U.S. patent application number 17/170176 was filed with the patent office on 2022-08-11 for fingerprint detection apparatus, system, and method.
This patent application is currently assigned to Japan Display Inc.. The applicant listed for this patent is Japan Display Inc.. Invention is credited to Chung-Kai CHEN.
Application Number | 20220253167 17/170176 |
Document ID | / |
Family ID | 1000006489672 |
Filed Date | 2022-08-11 |
United States Patent
Application |
20220253167 |
Kind Code |
A1 |
CHEN; Chung-Kai |
August 11, 2022 |
FINGERPRINT DETECTION APPARATUS, SYSTEM, AND METHOD
Abstract
A touch detecting system includes a detection panel that
generates a detection signal based on a received transmitter signal
and an object placed in proximity to a detection panel. The
detection signal includes information about the object. A receiver
circuit receives the detection signal and includes a control
circuit that determines, each time a transmitter start signal
becomes active, a delay time to add when generating an adaptive
control signal. The transmitter start signal indicates a start of
operation of the transmitter signal. A mixer circuit receives the
detection signal and the adaptive control signal, and outputs a
demodulated detection signal based on the detection signal and the
adaptive control signal. An output circuit receives the demodulated
detection signal and outputs an output detection signal that
includes the information about the object placed in proximity to
the detection panel.
Inventors: |
CHEN; Chung-Kai; (San Jose,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Japan Display Inc. |
Tokyo |
|
JP |
|
|
Assignee: |
Japan Display Inc.
Tokyo
JP
|
Family ID: |
1000006489672 |
Appl. No.: |
17/170176 |
Filed: |
February 8, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06F 3/04166 20190501;
G06F 3/0412 20130101; G06F 3/04182 20190501 |
International
Class: |
G06F 3/041 20060101
G06F003/041 |
Claims
1. A touch detecting system comprising: a transmitter circuit that
generates a transmitter signal; a detection panel that receives the
transmitter signal and generates a detection signal based on the
transmitter signal and an object placed in proximity to the
detection panel, the detection signal including information about
the object placed in proximity to the detection panel; and a
receiver circuit that receives the detection signal, the receiver
circuit including a control circuit that determines, each time a
transmitter start signal becomes active, a delay time to add when
generating an adaptive control signal, the transmitter start signal
indicating a start of operation of the transmitter signal, a mixer
circuit that receives the detection signal and the adaptive control
signal, and outputs a demodulated detection signal based on the
detection signal and the adaptive control signal, and an output
circuit that receives the demodulated detection signal and outputs
an output detection signal that includes the information about the
object placed in proximity to the detection panel, wherein the
control circuit includes an enable generating circuit that outputs
a comparator enable signal based on a first free running clock
signal and the transmitter start signal, a comparator circuit that
outputs a comparison output signal based on the comparator enable
signal and the detection signal, a delay determining circuit that
outputs a clock enable signal based on the comparison output and
the comparator enable signal, and a clock generator circuit that
outputs the adaptive control signal based on the clock enable
signal.
2. The touch detecting system according to claim 1, wherein each
time the transmitter start signal becomes active, the control
circuit determines a phase delay of the adaptive control signal
based on the detection signal, a first free running clock signal,
and the transmitter start signal.
3. The touch detecting system according to claim 1, further
comprising: at least one amplifier stage that amplifies the
detection signal for use by the mixer circuit.
4. The touch detecting system according to claim 1, further
comprising: a control circuit that determines to activate the
transmitter start signal when a touch detection operation should
begin.
5. The touch detecting system according to claim 1, where the
control circuit is configured to determine a different phase delay
of the adaptive control signal each time the transmitter start
signal becomes active.
6. The touch detecting system according to claim 1, wherein the
information about the object placed in proximity to the detection
panel includes information about a location of the object with
respect to the detection panel.
7. The touch detecting system according to claim 1, wherein the
object placed in proximity to the detection panel is a finger, and
the information about the object placed in proximity to the
detection panel includes information that uniquely identifies a
fingerprint of the finger.
8. The touch detecting system according to claim 1, wherein the
output circuit includes a low pass filter that receives the
demodulated detection signal and outputs a low pass filtered signal
as the output detection signal.
9. (canceled)
10. The touch detecting system according to claim 1, wherein the
enable generating circuit outputs the comparator enable signal to
become active on a predetermined cycle of the first free running
clock signal after the transmitter start signal becomes active.
11. The touch detecting system according to claim 1, wherein the
comparator circuit outputs the comparison output signal to become
active when the detection signal transitions from negative to
positive and inactive when the detection signal transitions from
positive to negative, the delay determining circuit outputs the
clock enable signal to become active after at least one cycle in
which the comparison output signal becomes active and inactive, and
the delay determining circuit outputs the clock enable signal to
become inactive after the transmitter start signal becomes
inactive.
12. The touch detecting system according to claim 1, wherein the
clock generator circuit outputs the adaptive control signal as a
second free running clock only while the clock enable signal is
active.
13. The touch detecting system according to claim 1, wherein the
enable generating circuit includes five D flip-flops connected in
series, with the transmitter start signal connected to a reset
input of each of the five D flip-flops connected in series, the
first free running clock signal connected to a clock input of each
of the five D flip-flops connected in series, an always active
signal connected to a D input of the first D flip-flop in the five
D flip-flops connected in series, a non-inverting output of each of
the first, second, third, and fourth of the five D flip-flops
connected in series being connected to a D input of the second,
third, fourth, and fifth D flip-flops connected in series,
respective, and a non-inverting output of the fifth D flip-flop
connected in series outputting the comparator enable signal.
14. The touch detecting system according to claim 1, wherein the
delay determining circuit includes two D flip-flops connected in
series, an always active signal connected to a D input of a first
of the two D flip flops connected in series, the comparator output
signal connected to a clock input of each of the two D flip-flops
connected in series, the comparator enable signal connected to a
reset input of each of the two D flip-flops connected in series, a
non-inverting output of the first D flip-flop of the two D
flip-flops connected in series is connected to a D input of the
second D flip-flop of the two D flip-flops connected in series, and
a non-inverting output of the second D flip-flop of the two D
flip-flops connected in series is output as the clock enable
signal.
