the quality of the acquired fingerprint image. Accordingly,
it would be an advance in the art to reduce the effects of noise,
parasitic capacitive coupling, and other effects in capacitive-type
fingerprint sensing circuits.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] 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 use of the accompanying drawings, in
which:
[0009] FIG. 1 is a high-level block diagram of one embodiment of a
fingerprint sensing area containing an array of fingerprint sensing
elements and interfacing with a fingerprint sensing circuit;
[0010] FIG. 2 is a partial cutaway profile view of a fingerprint
sensing area showing the interaction between a finger and
fingerprint sensing elements in a capacitive-type fingerprint
sensor, with a fingerprint ridge lying substantially over the
sensor gap;
[0011] FIG. 3 is a partial cutaway profile view of a fingerprint
sensing area showing the interaction between a finger and
fingerprint sensing elements in a capacitive-type fingerprint
sensor, with a fingerprint valley lying substantially over the
sensor gap;
[0012] FIG. 4 is a high-level block diagram of one embodiment of a
fingerprint sensing circuit for use with the present invention;
[0013] FIG. 5 is a timing diagram showing an example of various
signals that may exist within a fingerprint sensing circuit useable
with the present invention; and
[0014] FIG. 6 is a high-level block diagram showing an example of
an oscillator, multipliers, dividers, and the like that may be used
to generate the signals of FIG. 5.
DETAILED DESCRIPTION
[0015] 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
reducing noise in 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.
[0016] Consistent with the foregoing, an apparatus for reducing
noise 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 analog front end is
coupled to the fingerprint sensing area and is configured to
generate an analog response signal. An analog-to-digital converter
(ADC) samples the analog response signal and converts the sample to
a digital value, which may be received by a digital device such as
a processor or CPU. To reduce the amount of the noise that is
present in the analog response signal and therefore reflected in
the digital value, the digital device may be shut down while the
ADC is sampling the analog response signal.
[0017] In selected embodiments, the digital device is shut down by
turning off a clock signal to the digital device. In other
embodiments, the digital device is shut down by disabling the
digital device or turning off power to the digital device. In
certain embodiments, the digital device is configured to shut down
some interval prior to the time the ADC samples the analog response
signal. The interval may be selected to allow any noise to settle
out of the system prior to sampling the analog response signal.
[0018] In another embodiment in accordance with the invention, a
method for reducing noise in a fingerprint sensing circuit may
include providing a fingerprint sensing area onto which a user can
apply a fingerprint. An analog response signal associated with
finger activity over the fingerprint sensing area may then be
generated. The method may further include sampling the analog
response signal and converting the sample to a digital value. This
digital value may then be received by a digital device. To reduce
the amount of noise that is present in the analog response signal
during sampling and therefore reflected in the digital value, the
method may further include shutting down the digital device while
sampling the analog response signal.
[0019] In another embodiment in accordance with the invention, a
method for reducing noise in fingerprint sensing circuits includes
providing a fingerprint sensing area onto which a user can apply a
fingerprint and generating an analog response signal in response to
finger activity over the fingerprint sensing area. The analog
response signal is then sampled and converted to a digital value.
This digital value may then be received or processed by a CPU. To
reduce the amount of the noise that is present in the analog
response signal during sampling and reflected in the digital value,
the method may further include turning off a clock signal to the
CPU while sampling the analog response signal.
[0020] It will be readily understood that the components of the
present invention, as generally described and illustrated in the
Figures herein, may be arranged and designed in a wide variety of
different configurations. Thus, the following more detailed
description of the embodiments of the systems and methods of 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 selected embodiments of the invention.
[0021] Some of the functional units or method steps described in
this specification may be embodied or implemented as modules. For
example, a module may be implemented as a hardware circuit
comprising custom VLSI circuits or gate arrays, off-the-shelf
semiconductors such as logic chips, transistors, or other discrete
components. A module may also be implemented in programmable
hardware devices such as field programmable gate arrays,
programmable array logic, programmable logic devices, or the
like.
