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

Arpad's comments make a lot of sense to me. If we're trying to make
enhancements to IBIS to support SSN analyses and can't correctly predict the
device's behavior in light of VDDQ modulation, then what are we really going
to accomplish?

I think BIRD95 is headed in the right direction, but that we may be trying
to accomplish too many things at once. We're mixing together modeling of
the die-level power delivery system (ESR, ESL, bypass capacitance), modeling
the currents the buffer draws during operation and modeling of a device's
core operating current. That's a really ambitious goal, and I think we
might be better served by tackling some of these things individually (and
sequentially).

Why don't we focus first on extending the IBIS buffer model to correctly
represent current draw from the supplies as well as I/O pin behavior? We've
seen many times in other forums where people advocate (and are adamant
about) the use of SPICE models for evaluating SSN behavior. If the original
charter for IBIS was to provide a "black box" method for predicting a
buffer's behavior at its output terminal, why shouldn't we simply extend
that charter to include the modeling of behavior at the supply terminals as
well? An IBIS model that produced the same supply and output terminal
behavior as its SPICE counterpart could/should offer a significant advantage
in computation speed.

More importantly, if we define the IBIS buffer model's purpose as producing
the same currents and voltages at its interface pins [meaning supplies as
well as I/O pins] as its SPICE counterpart, then we have a reasonably well
defined method of establishing and validating the IBIS model's behavior -
namely, using the SPICE model with associated voltage and current probes.

I believe that even this task will be a challenging one. Getting the
model's behavior to scale accurately with varying supply references is new
algorithmic ground, and getting this to correlate against SPICE will not be
easy. Getting these algorithms implemented in commercial tools consistently
and within a reasonable time period won't be easy either - but it's worth
doing.

If we had an IBIS model that reproduced a corresponding SPICE buffer's
external behavior, then we should be able to drop that buffer into the same
circuit used to evaluate SSN using a SPICE buffer model and reproduce the
SPICE results everyone else uses as a reference. That will help IBIS
acceptance, and that keeps the IBIS ball rolling forward.

I don't believe we need to worry about modeling the die bypass capacitance
or parasitics at this point. This is an incredibly complex problem where
the methods are just being defined, and are sure to change over time. One
thing is sure, though - no matter how these parasitics are extacted and
modeled, they will probably be expressed as general purpose structure. If
we are able to model buffer characteristics correctly and combine that with
general purpose structure, we should be able to model whatever comes along.

I think all the IBIS tools have to innate ability to model general purpose
structure in their simulators anyway. Even those that don't provide a
general purpose syntax like SPICE to describe the netlist have the ability
built into their engines to support general structure - it's just a matter
or exposing it to the user.

Bottom line, I suggest we focus on getting IBIS models to accurately
reproduce SPICE behavior at ALL of their terminals. I've heard the same
arguments as Arpad - "yeah, but IBIS can't model this effect, or that
effect" wwwaaaayyyyyy too many times. I don't think the argument will
really ever end, but I'd like to win the "IBIS is equivalent" argument more
often than I currently do.

My $0.02.

Todd.

Todd Westerhoff
High Speed Design Specialist
Cisco Systems
1414 Massachusetts Ave - Boxboro, MA - 01719
email:twesterh@cisco.com
ph: 978-936-2149
============================================

"Always do right.
 This will gratify some people and astonish the rest."

- Mark Twain

-----Original Message-----
From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Muranyi,
Arpad
Sent: Tuesday, January 04, 2005 5:04 PM
To: ibis@eda.org; ibis-users
Subject: [IBIS] RE: BIRD95: Power Integrity Analysis Using IBIS

Mike (and Syed),

Nice comments! Your main concern raises a question in my mind regarding
on-die power delivery (passive) networks. This subject has popped up here
and there in discussions, but so far no one has raised it as an issue so
clearly and as importantly as you and Norio Matsui.

Up until now we have considered a buffer having a single point Power and GND
connection (unless it was using different supply voltages for clamps).
Except for some groupings done by the good old [Pin Mapping] keyword, these
supplies were all shorted together with all power and GND pads.

It seems that if these concerns you and Norio are raising are driven by a
common need, we would also need to consider how power is distributed between
the buffers and their sub-blocks.

However, if one of the primary purpose of doing BIRD95 is to make better SSN
simulations possible (including the power delivery networks), we would also
need to consider the so-called "gate modulation effect" for which I have
heard a lot a IBIS criticism since the mid 90's. (This effect is the one
that reduces the drive strength of the output stage due to the supply
voltage reduction which arises with power delivery noise). If we don't
account for that, no matter how well we know how the currents are
distributed between power GND and the output, they are still going to be
wrong...

Now, adding more than 4 supply nodes to a buffer is not possible with
[Model] or [External Model], only with [External Circuit].
This implies that describing predriver currents independently could only be
done with models using the new language extensions.
To me it seems that Syed's proposal is an attempt to provide an interim
solution using the legacy buffer modeling techniques.
The level of detail you and Norio brought up could only be addressed by the
new generation of models which will be written with those languages. I hope
that by then we will also figure out how to use ICM with IBIS to describe
the on-die passive interconnects to make it all work together.

