Subject: RE: clarification on IBIS min and max values

From: "Ingraham, Andrew" <[email-masked]>
Date: Tue, 14 Nov 2000 15:53:12 -0500

> I need some clarification on IBIS min and max values for CMOS circuits.
> For CMOS circuits, a high temperature is associated with "min" or
> slow-weak behaviour.  Similarly, a low temperature is associated with
> "max" or fast-strong behaviour.  
>  
> However, the package parasitics (L,R and C) have the biggest values
> corresponding to "max" and smallest values corresponding to "min".  Why
> aren't the smallest parasitics values associated with "max" or fast-strong
> behaviour instead?
 
A simulator does not need to set all parameters to their "min" values", or
"max" values, simultaneously.  It can mix-and-match any way it sees fit.

It is assumed that the package values are not (or might not be) correlated
with anything else ... that it is entirely reasonable to pair "max" L_pkg
with both "min" and "max" drive strengths, etc.  Both results may be
interesting to look at.  A given simulator might look at both, or maybe only
one.

For example, a fast-strong driver and min L might produce minimum delay, but
a fast-strong driver and max L might produce maximum overshoot and ringing,
which might even cause maximum delay (due to ringback).

The IBIS exception is Temperature, where it is assumed that the temperature
IS correlated with driver strength.  In this one case, "min" is whatever
temperature causes weakest drivers (high temperature if it is CMOS).

Andy

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From: Al Davis <[email-masked]>
Date: Wed, 15 Nov 2000 01:29:11 -0800

On Tue, 14 Nov 2000, Ingraham, Andrew wrote:
> A simulator does not need to set all parameters to their "min"
> values", or "max" values, simultaneously.  It can mix-and-match any
> way it sees fit.

Actually, although this was the original concept, the data really 
must be looked at as 3 data sets, but there is some crossing between 
min and max.  Some simulators may max-and-match, so they can do 
what-if calculations, but in doing so they are using inconsistent 
data.

Consider ....

1. If you take one pullup/pulldown, with a different set of 
waveforms, the end points will not match.

2. If you take one voltage_range or pullup_reference with a different 
set of waveforms, the end points will not match.

3. Rise/fall times are dependent on c_comp, and pullup/pulldown.

4. The only place temperature is used is in the transit time 
calculation which describes the nonlinear capacitance of the clamps.  
This is part of the load, so if you mismatch, waveforms could be off.

So, consider them to be data sets, but unfortunately, you can't just 
take a column.  It is twisted a bit.

al.

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From: Scott McMorrow <[email-masked]>
Date: Wed, 15 Nov 2000 08:27:15 -0800

Except that:

a) #3 is not necessarily true for slew rate controlled buffers

b) package parameters are not correlated to die parameters

c) it is possible to mix and match from one device to another.

regards,

scott

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
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