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PULSE
EEWeb.c
Issu
August 16, 20
Sam WurzelOctopart
Electrical Engineering Commun
EEWeb
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TABLE OF C ONTENTS
Sam Wurzel 4CO-FOUNDER AND CEO, OCTOPART INC.Interview with Sam Wurzel, internet industry entrepreneur.
The Righthand Side of the Weave Effect
InequalityBY MIKE STEINBERGER WITH SISOFT
Introduction to Touchscreens 11BY STEVE KOLOKOWSKY AND TREVOR DAVIS WITH CYPRESS
RTZ - Return to Zero Comic 16
How to properly manage differential skew on PC board traces due to local variations in board
material.
Kolokowsky and Davis provide an introduction to touchscreens and the components that make
them possible.
7
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INTERVIEW
Sam WurzelOctopart
Sum up Octopart in one
sentence.
Octopart solves the problem ofelectronic part search on the web
- we provide distributor stock and
pricing information, datasheets,
and advanced search features with
a focus on speed and simplicity.
What is your value
proposition?
Choosing and sourcing parts is
hard and time consuming. Octopart
exists to solve that problem - its the
easiest and fastest way to find parts
online.
Can you tell us about the early
start-up days at Octopart?
The early days were a lot of fun! In
the summer of 2006, Andres and I
were still in physics grad school.
Andres was in Berkeley and I was
in Boulder. Every day wed come
home from the lab and log into a
linux box in my living room to work
on Octopart. At that point we didnt
know anything about databases or
web technologies so most of what
we did was learn.
By early 2007 we had both quit
grad school and I had moved out
to Berkeley to work on Octopart
full time. For a while I was living
on Andres couch and we wouldliterally wake up, work on Octopart
all day and night and then fall
asleep. We had about 5 computers
stacked in Andres room and we
were running the site from his cable
modem connection. I remember the
first time we saw a search come in
that we couldnt directly trace to one
of us or our friends - I think it came
from Turkey. To this day still I dont
know how they found us.
Has the direction or vision of
Octopart changed from your
initial vision of the service?
Not really. From the beginning the
plan was to fix part search and
thats still the goal. The design
of the site and the access model
has not changed either. From the
beginning we wanted a clean
layout and we wanted everyone
to have access to complete
part information without anycumbersome registration process.
One of the frustrations we had with
existing sites was that they were
filled with distracting ads and they
required you to register or pay for
the service.
What has been the biggest
technical challenge in
developing Octopart?
The quantity of data we are dealing
with is very large and it changes
often. There are 15 million parts
in the database and we have to
keep all the pricing and availability
numbers are up to date. We also
have to make sure the data is
accurate which, given the scale of
the data, is challenging.
Also, the search aspect is technically
challenging. Maintaining full textand parametric search capability
over 15 million parts is tough.
Especially when there are many
different types of parts, each with
their own attributes.
Sam Wurzel, Andres Morey, and Harish Agarwal, Co-Founders of Octopart
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INTERVIEW
What has been the toughest
non-technical challenge you
have dealt with or are dealing
with?
Establishing ourselves in theindustry and communicating who
we are and what were trying to do
has been challenging. When we
started Octopart we did not have a
single contact within the electronics
industry. We just kind of jumped
into it. Weve made a lot of progress
on that front but theres still a lot of
work to do.
How do you entice users toOctopart?
We believe that if people find
Octopart useful they will tell their
friends and colleagues about it.
Our entire development process is
based around doing whats best for
users.
How do you differentiate
yourselves from other
competing search engines?
We take the approach of building
a full part database by combining
data from lots of different sources.
This gives us a few advantages over
other part search engines:
1. You get a full view of a single
part. You can see all of the
distributors of that part, all of
the images, all of the datasheetsand a complete set of part
attributes.
2. You can search by category
or do parametric search if
you dont know the exact part
number youre looking for.
3. We provide an API, http://
octopart.com/api which allows
anyone to develop applications
which leverage all of this part
information.