[0022] Modules may also be implemented in software for execution by
various types of processors. An identified module of executable
code may, for instance, comprise one or more physical or logical
blocks of computer instructions which may, for instance, be
organized as an object, procedure, or function. Nevertheless, the
executables of an identified module need not be physically located
together, but may comprise disparate instructions stored in
different locations which, when joined logically together, comprise
the module and achieve the stated purpose of the module.
[0023] Indeed, a module of executable code could be a single
instruction, or many instructions, and may even be distributed over
several different code segments, among different programs, and
across several memory devices. Similarly, operational data may be
identified and illustrated herein within modules, and may be
embodied in any suitable form and organized within any suitable
type of data structure. The operational data may be collected as a
single data set, or may be distributed over different locations
including over different storage devices, and may exist, at least
partially, merely as electronic signals on a system or network.
[0024] Reference throughout this specification to "one embodiment,"
"an embodiment," or similar language means that a particular
feature, structure, or characteristic described in connection with
the embodiment may be included in at least one embodiment of the
present invention. Thus, appearances of the phrases "in one
embodiment" or "in an embodiment" in various places throughout this
specification are not necessarily all referring to the same
embodiment.
[0025] Furthermore, the described features, structures, or
characteristics may be combined in any suitable manner in one or
more embodiments. One skilled in the relevant art will recognize,
however, that the invention can be practiced without one or more of
the specific details, or with other methods, components, etc. In
other instances, well-known structures, or operations are not shown
or described in detail to avoid obscuring aspects of the
invention.
[0026] The illustrated embodiments of the invention will be best
understood by reference to the drawings, wherein like parts are
designated by like numerals throughout. The following description
is intended only by way of example, and simply illustrates certain
selected embodiments of apparatus and methods that are consistent
with the invention as claimed herein.
[0027] Referring to FIG. 1, in selected embodiments, a fingerprint
sensor 10 useable with an apparatus and method in accordance with
the invention may include a fingerprint sensing area 12 to provide
a surface onto which a user can swipe a fingerprint. A dotted
outline of a finger 14 is shown superimposed over the fingerprint
sensing area 12 to provide a general idea of the size and scale of
one embodiment of a fingerprint sensing area 12. The size and shape
of the fingerprint sensing area 12 may vary, as needed, to
accommodate different applications.
[0028] In certain embodiments, the fingerprint sensing area 12 may
include an array of transmitting elements 16, such as a linear
array of transmitting elements 16, to assist in scanning lines of
"pixels" as a fingerprint is swiped across the fingerprint sensing
area 12. In this embodiment, the transmitting elements 16 are shown
as a linear array of conductive traces 16 connected to a
fingerprint sensing circuit 18. The transmitting elements 16 are
not drawn to scale and may include several hundred transmitting
elements 16 arranged across the width of a fingerprint, one
transmitting element 16 per pixel. A fingerprint image may be
generated by scanning successive lines of pixels as a finger is
swiped over the array. These lines may then be assembled to
generate a fingerprint image, similar to the way a fax image is
generated using line-by-line scanning.
[0029] In certain embodiments, the transmitting elements 16 are
configured to sequentially emit, or burst, a probing signal, one
after the other. As will be explained in more detail hereafter, the
probing signal may include a burst of probing pulses, such as a
burst of square waves. This probing signal may be sensed on the
receiving end by a receiving element 20. Like the transmitting
elements 16, the receiving element 20 is shown as a conductive
trace 20 connected to the fingerprint sensing circuit 18. Although
shown as a single receiving element 20, in other embodiments, pairs
of receiving elements 20 may be used to differentially cancel out
noise.
