Re: [SI-LIST] : Best type of models, edge rates & load

D. C. Sessions (dc.sessions@vlsi.com)
Wed, 21 Jul 1999 09:57:35 -0700

"Beal, Weston" wrote:
>
> D.C.,
>
> Normally I agree with most of your opinions and respect your experience, but
> think there's a flaw in your statement here. "Output timing should be to the
> 10% (initial) point of the edge. That's easy to plug in as Vmeas so that
> the timing numbers add up..." The problem here is that we need a time for
> rising and for falling. If we measured Tco at the 10% deviation for initial
> voltage we would have to measure to two different voltages. There is only
> one Vmeas parameter in IBIS. That's why IBIS authors usually pick 1/2*Vcc
> or something close. If the timing reference load is resistive, then Vmeas =
> 1/2*Vcc should work just fine, right?

We're working on that bit with the single Vmeas :-)
For now you're right.

No matter what you pick, the simulator needs to subtract out the given
V/T to get the start time for a transition, and then start the loaded
sim running until the receiver trips. The less that has to be removed
the more accurate the sims will be.

> -----Original Message-----
> From: D. C. Sessions [mailto:dc.sessions@vlsi.com]
> Sent: Tuesday, 20 July, 1999 4:33 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Best type of models, edge
> rates & load
>
> Mark Nass wrote:
> >
> > There has been some discussion recently about parameters
> of parts
> > specified into 40pf caps and accuracy of models. I
> generate this type
> > of data for our devices for our own use and our customers.
> So I am
> > curious as to what people think would be the optimal way
> to generate
> > Tco, Tsu, jitter parameters and IBIS models so that they
> would be confident
> > they were getting exactly what they needed for signal
> integrity and timing
> > analysis. Any feedback?
>
> If you had to pick just one load, it would be 50 ohms.
> Notice I didn't
> write "50 ohms and 30 pf" or anything like it. To get the
> best sims we
> need the high-frequency content of output waveforms, and
> artificially
> filtering it out by adding a lowpass is not exactly helpful.
>
> Of course, we need both the turnon and turnoff responses for
> both
> the pullup and pulldown paths, so that's four tables (rising
> into
> 50 ohm pullup, falling into 50 ohm pulldown, rising into 50
> ohm pulldown,
> and falling into 50-ohm pullup.
>
> Output timing should be to the 10% (initial) point of the
> edge. That's
> easy to plug in as Vmeas so that the timing numbers add up,
> and also
> easy enough to measure on a tester so that you're actually
> testing to
> the critical design parameter.
>
> No lumped caps anywhere (was this a surprise?)
>
> --
> D. C. Sessions
> dc.sessions@vlsi.com
>
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-- 
D. C. Sessions
dc.sessions@vlsi.com

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