15. The touch detecting system according to claim 1, wherein the
delay determining circuit includes a D flip-flop and a delay line,
an always active signal connected to a D input of the D flip-flop,
the comparator output signal connected to a clock input of the D
flip-flop, the comparator enable signal connected to an input of
the delay line, an output of the delay line connected to a reset
input of the D flip-flop, and a non-inverting output of the D
flip-flop is output as the clock enable signal.
16. The touch detecting system according to claim 15, wherein the
delay line produces a propagation delay of a signal from the input
of the delay line to the output of the delay line that is greater
than a propagation delay of the comparator.
17. A touch detecting receiver that receives a detection signal
indicating information about an object placed in proximity to the
detection panel from a detection panel and a transmitter start
signal from a controller indicating a start of operation of the
transmitter signal, the touch detecting receiver circuit
comprising: a control circuit that determines, each time the
transmitter start signal becomes active, a delay time to add when
generating an adaptive control signal; a mixer circuit that
receives the detection signal and the adaptive control signal, and
outputs a demodulated detection signal based on the detection
signal and the adaptive control signal; and an output circuit that
receives the demodulated detection signal and outputs an output
detection signal that includes the information about the object
placed in proximity to the detection panel, wherein the control
circuit includes an enable generating circuit that outputs a
comparator enable signal based on a first free running clock signal
and the transmitter start signal, a comparator circuit that outputs
a comparison output signal based on the comparator enable signal
and the detection signal, a delay determining circuit that outputs
a clock enable signal based on the comparison output and the
comparator enable signal, and a clock generator circuit that
outputs the adaptive control signal based on the clock enable
signal.
18. A method of detecting an object placed in proximity to a
detection panel, the method comprising: receiving a detection
signal from the detection panel, the detection signal including
information about the object placed in proximity to the detection
panel; receiving a transmitter start signal from a controller
indicating a start of operation of the transmitter signal;
determining, each time the transmitter start signal becomes active,
a delay time to add when generating an adaptive control signal;
generating the adaptive control signal; receiving, by a mixer
circuit, the detection signal and the adaptive control signal;
outputting, from the mixer circuit, a demodulated detection signal
based on the detection signal and the adaptive control signal;
outputting the demodulated detection signal as an output detection
signal that includes information about the object placed in
proximity to the detection panel; outputting a comparator enable
signal based on a first free running clock signal and the
transmitter start signal; outputting a comparison output signal
based on the comparator enable signal and the detection signal;
outputting a clock enable signal based on the comparison output and
the comparator enable signal; and outputting the adaptive control
signal based on the clock enable signal.
Description
FIELD OF THE INVENTION
[0001] The present disclosure relates generally to an apparatus and
system that detects a touch of a human and may also uniquely
identify the human by a fingerprint.
BACKGROUND OF INVENTION
[0002] The "background" description provided herein is for the
purpose of generally presenting the context of the disclosure. Work
of the presently named inventors, to the extent it is described in
this background section, as well as aspects of the description
which may not otherwise qualify as prior art at the time of filing,
are neither expressly nor impliedly admitted as prior art against
the present invention.
[0003] With more and more activities done online, biometric
identification is becoming increasingly important. One of the more
secure ways to implement biometric identification is fingerprint
matching. Capacitive fingerprint sensors are widely used in modern
electronic devices. The readout circuit of a capacitive touch and
fingerprint sensor may include a transmitter, amplifier, mixer,
lowpass filter and ADC (Analog to Digital Converter). First, the
mutual capacitance value is modulated to a TX frequency by a
transmitter and amplified by an amplifier. Next, the mixer
demodulates the signal back to base band and a lowpass filter
filters out noise at high frequency. Finally, the ADC converts the
analog signal in base band into a digital signal for post
processing in the digital domain.
SUMMARY OF INVENTION
[0004] This Summary is provided to introduce a selection of
concepts in a simplified form that are further described below in
the Detailed Description. This Summary is not intended to identify
key features or essential features of the claimed subject matter,
nor is it intended to be used to limit the scope of the claimed
subject matter. Furthermore, the claimed subject matter is not
limited to limitations that solve any or all disadvantages noted in
any part of this disclosure.
[0005] An embodiment of the invention includes touch detecting
system comprising: a transmitter circuit that generates a
transmitter signal; a detection panel that receives the transmitter
signal and generates a detection signal based on the transmitter
signal and an object placed in proximity to the detection panel,
the detection signal including information about the object placed
in proximity to the detection panel; and a receiver circuit that
receives the detection signal, the receiver circuit including a
control circuit that determines, each time a transmitter start
signal becomes active, a delay time to add when generating an
adaptive control signal, the transmitter start signal indicating a
start of operation of the transmitter signal, a mixer circuit that
receives the detection signal and the adaptive control signal, and
outputs a demodulated detection signal based on the detection
signal and the adaptive control signal, and an output circuit that
receives the demodulated detection signal and outputs an output
detection signal that includes the information about the object
placed in proximity to the detection panel.
[0006] According to another embodiment of the invention, each time
the transmitter start signal becomes active, the control circuit
determines a phase delay of the adaptive control signal based on
the detection signal, a first free running clock signal, and the
transmitter start signal.
[0007] According to another embodiment the invention further
comprises at least one amplifier stage that amplifies the detection
signal for use by the mixer circuit.
[0008] According to another embodiment the invention further
comprises a control circuit that determines to activate the
transmitter start signal when a touch detection operation should
begin.
[0009] According to another embodiment of the invention, the
control circuit is configured to determine a different phase delay
of the adaptive control signal each time the transmitter start
signal becomes active.
[0010] According to another embodiment of the invention, the
information about the object placed in proximity to the detection
panel includes information about a location of the object with
respect to the detection panel.