Octopart solvesthe problem ofelectronic partsearch on the
web - we providedistributor stock
and pricinginformation,
datasheets, andadvanced searchfeatures with afocus on speed
and simplicity.
Do you plan on integrating
social media into Octopart?
Sure, we already allow users to
leave comments about parts and
were thinking of other ways to
integrate social media in ways that
make sense. Getting help from
other engineers is one of the best ways to solve problems so were
thinking about ways to facilitate that
on Octopart.
Where do you see Octopart in
the next fve years?
I see Octopart as the repository for
all part data on the web. By making
that data available via API well open
the door to lots of new applications,
most of which we havent even
anticipated.
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The RighthandSide of theWeave Effect
Inequality
Mike SteinbergerLead Architect
Serial Channel Products
What Inequality?
Over the last several years, a lot has been written about
the introduction of differential skew into PC board tracesdue to local variations in the dielectric constant of the
board material. (For a small sampling, see [1], [2], [3],
[4]) These local variations are due to variations in the
percentage of glass cloth in the laminate, and can have
a measurable effect when the traces in a differential pair
run parallel to the fibers in the glass cloth. Hence the
term weave effect.
While several papers derive a maximum trace length as
a function of data rate, there is very little discussion of
the performance requirements those calculations were
based on, or the way those performance requirements
were derived.
As skilled and disciplined engineers, we write a
tolerance equation such as:
costs. Choosing that righthand value is an important part
of our job, so we should consider carefully how we makethat choice.
This is the inequality referred to in the title of this article,
and the purpose of this article is to offer some insight
into choosing a value for tmax, the righthand side of this
inequality.
Skew Modeling
For this study, the transmission model is shown in Figure
1.
For a 5 Gb/s transmission path, a 30 length of differentialFR4 trace was broken into ten segments, with the
differential skew inserted uniformly between these
segments. This model was chosen because it models
the distributed nature of the skew due to weave effect.
The conclusions of this study are insensitive with respect
to modeling approach, however. Lumping the differential
skew into a single transmission line at the end of the
transmission path produces essentially the same link
performance.
t t< maxskew t t tskew true complement / -and we choose a value for the righthand side that will
result in acceptable product yield while minimizing
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TECHNICAL ARTICLE
Figure 2 shows the eye height for the optimized solution
as a function of differential skew for seven different
combinations of equalizers. The transmit de-emphasis
consisted of four taps, including one pre-cursor tap, the
linear equalizer varied a single zero while keeping the
peak gain constant, and the DFE had five taps.
These results were generated using a performance
criterion which is a combination of eye width and eye
height; so one should not attempt to compare and contrast
the results from different combinations of equalizers
based solely on eye height. Also, the results will vary with
different linear equalizer designs or different numbers of
transmit or receive taps. The data in Figure 2 is valuable
in that it shows the effect of differential skew on a number
of different choices of equalizer. In particular, its clear
that when the differential skew becomes greater thana half a bit time (0.5 UI), bad things start to happen
regardless of which combination of equalizers is chosen.
Below a skew of 0.5UI, some combinations of equalizers
appear to be better able to compensate for the effects
of differential skew than others. Thus, the amount of
differential skew that can be tolerated is a function of
the equalization chosen as well as the performance
requirements of the channel.
The Case of the Creeping Suck-out
One can understand why 0.5UI differential skew is such afundamental limit by examining the unequalized transfer
functions. Figure 3 compares the transfer function for
0.44UI of differential skew to the transfer function for
0.61UI of differential skew. While both transfer functions
have a pronounced dip (i.e., suck-out) in them, the dip
for 0.44UI is centered more or less around the data rate
while the dip for 0.61UI is clearly below the data rate.
Effect of Equalization
In SiSofts Quantum Channel Designer, we have a
feature which optimizes the combined transmit and
receive equalization during the statistical analysis phase
of the simulation. The transmit de-emphasis, receiver
linear equalizer, and DFE can each be independently
included or excluded from the equalization solution,
resulting in a total of eight possible combinations of
equalizers. For a given channel configuration (e.g.,
differential skew) and combination of equalizers,the optimal performance is calculated in a couple of
seconds as part of the statistical analysis. This makes it
convenient to evaluate the optimal performance under a
wide range of conditions.
