RE: [IBIS-Users] RE: [IBIS] RE: IBIS model behavior between Vinh and Vinl input

From: Todd Westerhoff <twesterh_at_.....>
Date: Fri Jan 04 2008 - 10:26:59 PST
All,

I'm going to jump in the middle here, even though I didn't have the time to read this thread as
thoroughly as I would have like.

I'm confused by what Akhilesh is describing.  Akhilesh, are you saying your driver can't drive the
receiver's input pin beyond the VIL and VIH thresholds?  If that is truly the case, then you need to
consider the datasheet specifications for your receiving device very carefully.  It's not a matter
of what thresholds you have in an IBIS models you received (they may not be correct), nor is it a
matter of what behavior a HSPICE model of the input buffer might exhibit.  It's a matter of how the
part's performance is specified and guaranteed, and that's in the datasheet.

There can be a difference between the thresholds a signal is timed to, and the thresholds that a
switching signal must ultimately attain to ensure a valid switching event.  Distinguishing between
"signal quality" and "signal timing" thresholds in IBIS models can get pretty tricky, even for those
of us who've been at it a while.

What caught my eye was your initial description of the buffer as a "hysteresis buffer", although I
don't know if you meant the driving or receiving device.  If you meant the receiving device, it
could well be the case that the part is _timed_ to a voltage between the VIL and VIH thresholds, but
chances are, the signal still has to _attain_ VIL and VIH thresholds to be valid.

The device timing specifications are key to understanding what simulations to perform and how to
interpret the results.

Hope that helps,

Todd.

Todd Westerhoff
VP, Software Products
SiSoft
6 Clock Tower Place, Suite 250
Maynard, MA 01754
(978) 461-0449 x24
twesterh@sisoft.com
www.sisoft.com

-----Original Message-----
From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Tom Dagostino
Sent: Friday, January 04, 2008 1:02 PM
To: 'Muranyi, Arpad'; 'Akhilesh CHANDRA'; ibis@eda-stds.org; owner-ibis-users@eda.org;
ibis-users@eda.org
Subject: RE: [IBIS-Users] RE: [IBIS] RE: IBIS model behavior between Vinh and Vinl input

Arpad

I think we are in agreement.  I took Akhilesh's comment:

IBIS specs don't define that how logic X represent in IBIS simulation,
Due to this different simulator give different behavior for X. In my view we
will need to define how logic X represent in IBIS model simulation. It
will help to align all the simulator results.

To mean that he wanted to define X as a 1 or 0.  

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
503-430-1285 FAX
tom@teraspeed.com
www.teraspeed.com

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827 


-----Original Message-----
From: owner-ibis@server.eda.org [mailto:owner-ibis@server.eda.org] On Behalf
Of Muranyi, Arpad
Sent: Friday, January 04, 2008 9:41 AM
To: tom@teraspeed.com; Akhilesh CHANDRA; ibis@server.eda-stds.org;
owner-ibis-users@server.eda.org; ibis-users@server.eda.org
Subject: RE: [IBIS-Users] RE: [IBIS] RE: IBIS model behavior between Vinh
and Vinl input

Tom,

I may disagree with you on this one.  The 1 volt and 0 volt
representation
of "true" and "false" is just as "unreal" as an "X".  None of these are
actual
waveforms.  These are indicators which tells you whether the input
signal
was below, above, or between Vinh and Vinl.  I think it is plain wrong
to
have a "1" or a "0" on the output of the receiver when the signal is
between
Vinh and Vinl and it misleads the user of these tools thinking that they
have good signal integrity while they have problems.

I fully agree that digital parts are not made to operate with waveforms
between Vinh and Vinl.  Ant that is exactly the reason we should have an
output value of "X" to find out when the waveforms are in that region.
And that fact that you described that the "actual output ... will
change"
is exactly the reason for the "X", because we simply don't know what it
will be.  It is not guaranteed that it will be a "1" or a "0".  How do
you describe that if not with an "X"?

Arpad
========================================================================
=======

-----Original Message-----
From: Tom Dagostino [mailto:tom@teraspeed.com] 
Sent: Friday, January 04, 2008 1:34 AM
To: 'Akhilesh CHANDRA'; Muranyi, Arpad; ibis@server.eda-stds.org;
owner-ibis-users@server.eda.org; ibis-users@server.eda.org
Subject: RE: [IBIS-Users] RE: [IBIS] RE: IBIS model behavior between
Vinh and Vinl input

...
...
...

But having all simulators default to some arbitrary state when the input
is
between the reference levels is setting the user up for failure.  People
tend to assume if the simulator says X, X is real.  If they don't see X
in
the real circuit then there is something wrong.  Not all parts will
conform
to some arbitrarily designated state.

Digital parts are not made to operate with their inputs between Vinh and
Vinl.  Most parts have specifications that state the minimum slew rate
(input risetime/falltime) as the input to the part transitions between
the
two reference levels.  The actual output of an input buffer for a given
input voltage between Vinl and Vinh will change depending on the
internal
noise of the IC, the noise on the input, process, temperature, voltage,
phase of the moon, crosstalk, etc.

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
503-430-1285 FAX
tom@teraspeed.com
www.teraspeed.com

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827

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Received on Fri Jan 4 10:27:32 2008

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