[0011] According to another embodiment of the invention, the object
placed in proximity to the detection panel is a finger, and the
information about the object placed in proximity to the detection
panel includes information that uniquely identifies a fingerprint
of the finger.
[0012] According to another embodiment of the invention, the output
circuit includes a low pass filter that receives the demodulated
detection signal and outputs a low pass filtered signal as the
output detection signal.
[0013] According to another embodiment of the invention, the
control circuit includes: an enable generating circuit that outputs
a comparator enable signal based on a first free running clock
signal and the transmitter start signal; a comparator circuit that
outputs a comparison output signal based on the comparator enable
signal and the detection signal; a delay determining circuit that
outputs a clock enable signal based on the comparison output and
the comparator enable signal; and a clock generator circuit that
outputs the adaptive control signal based on the clock enable
signal.
[0014] According to another embodiment of the invention, the enable
generating circuit outputs the comparator enable signal to become
active on a predetermined cycle (e.g., a fifth cycle, a fourth
cycle, a sixth cycle or any other cycle) of the first free running
clock signal after the transmitter start signal becomes active.
[0015] According to another embodiment of the invention, the
comparator circuit outputs the comparison output signal to become
active when the detection signal transitions from negative to
positive and inactive when the detection signal transitions from
positive to negative, the delay determining circuit outputs the
clock enable signal to become active after at least one cycle in
which the comparison output signal becomes active and inactive, and
the delay determining circuit outputs the clock enable signal to
become inactive after the transmitter start signal becomes
inactive.
[0016] According to another embodiment of the invention, the clock
generator circuit outputs the adaptive control signal as a second
free running clock only while the clock enable signal is
active.
[0017] According to another embodiment of the invention, the enable
generating circuit includes five D flip-flops connected in series,
with the transmitter start signal connected to a reset input of
each of the five D flip-flops connected in series, the first free
running clock signal connected to a clock input of each of the five
D flip-flops connected in series, an always active signal connected
to a D input of the first D flip-flop in the five D flip-flops
connected in series, a non-inverting output of each of the first,
second, third, and fourth of the five D flip-flops connected in
series being connected to a D input of the second, third, fourth,
and fifth D flip-flops connected in series, respective, and a
non-inverting output of the fifth D flip-flop connected in series
outputting the comparator enable signal.
[0018] According to another embodiment of the invention, the delay
determining circuit includes two D flip-flops connected in series,
an always active signal connected to a D input of a first of the
two D flip flops connected in series, the comparator output signal
connected to a clock input of each of the two D flip-flops
connected in series, the comparator enable signal connected to a
reset input of each of the two D flip-flops connected in series, a
non-inverting output of the first D flip-flop of the two D
flip-flops connected in series is connected to a D input of the
second D flip-flop of the two D flip-flops connected in series, and
a non-inverting output of the second D flip-flop of the two D
flip-flops connected in series is output as the clock enable
signal.
[0019] According to another embodiment of the invention, the delay
determining circuit includes a D flip-flop and a delay line, an
always active signal connected to a D input of the D flip-flop, the
comparator output signal connected to a clock input of the D
flip-flop, the comparator enable signal connected to an input of
the delay line, an output of the delay line connected to a reset
input of the D flip-flop, and a non-inverting output of the D
flip-flop is output as the clock enable signal.
[0020] According to another embodiment of the invention, the delay
line produces a propagation delay of a signal from the input of the
delay line to the output of the delay line that is greater than a
propagation delay of the comparator.
[0021] Another embodiment of the invention includes a touch
detecting receiver that receives a detection signal indicating
information about an object placed in proximity to the detection
panel from a detection panel and a transmitter start signal from a
controller indicating a start of operation of the transmitter
signal, the touch detecting receiver circuit comprising: a control
circuit that determines, each time the transmitter start signal
becomes active, a delay time to add when generating an adaptive
control signal; a mixer circuit that receives the detection signal
and the adaptive control signal, and outputs a demodulated
detection signal based on the detection signal and the adaptive
control signal; and an output circuit that receives the demodulated
detection signal and outputs an output detection signal that
includes the information about the object placed in proximity to
the detection panel.
[0022] Another embodiment of the invention includes a method of
detecting an object placed in proximity to a detection panel, the
method comprising: receiving a detection signal from the detection
panel, the detection signal including information about the object
placed in proximity to the detection panel; receiving a transmitter
start signal from a controller indicating a start of operation of
the transmitter signal; determining, each time the transmitter
start signal becomes active, a delay time to add when generating an
adaptive control signal; generating the adaptive control signal;
receiving, by a mixer circuit, the detection signal and the
adaptive control signal; outputting, from the mixer circuit, a
demodulated detection signal based on the detection signal and the
adaptive control signal; and outputting the demodulated detection
signal as an output detection signal that includes information
about the object placed in proximity to the detection panel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] The scope of the present disclosure is best understood from
the following detailed description of exemplary embodiments when
read in conjunction with the accompanying drawings, wherein:
[0024] FIG. 1 is a block diagram of a first embodiment of a
fingerprint detection apparatus;
[0025] FIG. 2 is a circuit diagram of an example of an amplifier
stage;
[0026] FIG. 3 is a graph of an example of a detection signal in the
frequency domain;
[0027] FIG. 4 is a graph of an example of an amplified differential
detection signal in the frequency domain;
[0028] FIG. 5 is a graph of an example of a filtered detection
signal in the frequency domain;
[0029] FIG. 6A is a block diagram of a second embodiment of a
fingerprint detection apparatus;
[0030] FIG. 6B is a block diagram of a third embodiment of a
fingerprint detection apparatus;
[0031] FIG. 7 describes a functionality of an operation of the
second and third embodiments of the fingerprint detection
apparatus;
[0032] FIG. 8 is an example of an implementation of an enable
generating circuit;
[0033] FIG. 9 shows an example of a signal operation during a
beginning of operation of the enable generating circuit;
[0034] FIG. 10 shows an example of an implementation of the
comparator;
[0035] FIG. 11 shows an example of an implementation of a first
portion of a mixer;
[0036] FIG. 12 shows an example of an implementation of a second
portion of the mixer;
[0037] FIG. 13 shows an example of an implementations of a clock
generator circuit;
[0038] FIG. 14 shows an example of an operation of the clock
generator circuit;
[0039] FIG. 15 shows an example of an integrated circuit floor-plan
for the fingerprint detection apparatus;
[0040] FIG. 16 shows a first example of operation of the
fingerprint detector according to the second embodiment;
[0041] FIG. 17 shows a second example of operation of the
fingerprint detector according to the second embodiment;
[0042] FIG. 18 shows a third example of operation of the
fingerprint detector according to the second embodiment; and
[0043] FIG. 19 shows a fourth example of operation of the
fingerprint detector according to the second embodiment.