Differential Skew (UI)
EyeHe
ight(V)
0
0.25
0.2
0.15
0.1
0.05
0
0.2
PK+DFE
0.4 0.6 0.8 1.0
TX+PK+DFE
PK
TX+PK
DFE
TX+DFE
TX
Figure 2: Eye height vs. differential skew vs. optimized equalizer
configuration
Hertz (GHz)
DB
0
0.0
-10.0-20.0
-30.0
-40.0
-50.0
-60.0
-70.01.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
Transfer FunctionUnequalized BLUE: 0.44UI skew RED: 0.61UI skew
Figure 3: Unequalized transfer function for 0.44UI and 0.61UI
differential skew
A I
A I
TX1sisoft_serdes
SiSoft_TX
W1 W2 W3 W4
W5 W6 W7 W8
W9 W10 W11 W12
W13 W14 W15 W16
W17 W18 W19 W20
RX1SiSoft_RX
Figure 1: Circuit model for differential skew
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TECHNICAL ARTICLE
As the differential skew is increased, the location of the
dip moves even lower in frequency. The approximate
equation is:
criterion. While these choices may be appropriate for
some designs, many designs will come in at a much
lower cost if one explicitly calculates the performance
margin and accepts a slightly reduced yield due to
extremes in differential skew.
References
[1] Jeff Loyer, Richard Kunze, and Xiaoning Ye, Fiber
Weave Effect: Practical Impact Analysis and Mitigation
Strategies, paper 6-TA2, DesignCon2007.
[2] Scott McMorrow and Chris Heard, The Impact of
PCB Laminate Weave on the Electrical Performance of
Differential Signaling at Multi-Gigabit Data Rates, paper
6-TA3, DesignCon2005.
[3] Christopher White, Andrew Becker and Jim Fitzke,
Skew Impact Estimation on High Speed Serial
Channels Using Mathematical Analysis and Accurate
Lab Measurements, paper 7-WA2, DesignCon2010.
[4] Russell Dudek, John Kuhn and Patricia Goldman,
Opening Eyes on Fiber Weave and CAF, Printed Circuit
Design & Fab, http://pcdandf.com/cms/component/
content/article/ 220-2009-issues/6025-opening-eyes-on-
fiber-weave-and-caf, 01 April 2009.
About the Author
Michael Steinberger, PhD, has over 30 years experience
in the design and analysis of very high-speed electronic
circuits. Dr. Steinberger began his career at Hughes
Aircraft, designing microwave circuits. He then moved
to Bell Labs, where he designed microwave systems that
helped AT&T move from analog to digital long-distance
transmission. He was instrumental in the development of
high-speed digital backplanes used throughout Lucents
transmission product line. Prior to joining SiSoft, Dr.
Steinberger led a group of over 20 design engineers
at Cray, Inc. responsible for SerDes design, high-
speed channel analysis, PCB design, and custom RAM
design.
Figure 4 shows how well the various combinations of
equalizer cope with the two values of differential skew.
From Figure 4, its clear that while most of the
combinations of equalizer can cope fairly readily with a
dip at or above the data rate, they have a much harder
time coping with a dip thats below the data rate. They
DB
0
0.0
-10.0
-20.0
-30.0
-40.0
-50.0
-60.0
-70.0
-80.0
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
Transfer FunctionUnequalized BLUE: 0.44UI skew RED: 0.61UI skew
Figure 4: Equalized transfer functions for 0.44UI and 0.61UI of
differential skew
have at best a limited ability to increase the gain in thefrequency range where the dip occurs.
Choosing tmax
From Figure 2, its clear that the choice of the maximum
differential skew is going to be a function of the
equalization solution as well as the performance
requirements for the channel. The allowable differential
skew can go as high as 0.5UI, but accepting any value
higher than that is a bad idea.
