U.S. patent application number 14/565753 was filed with the patent office on 2015-04-02 for orthogonal multi-row touch panel stimulation.
The applicant listed for this patent is Broadcom Corporation. Invention is credited to Sumant Ranganathan, David A. Sobel, John S. Walley.
Application Number | 20150091869 14/565753 |
Document ID | / |
Family ID | 48743583 |
Filed Date | 2015-04-02 |
United States Patent
Application |
20150091869 |
Kind Code |
A1 |
Sobel; David A. ; et
al. |
April 2, 2015 |
Orthogonal Multi-Row Touch Panel Stimulation
Abstract
Control circuitry for a touch panel includes a touch panel
interface, a memory comprising scanning logic, and a controller in
communication with the memory and the touch panel interface. The
controller is operable, when the scanning logic is executed, to
energize a first and a second row in the touch panel
simultaneously, a first time; obtain a first signal measurement
along a column intersecting the first and second rows; energize the
first and the second row in the touch panel simultaneously, a
second time; obtain a second signal measurement along the column;
and determine a first pixel value and a second pixel value along
the column from the first signal measurement and the second signal
measurement.
Inventors: |
Sobel; David A.; (Los Altos,
CA) ; Ranganathan; Sumant; (Saratoga, CA) ;
Walley; John S.; (Ladera Ranch, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Broadcom Corporation |
Irvine |
CA |
US |
|
|
Family ID: |
48743583 |
Appl. No.: |
14/565753 |
Filed: |
December 10, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
13414985 |
Mar 8, 2012 |
8937606 |
|
|
14565753 |
|
|
|
|
61584491 |
Jan 9, 2012 |
|
|
|
Current U.S.
Class: |
345/174 |
Current CPC
Class: |
G06F 3/0446 20190501;
G06F 3/041 20130101; G06F 3/04166 20190501 |
Class at
Publication: |
345/174 |
International
Class: |
G06F 3/044 20060101
G06F003/044 |
Claims
1. A method comprising: performing a first measurement at a
receiver when a transmitter energizes a region of a touch panel
using a first waveform; performing a second measurement at the
receiver when the transmitter energizes the same region of the
touch panel using a second waveform, the second waveform orthogonal
in content to the first waveform; and combining the first and
second measurements to determine a pixel result on the touch
panel.
2. The method of claim 1, where: the first waveform comprises a
first Hadamard sequence; and the second waveform comprises a second
Hadamard sequence different from the first.
3. The method of claim 1, where the region comprises a selected
number of rows of the touch panel.
4. The method of claim 1, where the receiver is coupled to a column
of the touch panel.
5. The method of claim 1, where: the region is defined in a scan
map; and the method further comprises accessing the scan map to
determine the region.
6. The method of claim 1, where the first measurement and the
second measurement are performed at different times.
7. The method of claim 1, where further comprising characterizing a
touch blob on the touch panel responsive to the determined pixel
result.
8. A device comprising: a touch panel; and touch panel circuitry
comprising: a transmitter configured to: transmit a first waveform
over a portion of the touch panel; and transmit a second waveform
over the same portion of the touch panel, the second waveform
orthogonal to the first waveform; and a receiver configured to:
perform a first measurement when the transmitter transmits the
first waveform over the portion of the touch panel; and perform a
second measurement when the transmitter transmits the second
waveform over the portion of the touch panel; and measurement
circuitry configured to: combine the first and second measurements
to determine a combined measurement; and responsive to the combined
measurement, determine a pixel result on the touch panel.
9. The device of claim 8, further comprising memory configured to
store a scan map, the portion of the touch panel and first waveform
defined in the scan map; and where the measurement circuitry is
further configured to access the scan map to determine the portion
of the touch panel and first waveform.
10. The device of claim 8, where the transmitter is coupled to a
row of the touch panel.
11. The device of claim 8, where the receiver is coupled to a
column of the touch panel.
12. The device of claim 8, where the transmitter is configured to
transmit the first and second waveforms at different times.
13. The device of claim 8, where the measurement circuitry is
further configured to cause the transmitter to transmit the first
and second waveforms in response to a command to increase a
signal-to-noise ratio for the touch panel.
14. The device of claim 8, where the first and second waveforms
comprise Hadamard sequences.
15. The device of claim 8, wherein the measurement circuitry is
further configured to characterize a touch blob using the pixel
result.
16. A device comprising: a touch panel; a receiver coupled to the
touch panel, the receiver configured to perform multiple
measurements on the touch panel, the multiple measurements
including measurements of waveforms that are orthogonal in content;
and measurement circuitry coupled to the receiver, the measurement
circuitry is configured to, based on the orthogonal content of the
waveforms, determine a pixel result for the touch panel using the
multiple measurements.
17. The device of claim 16, where the measurement circuitry is
further configured to determine multiple pixel results using the
multiple measurements.
18. The device of claim 17, where the measurement circuitry is
further configured to characterize a touch blob responsive to the
multiple pixel results.
19. The device of claim 16, where the measurement circuitry is
configured to cause the receiver to perform the multiple
measurements in response to a command to increase signal-to-noise
ratio.
20. The device of claim 16, where: the measurement circuitry
comprises memory configured to store a scan map; and the waveforms
that are orthogonal in content are defined in the scan map.
Description
1. CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to, and incorporates by
reference, U.S. application Ser. No. 13/414,985 filed Mar. 8, 2012,
which claims priority to, and incorporates by reference, U.S.
Provisional Application Ser. No. 61/584,491 filed Jan. 9, 2012.
2. TECHNICAL FIELD
[0002] This disclosure relates to methods and apparatus for
capacitive touch screen devices.
3. BACKGROUND
[0003] Continual development and rapid improvement in portable
devices has included the incorporation of touch screens in these
devices. A touch screen device responds to a user's touch to convey
information about that touch to a control circuit of the portable
device. The touch screen is conventionally combined with a
generally coextensive display device such as a liquid crystal
display (LCD) to form a user interface for the portable device. The
touch screen also operates with a touch controller circuit to form
a touch screen device. In other applications using touch sensing,
touch pads may also be part of the user interface for a device such
as a personal computer, taking the place of a separate mouse for
user interaction with the onscreen image. Relative to portable
devices that include a keypad, rollerball, joystick or mouse, the
touch screen device provides advantages of reduced moving parts,
durability, resistance to contaminants, simplified user interaction
and increased user interface flexibility.
[0004] Despite these advantages, conventional touch screen devices
have been limited in their usage to date. For some devices, current
drain has been too great. Current drain directly affects power
dissipation which is a key operating parameter in a portable
device. For other devices, performance such as response time has
been poor, especially when subjected to fast motion at the surface
of the touch screen. Some devices do not operate well in
environments with extreme conditions for electromagnetic
interference and contaminants that can affect performance.
[0005] Further limitations and disadvantages of conventional and
traditional approaches will become apparent to one of ordinary
skill in the art through comparison of such approaches with aspects
of the present disclosure as set forth in the remainder of this
application and with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The system may be better understood with reference to the
following drawings and description. In the figures, like reference
numerals designate corresponding parts throughout the different
views.
[0007] FIG. 1 is an example of a block diagram of a portable
device.
[0008] FIG. 2 is an example top view of a portable device.
[0009] FIG. 3 is a simplified diagram of an example mutual
capacitance touch panel for use in the portable device of FIGS. 1
and 2.
[0010] FIG. 4 shows an example block diagram of the touch front end
of the portable device of FIG. 1.
[0011] FIG. 5 shows an example first sample asymmetric scan
map.
[0012] FIG. 6 shows an example second sample asymmetric scan
map.
[0013] FIG. 7 shows an example of a high-level architecture of the
touch front end of the portable device of FIG. 1.
[0014] FIG. 8 shows an example of a simplified capacitive touch
panel and related circuitry.
[0015] FIG. 9 illustrates an example baseline tracking filter or
use in a controller circuit for a portable device.
[0016] FIG. 10 shows an example first variance estimator in
conjunction with the baseline tracking filter of FIG. 9.
[0017] FIG. 11 shows an example second variance estimator in
conjunction with the baseline tracking filter of FIG. 9.
[0018] FIG. 12 shows an example smartphone that employs orthogonal
multi-row stimulation logic and asymmetric scanning logic.
[0019] FIG. 13 shows an example graph of number of rows stimulated
versus optimal column sum.
[0020] FIG. 14 shows an example of the logic that the orthogonal
multi-row stimulation logic may implement.
[0021] FIG. 15 shows an example of the logic that the asymmetric
scan logic may implement.
[0022] FIG. 16 shows an example of blob prediction.
[0023] FIG. 17 shows an example of scanning an 8 row by 9 column
touch panel with orthogonal signals.
DETAILED DESCRIPTION
[0024] Referring now to FIGS. 1 and 2, FIG. 1 shows a block diagram
of a portable device 100. FIG. 2 shows one embodiment of a portable
device 100 according to the block diagram of FIG. 1. As shown in
FIG. 1, the portable device 100 includes a capacitive touch panel
102, a controller circuit 104, a host processor 106, input-output
circuit 108, memory 110, a liquid crystal display (LCD) 112 and a
battery 114 to provide operating power.
[0025] FIG. 2 includes FIG. 2A which shows a top view of the
portable device 100 and FIG. 2B which shows a cross-sectional view
of the portable device 200 along the line B-B' in FIG. 2A. The
portable device may be embodied in a wide range of devices with a
touch sensitive display, including, as examples, a tablet computer,
a smart phone, music player, or a fixed device such as a kiosk.
[0026] The portable device 100 includes a housing 202, a lens or
clear touch surface 204 and one or more actuatable user interface
elements such as a control switch 206. Contained within the housing
are a printed circuit board 208 circuit elements 210 arranged on
the printed circuit board 208 and as are shown in block diagram
form in FIG. 1. The capacitive touch panel 102 is arranged in a
stack and includes a drive line 212, an insulator 214 and a sense
line 216. The insulator electrically isolates the drive line 212
and other drive lines arranged parallel to the drive line from the
sense lines 216. Signals are provided to one or more of the drive
lines 212 and sensed by the sense lines 216 to locate a touch event
on the clear touch surface 204. The LCD 112 is located between the
printed circuit board 208 and the capacitive touch panel 102.
[0027] As is particularly shown in FIG. 2A, the capacitive touch
panel 102 and the LCD 112 may be generally coextensive and form a
user interface for the portable device. Text and images may be
displayed on the LCD for viewing and interaction by a user. The
user may touch the capacitive touch panel 102 to control operation
of the portable device 100. The touch may be by a single finger of
the user or by several fingers, or by other portions of the user's
hand or other body parts. The touch may also be by a stylus gripped
by the user or otherwise brought into contact with the capacitive
touch panel. Touches may be intentional or inadvertent. In another
application, the capacitive touch panel 102 may be embodied as a
touch pad of a computing device. In such an application, the LCD
112 need not be coextensive (or co-located) with the capacitive
touch panel 102 but may be located nearby for viewing by a user who
touches the capacitive touch panel 102 to control the computing
device.