[0044] Further areas of applicability of the present disclosure
will become apparent from the detailed description provided
hereinafter. It should be understood that the detailed description
of exemplary embodiments is intended for illustration purposes only
and are, therefore, not intended to necessarily limit the scope of
the disclosure.
DETAILED DESCRIPTION
[0045] One difficulty when implementing a touch or fingerprint
detection process (hereinafter interchangeably referred to as
either fingerprint or touch detection) is the control of mixer
operation, because the mixer's control signal needs to be based on
propagation delay from transmitter to mixer input. However, the
value of the propagation delay is unknown before manufacture and
may depend on various and varying factors. A conventional solution
includes performing a one-time calibration after manufacture and
choosing the correct number of delay cells in series delay circuit
to match the phase difference between mixer's control signal and TX
(transmitter) signal to propagation delay. In this way, the mixer
control signal is essentially a delayed TX signal. But this
solution disadvantageously requires an additional testing step
during manufacture or setup, and the propagation delay can also
vary with age or different operating conditions.
[0046] An embodiment of the invention may advantageously generate
an adaptive mixer control signal without calibration and can
appropriately vary and/or determine a delay timing of the adaptive
control signal each time when fingerprint or touch sensing function
is required to thereby advantageously account for changes in age or
operating conditions.
[0047] FIG. 1 is a block diagram of a first embodiment of a
fingerprint detection apparatus including a detection panel 102, an
amplifier stage 112, a mixer 118, a low pass filter 124, and a
configurable delay circuit 128. The apparatus detects a fingerprint
of a human and may be used for uniquely identifying the human and
can be used to detect human touch in user interface
applications.
[0048] The detection panel 102 is a physical region of that may be
arranged in proximity to, or in direct contact with, a human finger
and includes a capacitive sensor having an array of capacitive
elements, which are represented in this example by equivalent
capacitors 104, 106, and 108, which may be used by the apparatus to
detect the proximity of the human finger and also detect the
proximity and relative locations of ridges and valleys of a
fingerprint on the human finger (hereinafter referred to as
fingerprint features). The detection panel 102 may form a portion
of a visual display element, for example, a transparent overlay on
any type of display, such as a liquid crystal display (LCD) or an
organic light emitting-diode display (OLED). In this example, the
detection panel 102 receives a time varying fingerprint transmitter
signal TX 100, shown by exemplary signal 130, which propagates
through the array of capacitive elements and is output as detection
signal 107. The propagating signal is affected by the relative
proximity and location of the fingerprint features.
[0049] The transmitter signal TX 100 is generated by a transmitter
circuit 138 to be a time varying balanced signal which is active
only when fingerprint detection is desired by the device that
includes the fingerprint detection apparatus. A frequency of the
signal TX 100 may be varied by the transmitter circuit 138, before
or during normal operation, to optimize a noise reduction strategy
as discussed further below. For example, the transmitter circuit
138 may change the frequency of signal TX 100, within a range of
possible frequencies that may be produced by the transmitter
circuit, to avoid a frequency of a noise source, such as a charger
or power supply noise. The amplifier stage 112 includes one or more
charge amplifier and programmable gain amplifier stages. The
amplifier stage 112 receives the detection signal 107 from the
detection panel 102 and a reference common mode voltage Vcm 110 to
generate an amplified differential detection signal 114, shown by
an exemplary waveform 116. The amplifier stage 112 increases the
amplitude of the detection signal 107, for example, to improve a
signal to noise ratio.
[0050] The configurable delay circuit 128 receives the transmitter
signal TX 100 and a register setting signal 132. The register
setting signal 132 indicates how many delay cells inside the
configurable delay circuit 128 are used and thus how much
propagation delay mixer control signal 134 has relative to TX
signal 100. Register setting signal 132 is implemented as a series
of 1s and 0s, or as an encoded number, or as a series of encoded
fields, including a predetermined indication of the number of delay
cells to be used that is selected at the time of manufacture.
[0051] The actual propagation delay resulting from the propagation
of the TX 100 signal through the detection panel 102 may be a
function of a variety of external factors, such as manufacturing
variations in the circuit and panel components (e.g., conductivity
of conductors, thickness of panel layers), environmental conditions
(e.g., temperature, humidity, and pressure), power supply voltages,
properties of the human finger (e.g., dirt, oil, moisture,
clothing, etc.), and circuit age or history (e.g., number of cycles
of use, elapsed time since manufacture). Thus, the information
corresponding to the propagation delay is determined by a
calibration process that may use dedicated or external test
equipment operating outside the normal operating mode. Since the
external factors noted above may change over time, the
configuration process may be required to be performed at various
times during the use and lifetime of the apparatus.