A couple of concluding thoughts:
A linear receive equalizer that has its gain peak above
half the data rate will tend to be a little more effective at
equalizing differential skew because it will be able to
increase the gain in the frequency range where a dip
could occur.
A lot of the published results only state acceptable
trace lengths for a very pessimistic choice of material
properties and a conservative choice of performance
f t2
1
dip skew
.
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INTRODUCTION TO
TechnologyOptions in aTouch-based World
TOUCHSCREENS
Ray SalemiVerification Consultant
Steve KolokowskySr Member of the Technical Staff
Trevor DavisDirector of Marketing & Applications
Every local mobile phone retailer, every electronics
store, and most every consumer electronics
company in the world is selling touchscreensand all of
them claim to be experts and to have the most advanced
or desirable technology. But do they? In fact, how is a
designer, engineer, or consumer even able to tell which
technology is right for their product development? What
are the key technologies even available in the market
and what is the benefit of one over another? How will
one technology last versus another in everyday use?
While it is true that there are many different touchscreen
technologies in the world, it is also true that the market
and application demands of different product segments
do demand different technology choices. Understanding
the benefits and limitations of current technologies will
help the consumer to make the most appropriate choice
in technology.
Touchscreen Components Revealed
While there are many different types of touchscreen
products, there is a relatively consistent set of
components that make a touchscreen product possible.
Whether the developer is making a new touchscreen-
enabled phone, Global Positioning System (GPS), touch-
enabled medical device, or virtually any other touch
product, there are five basic components to the system:
Figure 1: Touchscreen System Components
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TECHNICAL ARTICLE
Coverlens or Bezel
The coverlens bezel is the outward facing component
of the product. This is how the consumer interacts with
the product. In some products, this coverlens could
simply be a protective cover to prevent scratching and
damage, or it can actually be part of the touch sensing
system. In other technology systems it can actually hide
small cameras or infrared sensors that detect a persons
touch. Either way, much of the consumers perception
of the products look and feel will be determined by the
materials chosen for the coverlens.
Touch Controller
Once contact is initiated with a product, the electronics
in the system are activated for action. In todays
systems, the touch-controller is a small microcontroller-
based chip like the Cypress TrueTouch that is placedbetween the touch sensor and systems host controller.
This chip can either be located on a controller board
inside the system or it can be located on a flexible
printed circuit (FPC) affixed to the glass touch sensor.
This touch-controller takes information from the touch
sensor and translates it into information the systems
host controller can understand.
Touch Sensor
A touchscreen sensor is a clear glass or acrylic panel
with a touch responsive surface. This sensor is placedover a graphic display so that the touch area of the
panel covers the viewable area of the screen. There are
many different touch sensor technologies on the market
today, each using a different method to detect touch
input. Basic operation, however, remains the same as
these technologies all use an electrical current running
through the panel that, when touched, causes a voltage
or signal change. This voltage change is sensed by the
touch controller to determine the location of the touch on
the screen.
Display
Most touchscreen systems work on top of an Liquid
Crystal Display (LCD) or the newer Active Matrix
Organic Light Emitting Diode technology (AMOLED).
Displays for a touch-enabled product should be chosen
for the same reasons they would in a traditional system:
resolution, clarity, refresh speed, cost. One major
consideration for a touchscreen, however, is the level of
electrical emission. Because the technology in the touch
sensor is based on small electrical changes when the
panel is touched, an LCD that emits a lot of electricalnoise can be difficult to design around.
System Software
Without system software understanding of how to
interpret a touch signal, touch system hardware is
useless. The system software allows the touchscreen and
system controller to work together. Apples iOS exposed
gestures to everyone, providing zoom, swipe, and
rotate in many applications. Windows 7 has integrated
multi touch gestures into the core operating system and
Internet Explorer in several interesting ways. The shake
gesture minimizes all windows other than the active one.
Two finger tap is a quick zoom to allow selection of small
icons. Android provides developers with a very flexible
framework to define their own gestures, which will drive
more gesture innovation in the mobile device market.