[0028] Referring again to FIG. 1, the controller circuit 104
includes a digital touch system 120, a processor 122, memory
including persistent memory 124 and read-write memory 126, a test
circuit 128 and a timing circuit 130. In one embodiment, the
controller circuit 104 is implemented as a single integrated
circuit including digital logic and memory and analog
functions.
[0029] The digital touch subsystem 120 includes a touch front end
(TFE) 132 and a touch back end (TBE) 134. This partition is not
fixed or rigid, but may vary according to the high-level
function(s) that each block performs and that are assigned or
considered front end or back end functions. The TFE 132 operates to
detect the capacitance of the capacitive sensor that comprises the
capacitive touch-panel 102 and to deliver a high signal to noise
ratio (SNR) capacitive image (or heatmap) to the TBE 134. The TBE
134 takes this capacitive heatmap from the TFE 132 and
discriminates, classifies, locates, and tracks the object(s)
touching the capacitive touch panel 102 and reports this
information back to the host processor 106. The TFE 132 and the TBE
134 may be partitioned among hardware and software or firmware
components as desired, e.g., according to any particular design
requirements. In one embodiment, the TFE 132 will be largely
implemented in hardware components and some or all of the
functionality of the TBE 134 may be implemented by the processor
122.
[0030] The processor 122 operates in response to data and
instructions stored in memory to control the operation of the
controller circuit 104. In one embodiment, the processor 122 is a
reduced instruction set computer (RISC) architecture, for example
as implemented in an ARM processor available from ARM Holdings. The
processor 122 receives data from and provides data to other
components of the controller circuit 104. The processor 122
operates in response to data and instructions stored in the
persistent memory 124 and read-write memory 126 and in operation
writes data to the memories 124, 126. In particular, the persistent
memory 124 may store firmware data and instructions which are used
by any of the functional blocks of the controller circuit 104.
These data and instructions may be programmed at the time of
manufacture of the controller 104 for subsequent use, or may be
updated or programmed after manufacture.
[0031] The timing circuit 130 produces clock signals and analog,
time-varying signals for use by other components of the controller
circuit 104. The clock signals include digital clock signal for
synchronizing digital components such as the processor 122. The
time-varying signals include signals of predetermined frequency and
amplitude for driving the capacitive touch panel 102. In this
regard, the timing circuit 130 may operate under control or
responsive to other functional blocks such as the processor 122 or
the persistent memory 124.
[0032] FIG. 3 shows a diagram of a typical mutual capacitance touch
panel 300. The capacitive touch panel 300 models the capacitive
touch panel 102 of the portable device of FIGS. 1 and 2. The
capacitive touch panel 300 has N.sub.row rows and N.sub.col columns
(N.sub.row=4, N.sub.col=5 in FIG. 3). In this manner, the
capacitive touch panel 300 creates N.sub.row-times-N.sub.col mutual
capacitors between the N.sub.row rows and the N.sub.col columns.
These are the mutual capacitances that the controller circuit 104
commonly uses to sense touch, as they create a natural grid of
capacitive nodes that the controller circuit 104 uses to create the
typical capacitive heatmap. However, it is worth noting that there
are a total of (N.sub.row+N.sub.col)--or (N.sub.row+N.sub.col+2)
nodes if a touching finger or stylus and ground node in the
capacitive touch panel 300 are included. A capacitance exists
between every pair of nodes in the capacitive touch panel 300.
[0033] Stimulus Modes
[0034] The capacitive touch panel 300 can be stimulated in several
different manners. The way in which the capacitive touch panel 300
is stimulated impacts which of the mutual capacitances within the
panel are measured. A list of the modes of operation is detailed
below. Note that the modes defined below only describe the manner
in which the TFE 132 stimulates the panel.
[0035] Row-column (RC) mode is a first operating mode of a mutual
capacitive sensor. In RC mode, the rows are driven with transmit
(TX) waveforms and the columns are connected to receive (RX)
channels of the TFE 132. Therefore, the mutual capacitors between
the rows and the columns are detected, yielding the standard
N.sub.row.times.N.sub.col capacitive heatmap. In the example shown
in FIG. 3, RC mode measures the capacitors label Cr.sub.<i>,
C.sub.<j>, where <i> and <j> are integer indices
of the row and column, respectively. Generally, there is no
incremental value in supporting column-row (CR) mode, (e.g. driving
the columns and sensing the rows), as it yields the same results as
RC mode.
[0036] Self-capacitance column (SC) mode is a self-capacitance mode
that may be supported by the controller 102. In SC mode, one or
more columns are simultaneously driven and sensed. As a result, the
total capacitance of all structures connected to the driven column
can be detected.
[0037] In column-listening (CL) mode, the RX channels are connected
to the columns of the capacitive touch panel 102 and the
transmitter is turned off. The rows of the capacitive touch panel
102 will either be shorted to a low-impedance node (e.g. AC
ground), or left floating (e.g. high-impedance). This mode is used
to listen to the noise and interference present on the panel
columns. The output of the RX channels will be fed to a spectrum
estimation block (e.g. FFT block) in order to determine the
appropriate signal frequencies to use and the optimal interference
filter configuration, as will be described in further detail
below.
[0038] Timing Terminology
[0039] Some terminology is introduced for understanding the various
timescales by which results are produced within the TFE 132. The
TFE 132 produces a capacitive heatmap by scanning all desired nodes
of the capacitive touch panel 102 (e.g., all of the nodes, or some
specified or relevant subset of all of the nodes). This process may
be referred to as a frame scan; the frame scan may run at a rate
referred to as the frame rate. The frame rate may be scalable. One
exemplary frame rate include a frame rate of 250 Hz for single
touch and a panel size less than or equal to 5.0 inches in size. A
second exemplary frame rate is 200 Hz for single touch and a panel
size greater than 5.0 inches. A third exemplary frame rate is 120
Hz minimum for 10 touches and a panel size of 10.1 inches.
Preferably, the controller 104 can support all of these frame rates
and the frame rate is configurable to optimize tradeoff of
performance and power consumption for a given application. The term
scan rate may be used interchangeably with the term frame rate.
[0040] The controller circuit 104 may assemble a complete frame
scan by taking a number of step scans. Qualitatively, each step
scan may result in a set of capacitive readings from the receivers,
though this may not be strictly done in all instances. The
controller circuit 104 may perform each step scan at the same or
different step rate. For a row/column (RC) scan, where the
transmitters are connected to the rows and the receivers are
connected to the columns, it will take N.sub.row step scans to
create a full frame scan. Assuming a tablet-sized capacitive touch
panel 102 with size 40 rows.times.30 columns, the step rate may be
at least 8 kHz to achieve a 200 Hz frame rate.
[0041] For all mutual-capacitance scan modes a touch event causes a
reduction in the mutual capacitance measured. The capacitive
heatmap that is created by the TFE 132 will be directly
proportional to the measured capacitance. Therefore, a touch event
in these scan modes will cause a reduction in the capacitive
heatmap. For all self-capacitance scan modes, a touch event causes
an increase in the capacitance measured. The capacitive heatmap
that is created by the TFE 132 will be directly proportional to the
measured capacitance. Therefore, a touch event in these scan modes
will cause a local increase in the capacitive heatmap.
[0042] Referring now to FIG. 4, it shows a block diagram of the
touch front end (TFE) 132 of FIG. 1. In the illustrated embodiment,
the AFE 132 includes 48 physical transmit channels and 32 physical
receive channels. Additionally, some embodiments of the AFE 132 may
contain circuitry such as power regulation circuits, bias
generation circuits, and clock generation circuitry. To avoid
unduly crowding the drawing figure, such miscellaneous circuitry is
not shown in FIG. 4.
[0043] The TFE 132 includes transmit channels 402, a waveform
generation block 404, receive channels 406 and I/Q scan data paths
408. The transmit channels 402 and the receive channels 406
collectively may be referred to as the analog front end (AFE) 400.
The TFE 132 further includes, for the in-phase results from the I/Q
scan data path, a receive data crossbar multiplexer 410, a
differential combiner 412 and an in-phase channel assembly block
414. Similarly for the quadrature results, the TFE 132 includes a
receive data crossbar multiplexer 416, a differential combine 418
and an in-phase channel assembly block 420. The in-phase results
and the quadrature results are combined in an I/Q combiner 422. The
absolute value of the data is provided to a row and column
normalizer 424 and then made available to the touch back end (TBE)
134. Similarly, the heatmap phase information from the I/Q combiner
422 is provided to the TBE 134 as well.
[0044] The TFE 132 further includes a scan controller 426, read
control crossbar multiplexer 428 and transmit control crossbar
multiplexer 430. Further, the TFE 132 includes a spectrum
estimation processor 426 as will be described below in further
detail. The spectrum estimation processor 426 provides a spectrum
estimate to the TBE 134. The scan controller 426 receives high
level control signals from the TBE 134 to control which columns are
provided with transmit signals and which rows are sensed.
[0045] The receive data crossbar multiplexers 410, 416 and the
receive control crossbar multiplexer 428 together for a receive
crossbar multiplexer. These two multiplexers are used to logically
remap the physical receive TFE channels by remapping both their
control inputs and data outputs. As such, the control signals
routed to both multiplexers may be identical, as the remapping
performed by the receive data multiplexers 410, 416 and the receive
control multiplexer 428 needs to be identical.
[0046] The receive data crossbar multiplexers 410, 416 sit between
the output of the I/Q scan data path 408 and the heatmap assembly
blocks 414, 420. The purpose of the receive data crossbar
multiplexers 410, 416 is to enable the logical remapping of the
receive channels. This in turn allows for logical remapping of the
electrical connectors such as pins or balls which connect the
integrated circuit including the controller 104 to other circuit
components of the portable device 100. This will in turn enable
greater flexibility in routing a printed circuit board from the
integrated circuit including the controller 104 to the capacitive
touch panel 102.
[0047] Since the I/Q scan data path 408 outputs complex results,
the receive crossbar multiplexer may be able to route both the I
and Q channels of the scan data path output. This can easily be
achieved by instantiating two separate and identical crossbar
multiplexers 410, 416. These two multiplexers will share the same
control inputs.
[0048] The receive control crossbar multiplexer 428 sits between
the scan controller 426 and the AFE 400. It is used to remap the
per-channel receive control inputs going into the AFE 400. The
structure of the receive control crossbar multiplexer 428 may be
the same as for the receive data crossbar multiplexer 410, 416.
[0049] Since the Rx Ctrl crossbar is used in conjunction with the
Rx Data crossbar to logically remap the RX channels, it may be
programmed in conjunction with the Rx data crossbar. The
programming of the receive control multiplexer 428 and the receive
data crossbar multiplexers 410, 416 are not identical. Instead the
programming may be configured so that the same AFE to controller
channel mapping achieved in one multiplexer is implemented in the
other.
[0050] The scan controller 426 forms the central controller that
facilitates scanning of the capacitive touch panel 102 and
processing of the output data in order to create the capacitive
heatmap. The scan controller 426 operates in response to control
signals from the TBE 134.
[0051] Scan Controller Modes of Operation
[0052] The scan controller 426 may support many different modes. A
brief description of each mode is listed below. Switching between
modes is typically performed at the request of the processor 122
(FIG. 1), with a few exceptions noted below.