[0030] At the receiving element 20, a response signal may be
generated in response to the probing signal. The magnitude of the
response signal may depend on factors such as whether a finger is
present over the fingerprint sensing area 12 and, more
particularly, whether a ridge or valley of a fingerprint is
immediately over the gap 22 between a transmitting element 16 and
the receiving element 20. The magnitude of the signal generated at
the receiving element 20 may be directly related to the RF
impedance of a finger ridge or valley placed over the gap 22
between the corresponding transmitting element 16 and receiving
element 20.
[0031] By using a single receiving element 20 (or a small number of
receiving elements 20) and a comparatively larger number of
transmitting elements 16, a receiver that is coupled to the
receiving element 20 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. This design differs from many
conventional fingerprint sensors, which may employ a single large
transmitting element with a large array of receiving elements and
receivers. Nevertheless, the apparatus and methods described herein
are not limited to the illustrated transmitter and receiver design.
Indeed, the apparatus and methods disclosed herein may be used with
fingerprint 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 and receiving
elements.
[0032] As shown in FIG. 1, the fingerprint sensing area 12
(including the transmitting and receiving elements 16, 20) may be
physically decoupled from the fingerprint sensing circuit 18.
Positioning the sensing elements 16, 20 off the silicon die may
improve the reliability of the fingerprint sensor 10 by reducing
the sensor's susceptibility to electrostatic discharge, wear, and
breakage. This may also allow the cost of the sensor 10 to be
reduced over time by following a traditional die-shrink roadmap.
This configuration provides a distinct advantage over direct
contact sensors (sensors that are integrated onto the silicon die)
which cannot be shrunk to less than the width of an industry
standard fingerprint. Nevertheless, the apparatus and methods
disclosed herein are applicable to fingerprint sensors with sensing
elements that are located either on or off the silicon die.
[0033] Referring generally to FIGS. 2 and 3, in selected
embodiments, the transmitting and receiving elements 16, 20
discussed above may be adhered to a non-conductive substrate 30.
For example, the substrate 30 may be constructed of a flexible
polyimide material marketed under the trade name Kapton.RTM. and
with a thickness of between about 25 and 100 .mu.m. The Kapton.RTM.
polymer may allow the fingerprint sensor 10 to be applied to
products such as touchpads and molded plastics having a variety of
shapes and contours while providing exceptional durability and
reliability.
[0034] In selected embodiments, a user's finger may be swiped
across the side of the substrate 30 opposite the transmitting and
receiving elements 16, 20. Thus, the substrate 30 may electrically
and mechanically isolates a user's finger from the transmitting
element 16 and receiving element 20, thereby providing some degree
of protection from electrostatic discharge (ESD) and mechanical
abrasion.
[0035] The capacitive coupling between the transmitting element 16
and the receiving element 20 may change depending on whether a
fingerprint ridge or valley is immediately over the gap 22. This is
because the dielectric constant of a finger is typically ten to
twenty times greater than the dielectric constant of air. The
dielectric constant of the ridges of a finger may vary
significantly from finger to finger and person to person,
explaining the significant range of dielectric constants. Because a
fingerprint ridge has a dielectric constant that differs
significantly from that of air, the capacitive coupling between the
transmitting element 16 and receiving element 20 may vary
significantly depending on whether a ridge or valley is present
over the sensor gap 22.
[0036] For example, referring to FIG. 2, when a fingerprint ridge
32 is over the gap 22, the capacitive coupling between the
transmitting element 16 and receiving element 20 may be increased
such that the probing signal emitted by the transmitting element 16
is detected at the receiving element 20 as a stronger response
signal. It follows that a stronger response signal at the receiving
element 20 indicates the presence of a ridge 32 over the gap 22. On
the other hand, as shown in FIG. 3, the capacitive coupling between
the transmitting element 16 and receiving element 20 may decrease
when a valley is present over the gap 22. Thus, a weaker response
signal at the receiving element 20 may indicate that a valley 34 is
over the gap 22. By measuring the magnitude of the response signal
at the receiving element 20, ridges and valleys may be detected as
a user swipes his or her finger across the sensing area 12,
allowing a fingerprint image to be generated.