[0052] The configurable delay circuit 128 outputs a mixer control
signal 134, shown by exemplary signal 136, which is a delayed
version of the received transmitter signal TX 100. The configurable
delay circuit 128 delays the TX 100 signal by a duration of time
based on information provided in the register setting signal 132,
to produce the mixer control signal 134. For example, the
configurable delay circuit 128 includes a plurality of serially
connected delay elements that are enabled or disabled according to
the register setting signal 132 to vary a propagation delay applied
to the TX 100 signal before outputting the mixer control signal
134. The mixer control signal 134 has the same frequency as TX 100
and the phase of the mixer control signal 134 relative to TX 100
corresponds to the propagation delay through the detection panel
102 for proper detection of the fingerprint. For example, the
signal across capacitor 106, Cm, may be attenuated or even
completely lost when a phase error in the mixer control signal 134
approaches .pi./2.
[0053] The mixer 118 receives the amplified differential detection
signal 114 and the mixer control signal 134 from the configurable
delay circuit 128. The mixer 118 uses the mixer control signal 134
to provide control timing for demodulating the amplified
differential detection signal 114. Based on the received amplified
differential detection signal 114 and the mixer control signal 134,
the mixer 118 demodulates the amplified differential detection
signal 114 to output a demodulated detection signal 120, shown by
an exemplary waveform 122.
[0054] The low pass filter 124 is a filter that performs noise
reduction to remove undesirable signal components from the
demodulated detection signal 120 to produce a filtered detection
signal 126. The low pass filter 124 may be implemented as an analog
filter circuit or a digital filter circuit and is preferably
implemented as an analog filter circuit. The noise reduction is
advantageously performed as early as possible in the signal flow,
and advantageously, before the detection signal is converted into a
digital domain. In an alternative embodiment, where further noise
reduction is not needed, the low pass filter 124 may be
omitted.
[0055] Disadvantageous noise sources include noise resulting from
the signal path circuits themselves (e.g., thermal noise, 1/f
noise, etc.), signals outside the signal path (e.g., various clock
and high frequency signal used for other functions within a system
that includes the fingerprint detection apparatus), and the
external environment (e.g., visual display panel refresh/strobe
signals, processor signals, charger noise, etc.), at least because
these outside signal can be disadvantageously coupled into the
signal path anywhere between the charge amplifier and the
mixer.
[0056] The filtered detection signal 126 may be analyzed by another
system component (including, for example, an A/D converter and/or
logic circuit) to detect and/or identify a human fingerprint in
proximity to the detection panel 102 based, at least in part, on
the filtered detection signal 126.
[0057] FIG. 2 shows an example of the amplifier stage 112 including
a charge amplifier 208, capacitor 206, band-pass filter 212,
resistors 214, 216, 218, and 220, and programmable gain amplifier
(PGA) 222. Although the example of FIG. 2 shows only one stage each
of charge amplifier 208, band-pass filter 212, and PGA 222, the
embodiment may include more than one stage of one or more serially
connected stages of the charge amplifier 208, band-pass filter 212,
and PGA 222. In this example, the charge amplifier 208 receives Vcm
204 and detection signal 202 from the detection panel 102. The PGA
222 outputs the amplified differential detection signal 224
provided to the mixer 118. On the other hand, if signal to noise
ratio is adequate, band-pass filter 212 and PGA 222 can be omitted.
The first stage charge amplifier 208 may be necessary to convert
charge signal 107 into voltage signal 114. Alternatively, the
charge amplifier used in this example can be replaced by a
transimpedance amplifier or current conveyor circuits.
[0058] In the example of FIG. 2, the charge amplifier 208 converts
a single ended input detection signal 202 from the detection panel
102 into a differential signal in a first stage of the charge
amplifier 208. However, the amplification signal path of the
amplifier stage 112 may alternatively be implemented entirely as a
single ended amplifier by biasing all stages' positive inputs with
Vcm.
[0059] FIGS. 3-5 illustrate a fingerprint noise reduction strategy
of a fingerprint detection apparatus according to the present
disclosure.
[0060] FIG. 3 illustrates an exemplary graph of Power Spectral
Density (PSD) 302 versus frequency 310 for a content of a detection
signal 107 in a Touch Screen Panel (TSP) according to an embodiment
of the invention. The detection signal 107 includes the desirable
mutual capacitive signal Cm 306 that is modulated at a frequency of
signal TX 100, which may preferably be controlled and/or selected
to avoid a frequency of charger noise and low frequency 1/f noise.
The detection signal 107 in this example, also includes
disadvantageous noise interference 304 and 308.
[0061] FIG. 4 illustrates an exemplary graph of amplified
differential detection signal 114 output from the amplification
stage 112, which includes the mutual capacitance signal at the
frequency of TX 100 (406), and may attenuate noise above and below
the band pass frequency range 408.
[0062] FIG. 5 illustrates an exemplary graph of filtered detection
signal 126 output by the low-pass filter 124. The modulated mutual
capacitance signal 512 is down converted (i.e., shifted in
frequency domain) to be in the base band frequency range 504 by the
demodulation 508 performed by the mixer 118. The mixer also
upconverts all the low frequency noise in the signal. The low-pass
filter 124 passes the mutual capacitance signal at the base band
and attenuates/filters out the higher frequency signals outside of
base band frequency range 506.
[0063] In the first embodiment of a fingerprint detection apparatus
according to FIG. 1, the configurable delay circuit 128 produces
the mixer control signal 134 based on the register setting signal
132 which is determined by a calibration process. Such a process
may be time consuming, may cause the apparatus to be unavailable
periodically, and may result in an apparatus that is inaccurate at
times between calibrations. Further, such an embodiment may not
advantageously redetermine an appropriate delay to be applied to
the mixer control signal 134 each time touch detection is
required.
[0064] FIG. 6A illustrates a second embodiment of the fingerprint
detection apparatus including improvements that may advantageously
reduce or avoid the need for a calibration process. According to
the embodiment of FIG. 6A, the fingerprint detection apparatus
includes an improved mixer circuit 600, including mixer 624, and
low-pass filter 626.
[0065] Mixer 624 receives amplified differential detection signal
610, and the low-pass filter 626 outputs filtered detection signal
632. Mixer 624 and low-pass filter 626 may be implemented as
discussed above with respect to mixer 118 and low-pass filter 124,
respectively. Also, amplified differential detection signal 610,
demodulated detection signal 630, and filtered detection signal 632
correspond to the amplified differential detection signal 114,
demodulated detection signal 120, and filtered detection signal
126, respectively.