[0053] Active scan mode is considered the standard mode of
operation, where the controller 104 is actively scanning the
capacitive touch panel 102 in order to measure the capacitive
heatmap. Regardless of what form of panel scan is utilized, the
scan controller 426 steps through a sequence of step scans in order
to complete a single frame scan.
[0054] In single-frame mode, the controller initiates one single
frame scan at the request of the processor 122. After the scan is
complete, the capacitive heatmap data is made available to the
processor 122 and the scan controller 426 suspends further
operation until additional instructions are received from the
processor 426. This mode is especially useful in chip
debugging.
[0055] In single-step mode, the controller initiates one single
step scan at the request of the processor 122. After the scan is
complete, the outputs of the scan data path 408 are made available
to the processor 122 and the scan controller 426 suspends further
operation until additional instructions are received from the
processor 122. This mode is especially useful in chip testing and
debugging.
[0056] Idle scan mode is a mode initiated by the processor 122 in
order to run the controller 104 in a lower-performance mode.
Typically, this mode will be selected when the controller 122 does
not detect an active touch on the screen of the capacitive touch
panel 102, but still wants reasonably fast response to a new touch.
Therefore, the controller 122 is still active and capable of
processing the heatmap data produced by the TFE 132.
[0057] The primary differences between active scan mode and idle
scan mode are twofold. First, the frame rate in idle scan mode will
typically be slower than that used in active scan mode. Duty
cycling of the AFE 400 and other power reduction modes will be used
in order to reduce total power consumption of the controller 104
during idle scan. Second, the length of time used to generate a
single frame scan may be shorter in idle scan mode than in active
scan mode. This may be achieved by either shortening the duration
of a step scan or by performing fewer step scans per frame.
Reducing total frame scan time can further reduce power at the
expense of reduced capacitive heatmap signal to noise ratio
(SNR).
[0058] Spectrum estimation mode is used to measure the interference
and noise spectrum coupling into the receive channels. This
measurement is then analyzed by the processor 122 to determine the
appropriate transmit frequency and calculate the optimal filter
coefficients for the filters within the scan data path 408. This
mode is typically used with the Column Listening mode.
[0059] In spectrum estimation mode, most of the blocks of the TFE
132 in FIG. 4 are disabled. The scan controller 426, the AFE 400,
and the spectrum estimation preprocessor 432 may be used. The
transmit channel 402 of the AFE 400 is powered down, and the
receive channel 406 of the AFE 400 records the background noise and
interference signals that couple into the capacitive touch panel
102. The receive data from all of the channels of the AFE 400 are
routed to the spectrum estimation preprocessor 432, which performs
mathematical preprocessing on this data. The output of the spectrum
estimation preprocessor 432 will be an N-point vector of 16-bit
results, where N is approximately 200. The output of the spectrum
estimation preprocessor 432 is handed off to the processor 122 for
further analysis and determination of the appropriate transmit
frequency to use. This process is described in greater detail
below.
[0060] In addition to the functional modes described above, the
controller 104 may have a set of sleep modes, where various
functional blocks in the controller 104 are disabled and/or powered
down completely.
[0061] A frame scan includes of a series of step scans. The
structure of each step scan may be identical from step scan to the
next within a given frame scan; however, the exact values of
control data vary from step scan to step scan. Furthermore, the
operation of a given frame scan may be determined by configuration
parameters and may or may not affected by data values measured by
the receive channel. One example of the frame scan logic that the
controller circuit 104 may implement is shown below.
TABLE-US-00001 // Initialization Set DDFS parameters; Clear
heatmap_memory; // Step scan loop For step_idx = 1 to
num_step_scans { // Configure circuits according to step_idx Set
scan_datapath_control to scan_datapath_parameters[step_idx]; Assert
Rx_reset and wait TBD clock cycles; Set AFE_control_inputs to
AFE_parameters[step_idx]; Deassert Rx_reset and wait TBD clock
cycles; // Run step scan and collect data Send start signal to DDFS
and scan data path; Wait for TBD clock cycles for step scan to
complete; Pass datapath_results[step_idx] to heatmap assembly block
// Incremental heatmap processing } // step_idx loop
[0062] The incremental heatmap processing operation is described in
greater detail below.
[0063] Multi-Transmit Support and Block Stimulation of the
Panel
[0064] In order to achieve improved SNR in the capacitive heatmap,
the controller circuit 104 provides support for multi-transmit
(multi-Tx) stimulation of the capacitive control panel 102.
Multi-Tx simulation (or Multi-Tx) means that multiple rows of the
panel are simultaneously stimulated with the transmit (Tx) signal,
or a polarity-inverted version of the Tx signal, during each step
scan. The number and polarity of the rows stimulated, may be
controlled through control registers in the AFE 400. The number of
rows simultaneously stimulated during multi-Tx is defined as a
parameter N.sub.multi. N.sub.multi may be a constant value from
step-to-step within a given frame and also from frame-to-frame.
[0065] If N.sub.multi rows are simultaneously stimulated during a
step scan, it will take at least N.sub.multi step scans to resolve
all the pixel capacitances being stimulated. Each receiver has
N.sub.multi capacitances being stimulated during a scan step. Hence
there are N.sub.multi unknown capacitances, requiring at least
N.sub.multi measurements to resolve these values. During each of
these N.sub.multi steps, the polarity control of the Tx rows will
be modulated by a set of Hadamard sequences. Once this set of
N.sub.multi (or more) step scans is complete, the next set of
N.sub.multi rows can be stimulated in the same fashion, as
N.sub.multi will almost always be less than the number of actual
rows in the capacitive touch panel 102.
[0066] In this way, the processing of the entire capacitive touch
panel 102 occurs in blocks, where N.sub.multi rows of pixels are
resolved during one batch of step scans, and then the next
N.sub.multi rows of pixels are resolved in the next batch of step
scans, until all the panel rows are fully resolved.
[0067] In most scenarios, the number of panel rows will not be an
exact multiple of N.sub.multi. In these situations, the number of
rows scanned during the final block of rows will be less than
N.sub.multi However, N.sub.multi scan steps may be performed on
these remaining rows, using specified non-square Hadamard
matrices.
[0068] Differential Scan Mode
[0069] Differential scan mode is an enhancement to normal scanning
mode, whereby the frame scan operation is modified to exploit the
correlation of the interference signal received across adjacent
receive channels. In this mode, the normal frame scan methodology
is performed; however the number of step scans used to assemble a
single frame is doubled. Conceptually, each step scan in the scan
sequence becomes two step scans: the first is a single-ended or
normal step scan with the default values for the AFE control
registers, and the second is a differential step scan.
[0070] Given N.sub.RX receive channels, the differential scan mode
yields a total of 2N.sub.RX receiver measurements per aggregate
scan step. (e.g. N.sub.RX single-ended measurements and N.sub.RX
differential measurements.) These 2N.sub.RX measurements are
recombined and collapsed into N.sub.RX normal measurements in the
Differential Combiner block 412, 418 shown in FIG. 4.
[0071] FIG. 7 shows a high-level architecture 700 of the analog
front end. The architecture 700 includes a transmit channel 702
providing signals to columns of the capacitive touch panel 102 and
a receive channel 704 sensing signals from the capacitive touch
panel 102. The transmit channel 702 includes a digital to analog
converter 706, polarity control circuits 708 and buffers 710. The
receive channel 704 includes a pre-amplifier 712 and analog to
digital converter 714.
[0072] All transmit channels may be driven by a shared transmit
data signal labeled TxDaC in FIG. 7. Each physical transmit channel
may also receive a common transmit digital to analog converter
clock signal, labeled TxDacClk, to drive the transmit digital to
analog converter 706. The clock signal will come directly from a
frequency locked loop block within the TFE 132, and this clock
signal will also be routed to the digital portion of the TFE
132.
[0073] Each physical transmit channel may also have its own set of
channel-specific TxCtrl bits that appropriately control various
parameters of the transmit channel, such as enable/disable,
polarity control, and gain/phase control). These TxCtrl bits are
not updated at the TxDacClk rate, but rather are updated between
subsequent step scans during the frame scan operation.
[0074] A control signal controls the transmit polarity of each of
the 48 transmit channels. As will be described in greater detail
below, the polarity of the transmit outputs may be modulated in an
orthogonal sequence, with each transmit output having a fixed
polarity during each scan step during a frame scan.
[0075] All receive channels will receive a set of common clock
signals. These clock signals are provided directly from a frequency
locked loop block within the TFE 132, and this clock signal is also
routed to the digital portion of the TFE 132. The clock signals
routed to the RX channels include the signal RxADCClk which drives
the RxADC. A typical clock frequency for this signal is 48 MHz.
[0076] Each physical receive channel will also have its own set of
channel-specific receive control bits, labeled RxCtrl in FIG. 7,
that appropriately control various parameters of the receive
channel, such as enable/disable and gain control. These receive
control bits are updated between subsequent step scans during the
frame scan operation.
[0077] Additionally, there may be a shared set of control settings,
labelled RxCtrlUniv in FIG. 7, that will control all receive
channels simultaneously. These registers are primarily composed of
generic control bits that will remain constant for a given
implementation of the controller 104.
[0078] There are also one or more reset lines labeled RxReset that
are common to all reset channels. These reset lines may be asserted
in a repeatable fashion prior to each scan step.
[0079] Waveform Generation
[0080] The waveform generation block (WGB) 404 in FIG. 4 generates
the transmit waveform for the TX channels 402. The WGB 404
generates a digital sine wave. Additionally, WGB 404 may generate
other simple periodic waveforms; such as square waves having edges
with programmable rise and fall times.
[0081] The primary output of the WGB 404 is the data input to the
transmit channels 402 labelled TxDAC in FIG. 4. The WGB 404
receives as input signals a clock signal labelled TxDacClk and a
signal labelled Start in FIG. 4. Upon receiving the Start signal
from the scan controller 426, the WGB 404 begins producing digital
waveforms for the duration of a single step scan. At the conclusion
of the step scan, the WGB 404 ceases operation and waits for the
next start signal from the scan controller 426.
[0082] The WGB 404 may have some amount of amplitude control, but
the WGB 404 will typically be operated at maximum output amplitude.
Therefore, the performance requirements listed below only need to
be met at max output amplitude. All signal outputs may be in two's
complement format. The WGB 404 may also provide arbitrary
sine/cosine calculation capabilities for the scan data path 408 and
spectrum estimation preprocessor 432.
[0083] The following table lists typical performance for the WGB
404.
TABLE-US-00002 Specification Min Nom Max Comment Clock rate 8 MHz
Will operate at TxDacClk rate Output frequency 0 Hz -- 2 MHz
Frequency ctrl resolution -- 15 bits -- Desired resolution of ~61
Hz. Can be different. # of output bits -- 8 -- Output amplitude 50%
100% 100% amplitude amplitude amplitude Amplitude ctrl resolution
-- 7 bits -- Corresponds to 1% stepsize in amplitude control. DC
bias control 0 0 0 All outputs should be balanced around 0 Output
THD -40 dBFs Sine wave mode only Rise/fall time 1 calk -- 256 calk
Square-wave mode cycle @ cycles @ only. Independent 8 MHz 8 MHz
control of rise time vs. fall time NOT required.