[0037] Referring to FIG. 4, in certain embodiments, a fingerprint
sensing circuit 18 useable with an apparatus and method in
accordance with the invention may include a transmitter clock 40
configured to generate an oscillating signal, such as an
oscillating square-wave signal. Scanning logic 42 may be used to
sequentially route the oscillating signal to buffer amplifiers 46,
one after the other, using switches 44. The buffer amplifiers 46
may amplify the oscillating signal to generate the probing signal.
As shown, the buffer amplifiers 46 may sequentially burst the
probing signal 48 to each of the transmitting elements 16, one
after the other. A response signal, generated in response to the
probing signal 48, may be sensed at the receiving element 20 and
may be routed to a variable-gain amplifier 50 to amplify the
response signal. The amplified response signal may then be passed
to a band pass filter 52 centered at the frequency of the
transmitter clock 40.
[0038] The output from the band pass filter 52 may then be supplied
to an envelope detector 54, which may detect the envelope of the
response signal. This envelope may provide a baseband signal, the
amplitude of which may vary depending on whether a ridge or valley
is over the sensor gap 22. The baseband signal may be passed to a
low pass filter 56 to remove unwanted higher frequencies. The
variable-gain ampifier 50, band pass filter 52, envelope detector
54, low pass filter 56, as well as other analog components may be
collectively referred to as an analog front end 57.
[0039] The output from the low pass filter 56 may be passed to an
analog-to-digital converter (ADC) 58, which may convert the analog
output to a digital value. The ADC 58 may have, for example, a
resolution of 8 to 12 bits and may be capable of resolving the
output of the low pass filter 56 to 256 to 4096 values. The
magnitude of the digital value may be proportional to the signal
strength measured at the receiving element 20. Likewise, as
explained above, the signal strength may be related to the
capacitive coupling between the transmitting element 16 and
receiving element 20, which may depend on the RF impedance of the
feature over the sensor gap 22.
[0040] The resulting digital value may be passed to a CPU 60 or
other digital components, which may eventually pass digital
fingerprint data to a host system 62. The host system 62, in
selected embodiments, may process the fingerprint data using
various matching algorithms in order to authenticate a user's
fingerprint.
[0041] In addition to processing the digital data, the CPU 60 may
control the gain of the variable-gain amplifier 50 using a
digital-to-analog converter (DAC) 64. The gain may be adjusted to
provide a desired output power or amplitude in the presence of
variable sensing conditions. For example, in selected embodiments,
the gain of the variable-gain amplifier 50 may be adjusted to
compensate for variations in the impedance of different fingers. In
selected embodiments, the CPU 60 may also control the operation of
the scanning logic 42.
[0042] Referring to FIG. 5, a timing diagram is illustrated to
describe various signals that may occur within the fingerprint
sensing circuit 18 of FIG. 4. In selected embodiments, the
fingerprint sensing circuit 18 may include a phase-locked loop
(PLL) output 70 which may provide the master reference signal to
drive many of the components in the fingerprint sensing circuit 18.
In certain embodiments, the PLL output 70 may be derived from a
lower frequency crystal input that is multiplied to generate the
PLL output 70. As will be explained in more detail in association
with FIG. 6, the PLL output 70 may be divided down to provide
clocking signals to different components in the fingerprint sensing
circuit 18.
[0043] A pixel clock signal 72 may control the amount of time each
transmitting element 16 is transmitting the probing signal 48. For
example, a rising edge 74a of the pixel clock signal 72 may cause a
first transmitting element 16 in the array to begin emitting the
probing signal 48. The next rising edge 74b may cause the first
transmitting element 16 to cease transmitting and cause the next
transmitting element 16 to begin emitting the probing signal 48.