However, the gate modulation effect could still be described relatively well
within the legacy IBIS framework with a few simple parameters. For an
example we could look at how HSPICE's B-element does it with the "spu_scal"
and "spd_scal" parameters. These were implemented on our request to do just
that, but there are no IBIS keywords or subparameters to transfer these
coefficients in an IBIS file. I wonder whether we should include these
features in BIRD95?

Regarding #4, current should be positive into the power node, into the I/O
node, but OUT OF the GND node.

Regarding #6, very good point, but it all depends on whether the diff pair
is just two independent single ended buffers driven in the opposite
directions, or a true (current source + current steering
transistors) construction. Legacy IBIS can't do the second one, period. So
how would someone translate a measurement done on a true diff buffer to a
model that can only do two single ended buffers driven in opposite
directions? I am not saying that this is impossible, but I don't see an
immediate solution, and I didn't spend time on thinking about it yet.

There is another issue that I ran into recently in connection with
differential buffers when they are designed with bipolar transistors.
It is a pain taking IV curves of these, because due to the nature of the
bipolar output transistors the two outputs are much less independent as in
the case with MOSFET outputs. (The nature of bipolar transistors is such
that the output voltage will have a great effect of the bias levels of the
predrivers, and thereby other half of the output). I don't know what we can
or should do with these animals in an IBIS I-V curve sense. This is not
necessarily a BIRD95 problem, but since you brought up your question, I had
to mention it.

Regarding #7, I feel these tables should be under subparameters inside the
[xxx Waveform] keywords. That way there would be no confusions. (I hope).

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

-----Original Message-----
From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Mike
LaBonte
Sent: Wednesday, December 29, 2004 8:34 AM
To: ibis@eda.org; 'ibis-users'
Subject: RE: [IBIS] Re: [IBIS-Users] BIRD95: Power Integrity Analysis Using
IBIS

BIRD95 reminds us of the difficulties associated with the current profile
issue, but hopefully this time we can work it through to a conclusion. My
comments:

1) Norio Matsui may have raised the most important issue, the fact that
there may not be exactly one bypass capacitor and one pre-driver section
associated with one buffer, all sharing current from one supply. In fact
this case seems quite unlikely. Scaling the total bypass capacitance,
pre-driver current and output current for an arbitrary slice of a component
down to one of it's buffers should a model that is likely not very accurate
for any single buffer, or for any collection of buffers unless the
collection happens to be equal to those in the slice. And a single current
profile still doesn't solve the correct apportionment of current to paths
from distinct power supply voltages.

2) Accepting for the moment that we are attempting to represent multiple
current paths with a single table here, why not specify ground current
instead of power current? Isn't it more likely that all of these currents
flow through a single ground plane than through a single power plane?

3) Although the IBIS specification limits itself with regard to explaining
how a simulator might use the data, I feel that some effort might be
warranted here to advance the cause of consistency among simulators. For
starters, should we have some words of wisdom explaining that the current
out of the pad can be determined, and represents the difference between the
current into the power supply (if there is only one) and out through ground
(if there is only one). Digging deeper, how will the simulator decide how
much of the current passing through ground is "crowbarred" through the
pullup and pulldown, versus passing through the other current path formed by
the bypass capacitors and pre-drivers? This has a bearing on the turn-on/off
rates chosen for the pullup/pulldown. Mainly I would like to see some
discussion of this from simulator people, to validate that the simulation
outcome is deterministic. Even if there has been discussion already within
the IBIS committee, it would be good to make a record in the IBIS
specification for future implementers.

4) I think the text should clarify that positive values represent current
flowing into the buffer.

5) Although it may seem obvious, maybe the specification should clarify that
in the case of open_XXX models, where current is supplied by the test
fixture, that current is not to be accounted for in the [Series Composite
Current].

6) What would the [Series Composite Current] profile look like for a
differential output buffer? Presumably these must be taken as a pair, and
the total current for the pair ideally remains nearly static. Should we
require the measurements to be taken this way, or should we allow model
makers to provide separate P and N models with different current profiles?

7) The proposed [Series Composite Current] keyword associates itself with a
preceding [XXX Waveform] keyword, and is thus position dependent. Some
existing IBIS keywords are position dependent. For example, [Ramp] is part
of a second level of hierarchy under [Model]. But I think [Series Composite
Current] would be a third level of hierarchy, being under [XXX Waveform],
which is under [Model]. Is this OK, or would it be better to simply add 3
more columns to the [XXX Waveform] tables?

8) Can the [Series Composite Current] keyword follow only some [XXX
Waveform] tables and not others?

9) The text describes [Series Composite Current] as current flowing through
R_VDD, but the circuit diagram calls it R_VDDQ.

10) More nit picking: I had to modify the ASCII circuit diagram to make it
line up right.

It's great to be discussing this important issue again. Overall I am left
wondering if the various current paths can reasonably be merged into a
single table.

Mike LaBonte

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Received on Tue Jan 4 20:19:19 2005