Figure 2: Four Primary Touchscreen Technologies
1. Surface Capacitance 2. Infrared Touchscreen 3. Resistive Touch 4. Capacitive Touch
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TECHNICAL ARTICLE
Primary Touchscreen Technology Options
Resistive Touchscreens
Resistive touchscreens remain the most common
touchscreen technology. They are used in high-traffic
applications and they are immune to water or other
debris on the screen. Resistive touch screens are
usually the lowest cost touchscreen solution. Because
they react to pressure, they can be activated by a finger,
gloved hand, stylus or other object like a credit card
or fingernail. The Nintendo DS uses a 4-wire resistive
touchscreen that can be use with a plastic stylus.
Surface Capacitive Touchsreens
Surface Capacitive technology works with a glass or
plastic cover lens up to several millimeters thick. This
provides a clearer and more durable display thanthe flexible plastic cover typically used in a resistive
touchscreen. In a surface capacitive display, sensors
in the four corners of the display detect capacitance
changes due to touch. These touchscreens can only
be activated by a finger or other conductive object.
Touchscreen slot machines and poker machines are
two main applications for these screens. The main
disadvantage of surface cap screens is accuracy.
Typical position error is 1-1.5% of screen size, which is
plenty for selecting a card or starting a slot machine.
Infrared or Camera-based Touchscreen
IR touchscreen technology does not require any
changes to the display stackup, since it works in front
of the screen. This makes it ideal for vandal-resistant
applications. While surface capacitance systems
observe disruption in electrical signals, IR touchscreens
observe disruption in IR signals that cross the plane of
the display. IR systems are used almost exclusively on
kiosk or large form factor displays because of their bulky
profile and large power requirement. A touch signal
can be detected from almost any object that disrupts
the IR beam which makes IR technology more ideal forglove or passive stylus use (though accuracy for stylus
is quite low). The Microsoft Surface interface uses IR
cameras to detect multiple touches without projecting
IR beams above the touch surface.
Projected Capacitive Touchscreens
Projected capacitive touchscreens are the latest entry to
the market. This technology also offers superior optical
clarity, but it has significant advantages over surface
capacitive screens. Projected capacitive sensors require
no positional calibration and provide much higher
positional accuracy. Projected capacitive touchscreens
are also very exciting because they can detect multiple
touches simultaneously. Apples iPod Touch and iPhone
use this type of touchscreen.
Touchscreen Technology Revealed
The most widely used touchscreen technologies in
consumer electronics today are resistive and capacitive.
As these are the two most common, we will focus further
technology discussion here. In fact, most people have
interacted with resistive touchscreens while using an
ATM at the bank, or at the credit card checkout in most
stores. Projective capacitance touchscreens, on the
other hand, are most known for their use in the mobilehandset application. Both resistive and capacitive
technologies have a strong electrical component, both
use ITO (Indium-Tin-Oxide, a clear conductor) as their
primary technology component, and both are used in
high volume all over the world.
A resistive touchscreen consists of a flexible top layer,
then a layer of ITO (Indium-Tin-Oxide), an air gap, and
then another layer of ITO. The panel typically has either
four, five, or eight wires attached to the ITO layers: one
on the left and right sides of the X layer, and one on the
top and bottom sides of the Y layer
A touch is detected when the flexible top layer is pressed
down to contact the lower layer. The location of a touch
is measured in two steps: First, the X right is driven to
a known voltage, and the X left is driven to ground and
the voltage is read from a Y sensor. This provides the X
coordinate. This process is repeated for the other axis to
determine the exact finger position.
Resistive touchscreens also come in 5-wire, and 8-wire
versions. The 5-wire version replaces the top ITO layer
with a low-resistance conductive layer that provides
better durability. The 8-wire panel was developed to
enable higher resolution by enabling better calibration of
the panels characteristics.
Some of the benefits of resistive technology are that they
can easily be used for larger size displays (10 inches +)
and can be used to detect the touch of any non-conductive
pressure. This makes resistive touchscreens the current
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TECHNICAL ARTICLE
default for products that need the use of stylus pen input.
