Because of fundamentally analog SI (signal-integrity) issues that accompany today's higher data rates, digital electronics is now as much analog as it is digital. As SI expert Eric Bogatin, PhD, chief technology officer of GigaTest Laboratories put it several years ago, "There are only two kinds of EEs: those who have had SI problems and those who will." Still, although you mustn't become complacent, system architects, IC gurus, and test-instrument designers are hard at work making digital design's transition to SI's ana-log world as painless as possible. Even so, most EEs will experience some discomfort. According to the classical view, the days when you could ignore SI ended when bus-clock rates passed approximately 50 MHz. At that point, give or take a few megahertz, when you designed buses or interconnects, you had to start taking terminations seriously and stop thinking of reflections as just a little overshoot and ringing on waveform edges at state changes. Today, clock rates have leapt upward by an additional factor of at least 50 to 2.5 GHz or more (equivalent to clock periods of 400 ps or less). At such speeds, terminations and reflections are only two of the SI issues on a long list that you must grapple with.
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