RE: [IBIS] BIRD95: Power Integrity Analysis Using IBIS

A few more comments on BIRD95:

1) Maybe the description should mention the desirability of using the
C_comp_pullup and C_comp_pulldown parameters when [Series Composite Current]
is used. Those parameters allow the simulator to apportion dynamic current
flow to the correct power/ground terminals as the pad voltage changes.

2) Since IBIS buffer models and many SPICE buffer models have separate
terminals for clamp power supplies, and since use of C_comp_power_clamp and
C_comp_gnd_clamp will cause the simulator to direct some current flows
between the pad and those terminals, is the C_comp_power_clamp current to be
included in [Series Composite Current]? The fact that the simulator easily
determines power clamp current seems to make it unnecessary to include that
current in [Series Composite Current], but what if the black box SPICE
buffer model has the clamps built-in? Then there would be no way to separate
the currents.

3) It may be best to clarify whether or not [Series Composite Current]
includes power clamp supply current, even that which flows through
C_comp_power_clamp. Note that built-in termination resistance currents would
show up in [Series Composite Current] if the resistance is in the power
clamp, but not if it is in the ground clamp. Assuming these terminators are
always outside the SPICE buffer model, it would be possible to state that
resistive currents are excluded from [Series Composite Current]. In other
words, can we say that inside the IBIS model [Series Composite Current]
includes currents that pass through the pullup and C_comp_power_clamp, but
not current that passes through power clamp? This gets complicated quickly!

4) Note that some of the debate relates to assumptions about what circuitry
is inside the black box SPICE model. This BIRD gives us a forum for
discussing those assumptions, but any effective change in requirements for
what is included or not included in SPICE buffer models today probably
presents a barrier to production of models using this keyword. It would help
me if the diagram in BIRD95 drew a box around the portion that constitutes
the black box buffer, with now-familiar 7 terminals at the perimeter.

5) Why is it called [Series Composite Current]? If this is the series
current into the power port of the buffer what would be called the shunt
current? Inside the buffer we might regard this current as a composite of
pullup, predriver, and various capacitance currents. But treating the buffer
as a black box, isn't there only one I/O (excluding clamp) power terminal?
Could we call it [Power Current]? That would also make for a neater syntax
if there ever is a [Ground Current] keyword.

I would also like to suggest that that comments of those developing IBIS
simulators that perform PDS analysis today would be especially helpful in
this important effort.

Mike LaBonte

-----Original Message-----
From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Mirmak,
Michael
Sent: Saturday, December 25, 2004 1:59 AM
To: ibis@eda.org; ibis-users@eda.org
Subject: [IBIS] BIRD95: Power Integrity Analysis Using IBIS

The enclosed BIRD, Power Integrity Analysis Using IBIS, is submitted on
behalf of Syed Huq, Vinu Arumugham and Zhiping Yang of Cisco Systems.
It will be discussed at the January 7, 2005 IBIS Open Forum teleconference.

- Michael Mirmak
  Intel Corp.
  Chair, EIA IBIS Open Forum

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Received on Fri Jan 7 11:31:40 2005