RE: [IBIS-Users] IBIS model behaviour with variable voltage source.

Amit,

To expand on Arpad's answer a bit, the behavior of an IBIS model connected to a non-ideal supply
might match the behavior of its corresponding transistor level model, and it might not.  Even more
baffling, the IBIS power behavior might match its transistor level counterpart in one IBIS
simulator, but not in a different simulator.

This occurs because the original IBIS specification dealt with the behavior of the I/O pin only and
excluded the behavior of the power pins.  The original assumption was that all rail current went
through the I/O pad, but that isn't quite true.  Rail current consumed by the pre-driver circuits
doesn't go through the I/O pad, and any current shunted between power and ground ("crowbar" current)
during a switching event doesn't go through the I/O pad either.

BIRD95 was developed to give IBIS simulators the additional information they need to correctly
predict current demands on the power rails, allowing a simulator to reasonably predict switching
currents without requiring the use of a more complex (and therefore slower) transistor level model.
BIRD98 was developed to be used in conjunction with BIRD95, allowing the effect of power supply
fluctuations on the I/O behavior to be modeled (again, without requiring the use of a transistor
level model).

As a practical point, I don't know of any semiconductor vendors that are supplying models based on
these BIRDs, nor do I think support is currently available in EDA tools.  Thus, as an immediate and
practical solution, I would recommend that you do exactly what you're already doing - test the
transistor level model's behavior against its IBIS counterpart, quantify the difference and decide
how best to proceed with a given analysis.

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-users@eda.org [mailto:owner-ibis-users@eda.org] On Behalf Of Muranyi, Arpad
Sent: Friday, January 04, 2008 1:22 PM
To: ibis-users
Subject: RE: [IBIS-Users] IBIS model behaviour with variable voltage source.

Amit,

This is a difficult thing to model.  Please look at
BIRD95 and some related presentations from Sigrity
on the IBIS web site.  Unfortunately the web site is
down at the moment, so I can't give you specific links.

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

-----Original Message-----
From: owner-ibis-users@server.eda.org
[mailto:owner-ibis-users@server.eda.org] On Behalf Of Amit KUMAR
Sent: Thursday, January 03, 2008 9:44 PM
To: ibis-users
Subject: [IBIS-Users] IBIS model behaviour with variable voltage source.

Hello All,

I have an IBIS model which reproduces the Spice waveform for all cases.
But when i connect an inductor to the power supply and then connect it 
to IBIS model then the results
i get for IBIS models and for spice do not match...
Is this observation a limitation of IBIS Models?

Thanks a lot
Amit


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