[0066] The improved mixer circuit 600 includes an enable generating
circuit 606, a comparator 608, D flip-flops 616 and 618,
collectively forming a phase detector 612. The improved mixer
circuit 600 also includes a clock generator circuit 620 and the
original mixer 624 itself.
[0067] The enable generating circuit 606 receives a signal TX_Start
602 and free-running clock CLK 604. TX_Start 602 is provided from a
controller and becomes active at a time when fingerprint detection
is required and determined by a higher level function performed by
the controller. For example, TX_Start 602 may become active under
the control of the controller when a user attempts to unlock a
smartphone. CLK 604 is a free-running clock signal. The enable
generating circuit 606 outputs a comparator enable signal EN_COMP
636, which is normally low (inactive), and remains low for at least
1 TX period after TX starts, to allow the mixer input signals to
stabilize.
[0068] The comparator 608 receives the differential detection
signal 610 and EN_COMP 636. The comparator 608 outputs a comparison
output signal 638 that is high when the differential detection
signal 114 is positive and low when differential detection signal
114 is negative.
[0069] A D input of the D flip-flop 616 receives VDD 614 (i.e.,
always active signal). A clock input of the D flip-flop 616
receives comparator 608, and a RST input of the D flip-flow 616
receives EN_COMP 636. A D input of the D flip-flop 618 receives a
non-inverted Q output from D flip-flop 616. The CLK and RST inputs
of the D flip-flop 618 are tied to the same signals as the
corresponding CLK and RST inputs of the D flip-flop 616. The
comparator 608 is used to sense the positive input crossing, not
input states, and therefore, two D flip flops are provided to
ignore the first comparator output going high. Further, two D flip
flops are used to ignore a comparator going high immediately after
EN_COMP goes high, in the event that positive comparator input inp
1010>negative comparator input inn 1012 at that moment (See FIG.
10).
[0070] An enable input EN of the CLK Generator 620 receives the
non-inverted Q output from D flip-flop 618. An output of the CLK
Generator 620 is provided as an adaptive mixer control signal 640
to the mixer 624.
[0071] FIG. 6B illustrates an alternative implementation of the
fingerprint detection apparatus in FIG. 6A. According to the
embodiment in FIG. 6B, a delay cell 617 is inserted between the
output of the enable generating circuit 606 and the RST input of
the D flip-flop 616. The delay cell 617 adds a delay of duration D.
This delay duration D is preferably longer than the comparator
608's propagation delay, which when determined by Slew Rate (SR)
is
V O .times. H - V O .times. L 2 .times. S .times. R
##EQU00001##
and when determined by linear response is
.tau. c .times. ln ( 1 1 - V O .times. H - V O .times. L 2 .times.
A V .function. ( 0 ) .times. V i .times. n ) . ##EQU00002##
The delay D is preferably as short as possible, and longer than the
comparator's propagation delay. The delay D is optimally less than
1 TX period. According to this embodiment, even if comparator 608's
output 638 becomes HIGH immediately after its enable signal 636
becomes HIGH, this comparator output 638's rising edge will be
ignored because D flip-flop 616's RST is still LOW due to the delay
cell 617. The embodiment of FIG. 6B may result in a phase
detector's response time that is advantageously shorter than the
embodiment of FIG. 6A, because instead of two, only one D flip-flop
is used.
[0072] The fingerprint detector apparatus according to FIG. 6A or
FIG. 6B automatically generates an automatic mixer control signal
640 (aka, adaptive control signal) for controlling the operation of
a mixer without requiring any calibration process. The adaptive
mixer control signal 640 is generated each time after TX starts and
phase detector 612 senses the current propagation delay from 100 to
114, so compared with the traditional one time calibration process
done in the testing facility after manufacture, it can take
variations such as temperature change and circuit aging into
account.
[0073] An operation of an alternative embodiment of an improved
mixer circuit is described functionally with respect to the example
of FIG. 7. Such an embodiment may be implemented using a
programmable processing circuit, logic circuits, or other circuits,
for example a circuit including components as in the example of
FIG. 6. According to this alternative embodiment, the signal TX 100
is first generated by the transmitter circuit 138 and received by
the improved mixer circuit when the device that includes the
fingerprint detection apparatus (e.g., a cell phone, a touch panel,
etc.) determines to start fingerprint detection. A higher level
function in a controller determines that fingerprint or touch
detection is required, and transmitter signal TX 100 starts in step
S702. Operation waits at step S704 a predetermined amount of time
until the mixer inputs become stabilized. The predetermined amount
of time is set to at least one TX period. For example, if the
slowest TX frequency is 100 kHz, the predetermined amount of time
is set to at least 10 us. The predetermined amount of time is
determined in the design phase, or, if configurable, may be set in
a register setting. In step S706, the phase detector in the
improved mixer circuit starts sensing mixer input zero crossings,
and in step S708 after a mixer input zero crossing is sensed, a
clock at TX frequency starts. In step S710, the TX frequency clock
in S708 is provided to the mixer as its control signal, and mixer
starts to rectify its input signal. In step S706, the comparator is
enabled and starts sensing, and in step S708, the clock generator
starts generating a clock at TX frequency after comparator senses
mixer input zero crossing. Thus, the automatically generated mixer
control signal in S710 is able to rectify mixer input signal, i.e.,
make mixer output all positive or negative.