[0084] In FIG. 4, the differential combiner blocks 412, 410 provide
the capability to operate in differential mode, where the receive
channels 406 alternate step scans between single-ended measurements
and differential measurements. The purpose of the differential
combiner blocks 412, 418 is to combine the N.sub.RX single-ended
measurements and (N.sub.RX-1) differential measurements into a
single set of N.sub.RX final results for use in the heatmap
assembly blocks 414, 420 that follow.
[0085] The differential combiner blocks 412, 418 are akin to a
spatial filter. Let the vector, c, be an N.sub.rx-by-1 vector of
the capacitances to estimate. In differential mode, you have a
vector, s, of single-ended measurements and a vector, d, of
differential measurements. Hence, an estimate of c, called
c.sub.est, is sought by optimally recombining s and d. Determining
the optimal recombination requires substantial computation, but
simulations have shown that the following recombination scheme
works to within roughly 0.5 dB of optimal performance over the
expected range of operating conditions:
c.sub.est,n=a.sub.1s.sub.n-2+a.sub.2s.sub.n-1+a.sub.3s.sub.n+a.sub.2s.su-
b.n+1+a.sub.1s.sub.n+2+b.sub.1d.sub.n+1+b.sub.2d.sub.n-b.sub.2d.sub.n+1-b.-
sub.1d.sub.n+2
[0086] where the subscript n indicates result from the n.sup.th
receiver channel, and 0.ltoreq.n.ltoreq.N.sub.RX-1.
[0087] Furthermore, the coefficients are subject to the following
constraints:
0.ltoreq.a.sub.1,a.sub.2,a.sub.3.ltoreq.1
a.sub.3=1-2a.sub.1-2a.sub.2
b.sub.1=a.sub.1
b.sub.2=a.sub.1+a.sub.2
[0088] Given these constraints, it can be observed that the math
operation listed above can be collapsed into two multiplication
operations:
c.sub.est,n=s.sub.n+a.sub.1(s.sub.n-2-2s.sub.n+s.sub.n+2+d.sub.n-1+d.sub-
.n-d.sub.n+1-d.sub.n+2)+a.sub.2(s.sub.n-1-2s.sub.n+s.sub.n+1+d.sub.n-d.sub-
.n+1)
[0089] The equations above assume that the data exists for 2
receivers on either side of the nth receiver. (e.g.
2.ltoreq.n.ltoreq.N.sub.RX-3) Therefore, the equations above may be
modified for the two outer edge receive channels on either side.
The modifications are quite simple. First, replace any non-existent
s.sub.k term with the nearest neighboring s.sub.j term that does
exist. Second, replace any non-existent d.sub.k term with 0.
Putting these rules together and expressing the mathematics in
matrix form, we get:
c est = [ a 1 + a 2 + a 3 a 2 a 1 0 0 - b 2 - b 1 0 0 a 1 + a 2 a 3
a 2 a 1 0 b 2 - b 2 - b 1 0 a 1 a 2 a 3 a 2 a 1 b 1 b 2 - b 2 - b 1
a 1 a 2 a 3 a 2 a 1 b 1 b 2 - b 2 - b 1 0 a 1 a 2 a 3 a 2 + a 1 0 b
1 b 2 - b 2 0 0 a 1 a 2 a 3 + a 2 + a 1 0 0 b 1 b 2 ] [ s 0 s N RX
- 1 d 1 d N RX - 1 ] ##EQU00001##
[0090] Lastly, while the optimal values of {a1, a2, a3, b1, b2} are
dependent upon the precise noise and interference environment, it
has been found that the following values for these parameters
operate near optimal performance for the expected range of
operating environments:
a.sub.1=1/8
a.sub.2= 7/32
a.sub.3= 5/16
b.sub.1=1/8k.sub.ADC
b.sub.2= 11/32k.sub.ADC
[0091] The parameters b.sub.1 and b.sub.2 above are dependent upon
another parameter, k.sub.ADC. The new parameter, k.sub.ADC, is
dependent upon the value of receive channel analog to digital
converter gain (Rx_AdcGain) used during the differential
measurement step, as detailed in the table below:
TABLE-US-00003 Rx_AdcGain<1:0> used during differential
measurement step k.sub.ADC 00 1 01 3/4 10 1/2 11 3/8
[0092] These a and b coefficients should be programmable by a
control source such as firmware that is part of the controller 104,
but the default values should be those listed above. The table
below indicates the suggested bit width for each coefficient:
TABLE-US-00004 Coefficient Bit width a.sub.1 5 a.sub.2 5 a.sub.3 5
b.sub.1 6 b.sub.2 8
[0093] The heatmap assembly blocks (HAB) 414, 420 take the step
scan outputs from the scan data path 408 or differential combiners
412, 418, if used, and assembles the complete capacitive heatmap
that is the major output of the frame scan operation. In order to
do so, it may mathematically combine all of the step scan outputs
in the appropriate manner to create estimates of the capacitance
values of the individual capacitive pixels in the capacitive touch
panel 102.
[0094] As shown in FIG. 4, there are two separate and identical
instantiations of the HAB. A first HAB 414 is for the I-channel
data and a second HAB 420 is for the Q-channel data. Each HAB 414,
418 operates on the either the I-channel or Q-channel data in order
to create either an I-channel or a Q-channel capacitive
heatmap.
[0095] In order to demonstrate the mathematics that may apply for
heatmap assembly, an example 4.times.5 capacitive touch panel 800
is illustrated in FIG. 8. In this example, only the capacitive
pixels in column 1 are analyzed, but the same principle can be
easily extended to each of the five columns in the example
capacitive touch panel 800. In particular, the output of receive
column j is only affected by capacitance pixels in column j.
[0096] The example capacitive touch panel 800 includes a touch
panel 802, a transmit digital to analog converter (TxDAC) 804,
transmit buffers 806, 808, 810, 812, and a receive analog to
digital converter 814. The transmit buffers 806, 808, 810, 812 each
have an associated multiplier 816, 818, 820 822, respectively. The
multipliers 816, 818, 820 822 operate to multiply the applied
signal from the TxDAC by either +1 or -1.
[0097] In the example of FIG. 8, a single TxDAC waveform is sent to
all four transmit buffers 806, 808, 810, 812. However, each buffer
multiplies this waveform by either +1 or -1 before transmitting it
onto the row of the touch panel 802. For a given step scan
(indicated by the subscript "step_idx"), each value of Hi, step_idx
is held constant. But for subsequent step scans in the scan
sequence, these values may change. Therefore, at a given step
index, the voltage received at m.sup.th Rx channel is:
V step _ idx , m = V TX RxGain m n = 0 NumRows - 1 H n , step _ idx
C n , m ##EQU00002##
[0098] where V.sub.TX is the amplitude of the transmit signal and
RxGain.sub.m is the gain of the receive channel m. In order to
simplify the analysis, these two parameters are assumed to be equal
to 1 and ignored in subsequent calculations.
[0099] As can be seen by this equation above,
V.sub.step.sub.--.sub.idx,m is based on NumRows (e.g. 4) unknown
values, C.sub.n,m, with n=0 to 3 in this example. Therefore, if
four independent step scans are performed with four independent H
sequences applied to the four transmit buffers 806, 808, 810, 812,
the relationship between V and C can be inverted in order to
estimate the C values from V. In matrix form, this can be
written:
V m = H C m ##EQU00003## V m = [ V 0 , m V 1 , m V NumSteps - 1 , m
] ##EQU00003.2## H NumSteps , NumRows = [ H 0 , 0 H 0 , NumRows - 1
H NumSteps - 1 , 0 H NumSteps - 1 , NumRows - 1 ] ##EQU00003.3## C
m = [ C 0 , m C 1 , m C NumRows - 1 , m ] ##EQU00003.4##
[0100] In this formulation, the column vector C.sub.m represents
the capacitance of the capacitive pixels in the m.sup.th column of
the capacitive touch panel. H is a NumSteps.times.NumRows matrix,
where the n.sup.th column of the H-matrix represents the
multiplicative sequence applied to the n.sup.th transmit row. The
optional superscript of H indicates the dimensions of the H matrix.
V.sub.m is a column vector, where the n.sup.th entry in the matrix
is the n.sup.th step scan output of m.sup.th RX channel.
[0101] In the present application, H is a special form of matrix,
called a modified Hadamard matrix. These matrices have the property
that:
H.sup.TH=NumStepsI
[0102] where I is the NumRows.times.NumRows identity matrix, and
H.sup.T is the transpose of H.
[0103] Given the formulation above, and the properties of the
H-matrix, the relationship from C.sub.m to V.sub.m can be inverted
in order to extract out the values of the C.sub.m vector from the
V.sub.m measurements. Using the terminology defined above:
C m = 1 NumSteps H T V m ##EQU00004##
[0104] In the example above, the panel had four rows and the value
of NumSteps (equivalently N.sub.multi) was also set to four.
Therefore, all panel rows were stimulated during every step scan.
In general, the number of panel rows will be larger than the value
of N.sub.multi. In that case, the panel stimulation is broken up
into blocks. During each block of N.sub.multi step scans,
N.sub.multi adjacent rows are stimulated with the Hadamard polarity
sequencing described above.
[0105] The heatmap assembly block 414, 420 works on each block of
N.sub.multi scans independently in order to create the complete
heatmap output. For instance, if there were twelve panel rows and
N.sub.multi were set to four, then the first four step scans would
be used to stimulate and assemble the first four rows of the
capacitive heatmap; the next four step scans would be for the fifth
through eighth panel rows; and the last four step scans would be
for the ninth through twelfth rows. Therefore, for each block of
N.sub.multi rows, the heatmap assembly block operates in the exact
same manner as defined above. However, the outputs of the HAB 414,
420 are mapped to the subsequent rows in the complete capacitive
heatmap.
[0106] The heatmap assembly block 414, 420 is capable of assembling
a 32-column-wide heatmap, as there are a total of 32 receiver
channels implemented in one embodiment. However, in many cases, the
capacitive touch panel used will not have 32 columns, and hence not
all 32 receive channels are used.
[0107] Mathematical Extensions for Asymmetric Panel Scanning
[0108] As described above, the controller 104 preferably has the
capability to perform asymmetric panel scans, where the firmware
supporting operation of the controller 104 has the capability to
define the number of times each row is to be scanned. Given the
formulation for asymmetric panel scanning outlined above, the
changes to the heatmap assembly operation in order to support this
feature are minimal.
[0109] As described above, the heatmap is assembled in a blocks of
N.sub.multi rows. In asymmetric scanning, N.sub.multi can vary on a
block-by-block basis. Therefore, the old equation of:
C m = 1 NumSteps H T V m ##EQU00005##
[0110] is still valid. However, with asymmetric scanning, the
dimensions of C, V, and H and the value of NumSteps change on a
block-by-block basis.
[0111] The I/Q combiner 422 shown in 4 is used to combine the I-
and Q-channel heatmaps into a single heatmap. The primary output of
the I/Q combiner 422 is a heatmap of the magnitude (e.g.
