[SI-LIST] : Reflections on clock line

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From: Bob Perlman (bobperl@best.com)
Date: Wed Apr 05 2000 - 13:42:44 PDT

Hi -

You left out some important information, such as the type of driver
you're using and the impedance of the trace (I assume it's a fixed
impedance, but perhaps not). Even without that information, I
suspect you're experiencing two problems:
1) You're getting reflections from the unterminated end of the net.

2) The driver's output impedance is too high.

If the driver impedance is indeed too high, you have, in effect, a
series-terminated, daisy-chained clock net, a bad combination.
Even with a termination at the end of the line, you'll see voltage
plateaus for all but the last load.

Just to give you an idea of what a simulator would tell you about
such a net, I've run a Hyperlynx simulation with a 40-ohm driver and
a daisy-chained net with equally-spaced 6pf loads. (In my
simulation, the driver impedance is a pretty good match for the
loaded net impedance, so the load at the end of the net looks just
fine. If the driver impedance is made lower, you'll see some
overshoot/undershoot at the end load.)

There are at least two solutions:
 - use a stronger (i.e., lower-impedance) driver, and terminate at the
end of the net.
 - use series termination, and drive each load with its own driver. It
may actually be possible to drive multiple loads with each driver, as
long as all loads are at the end of the clock nets.

Want a more in-depth explanation? Go to my web site (see
signature below), where you'll find a paper and presentation on
series termination that address some of these issues in more

Good luck,
Bob Perlman

> I have a 20in long 50MHz clock line which connected to 8 loads(5.5pF
> per load). (Vcc=3.3) I used daisy-chain to connect all loads but the
> waveform of the nearest receiver (respective to the driver) only swing
> between 1V and 2V. The waveform of the furthest receiver overshoot
> above 4V. I think reflection is the main reason and I had put an end
> termination but there is no improvement for the nearest receiver
> waveform. Anybody have any ideal to solve this problem? I appreciate
> any advice or suggestions.
> Thanks.
> Regards,

Bob Perlman
Cambrian Design Works
Digital design and signal integrity consulting


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