This process may continue for each transmitting element 16 in the
array to generate a "line" of fingerprint data. Successive lines
may be scanned in this manner and the lines may be combined to
generate a fingerprint image as previously discussed. The interval
between the rising edges 74a, 74b may be referred to as the "pixel
period" 76.
[0044] Analog response signals 78a, 78b may represent the signal
that is output from the analog front end 57 described in FIG. 4. A
first analog response signal 78a may reflect the response that
occurs when a ridge transitions to a valley over the sensing gap
22. A second analog response signal 78b may reflect the response
that occurs when a valley transitions to a ridge over the sensing
gap 22.
[0045] As shown, the first analog response signal 78a may initially
have a magnitude 80a when a ridge is placed over the sensor gap 22
(reflecting greater capacitive coupling between the transmitting
and receiving elements 16, 20). When the ridge is removed from the
sensor gap 22 (i.e., a valley is placed over the sensor gap 22),
the magnitude 82a of the signal 78a may become smaller (reflecting
reduced capacitive coupling between the transmitting and receiving
elements 16, 20). The gradual transition between the peak
magnitudes 80a, 82a may be the result of time constants of analog
components in the fingerprint sensing circuit 18. That is, due to
the frequency response of analog components, such as band pass and
low pass filters 52, 56, some time may be needed for the signal 78a
to settle at a new level. After the signal 78a has transitioned
from one peak value 80a to the other 82a, the signal 78a may reach
a substantially steady state level 84a. Sampling is ideally
conducted during this period.
[0046] Similarly, the second analog response signal 78b may
initially have a magnitude 80b when a valley is placed over the
sensor gap 22 (reflecting lower capacitive coupling between the
transmitting and receiving elements 16, 20). When a ridge is placed
over the sensor gap 22, the magnitude 82b of the signal 78b may
become larger (reflecting increased capacitive coupling between the
transmitting and receiving elements 16, 20). The signal may
transition between the peak magnitudes 80b, 82b, reflecting time
constants in the fingerprint sensing circuit 18, after which the
signal 78b may reach a substantially steady state level 84b.
[0047] An ADC clock signal 86 may be used to clock the ADC 58 in
the fingerprint sensing circuit 18. In this example, the ADC 58
uses three clock cycles 88 to sample the voltage of the analog
response signal 78a, 78b and eight clock cycles to convert the
sample to a digital value. In certain embodiments, the ADC 58 may
open a gate and store the sample on a capacitor for about three
clock cycles. After the three clock cycles have passed, the ADC 58
may close the gate and convert the sample to a digital value. In
certain embodiments, the ADC 58 may use one clock cycle to generate
each bit of the digital value. The number of clock cycles used for
sampling and converting is presented only by way of example and is
not intended to be limiting. As shown, sampling may be performed
during the steady state period 84a, 84b (i.e., the peak response
period) to generate the most accurate sample.
[0048] In selected embodiments, an ADC convert pulse signal 92 may
be used to instruct the ADC 58 to begin sampling. For example, when
an edge, such as a rising edge 94, of the ADC convert pulse signal
92 is detected by the ADC 58, the ADC 58 may begin to sample the
analog response signal 78a, 78b and convert the sample to a digital
value.
[0049] A CPU clock signal 96 may be used to clock the CPU 60 and
other digital components in the fingerprint sensing circuit 18. As
shown, in selected embodiments, the clock signal 96 may be
configured to operate during an initial portion of the pixel period
76 (the period of minimum noise sensitivity) but may be shut down
during the remainder of the pixel period 76. This may be performed
to reduce system noise (e.g., switching noise produced by the CPU
60 or other digital components when they change state) in the
fingerprint sensing circuit 18 when the ADC 58 is sampling the
analog response signal 78. This will ideally reduce the amount of
noise that is reflected in the digital value, allowing a clearer
fingerprint image to be generated.
[0050] The signal 98 may represent noise that is generated in the
fingerprint sensing circuit 18 when the CPU clock 96 is turned on.