Sqrt[I.sup.2+Q.sup.2]). This is the heatmap that is handed off to
the touch back end 134.
[0112] The row/column normalizer 424 shown in FIG. 4 is used to
calibrate out any row-dependent or column-dependent variation in
the panel response. The row/column normalizer 424 has two static
control input vectors, identified as RowFac and ColFac. RowFac is
an Nrow-by-1 vector, where each entry is 1.4 unsigned number (e.g.
LSB= 1/16. Range is 0 to 31/16). ColFac is an Ncol-by-1 vector,
where each entry has the same dimensions as RowFac.
[0113] If the input data to the Row/Column Normalizer block is
labeled as HeatmapIn(m,n), where m is the row index and n is the
column index, the output of the block should be:
HeatmapOut(m,n)=HeatmapIn(m,n)RowFac(m)ColFac(n)
[0114] In one embodiment, the controller 104 has the capability to
allow RowFac and ColFac to be defined either by OTP bits or by a
firmware configuration file. The OTP settings will be used if the
manufacturing flow allows for per-module calibration, thus enabling
the capability to tune the controller 104 on a panel-by-panel
basis. If RowFac and ColFac can only be tuned on a per-platform
basis, then the settings from a firmware configuration file will be
used instead.
[0115] Spectrum Estimation
[0116] The spectrum estimation preprocessor 432 operates to
determine the background levels of interference that couple into
the receive channels 406 so that the controller 104 may
appropriately select transmit frequencies that are relatively quiet
or interference free.
[0117] The spectrum estimation preprocessor 432 will generally only
be used during SEM mode, so it is not part of the standard
panel-scan methodology. Instead, the spectrum estimation
preprocessor 432 will be used when conditions indicate that SEM
should be invoked. At other times, the spectrum estimation
preprocessor 432 can be powered down.
[0118] Baseline Tracking and Removal Filter
[0119] A touch event should be reported when the measured
capacitance of a capacitive pixel (or group of pixels) changes by a
large enough amount in a short enough period of time. However, due
to slow environmental shifts in temperature, humidity or causes of
drift, the absolute capacitance of a pixel (or group of pixels) can
change substantially at a much slower rate. In order to
discriminate changes in pixel capacitance due to a touch event from
changes due to environmental drift, a baseline tracking filter can
be implemented to track the changes in the baseline (e.g.
"untouched" or "ambient" value of the capacitance), and simple
subtraction of the baseline capacitance from the input capacitance
will yield the change in capacitance due to the touch event.
[0120] FIG. 9 illustrates a baseline tracking filter 900. The
filter 900 includes a low-pass filter (LPF) 902, a decimator 904
and a combiner 906. The input signal to the filter 900 is provided
to the combiner 906 and the decimator 904. The output signal of the
decimator is provided to the input of the LPF 902. The output of
the LPF 902 is combined with the input signal at the combiner 906.
The LPF 902 has an enable input for controlling operation of the
filter 900.
[0121] The LPF 902 in the baseline tracking filter 900 is used to
improve the estimate of the baseline capacitance value. One
embodiment uses a simple finite impulse response (FIR) moving
average filter of length N (aka "comb filter"), such as:
H N ( z ) = 1 N 1 - z - N 1 - z - 1 = 1 N n = 0 N - 1 z - n
##EQU00006##
[0122] Another embodiment a 1-tap infinite impulse response (IIR)
filter, also referred to as a modified moving average, with
response:
H k ( z ) = 1 k 1 - ( 1 - 1 k ) z - 1 ##EQU00007##
[0123] The FIR embodiment of the filter 902 may be used upon
startup and recalibration of the baseline value, as it can quickly
acquire and track the baseline value. The IIR embodiment of the
filter 902 should be used once the baseline value is acquired, as
it can be a very computationally efficient means to implement a
low-pass filter, particularly if k is chosen to be a power of 2. By
increasing the value of k, one can set change the signal bandwidth
of the filter to arbitrarily small values with minimal increase in
computational complexity.
[0124] Filter 900 has two outputs, labeled "Out" and "Baseline" in
FIG. 9. The Baseline output is the estimate of the current baseline
(aka "ambient" or "untouched") capacitance of the particular panel
pixel(s) being scanned, and the "Out" output is the
baseline-corrected value of that capacitance measurement. The "Out"
value is what should be used in the subsequent touch-detection
logic.
[0125] The LPF 902 in FIG. 9 has an enable signal in order to shut
down the LPF 902 when a touch event is detected. This is provided
so that the baseline output is not corrupted by spurious data, most
likely from a touch event. If the enable signal is low, the LPF 902
will hold its previous output without updating its output with the
incoming data, effectively ignoring the incoming data. Once the
enable signal is high, the LPF 902 will continue to update its
output with the incoming data. Logic for generating the enable
signal is detailed in the following equation:
Enable=Out.ltoreq.PosLPFThresh)&&(Out.gtoreq.NegLPFThresh)
[0126] where PosLPFThresh and NegLPFThresh are configurable
parameters.
[0127] In a mutual-capacitance scan mode, where a touch event
causes a reduction in the input data, the NegLPFThresh should be
set to k.sub.T*TouchThresh, where 0<k.sub.T<1 and TouchThresh
is the touch-detection threshold defined below. These may both be
programmable parameters. In a mutual-capacitance scan mode, there
is no expected physical mechanism that would cause the input data
to exhibit a positive transient. Therefore, PosLPFThresh may be a
programmable parameter used to filter out spurious data, should an
unexpected positive transient occur.
[0128] Programmable Update Rate
[0129] The timescale of most baseline drift phenomena will be far
slower than the frame rate of the touch panel scan. For instance,
observed baseline drift devices had timescales on the order of 1
hour or longer, whereas the frame rate of a current device may be
on the order of 200 frames/second. Therefore, in order to reduce
the computation for baseline tracking, the controller circuit 104
shall have the capability to scale the update rate of the baseline
tracking filter 900. The device may do this by using the decimator
904 to decimate the data fed to the filter 900, so that the filter
900 only operates on every N_BTF_decimate frames of heatmap data,
where N_BTF_decimate is a programmable parameter. Therefore, the
Baseline signal in FIG. 9 will update at this slower rate. However,
the baseline corrected output signal ("Out" in FIG. 9) may be
calculated for every frame.
[0130] Baseline tracking needs to exercise special care when
spectrum estimation mode (SEM) is invoked. SEM may cause a
configuration change in the analog front end which in turn will
alter the gain in the transfer function (e.g. from capacitance
values to codes) of the touch front end. This, in turn, may cause
abrupt changes in the capacitive heatmap to occur that could be
accidentally interpreted as touch events.
[0131] A touch event is detected when the baseline-corrected output
exhibits a significant negative shift. The shift in this output may
be larger than a programmable parameter, called TouchThresh.
Furthermore, since the controller circuit 104 may scan a panel at
upwards of 200 Hz and a human finger or metal stylus moves at a
much slower timescale, a programmable amount of debounce, dubbed
TouchDebounce, should also be included. Therefore, before a touch
is recognized, the output of the baseline filter may be more
negative than TouchThresh for at least TouchDebounce frames. It is
likely that TouchDebounce will be a small value, in order that the
total touch response time is faster than 10 ms.
[0132] Heatmap Noise Estimation
[0133] The touch back end 134 requires an estimate of the noise
level in the capacitive touch panel 102 in order to properly
threshold the touch blobs during the detection process. The noise
level can be detected by observing noise at the output of the
baseline tracking filter as shown in FIG. 10. FIG. 10 shows a first
variance estimator 1000 in conjunction with the baseline tracking
filter 900 of FIG. 9. In FIG. 10, the baseline tracking filter 900
has its Out output coupled to an input of the variance estimator
1000. The variance estimator 1000 includes a decimator 1002, a
signal squarer 1004 and a low-pass filter 1006. The variance
estimator 1000 in this embodiment is simply a mean-square
estimator, as the output of the baseline tracking filter 900 is
zero-mean. Hence the mean-square is equal to the variance.
[0134] In order to lower the computational requirements for the
variance estimator 900, the data entering the variance estimator
can be decimated in the decimator 1002 by the factor,
N_VAR_decimate. The low-pass filter 1006 in the variance estimator
1000 may either be a comb-filter or a modified-moving-average
filter. The length of the response of the filter 1006 may be a
programmable parameter, averaging data over as many as 100 or more
frames. In order to lower memory requirements, the MMA filter may
be preferred.
[0135] As with the baseline tracking filter 900, the LPF 1006 in
the variance estimator 1000 has an input for an enable signal. The
enable signal is low when the pixel in question is being touched.
Otherwise, the variance estimate will be corrupted by the touch
signal. When the enable signal is low, the LPF 1006 should retain
state, effectively ignoring the data coming into the variance
estimator 1000.
[0136] The output of the variance estimator 1000 is the variance of
one single pixel in the capacitive touch panel 102. Therefore, this
provides an independent variance estimate of each pixel in the
panel. To get an estimate of the variance across the panel 102, the
controller circuit 104 may average the per-pixel variances across
the entire frame.
[0137] Alternately, if only a single per-frame variance estimate is
needed, the controller circuit 104 can follow the approach shown in
FIG. 10. FIG. 11 shows a second variance estimator 1100 in
conjunction with the baseline tracking filter 900 of FIG. 9. In
FIG. 11, all the per-pixel baseline tracking filters are grouped as
baseline tracking filters 900, on the left in the figure. All the
baseline-corrected outputs from the baseline tracking filters 900
are passed to the variance estimator 1100.
[0138] Like the variance estimator 1000 of FIG. 10, the variance
estimator 1100 includes a decimator 1102, a signal squarer 1104 and
a low-pass filter 1106. The variance estimator 1100 further
includes a summer 1108. The variance estimator 1100 combines the
outputs of the baseline tracking filters 900 into a single value by
summing the baseline-corrected outputs across the entire frame in
the summer 1108. This averaged value is then passed to the same
square-and-filter estimator that was described above, formed by the
signal squarer 1104 and the low-pass filter 1106. Assuming that the
noise is uncorrelated from pixel-to-pixel, the output of the
variance estimator 1100 is equal to the sum of all the pixel
variances reported by the block diagram in FIG. 10. In order to
generate the average pixel variance across the panel, this result
may be divided by the total number of pixels in the capacitive
touch panel 102. To generate an estimate of the standard-deviation
of the noise, the controller circuitry 104 may take the square root
of the variance.
[0139] In one implementation, the controller circuit 104 implements
orthogonal multi-row stimulation of the touch panel. One specific
example is described with reference to FIG. 12 in the context of a
smartphone 1200 (although the techniques may be implemented in any
device with a touch panel). The smartphone 1200 includes a
transceiver 1202 and the control circuitry 104, including one or
more processors 1204, a memory 1206, and a user interface 1208. The
transceiver 1202 may be wireless transceiver, and the transmitted
and received signals may adhere to any of a diverse array of
formats, protocols, modulations, frequency channels, bit rates, and
encodings Thus, the transceiver 1202 may support the
802.11a/b/g/n/ac standards, the 60 GHz WiGig/802.11TGad
specification, Bluetooth, Global System for Mobile communications
(GSM), Time Division Multiple Access (TDMA), Frequency Division
Multiple Access (FDMA), Code Division Multiple Access (CDMA), or
other wireless access techniques or protocols.