[0074] In embodiments according to FIGS. 6A, 6B, and 7, a delay
time, which is measured from when the TX_Start goes active until
the mixer begins to demodulate its input signal, is determined each
time when a higher level function determines that a touch or
fingerprint detection action should be initiated (i.e., each time
the TX_Start signal goes active). The value of this delay time
depends on the relative position of EN_COMP's rising edge to mixer
input signal 114, so it is not the same every time. Furthermore,
this delay time may be different each time touch or fingerprint
detection action should be initiated, because EN_COMP doesn't go
active at the same time each time TX_Start goes active. For
example, in FIGS. 8 and 9, the delay time from TX_Start active to
EN_COMP rising edge is any time between 16 to 20 us. In FIG. 16,
EN_COMP goes active when mixer input is high and in FIG. 17,
EN_COMP goes active when mixer input is negative. Therefore, the
delay times from TX_Start to mixer starts to demodulate in FIGS. 16
and 17 are different.
[0075] FIG. 8 shows an example of an implementation of an enable
generating circuit 800 corresponding to the enable generating
circuit 606. The enable generating circuit 800 includes D
flip-flops 808, 810, 812, 814, and 816, each having a RST input
connected to TX_Start 806, and each having a CLK input connected to
a CLK 804 (e.g., a 250 kHz free-running clock). The D input of D
flip-flop 808 is tied to VDD 802 (always active/high). The
non-inverting Q output of each of D flip-flops 808, 810, 812, and
814 is connected to the D input of D flip-flop 810, 812, 814, and
816, respectively. The non-inverting Q output of D flip-flop 816 is
output as EN_COMP 818. There are 5 D flip-flops in FIG. 8, but
different number of D flip-flops can also be used. The number of D
flip-flops can either be determined in the design phase or made
configurable using switches and register settings, or, for example,
made configurable like the configurable delay circuit 128.
Alternatively, EN_COMP can also be generated with a series of delay
cells. FIG. 9 shows an example of a signal operation during a
beginning of operation of the enable generating circuit 606. In
this example, TX_Start 902 becomes active (high) when an apparatus
that includes the fingerprint detection device indicates that
fingerprint detection should begin. Signal 904 is continuously
operating in this example at 250 kHz. The 250 kHz frequency does
not need to be the same as the TX frequency. At the fifth rising
edge of 904 after TX_Start 902 becomes active, EN_COMP becomes high
at time 914 (between 16 us and 20 us according to this example).
Alternatively, EN_COMP can be made to go high on any other
predetermined rising edge of 904 after TX_Start 902 becomes
active.
[0076] FIG. 10 shows a possible implementation of the comparator
608, including transistors 1014 and 1016, and bias current source
1002 connected to VDD 1004 and VSS 1006. Negative (i.e., inverting)
input 1012 and positive (i.e., non-inverting) input 1010 receive a
differential input signal, and the comparator output is provided at
node 1008. The comparator has hysteresis using internal positive
feedback to function properly in a noisy environment. To reduce
power consumption, the bias current source may be disabled after
EN_CLK is high, for example, based on counters or logic gates.
[0077] FIG. 11 shows a possible implementation of a first portion
of the mixer 624 and includes inverters 1104 and NAND gates 1106
arranged to generate the non-overlapping outputs Ph1 and Ph2, and
their inverse Ph1b and Ph2b respectively from input signal
1102.
[0078] FIG. 12 shows a possible implementation of a second portion
of the mixer 624 and includes transistors 1228 connected to
positive input 1222 and negative input 1220 that receive the
amplified differential detection signal 610 and generate inverting
output 1224 and non-inverting output 1226 to output the demodulated
detection signal 630. Switching of the transistors 1228 is
controlled by signals Ph1, Ph2, Ph1b, and Ph2b as shown in FIG.
11.
[0079] FIG. 13 shows an implementation of a clock generator circuit
620 that includes a free-running oscillator 1302 (e.g., 32 MHz
oscillator), and a logic circuit 1306 including D flip-flops 1308,
1310, 1312, and 1314. The non-inverting Q output of the last of the
serially connected D flip-flops outputs automatic mixer control
signal 1316 to the mixer 624. The free-running clock 1302 output
signal runs continuously and therefore is present before EN_CLK
goes high. The logic circuit 1306 implements a frequency divider
using the free running clock 1302 as an input clock, having a
frequency selected to be readily available within the apparatus
that includes the fingerprint detector, fast enough to minimize the
delay between EN_CLK going high and the start of the automatic
mixer control signal provided to the mixer, and without being too
fast to result in excessive power consumption, cost increases and
increased risk of stray signal noise. The free-running clock 1302
in this example is preferably 32 MHz, resulting in max delay
between EN_CLK going high and the start of the adaptive control
signal provided to the mixer of 31.25 ns.
[0080] FIG. 14 shows an example of an operation of the clock
generator circuit 620. According to this example, a free running
clock 1402 operates at, for example, 32 MHz. After input signal
EN_CLK 1404 goes high, on the next rising edge of the free running
clock 1402, divided down clocks 1406 (i.e., the non-inverting Q
output of D flip flop 1308) and 1408 (i.e., the non-inverting Q
output of D flip flop 1314) start running. The output automatic
mixer control signal 1408 remains high (active) for four clock
cycles of the half frequency. According to this example, the delay
1410 from EN_CLK 1404 going high to the output automatic mixer
control signal 1408 going high is less than or equal to 31.25
ns.
[0081] The fingerprint detection apparatus may advantageously be
implemented on different portions of the same integrated circuit
(e.g., a sensor Analog Front End (AFE) integrated circuit or a
system on chip (SOC) integrated circuit) for a more efficient
layout and to reduce the risk of high frequency signal portions
corrupting other portions with coupled signal noise.
[0082] FIG. 15 shows an integrated circuit floor-plan that locates
different portions of the fingerprint detection apparatus in
different areas of the same integrated circuit 1514. According to
this example, a detection panel 1502 includes an array of
capacitive and resistive elements arranged in rows and columns.
Each column is driven by a time varying fingerprint transmitter
signal TX1-TX40, and a received signal RX1-RX40 (e.g., each
corresponding to detection signal 107) is output. The integrated
circuit 1514 includes four phase detectors 1504, the output of each
phase detector 1504 is shared by 10 RX channels. Each phase
detector 1504 takes inputs from one channel only. Therefore, no
dedicated multiplexing device is needed. This is because nearby
channels are expected to have very similar propagation delay, so
nearby channels can share the phase detector's output EN_CLK, too.
