Re: [SI-LIST] : RE: Edge rates

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From: D. C. Sessions (si-list@lumbercartel.com)
Date: Sat Apr 28 2001 - 07:33:31 PDT


On Friday 27 April 2001 11:59, Doug Hopperstad wrote:
> I have a question regarding edge rates. Is there a standard for the edge
> rate points, i.e. 90%/10% or a specific voltage point like 2v, 0.8 v? I have
> some data sheets that indicate the edge rates are determined by the percent
> method and other data sheets (SSTL) that call out for ViH/ViL point for the
> markers. My Scope is defaulted at the 90/10 percent setting and if I use it,
> I get fairly good timing numbers of ~ 600pS. However, the vendor of the
> component is calling out to use Vref +/- 0.35 volts with Vref normally at
> 1.25 volts. If I measure the edges in the lab with that method, my timing is
> an insane 180pS per the 0.7 volt delta. My overshoot is not reflective of
> the faster edge rates.
>
> Can anyone provide some insight into the different procedures called out in
> the design specification books?

Well, for starts you can forget about 0.8/2.0 unless you're actually using
bipolar devices with Schottky inputs. Which I doubt.

The SSTL (and other recent JEDEC) specs call for edge rates between
the AC thresholds because that's what the receivers actually care about.
In general, there's an input amplitude/risetime combination that gets about
as much receiver performance as you're going to; after that something else
becomes the dominant performance limiter. Receiver delay becomes very
messy to estimate once you violate that condition; the IBIS committee tried
for almost two years to come up with a way to characterize input delays and
effectively gave up in favor of what amounts to circuit simulation (still not
complete.)

All of which is irrelevant to "edge rate" as a signal-integrity issue.
Despite other comments, that really does depend primarily on percentages
since you're trying to determine the timescale of a linear process (that is,
the time answer is independent of voltage scaling.)

-- 
| The race is not always to the swift, nor the battle to the strong. |
| Because the slow, feeble old codgers like me cheat.                |
+--------------- D. C. Sessions <dcs@lumbercartel.com> --------------+

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