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

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From: Ritchey Lee (leeritchey@earthlink.net)
Date: Tue May 01 2001 - 04:32:05 PDT


I think that the reason for two different views on this (10%-90% vs two
voltage
levels) is this:

When delivering signals to inputs for purposes of performing switch,
what
matters is how fast the signal transits the "forbidden zone" on an
input. This
gives rise to the +- values around Vthres. A better way to specify this
is slew
rate and some IC manufacturers specify slew rate or variable slew rate.

When calculating cross talk and reflections, the whole transition is
important.
This is better characterized with the 10% to 90% values.

So, both methods are important. Just depends on what part of the
analysis is
going on.

Lee

Larry Miller wrote:

> In his Digital Black Magic book Dr Howard Johnson made a pretty good case
> that from a signal integrity point it does not matter that much how you call
> it. (I think there is a whole Appendix on it.)
>
> For 2.5 and 3.3v logic I usually use 2.0 volts as the minimum logic "1"
> reading and 0.8v for the maximum logic "0". The ASIC guys seem to use this,
> and it is at least very comparable between measurements for many kinds of
> signals (I try to avoid percentages as they violate the Law of The Shifting
> Baseline (it takes a 100% increase to offset a 50% decrease)), especially if
> you are going to connect the signals together.
>
> Larry Miller
>
> -----Original Message-----
> From: Doug Hopperstad [mailto:doug.hopperstad@qlogic.com]
> Sent: Friday, April 27, 2001 12:00 PM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : RE: Edge rates
>
> 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?
>
> Doug Hopperstad
>
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