October 9, 1997

Ringing in a new era

Howard Johnson, PhD

Has there been any change in the laws of physics?
Has ringing just gone away?

The time has come. After more than 50 years of progress in digital electronics, we have reached a breakthrough. I am ready now, today, right here, to declare the start of a new era. Just to make things official, here's my statement:

"Be it known in all corners of the globe that from this day forward, there is absolutely, completely, totally no excuse whatsoever for system problems, glitches, data errors, or other artifacts related to ringing in digital signals."

Before we all throw our pocket protectors into the air and shout, "Hooray!", let's take a close look at the wording of my pronouncement. After all, you may be wondering, has there been any change in the laws of physics? Has ringing just gone away? Can we henceforth just ignore it? The answers to these questions are, emphatically, "No, no, and no."

I didn't say ringing would no longer exist. Nor did I imply that it would not occasionally raise its ugly head to munch on a tender signal or two. Far from it. All I said was that there would no longer be any excuse for problems caused by ringing. I feel confident making this statement because ringing is a preventable system problem.

The most important thing to realize about ringing is that it is a deterministic, predictable system artifact that you can simulate with incredible accuracy. In the world of ringing, simulation is the ideal way to prevent problems. Why? Because if a problem can be simulated, it is by definition preventable--if a circuit won't work, nobody will build it.

Please don't misinterpret this statement as a blanket endorsement of simulation for all digital-design problems. In some applications, today's simulation technology just doesn't work. For example, try simulating the crosstalk on traces that pass over a split plane boundary. In that sort of complex, 3-D electromagnetic-field application, most simulators don't have enough muscle to do the job. However, the simulation of ringing on pc-board traces--in the presence of a solid ground plane with known source and load impedances and with a known rise time--is a well-known, easily calculable problem. If the simulation says a circuit is OK, it will probably work.

Computer simulation of ringing, even with all its warts, is far better than the simple rules of thumb I see in design shops all around the world. Old rules such as "3-in. trace stubs are OK," which may have served well in the past, don't even come close to cutting it with today's superfast digital logic. If you really want to know how far you can push a trace, simulate it.

I can't tell you how many engineers have contacted me with some weird bus configuration, wanting to know if it's going to work. My answer: "If you have to ask, simulate it."

But what if you don't have a simulator? That brings me to my last point, the "no-excuse" part. Simulation technology is widespread, easy to use, and cheap. If, by using a simulator, you can save one design spin on one circuit board, the simulator pays for itself. If you don't have a simulator, get one. If you already have one but aren't satisfied with it, here are some sources that may interest you:

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