There are, however, several drawbacks to resistive
technology. The flexible top layer scratches easily, has
only 75-80 percent clarity, and the resistive touchscreen
measurement process has several error sources. If
the ITO layers are not uniform, the resistance will not
vary linearly across the sensor. Measuring voltage
to 10 or 12-bit precision is required, which is difficult
in many environments. Many of the existing resistive
touchscreens require periodic calibration to realign the
touch points with the underlying LCD image.
Conversely, projected capacitive touchscreens have no
moving parts. The only thing between the LCD and the
user is ITO and glass, which have nearly 100 percent
optical clarity. The projected capacitance sensing
hardware consists of a glass or acrylic top layer,
Figure 3: Resistive Sensing Circuit
Figure 4: Capacitive Sensing Circuit
followed by an array of X and Y sensors that are either
deposited or etched in an ITO layer in either a single
layer (lowest cost) or in separate layers depending on
the manufacturers process. The panel will have a wire
for each X and Y sensor, so a 10 x 14 panel will have 24
connections, while a 12 x 20 panel will have 32 sensor
connections.
As a finger or other conductive object approaches the
screen, it creates a capacitor between the sensors and thefinger. This capacitor is small relative to the others in the
system (about .5pF out of 20pF), but it is measurable by
several techniques that typically involve rapidly charging
an in-circuit capacitor and measuring the discharge time
through a resistor. Two sensing types are commonly
used, mutual capacitive and self-capacitive sensing. Self
cap senses the increase in self-capacitance of a sensor
as a finger touches the screen. Mutual cap measures
the decrease in capacitive coupling between a transmit
sensor and a receive sensor as shown in Figure 3 and 4.
A projected capacitive sensor array is designed so that
a finger will interact with more than one X sensor and
more than one Y sensor at a time. This enables software
to accurately determine finger position to a very fine
degree through interpolation. Since projected capacitive
panels have multiple sensors, they can detect multiple
fingers simultaneously, which is impossible with other
technologies. This enables exciting new applications
based on multiple finger presses.
Because capacitive sensing based solutions do not
sense through pressure, they are much more durablethan resistive technology. They also can be based on a
much harder cover lens material which is very pleasing
to the touch which gives the user an enjoyable touch
experience. And because there are fewer layers of
material than a resistive panel, and the etched ITO of a
capacitive sensor is colorless, capacitive touch solutions
have much better transmissivity and have a much sharper
visual image.
So, despite the fact that touch sensors are shipped in
many different product categories and product types,
their basic construction is quite similar from product toproduct. The underlying technology, however, can be quite
different and can deliver very different user experiences.
Depending on the end product requirement, a designer
may choose to use one technology choice over another. If
large screen form factor is required, perhaps IR sensing
is ideal. For low cost, stylus Point of Sale terminals where
a pen input is critical, a resistive screen might be best.
For a wall mounted customer information kiosk, surface
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TECHNICAL ARTICLE
capacitive touchscreens might be a good choice. And
for ultra portable handsets or mobile devices with
multi-touch input, projected capacitance might be
the best option. In the end, it is the user experience
and product requirements that dictate the technology
choice for touchand now you know your options.
About the Authors
Steve Kolokowsky is currently working on
touchscreen solutions for Cypress Semiconductor. He
has over 20 years of experience creating embedded
solutions and software. Steve has been involved with
Cypress TrueTouch solutions and USB solutions
including Cypress best-selling USB mass storage
chip, the AT2LP. Prior to Cypress, Steve worked for
Cirrus Logic creating DSP tools and development kits.
Steve has written over 40 technical articles that have
been published in at least six languages. He has over 10
patents issued and several more applications pending.
Trevor Davis is currently the Director of Marketing
& Applications for Cypresss Consumer andComputation Division (CCD) focused on User
Interface in consumer products. Trevor received his
undergraduate degree from the United States Air Force
Academy and also holds his Masters in Business
Administration. Trevor has worked in high technology
positions for the military, nonprofit, and commercial
sectors for the past 15 years and is fascinated by the
speed of innovation in User Interface products.
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