EEWeb Pulse - Volume 7

PULSE EEWeb.comIssue7

August 16, 2011

Sam WurzelOctopart

Electrical Engineering Community

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TABLE O

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TSTABLE OF CONTENTS

Sam Wurzel 4CO-FOUNDER AND CEO, OCTOPART INC.Interview with Sam Wurzel, internet industry entrepreneur.

The Righthand Side of the Weave Effect Inequality BY 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


INTERVIEWFEA

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Sam WurzelOctopart

Sum up Octopart in one sentence.

Octopart solves the problem of electronic 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 - it’s 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 we’d come home from the lab and log into a linux box in my living room to work on Octopart. At that point we didn’t 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 would literally 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 couldn’t directly trace to one

of us or our friends - I think it came from Turkey. To this day still I don’t 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 that’s 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 any cumbersome 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 text and 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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What has been the toughest non-technical challenge you have dealt with or are dealing with?

Establishing ourselves in the industry and communicating who we are and what we’re 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. We’ve made a lot of progress on that front but there’s still a lot of work to do.

How do you entice users to Octopart?

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 what’s 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 datasheets and a complete set of part attributes.

2. You can search by category or do parametric search if you don’t know the exact part number you’re 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 solves the problem of electronic part search on the

web - we provide distributor stock

and pricing information,

datasheets, and advanced search features with a focus on speed and simplicity.

Do you plan on integrating social media into Octopart? Sure, we already allow users to leave comments about parts and we’re 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 we’re thinking about ways to facilitate that on Octopart. Where do you see Octopart in the next five years?

I see Octopart as the repository for

all part data on the web. By making that data available via API we’ll open the door to lots of new applications, most of which we haven’t even anticipated. ■

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The RighthandSide of the

Weave EffectInequality

Mike SteinbergerLead ArchitectSerial Channel Products

What Inequality?Over the last several years, a lot has been written about the introduction of differential skew into PC board traces due 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 make that 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 ModelingFor this study, the transmission model is shown in Figure 1.

For a 5 Gb/s transmission path, a 30” length of differential FR4 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 t t t< maxskew skew true complement/ -t t t t t< maxskew skew true complement/ -and we choose a value for the righthand side that will result in acceptable product yield while minimizing


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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, it’s clear that when the differential skew becomes greater than a 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-outOne can understand why 0.5UI differential skew is such a fundamental 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 EqualizationIn SiSoft’s 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)

Eye H

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Figure 2: Eye height vs. differential skew vs. optimized equalizer configuration

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Figure 3: Unequalized transfer function for 0.44UI and 0.61UI differential skew

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Figure 1: Circuit model for differential skew


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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 AuthorMichael 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 Lucent’s 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, it’s 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 that’s below the data rate. They

Hertz (GHz)

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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 the frequency range where the dip occurs.

Choosing tmaxFrom Figure 2, it’s 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

ft21

dipskew

.

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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 touchscreens—and 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 RevealedWhile 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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Coverlens or BezelThe 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 person’s touch. Either way, much of the consumer’s perception of the product’s look and feel will be determined by the materials chosen for the coverlens.

Touch ControllerOnce contact is initiated with a product, the electronics in the system are activated for action. In today’s systems, the touch-controller is a small microcontroller-based chip like the Cypress TrueTouch™ that is placed between the touch sensor and system’s 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 system’s host controller can understand.

Touch SensorA touchscreen “sensor” is a clear glass or acrylic panel with a touch responsive surface. This sensor is placed over 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.

DisplayMost 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 electrical noise 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. Apple’s 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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Primary Touchscreen Technology Options

Resistive TouchscreensResistive 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 TouchsreensSurface Capacitive technology works with a glass or plastic cover lens up to several millimeters thick. This provides a clearer and more durable display than the 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 TouchscreenIR 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 for glove 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 TouchscreensProjected 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. Apple’s iPod Touch and iPhone use this type of touchscreen.

Touchscreen Technology RevealedThe 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 mobile handset 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 panel’s 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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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 manufacturer’s 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 the finger. 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 durable than 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 to product. 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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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 touch—and now you know your options.

About the AuthorsSteve 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 Cypress’s Consumer and Computation 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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EEWeb Pulse - Volume 7

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