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Terminator Crazy
(Originally published in
Electronic Design
Magazine, October, 1996)
Have you ever had
to add terminators to a high-speed board in order to get
it to work? If your answer is ``yes,'' do not
despair--you are not alone. In my high-speed design
workshops, I hear engineers constantly gripe about
terminators. Why are terminators so frequently needed?
The root cause is the fantastic improvements in rise/fall
times we are now enjoying in modern logic families.When I started working
with digital logic, a 10 ns rise time was considered
fast. Today, the situation is completely different. Now
there are logic families that can transition ten or
twenty times within 10 ns. For example, the output rise
time specified on the PCI bus can be as fast as 500 ps.
When working with such fast drivers, a rising-edge
waveform has plenty of time to finish its business, roll
over, and smoke a cigarette long before it smashes into
the far end of a printed-circuit board trace. If the far
end is unterminated, the signal will overshoot, reverse
course, and then come roaring back toward the driver. At
the driver end, the returning signal will bounce around
some more, settling down only after making several
round-trip tours of the neighborhood. In other words, an
unterminated trace with a fast driver will ring like
crazy.
Because the
drivers available today are getting so much faster, the
percentage of traces susceptible to ringing is
skyrocketing. To cure ringing problems, most fast boards
are sprinkled with terminating resistors. The terminators
are selected to have an impedance in the 10 to 100 ohm
range, something that (hopefully) matches the actual
transmission characteristics of the printed-circuit
board. When a fast signal hits a terminator, it acts like
a sponge, absorbing just enough of the signal energy to
prevent reflections. Traces with properly engineered
terminators don't ring, overshoot, or undershoot.
How do you know
when a terminator is needed? The ratio of trace delay to
rise time is the first clue. Terminations are almost
always required when the trace delay exceeds the logic
rise time. Many people take an even more conservative
approach, installing terminations when trace delay
exceeds one-fourth, or even one-sixth, of the logic rise
time.
The second most
important factor in the ringing equation is capacitive
loading. A capacitively loaded line, even when short,
sometimes requires a terminator. This happens because the
capacitance gives the line an apparent delay longer than
the bare unloaded line. When the apparent delay goes up,
so does the need for a terminator. At the same time,
capacitive loading lowers the apparent characteristic
impedance of a short line. Sometimes as little as 10 or
20 pF can make a noticeable difference. The best
terminating value is a function of line length, line
impedance, and capacitive load.
Every designer
should establish clear rules for when terminators are
required. These rules put the designer in control,
actively managing signal integrity instead of the other
way around. The rules should take into account the trace
impedance, length, and topology, as well as the driver
rise time, its output impedance, the position and size of
capacitive loads, and some measure of the tolerance for
overshoot and undershoot. Use a trace simulator, or
actual laboratory measurements, to double-check all
calculations. With a little planning, ringing need not
become a serious problem.
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