For example, the top 1504 in FIG. 15 can take inputs from RX5 and
RX1-RX10 all use the same adaptive mixer control signal 1516. The
integrated circuit 1514 also includes four frequency dividers 1508,
each connected to send an adaptive mixer control signal 1516 and
receive EN_CLK 1518 to/from a corresponding one of the phase
detectors 1504. Alternatively, each RX can also have its own phase
detector 1504 and frequency divider 1508, if circuit complexity is
not a concern. Additionally, the integrated circuit 1514 includes a
single free-running clock oscillator 1510 (e.g., corresponding to
oscillator 1302) shared by all the frequency dividers 1508.
Although the number of TX and RX channels in this example is 40,
the invention also applies to other numbers of TX and RX channels,
for example greater than 40 or less than 40.
[0083] Each phase detector 1504 includes circuitry corresponding to
the functionality of the phase detector 612 in FIG. 6. Each
frequency divider 1508 and a single free-running clock oscillator
1510 include circuitry corresponding to the functionality of the
clock generator circuit 620 in FIG. 6.
[0084] According to this floor-planning example, only EN_CLK and
the adaptive mixer control signal provided to the mixer are routed
across longer distances in the integrated circuit, and the high
frequency free-running clock (e.g., 32 MHz clock 1402 in FIG. 14)
is not routed over long distances on the integrated circuit.
Further, according to this example, each channel (i.e., each RX
row) has a dedicated corresponding mixer, but multiple channels can
share one phase detector and one frequency divider. In particular,
the output of only one receive channel in a plurality of receive
channels (e.g., only receive channel RX5 out of receive channels
RX1-RX10) is provided to the phase detector, while the other
outputs of receive channels RX1-4 and RX6-10 remain unconnected.
Furthermore, a same frequency divider output signal may be provided
in common to a plurality of RX channels (e.g., the output of one
frequency divider is provided in common to the inputs of all of
receive channels RX1-RX10, while a different frequency divider
output is provided in common to all of receive channels RX11-20).
This floor plan places frequency divider 1508 next to free running
clock oscillator 1510, so the area of region 1512 that has high
frequency signals can be minimized. This floor-planning strategy is
especially useful when chip area is big and may advantageously
allow signals to be routed so that a fast running clock is not
distributed throughout all portions of the integrated circuit.
[0085] Also, a single free-running clock oscillator 1510 is shared
by all four frequency dividers 1508. The floor plan according to
this example can minimize interference to analog circuits (e.g.,
amplifier stage 112) by signals propagating in the high frequency
components in the area of region 1512.
[0086] FIGS. 16-19 show examples of operation of the fingerprint
detector according to the second embodiment in which a TX 1602 is
provided to a detection panel 102. In these examples, signal 1604
corresponds to the CA Diff Out signal 210 in FIG. 2. In FIG. 16,
after propagating through the detection panel 102 and an amplifier
stage 112, an amplified differential detection signal 1606
(corresponding to amplified differential detection signal 114) is
generated. EN_COMP 1612 goes high at time 1617. Adaptive mixer
control signal 1616 (corresponding to adaptive mixer control signal
640) starts cycling at time 1614, and runs at the same frequency as
TX 1602, but with phase delayed to correspond to the zero crossing
times of the amplified differential detection signal 1606. First
zero crossing 1608 and second zero crossing 1610 indicate the first
and second zero crossings after EN_COMP 1612 goes high. In this
example, first zero crossing 1608 is actually a false zero
crossing, because amplified differential detection signal 1606 is
not zero at time 1617. However, because there are two D flip flops
616 and 618 in series, instead of one, the false zero crossing at
1617 is ignored by clock generator circuit 620 and the adaptive
mixer control signal 1616 doesn't begin until time 1614. A
demodulated detection signal 1620 (corresponding to demodulated
detection signals 630 and 120) is output by the mixer and provided
to the low-pass filter 626. According to this example, the input to
the mixer, amplified differential detection signal 1606, cycles at
the frequency of TX 1602, but with a phase shift caused by
propagation delays through the detection panel. EN_COMP 1612 goes
active when the positive comparator inp 1010 is greater than
negative comparator inn 1012 but the counter makes sure that the
adaptive control signal provided to the mixer starts at the time of
the comparator positive input zero crossing. Low Pass Filter output
waveform 1622 shows the output of the LPF 626 in this example.
[0087] In the example of FIG. 17, amplified detection signal 1706,
EN_COMP 1712, adaptive mixer control signal 1716, and demodulated
detection signal 1720 are shown in a condition where positive
comparator inp 1010 is less than negative comparator input inn 1012
when EN_COMP 1712 goes HIGH. Low Pass Filter output waveform 1722
shows the output of the LPF 626 in this example.
[0088] In the example of FIG. 18, amplified detection signal 1806,
EN_COMP 1812, automatic mixer control signal 1816, and demodulated
detection signal 1820 are shown in a condition where the mixer
input signal 610 (equivalent to 114 or 224) has an incorrect
frequency, e.g., in a case where a strong noise is coupled into the
signal path (e.g., signals 100, 107, 114 in FIG. 1, and signals
202, 210, 224 in FIG. 2) and overwhelms the desired signal. The
signal cannot be rectified in this case, and the LPF 626
advantageously filters out the noise, as shown by Low Pass Filter
output waveform 1822, which shows the output of the LPF 626 in this
example.
[0089] In the example of FIG. 19, amplified detection signal 1906,
EN_COMP 1912, automatic mixer control signal 1916, and demodulated
detection signal 1920 are shown in a condition where the mixer
input signal 610 (e.g., equivalent to signal 114 or 224) is not
periodical, so it can't be rectified, and will instead be
advantageously filtered out by the LPF 626, as shown by the Low
Pass Filter output waveform 1922.
* * * * *