RE: [IBIS] BIRD95.3: Power Integrity Analysis using IBIS

Hello,

I think Mike raises a good point.  This comment actually reminded me
to something else that I have seen in the past.  When the incoming
signal causes the input receiver to switch, there will be some
current flowing, similar to the pre-driver current we have seen
in the Sigrity presentation in the last Summit.

This BIRD's I-t tables are all supposed to be time correlated with
the V-t curves of drivers.  There is no provision in this BIRD for
inputs.  Are we ignoring something that may rotationally be an
important effect for power delivery noise simulations?

If we decided to address this, what should the I-t curves be time
correlated with for receivers?  The only thing I can think of
right now is the threshold crossing.

Does anyone feel that we should make provisions for accounting
for the switching currents of receivers also?

Regarding the "sinking feeling" comments on the bottom, are you
saying that you are an *-AMS advocate vs. macro modeling?

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

-----Original Message-----
From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Mike
LaBonte
Sent: Wednesday, March 30, 2005 8:14 AM
To: 'ibis'
Subject: RE: [IBIS] BIRD95.3: Power Integrity Analysis using IBIS

I find this paragraph of BIRD95.3 worthy of discussion:

|               These keywords might not have any effect for the
Model_type
|               settings of Input, Input_diff, Terminator, Series, and
|               Series_switch.  These settings do not have a driver mode
of
|               operation.

The analysis that lead to it is given:

6) Z_VDDDQ subparams and placement under Model_type - Bob Ross/John
Angulo  Added statement that the new keywords might not be used for
Input, Terminator,  Series, Series Switch, Input_diff model_type.  The
positioning of [Composite  Current] under the [Rising Waveform] and
[Falling Waveform] keywords already  constrain that it is used only
where the waveform tables are used.

This implies that the RLC+G circuit connecting the power and ground
rails inside the black box might not be present in an input model. I
don't see why. That predriver and bypass circuitry exists in inputs, and
will cause power and ground currents when the power-to-ground voltage
changes due to external influence, even in an input. Is the simulator
expected to selectively ignore the elements based on whether or not an
IO or 3-state buffer is enabled?

My opinion is that the IBIS specification should not use the word
"might" in any place that "might" cause 2 simulators to behave
differently. The elements should be absolutely present for almost any
model type if they are specified. The only exceptions would be the
Series types, which do not have power and ground connections; all BIRD95
keywords should be illegal for those.

My apologies if I am jumping in late on something that has already been
discussed. I saw nothing about this change on the reflector.

While I'm at it I just have to mention the sinking feeling I get when I
read of new IBIS keywords that predefine structural elements that are
simply R, L, C, and G. It's just a shame that we are still going down
that path and not prescribing flexible circuit models, especially
considering that simulators will probably just apply the keyword values
to a template in the tool's own flexible macromodel-type language.

Mike LaBonte

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