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(Reprinted with Permission) Dr. Howard Johnson is president of Signal Consulting Inc., a high-technology consulting firm specializing in solving high-speed digital design problems. He regularly presents technical workshops for digital engineers, including courses for Oxford University and UC-Berkeley. He is the author of "High-Speed Digital Design: A Handbook of Black Magic" (Prentice-Hall, 1993) ISBN: 0-13-395724-1. He can be reached at (806) 556-0800; fax (206) 881-6149; e-mail <howiej@sigcon.com>. Signal Consulting The Art of High-Speed Digtial Design http://www.sigcon.com |
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| Ernie
fidgeted uncomfortably
on the big leather sofa, listening to the others drone on. His tie was tight. He wasn't often invited into the corner office, and he knew why. It's because I'm not like the others, and because I can't keep covering things up.... Finally, Ernie couldn't stand it any longer. uot;Jim, you're the presi- dent, and you've got to know," he blurted. "There's some sort of glitch on the main processor board. It might be a crosstalk effect or a noise problem. We just don't know. We'll have to hold up shipments until we get it sorted out." Jim turned white, then red. The veins began to stand out on his forehead. He hesitated, and then stood up and thundered, "So that's what you guys are trying to tell me?" He was used to problems, but this was absolutely unbelievable. "Let's see, gentlemen. We have 145 manufacturing people, 35 tech- nicians, and a warehouse staff wait- ing for this product. Our sales team is trained, advertising is in the works, and I have four press inter- views starting tomorrow. The comp- any burn rate is about $100,000 per day. If this problem takes a week, we lose five hundred grand. That's more than you're worth, Ernie. Why didn't you tell me about this before!! How could we get this far and not know there was a prob- lem???" Ernie's lips trembled. He tried to make everyone understand. "Hey, look, I did everything I could. The simulator said everything was per- fect. The timing was all checked out. How was I supposed to know we'd have crosstalk." How was I supposed to know that fast signals wouldn't go through that con- nector?" |
| "They don't even teach crosstalk
and ringing in college. Those are the subjects we're supposed to learn through experience. That's just the way it works." It's cruel how our educational sys- tem has failed us. Twenty years ago, all computer designers went through a common electrical engi- neering curriculum. This included basic analog circuits, transmission I lines, and linear systems theory. Trouble is, the computer hardware of that era was so slow that few people needed to know anything I about analog circuits to make their systems function. Take a look in the first edition of Texas Instruments logic catalog (if you can still get a hold of one). The typical LS-TTL logic gate had a rise/fall time of about 20 ns. That's very slow by to- day's standard. It doesn't take an analog guru to plug together a lot of LS-TTL logic. As a result, colleges and univer- sities felt they could safely drop the old analog curriculum requirements, in favor of newer, more modern computer science classes. And, there were even those who said that analog design was becoming ir- relevant. Not that I have anything against computer science. It's a wonderful discipline. Without it we wouldn't have the processor sophistication we now have, or have nearly as many people trained to work with complex computer architectures. I can't blame the educators for their decisions. A modern univer- sity has a lot on its plate. There are too many subjects to teach, and too few hours available to teach them. |
| Today,
modern processors are
clocked at over 100 MHz. Plain vanilla chip outputs sport sub- nanosecond rise times. Analog ef- fects are beginning to dominate system design. Digital designers without basic analog training operate at a serious disadvantage. In reaction to this trend, some col- leges and universities are reintro- ducing analog material in their computer science curriculum. I par- ticularly applaud Clayton Paul at the University of Kentucky and Henry Ott of Bell Labs for educat- ing digital engineers about the tough new world of electromagnetic compatibility (EMC) standards. Other courses of importance to digital engineers include basic cir- cuit theory, ELM waves, and trans- mission lines. A well-rounded dig- ital engineer should understand enough circuit theory to recognize the ways two adjacent digital sig- nals will behave like a loosely cou- pled, single-turn transformer. He or she should know enough E&M wave theory to understand why a ground plane is better for high- speed boards than a loose mesh of ground traces. All digital engineers should have a grasp of signal propa- gation, superposition, and reflec- tions on transmission lines. A proper understanding of basic signal integrity and EMC is crucial to the continuing evolution of dig- ital technology. Engineers with a solid background in signal integrity and EMC EE will greatly enhance their chances for career success. Dr. Howard Johnson is
president of
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