By shutting off the CPU clock 96 at an appropriate time, noise may
be allowed to settle out of the system prior to sampling the analog
response signal 78. Thus, in selected embodiments, the CPU clock 96
may be shut down some interval prior to the time the ADC 58 samples
the analog response signal 78 in order to allow time for noise to
settle out of the system.
[0051] In alternative embodiments, instead of shutting down the CPU
clock 96 while sampling the analog response signal 78, other
actions may be taken to reduce noise in the fingerprint sensing
circuit 18. For example, the CPU 60 itself may be disabled or shut
down when sampling the analog response signal 78, and then
re-enabled or re-started once sampling is complete.
[0052] Referring to FIG. 6, in order to generate the signals
described in FIG. 5, in selected embodiments, an oscillator 100,
such as a crystal oscillator 100, may be used to provide a stable
clock signal 102. In this example, the oscillator produces a 24 MHz
output signal. A frequency multiplier 104 or phase-locked loop
circuit 104 may multiply the oscillator frequency to generate the
PLL output 70, as previously discussed. In this example, the PLL
output 70 is a 144 MHz signal. This output 70 may provide a master
reference signal to drive many of the components in the fingerprint
sensing circuit 18.
[0053] As mentioned, the PLL output 70 may be divided to generate
clocking signals to clock different components in the fingerprint
sensing circuit 18. For example, in selected embodiments, a
frequency divider 106 may divide the PLL output frequency 70 to
generate a probing signal frequency 108, in this example 18 MHz.
This signal 108 may be divided further to generate the pixel clock
signal 72, in this example a 1 MHz signal.
[0054] Similarly, the PLL output 70 may be divided to provide clock
signals 96, 86 for the CPU 60 and ADC 58, respectively. In this
example, a divider 112 may divide the PLL output 70 to generate a
CPU clock signal 96, in this example a 48 MHz signal 96. Similarly,
a divider 114 may divide the PLL output 70 to generate an ADC clock
signal 86, in this example a 12 MHz signal 86.
[0055] In selected embodiments, the CPU clock 112 and ADC clock 114
may receive input from the pixel clock 72. For example, a rising or
falling edge of the pixel clock 72 may cause the CPU clock 112 and
ADC clock 114 to begin outputting the CPU clock signal 96 and ADC
clock signal 86, respectively. In selected embodiments, the CPU
clock 112 and ADC clock 114 are programmable with respect to how
many clock pulses to output and a delay before outputting the clock
pulses. Thus, when the CPU clock 112 and ADC clock 114 receive a a
rising or falling edge from the pixel clock 110, they may output a
specified number of clock pulses starting at a specified time as
determined by the delay. In this way, the CPU clock 96 may be shut
down during selected portions of the pixel period 76, and more
particularly while the ADC 58 is sampling the analog response
signal 78.
[0056] It should be recognized that the components and frequencies
illustrated in FIG. 6 simply represent one embodiment of an
apparatus and method for implementing the invention as discussed
herein. Thus, the illustrated components and frequencies are
presented only by way of example and are not intended to be
limiting. Similarly, the signals illustrated in FIG. 5 are
presented only by way of example and are not intended to be
limiting.
[0057] Similarly, apparatus and methods in accordance with the
invention are applicable to a wide variety of fingerprint sensing
technologies are not limited to the fingerprint sensing technology
disclosed herein. Indeed, the apparatus and methods may be
applicable to a wide variety of capacitive, optical, ultrasonic,
and other fingerprint sensing technologies where noise may be a
concern. Each of these technologies may benefit from reduced noise
by turning off clocks to various components during sampling, or by
shutting down or disabling selected components during sampling.
Thus, apparatus and methods are not limited to any specific type of
fingerprint sensor but may be applicable to a wide variety of
fingerprint sensing technologies.
[0058] 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.
* * * * *