[0140] The processor 1204 executes the orthogonal multi-row
stimulation ("OMS") logic 1210. The OMS logic 1210 may be part of
an operating system, an application program, firmware, or other
logic. The user interface 1208 includes a capacitive touch panel
1212 which is divided into a grid of rows 1214 and columns 1216
(and as also shown, for example, in FIG. 3). For the sake of
illustration, the transmitters are connected to the rows 1214 and
the receivers are connected to the columns 1216, however, that
arrangement may be reversed. At each intersection of row and column
(referred to as a pixel), there is a capacitance that is impacted
by the presence or absence of a finger, stylus, or any other
conductive touching object. The capacitance value at each pixel
provides the "heat map" of capacitance across the touch panel 1212,
and may be envisioned as a contour map of the touch panel 1212 in
which the values are the capacitance values at each pixel.
[0141] Traditionally, to obtain the capacitance value at each
pixel, each row of the touch panel 1212 is sequentially energized
(i.e., one at a time) by its transmitter, and the receivers across
the row, for each column, determine the capacitance values for the
pixels in the energized row. Thus, the touch panel 1212 is scanned
one row at a time. Accordingly, if there are `n` rows, it takes `n`
scans to image the entire touch panel 1212. Assuming for the sake
of illustration that there is a one millisecond time window in
which to image the entire touch panel 1212 (for a frame rate of
1000 frames per second), and there are 10 rows, then each row
receives energy for some fraction of the total scan time window,
e.g., 1/10th of a millisecond. If a faster frame rate is desired,
then the duration of each row scan must be reduced to less than
1/10th of a millisecond. Doing so, however, decreases the signal to
noise ratio (SNR) because less time is spent sending power into any
particular pixel. Accordingly, there is a tradeoff between frame
rate and SNR with single row scanning.
[0142] The OMS logic 1210, however, energizes multiple rows
simultaneously. The multiple rows may be consecutive or
non-consecutive. In addition, the OMS logic 1210 energizes the rows
in a linearly independent and orthogonal way that allows the OMS
logic 1210 to recover the individual capacitance values at the
multiple pixels over the multiple rows on the same column. The OMS
logic 1210 may energize the touch panel 1210 through the touch
panel interface 1218. The touch panel interface 1218 may be
implemented as described above (e.g., with regard to the front end
132 and back end 134 and FIG. 7, including polarity and amplitude
control).
[0143] Taking a specific example using two rows, the OMS logic 1210
may energize, during a first scan cycle, the first row and the
second row with positive polarities, resulting in a first receiver
measurement along the column of Row1+Row2. On the second scan
cycle, the OMS logic 1210 may energize the first row with a
positive polarity and the second row with a negative polarity,
resulting in a second receiver measurement along the column of
Row1-Row2. With two variables and two unknowns, these two equations
can be solved to obtain the individual values of Row1 and Row2:
Row1=(first measurement+second measurement)/2; and Row2=(first
measurement-second measurement)/2.
[0144] Note that although two measurements are obtained to
determine the two pixel values, one benefit is that the signal to
noise ratio (SNR) is increased. One reason for the increase is that
every measurement will include some noise, and the averaging of the
measurements tends to average out the random noise, resulting in
less noise influencing the results. This signal is coherent and
does not average out.
[0145] As a result, the OMS logic 1210 achieves a higher SNR at the
same frame rate as the single row at a time scanning technique.
Furthermore, the OMS logic 1210 may achieve approximately the same
SNR as the single row scanning technique by reducing the amount of
time spent driving each row. Thus, the OMS logic 1210 facilitates
higher SNR measurements at the same frame rate, or about the same
SNR at faster frame rates, compared to the single row at a time
scanning technique. The OMS logic 1210 may execute in response to a
command or request (from a host processor, for example) to increase
SNR.
[0146] In support of the OMS logic 1210, the memory 1206 may store
OMS parameters 1219 that determine the frame rate, desired SNR,
drive time for the rows, or other parameters under which the OMS
logic 1210 operates. The control circuitry 104 may drive the touch
panel with sinusoidal waveforms, square wave waveforms, or other
preferably orthogonal waveform types.
[0147] The OMS logic 1210 may scan the touch panel 1212 in
consecutive rows forming blocks. For example, rows 0 then 1, then
rows 2 and 3, and so on. Alternatively, the OMS logic 1210 may scan
the touch panel 1212 at non-consecutive rows. The OMS logic 1210
may scan as many rows simultaneously as the OMS logic 1210 has
orthogonal signal sets. For example, given 48 orthogonal signal
sets, the OMS logic 1210 may scan up to 48 rows simultaneously. In
one implementation, the OMS logic 1210 supports scanning 1 to 16
rows simultaneously, but other implementations may scan more than
16 rows at a time.
[0148] In one implementation, the touch panel interface 1218
includes an output driver for each row 1214. Each driver may use a
sinusoidal waveform of the same frequency or different frequencies,
or more generally use orthogonal waveforms to drive different rows.
The touch panel interface 1218 may further include independent gain
control on each driver output in order to control the polarity and
gain of the waveforms. Thus, the orthogonality of the waveforms may
be accomplished by polarity control, amplitude control (e.g.,
amplitude modulation), phase control, waveform shape control, or in
any other way for building orthogonal signals.
[0149] In one implementation, the OMS logic 1210 scans the touch
panel 1208 using Hadamard codes implemented using amplitude
modulation. In this regard, the amplitude modulation creates
signals that are +1 or -1 waveforms (e.g., as opposed to complex
valued waveforms). With respect to the signal measured at the
receivers, the driving signals are additive (e.g., driving with +1,
+1 signals results in about twice the signal (e.g., a +2 signal) at
the receiver, and driving with +1, -1 signals results in very low
level of signal at the receiver). One goal is to maximize the
dynamic range at the receiver. In that regard, it is desired for
the useful part of the signal to take up the dynamic range of the
receiver. Accordingly, for some of the higher order multi-row
stimulations, the OMS logic 1210 employs signal sets that (when
arranged in matrix form) have minimized absolute value of the
column sums, so that the gain on the receiver pre-amps in the touch
panel interface 1218 do not need to be reduced significantly and
expose the signal to quantization noise that detrimentally impacts
the receiver measurements. The orthogonal signal sets may be
represented by Hadamard matrices or modified Hadamard matrices. The
Hadamard matrices have the property that H*H'=N*I (for N rows), in
other words, the rows are orthogonal.
[0150] The H4 Hadamard matrix, below, shows one alternative, with a
maximum absolute value of the column sum of 4. The second H4
matrix, H'4, below is a modified Hadamard matrix in which the
maximum absolute value of column sum is 2. Accordingly, the peak
amplitude received is 2. Therefore, the amplifier gain can be
increased, which beneficially increases dynamic range and SNR for
the subsequent analog to digital conversion. Such matrices may be
found by a brute force or exhaustive search for any number of
rows.
H 4 = [ + 1 + 1 + 1 + 1 + 1 - 1 + 1 - 1 + 1 + 1 - 1 - 1 + 1 - 1 - 1
+ 1 ] H 4 ' = [ - 1 - 1 - 1 - 1 + 1 - 1 + 1 - 1 + 1 + 1 - 1 - 1 + 1
- 1 - 1 + 1 ] ##EQU00008## Max ( ColSum [ H 4 ] ) = 4 Max ( ColSum
[ H 4 ' ] ) = 2 ##EQU00008.2##
[0151] FIG. 13 shows a graph 1300 of the number of sequences used
versus lowest maximum absolute value column sum 1302. Finding the
matrices with lowest maximum absolute value of column sums keeps
the absolute value of the column sum (and therefore the amplitude
at the receiver) from growing linearly or faster than linearly, and
instead grows only at about the square root of the number of
sequences used.
[0152] Turning ahead to FIG. 17, that Figure shows an example of a
touch panel grid 1700 scanned using the H'4 matrix. In this
example, the touch panel grid 1700 includes eight rows 1702 and
nine columns 1704. The pixels in each of the nine columns that are
energized at the same time are labeled A, B, C, and D. As
illustrated in FIG. 17 the OMS logic 1210 may drive four rows (all
the way across the nine rows) simultaneously on the first scan with
-1, +1, +1, +1 signals; then the same four rows on the second scan
with -1, -1, +1, -1 signals; then the same four rows on the third
scan with -1, +1, -1, -1 signals; then the same four rows on the
fourth scan with -1, -1, -1, +1 signals. After taking the receiver
measurements after each scan, the OMS logic 1210 processes the four
receiver measurements for each of the nine columns to obtain the
four individual pixel values A, B, C, and D in each of the nine
columns by solving for four variables using the four orthogonal
measurements -A+B+C+D; -A-B+C-D; -A+B-C-D; -A-B-C+D.
[0153] An example set of matrices that the OMS logic 1210 may use
is given below (transposed with respect to the examples noted
above), along with a table showing the maximum row sum. Note that
the Hadamard matrices are generally of size 4k, (e.g., 8), but that
any number of rows (e.g., 7) may be scanned. In the case where
fewer rows are scanned than the matrix supports, the OMS logic 1210
may determine which rows or columns from the Hadamard matrices to
apply, giving consideration to minimizing the amplitude of the
received signal. Examples are given below for the rows to choose
for non integer multiples of 4 rows (e.g., scanning 6 rows with an
8.times.8 matrix, use the first six rows of the 8.times.8 matrix
noted below). As another specific example, scanning a touch panel
with 15 rows, the OMS logic 1210 may use a 4.times.4 matrix to scan
the first, second, and third sets of four rows, then obtain four
measurements again over the last three rows using the first three
rows of the 4.times.4 matrix to scan the remaining three rows
(thereby taking four measurements to solve for three variables, but
gaining SNR though an extra measurement is made).
[0154] As noted above, the OMS logic 1210 may use Hadamard
sequences to modulate the polarity of the transmitters during
individual step scans. The format for a Hadamard matrix is:
H NumSteps , NumRows = [ H 0 , 0 H 0 , NumRows - 1 H NumSteps - 1 ,
0 H NumSteps - 1 , NumRows - 1 ] ##EQU00009##
[0155] The superscript of H (if shown) is used to depict the
dimensions of the Hadamard matrix. Using this terminology, each
column in the matrix H represents a single Hadamard sequence.
Properties of Hadamard matrices, include:
[0156] All entries in the H matrix are either +1 or -1.
[0157] All columns are orthogonal to every other column.
(Similarly, all rows are orthogonal to every other row.).
[0158] Valid values of NumSteps is 1, 2, or any integer multiple of
4.
[0159] NumRows.ltoreq.NumSteps.
[0160] Given this notation, HNumSteps,NumRows(j) is the jth column
vector of HNumSteps,NumRows.
[0161] Below is a list of Hadamard matrices that the OMS logic 1210
may support. The matrices may be pre-stored in the memory 1206. In
the matrices below, only the polarity of the entries is shown;
therefore `+` indicates a value of +1 and `-` indicates a value of
-1.
[0162] As will be detailed in the matrices below, given a matrix
H.sup.N,N, where N is a multiple of 4, one can construct
H.sup.N,N-1, H.sup.N,N-2, H.sup.N,N-3 by choosing the first N-1,
N-2, or N-3 columns of the full N-by-N matrix. Therefore, only
H.sup.N,N need be stored on-chip (e.g., in the memory 1206).
[0163] The table below for each H.sup.N,k matrix shows the maximum
of the absolute value of the row-sums of the matrix. The control
circuitry 104 may use the maximum sum parameter for dynamic range
scaling within the analog front end to the touch panel 1212.
H 1 , 1 and H 2 , : H 1 , 1 = [ + ] H 2 , 2 = [ + + + - ] H 2 , 1 =
[ + + ] a ) H 4 , 4 H 4 , 4 = [ + - + + + + + - + - - - + + - + ] H
4 , 3 = [ H 4 , 4 ( 1 : 3 ) ] b ) H 8 , 8 H 8 , 8 = [ - + + + + + -
+ - - + - + - - - - + - - + + + - - - - + + - + + - + + + - - + - -
- + - - + + + - + - - - - - + - - - + - + - - ] H 8 , 7 = [ H 8 , 8
( 1 : 7 ) ] H 8 , 6 = [ H 8 , 8 ( 1 : 6 ) ] H 8 , 5 = [ H 8 , 8 ( 1
: 5 ) ] c ) H 12 , 12 H 12 , 12 = [ - + - + - - - - + - + - - - + +
+ + + - + - - - - + + - - + + + + - + + - - - - + - + + - - + - - +
+ + + + - + - + + - - + - - - + + - - + - - - + - + + - + + + + - +
- - - + - + - + - - - + - - + + - - + - - + + + - - + - - - - + + +
- - - + + - + - - - - - - + - - - - + + - - + + + + ] H 12 , 11 = [
H 12 , 12 ( 1 : 11 ) ] H 12 , 10 = [ H 12 , 12 ( 1 : 10 ) ] H 12 ,
9 = [ H 12 , 12 ( 1 : 9 ) ] d ) H 16 , 16 ##EQU00010## H 16 , 16 =
[ - + - + - - + + + + + + + - - + - - - - - + + - + - + - + + - - -
+ + - - - - - + + - - + - + - - - + + - + - + + - - + + + + + - + -
+ + + - - + + + + - + + - - - - - + - - + + - + - - - + + - + + - +
+ + + + + - - - + - + - - + + + - + - + - - + - - - - - + - + - - +
+ - - - - - + + - - - - - - + + - - + - + - - + + - + + - - - - - -
- + + - + - + - - + + - + - + - + + - - - - - - + - + + + - - - - -
- + - - + - - - - + - - + - + - + + + - - - + + - + + + + - - + + +
- + - - - + + + - + - - + + - + + + + ] H 16 , 15 = [ H 16 , 16 ( 1
: 15 ) ] H 16 , 14 = [ H 16 , 16 ( 1 : 14 ) ] H 16 , 13 = [ H 16 ,
16 ( 1 : 13 ) ] ##EQU00010.2##
[0164] The matrices listed above have column sums as follows:
TABLE-US-00005 H.sup.1,1 1 H.sup.2,2 2 H.sup.4,3 3 H.sup.4,4 2
H.sup.8,5 3 H.sup.8,6 4 H.sup.8,7 5 H.sup.8,8 4 H.sup.12,9 3
H.sup.12,10 4 H.sup.12,11 5 H.sup.12,12 4 H.sup.16.13 5 H.sup.16,14
6 H.sup.16,15 5 H.sup.16,16 4
[0165] Note also that the OMS logic 1210 may use complex Hadamard
matrices that can be constructed for any N, as shown in the example
below 6.times.6 Fourier matrix:
H 6 = [ 1 1 1 1 1 1 1 - 1 j - j - j j 1 j - 1 j - j - j 1 - j j - 1
j - j 1 - j - j j - 1 j 1 j - 1 - 1 j - 1 ] ##EQU00011##
[0166] In that regard, the touch panel interface 1218 may include
two digital to analog converters that generate the inphase and
quadrature (I/Q) signals. Thus, the OMS logic 1218 may effective
drive the touch panel 1212 with a complex H-matrix with +1/-1/j/-j
entries, and this can extend to matrices of size 2k. The I/Q
signals can be sinewaves, for example. Column sums can be further
reduced using I/Q signals as shown in the table below:
TABLE-US-00006 Optimal sum with Optimal sum with N Complex H real H
4 2 2 6 2.83 4 8 2.83 4 10 3.16 4 12 4 4
[0167] Furthermore, the matrices may have zero-valued entries
without adding any hardware complexity, though there may be a
slight SNR penalty from lowered signal energy. The example below
shows an example of a 6.times.6 matrix including zeros.
H 6 = [ - 1 - 1 - 1 - 1 - 1 0 1 - 1 1 - 1 0 1 1 1 - 1 0 - 1 1 1 - 1
0 1 - 1 - 1 1 0 - 1 - 1 1 - 1 0 1 1 - 1 - 1 - 1 ] ##EQU00012##
[0168] FIG. 14 shows an example of the logic 1400 that the OMS
logic 1210 may implement. The OMS logic 1210 may drive a first and
a second row in the touch panel simultaneously, a first time
(1402). The OMS logic 1210 may obtain a first signal measurement
along a column intersecting the first and second rows (1404). The
OMS logic 1210 may then energize the first and the second row in
the touch panel simultaneously, a second time (1406) and obtain a
second signal measurement along the column (1408). The OMS logic
1210 may then determine a first pixel value and a second pixel
value along the column from the first signal measurement and the
second signal measurement (1410). The logic 1400 may be extended to
cover any number of simultaneously scanned rows (e.g., 4 or 8 rows
at a time) using the matrices described above.
[0169] The memory 1206 may also store asymmetric scanning (AS)
logic 1220 that operates as directed or configured by the AS
parameters 1222. The AS logic 1220 may be part of or called by the
OMS logic 1210, as examples, or may be a separate program or set of
firmware instructions executed by the processor 1204. The AS logic
1220 helps address the fact that capacitive touch panels may have
many input/output (I/O) lines to be scanned across the full touch
panel, but that, in many cases, only a subset of the entire touch
panel has "interesting" activity present within it. Accordingly,
the AS logic 1220 may avoid the resource cost of scanning the
entire touch panel 1212, and thereby save power and increase
responsiveness and effective frame rate. Furthermore, touch
activity from a small object (such as a stylus or small finger)
would have reduced signal strength, and the AS logic 1220 may
beneficially allocate additional scan time to that region to better
read and facilitate better interpretation of the low signal
strength signals.
[0170] In that regard, the AS logic 1220 may recognize the regions
in which certain activity of interest is occurring on the touch
panel 1212, and adaptively reconfigure the scanning to spend more
or less time in such regions. As a result, the AS logic 1220 helps
provide high SNR and high effective frame rate at low power because
the AS logic 1220 may scan just a subset of the full touch panel
1212 to focus on activity of interest.
[0171] The AS logic 1220 may be configured (e.g., through the AS
parameters 1222) to determine that certain categories of activity
on the touch panel 1212 warrant focused attention through
asymmetric scanning (e.g., increased SNR on demand for specific
areas). In one embodiment, any interaction (e.g., by touch or
hover) qualifies as a category for focused attention, while areas
that are not presently touched qualify as a category for which no
focused attention is needed. However, the AS logic 1220 may still
scan those areas to determine when the currently untouched areas
become touched. Additionally or alternatively, areas on the touch
panel 1212 with low SNR (e.g., the signal is low or the noise is
high) may qualify as focused attention category areas. Thus, for
example, the AS logic 1220 may attempt to compensate for high noise
in certain areas of the touch panel 1212 by giving them focused
attention. As another example, the AS logic 1220 may attempt to
better determine activity for low signal areas of the touch panel
1212, such as might result from a `hover touch` (e.g., an
interaction with the touch panel 1212 without physical contact on
the touch panel 1212) or the use of a fine tip stylus. In other
words a touch on a capacitive touch panel may be a matter of signal
strength and proximity rather than a matter of physical deformation
or actual contact with the touch panel. A further benefit of the
asymmetric scan technique is that areas of the panel that have no
interesting activity may not need to be scanned as frequently, as
long, or with as much energy, leading to frame rate improvements,
and more generally to the flexibility to balance performance,
accuracy, frame rate, and power consumption for the overall panel
scanning task.
[0172] In one implementation, the AS logic 1220 supports asymmetric
panel scanning by scanning any region of interest (e.g., particular
rows on the touch panel 1212) more frequently than other rows. Note
that although an entire row at a time may be driven, if only
particular pixels in the row are of interest (e.g., are included in
the region of interest), then the control circuitry 104 may power
down the receivers for the columns in which the pixels exist to
save power and computational resources to determine pixel values in
columns that are not of interest. The achievable SNR of a pixel in
a given row is directly proportional to the number of step scans
over which its row is stimulated. By default, this value (stored,
e.g., as an AS parameter 1222) is equal to "Nmulti," but under
certain scenarios the AS logic 1220 may determine that additional
SNR is warranted for a specific region(s) of the panel meeting
certain criteria such as those identified above.
[0173] Accordingly, the AS logic 1220 may determine that certain
rows in the touch panel 1212 will be stimulated with additional
scan steps beyond the number given by the Nmulti parameter for the
entire frame scan. The number of scan steps for the rows forms an
asymmetric scan map 1224 for the touch panel 1212. Furthermore, the
AS logic 1220 may follow selected rules, any of which may be made
mandatory or optional, and which may be represented in the AS
parameters 1222 (or in other ways) when determining the asymmetric
scan map 1224. Example rules include:
[0174] Rule 1) The AS logic 1220 should perform asymmetric scan on
a row-by-row basis. In other words, all pixels in a given row shall
be stimulated in an identical fashion.
[0175] Rule 2) The number of scan steps that any given row can be
stimulated (given, for example by the "Nscan" parameter in the AS
parameters 1222) may be equal to one of the following values: {0,
1, 2, 4, 8, 12, 16}.
[0176] Rule 3 (optional) The number of rows simultaneously
stimulated (i.e., Nmulti) may be equal to an integer multiple of
the number of step scans that those rows should be stimulated
(i.e., as given by the Nscan parameter among the AS parameters
1222). In the asymmetric scan maps 1224, this manifests as the
constraint that there be an integer multiple of Nscan adjacent rows
with the same value of Nscan.
[0177] Some examples asymmetric scan maps 1224 and resultant
control flows are described next. In view of Rule 1, the scan map
1224 may be specified as a 1-dimensional vector. For the purposes
of illustration, however, the scan maps 1224 are shown as a 2-d
matrix representing the two dimensional touch panel.
[0178] FIG. 5 shows an example of a scan map 500. The eight rows
are labeled 0-7 and the five columns are labeled 0-4. In each pixel
(e.g., pixel 502), there is a number, Nscan, (e.g., "2") indicating
the number of step scans that the AS logic 1220 will perform in
that pixel. The scan map 500 follows the 3 rules outlined above. It
can be seen by inspection that Rules 1 and 2 are observed.
Furthermore, it can be seen that optional Rule 3 is observed, as
there are 4 adjacent rows that have Nscan=4 (the rows 504) and four
adjacent rows that have Nscan=2 (the rows 506 and 508).
[0179] Based on the example scan map 500, the AS logic 1220 may
implement the following step scan instructions for scanning the
touch panel 1212:
TABLE-US-00007 Hadamard sequence used (see above concerning the OMS
logic 1210 for more details on Step Scan Number Tx Rows Active the
Hadamard matrices) 1 0, 1 H2,2 (1) 2 0, 1 H2,2 (2) 3 2, 3 H2,2 (1)
4 2, 3 H2,2 (2) 5 4, 5, 6, 7 H4,4 (1) 6 4, 5, 6, 7 H4,4 (2) 7 4, 5,
6, 7 H4,4 (3) 8 4, 5, 6, 7 H4,4 (4)
[0180] In the table above, the AS logic 1220, for step scans 1 and
2, performs an Nmulti=2 scan of rows 0 and 1. The AS logic 1220
also, for steps 3 and 4, performs the same scan on rows 3 and 4.
The AS logic 1220, for steps 5 to 8, performs an Nmulti=4 scan of
rows 4 to 7. In other words, the scan map 500 results in the AS
logic 1220 performing an asymmetric scan of the touch panel 1212 in
a very efficient manner. The AS logic 1220 has effectively
partitioned the touch panel 1212 into sub-panels, and uses the OMS
logic 1210 noted above to perform the scanning of each sub-panels
(e.g., each set of rows 504, 506, and 508).
[0181] FIG. 6 shows another example of a scan map 600 that the AS
logic 1220 may implement, although it may be somewhat less
efficient than the scan map 500. The scan map 600 includes row sets
602, 604, 606, and 608.
[0182] In the example shown in FIG. 6, the scan map 600 is modified
slightly compared to the scan map 500. In particular, the Nscan
parameter for the row that is part of row set 606 is reduced from 4
to 2. The scan map 600 follows the Rules 1 and 2 above, but does
not follow the optional Rule 3, as there are five adjacent rows
with Nscan=2 and three adjacent rows with Nscan=4.
[0183] The AS logic 1220 may execute more scan sequence steps for
the scan map 600 than in the example of FIG. 5. One example is:
TABLE-US-00008 Step Scan Number Tx Rows Active Hadamard sequence
used 1 0, 1 H2,2 (1) 2 0, 1 H2,2 (2) 3 2, 3 H2,2 (1) 4 2, 3 H2,2
(2) 5 4 H2,1 (1) 6 4 H2,1 (2) 7 5, 6, 7 H4,3 (1) 8 5, 6, 7 H4,3 (2)
9 5, 6, 7 H4,3 (3) 10 5, 6, 7 H4,3 (4)
[0184] In this example, not following optional Rule 3 resulted in
the AS logic 1220 using a larger number of scan steps (10) than in
the previous example (8).
[0185] Turning ahead to FIG. 15, that FIG. 15 shows an example of
logic 1500 that the AS logic 1220 may implement. The AS logic 1220
retrieves the AS parameters 1222 and scan map 1224, including as
examples, Nmulti, and Nscan (1502). The AS logic 1220 may also
receive an identification or determine one or more regions of
interest on the touch panel 1212 for asymmetric scanning (e.g., to
increase SNR in that region) (1504). The number of step scans per
row may be increased in an attempt to increase the SNR to any
desired level (e.g., to 2 step scans, 4, 8, 12, 16, and so on
according to the orthogonal signal options noted above with respect
to the OMS logic 1210). In some implementations, the SNR increase
is proportional to the number of step scans used. As a result, it
is possible to determine the number of steps scans needed to
achieve a target SNR. The AS logic also determines the orthogonal
signals for driving the touch panel 1212 (1506). For example, the
AS logic 1220 may use the Hadamard matrices identified above to
drive the sets of rows as identified in the scan map. The scan map
may be constructed to provide additional step scans in the regions
of interest on the touch panel 1212. The AS logic 1220 then
executes asymmetric scanning of the touch panel 1212 using the
orthogonal signals according to the scan map and the AS parameters
1222 (1508) to focus on the region(s) of interest.
[0186] In some implementations, the AS logic 1220 collects
asymmetric scan requests from the host or other higher level logic,
while the control circuitry 104 is performing regular, non
asymmetric scans of the touch panel 1212. For each area where
interaction with the touch panel 1212 is detected (e.g., an area
referred to as a "blob" due to decreases in raw capacitance value
(or increases in delta capacitance value, untouched minus touched)
in the heat map across multiple pixels at or near where an
interaction with a finger or stylus (as examples) is occurring),
the AS logic 1220 may determine the SNR (e.g., using noise
estimates from a non active region of the touch panel 1212). The AS
logic 1220 may then, if the SNR is below a predefined target SNR
(e.g., because the signal is too low), request (e.g., initiate)
increased SNR in the area of the blob through asymmetric scanning.
If the SNR then increases past a second predefined threshold, then
the AS logic 1220 may stop the asymmetric scan in the blob region,
e.g., because the SNR is now sufficient to track the interaction
without additional focus through asymmetric scanning.
[0187] The AS logic 1220 (or other logic in the system) may also
predict the blob position and request increased SNR in the
predicted position. For example, the AS logic 1220 may use motion
estimation for the blob over time to estimate velocity and thus the
next predicted position. Between scans of the touch panel 1212, the
blob may be recognized using a shape recognition algorithm, or by
closeness to the last blob position (because blobs do not tend to
move quickly in comparison to the scan frame rate). For example,
the predicted position may be the predicted position in one to two
frames (or some other configurable parameter in the AS parameters
1222) in the future. The predicted position may be identified by
rows, with a configurable parameter number of additional rows above
and/or below the predicted position. Another parameter, a
capacitance threshold, may determine whether any part of the touch
panel area belongs to the blob. The AS logic 1220 may also find the
connected components of touch panel areas above the capacitance
threshold to identify the blob. The SNR of the blob, for
determining when to start or stop asymmetric scanning, may be
determined by the peak (highest delta in capacitance value) pixel
in the blob, squared, and dividing by the noise variance estimate.
The AS logic 1220 may also communicate with other logic in the
system, to inform the other logic about how the asymmetric scanning
was done. For example, the AS logic 1220 may communicate the number
of scan steps per row, the measured capacitance data, noise
estimates, or any other data to other logic in the system. In some
implementations, the system may perform its analysis not on raw
capacitance values, but on delta capacitance: the difference
between capacitance when the touchscreen is not touched, and the
current reading. The current reading (the raw capacitance)
decreases in the presence of a touch.
[0188] FIG. 16 shows blob position prediction using an example of
series 1600 of frames 1602, 1604, 1606 (of 15 columns and 10 rows)
over time in which a blob 1608 exists. The capacitance measurements
(e.g., delta capacitance) of the pixels in the blob are noted in
the frame 1600. The values are for illustration only and not meant
to be representative of actual measurements. The AS logic 1220 has
predicted the velocity vector 1610 (in this example, one pixel to
the right and down) and has predicted a next location 1612 for the
blob. The AS logic 1220 has established buffer rows 1614, 1616,
1618 that are also asymmetrically scanned as a guard on the
estimated position 1612 to protect against unexpected changes in
velocity or next position of the blob 1608. The predicted next
location 1612 and the buffer rows 1614-1618 thus identifying the
rows over which asymmetric scanning may occur in an upcoming frame.
The AS logic 1220 may establish any number of buffer rows or
columns above the predicted location, below the predicted location,
to the left or to the right, or any combination thereof. Column
processing logic may be enabled for the buffer columns, and
disabled for the remaining columns, for example. The number of
buffer rows may increase as the predicted velocity vector
increases.
[0189] One motivation for performing blob position prediction is
system processing delays. In particular, after the AS logic 1220
receives a request for asymmetric scan, the blob of interest may
have moved. Therefore the region of interest may not be exactly the
same region of interest originally specified for asymmetric scan.
Accordingly, when position prediction is enabled (e.g., as set by
an activation parameter in the AS parameters 1222), the AS logic
1220 may take the blob movement into account by predicting where
the blob will be by the time that asymmetric scan starts. In other
words, given a prediction of blob motion (e.g., a velocity vector)
and the time until the scan starts, the AS logic may determine an
adjusted region of interest that is predicted to include the blob
when the asymmetric scan is actually carried out.
[0190] After the asymmetric scan, the results are returned to the
asymmetric scan requesting logic. Per row, the requesting logic
thus knows what the noise estimate is, as it may vary across the
rows as a result of the asymmetric scan. The requesting logic may
then determine the blob shape and location based on the SNR as
noted above. When the SNR is above a threshold, then the requesting
logic may end the asymmetric scan. The requesting logic may also
implement debouncing to prevent reactivation of asymmetric scan
between individual frames. In one implementation, the requesting
logic may wait to have two or three frames (or some other
configurable debouncing parameter number) with strong SNR before it
ends asymmetric scan, or some configurable number of frames with
low SNR before it begins asymmetric scan. When there are multiple
blobs on the same row, the AS logic 1220 may select the number of
step scans needed to try to achieve a desired level of SNR for any
particular blob (e.g., to increase the SNR for the weakest blob to
a desired level).
[0191] The methods, devices, and logic described above may be
implemented in many different ways in many different combinations
of hardware, software or both hardware and software. For example,
all or parts of the system may include circuitry in a controller, a
microprocessor, or an application specific integrated circuit
(ASIC), or may be implemented with discrete logic or components, or
a combination of other types of analog or digital circuitry,
combined on a single integrated circuit or distributed among
multiple integrated circuits. All or part of the logic described
above may be implemented as instructions for execution by a
processor, controller, or other processing device and may be stored
in a tangible or non-transitory machine-readable or
computer-readable medium such as flash memory, random access memory
(RAM) or read only memory (ROM), erasable programmable read only
memory (EPROM) or other machine-readable medium such as a compact
disc read only memory (CDROM), or magnetic or optical disk. Thus, a
product, such as a computer program product, may include a storage
medium and computer readable instructions stored on the medium,
which when executed in an endpoint, computer system, or other
device, cause the device to perform operations according to any of
the description above.
[0192] The processing capability of the system may be distributed
among multiple system components, such as among multiple processors
and memories, optionally including multiple distributed processing
systems. Parameters, databases, and other data structures may be
separately stored and managed, may be incorporated into a single
memory or database, may be logically and physically organized in
many different ways, and may implemented in many ways, including
data structures such as linked lists, hash tables, or implicit
storage mechanisms. Programs may be parts (e.g., subroutines) of a
single program, separate programs, distributed across several
memories and processors, or implemented in many different ways,
such as in a library, such as a shared library (e.g., a dynamic
link library (DLL)). The DLL, for example, may store code that
performs any of the system processing described above. While
various embodiments of the invention have been described, it will
be apparent to those of ordinary skill in the art that many more
embodiments and implementations are possible within the scope of
the invention. Accordingly, the invention is not to be restricted
except in light of the attached claims and their equivalents.
* * * * *