Subject: accuracy BIRD

From: [email-masked]
Date: Thu, 11 Mar 1999 17:23:01 -0600

The IBIS Accuracy Subcommittee has been considering submitting a new BIRD
that would allow for a wider variety of golden waveforms to be included in
an IBIS datasheet (model).  Right now these new golden waveforms would
correspond to the additional test loads described in section 3 of the IBIS
Accuracy Specification, but we will certainly be adding more test loads to
the list as we examine different kinds of I/O buffers and their associated
electrical behavior.  At the 2/26 IBIS Users Group conference call, Bob
Ross suggested we run this by the IBIS reflector to solicit your input, so
here it is.  The two main questions are:

1)  Is it necessary to describe the test loads in a machine-readable
fashion, similar to the distributed package model and EBD options already
present in IBIS?  Or should we opt for the more simple approach of adding a
waveform with comments describing the test load?

2)  How does one distinguish between a golden waveform (w/ package) and a
regular old VT table (w/o package) in IBIS?

For those of you who were not at the DesignCon99 Summit, the IBIS Accuracy
Subcommittee presented a two-stage approach to documenting the correlation
results of an IBIS datasheet.  Both stages involve embedding a golden
waveform, derived from SPICE, into the IBIS datasheet.  The semiconductor
vendor can correlate the lab data to these golden waveforms and report the
correlation results in the "accuracy trailer," a comment section appended
to the IBIS datasheet.  Using these same golden waveforms, the user can
correlate behavioral simulations and take up any significant discrepancies
with the simulator vendor.  Question 1 above refers to automating the
second step of the correlation process.  Of course, the user can always
capture the test load manually using the simulator's schematic capture
tool.

Greg Edlund
Advisory Engineer, Critical Net Analysis
IBM
3650 Hwy. 52 N, Dept. HDC
Rochester, MN 55901
[email-masked]

---

From: "Chris Rokusek" <[email-masked]>
Date: Thu, 11 Mar 1999 18:38:42 -0800

Greg,

Golden Waveforms are a great idea!

>waveform with comments describing the test load?

A "machine readable" format is prefered in order to facilitate automated
validation.

You mention using SPICE as the waveform creator so are transmission lines
and package info really needed?

If you're going to standardize the golden waveform loads, I suggest:

  1) No Transmission lines (we're not testing the accuracy of T-Line
modeling or
     T-line extraction parameters are we?).

  2) No package parasitics (or they are spelled out explicity within the
waveform).

  3) A Shunt R to the Positive Rail, the Negative Rail and halfway in
between.  R should be
     near the impedence of the Transmission line the driver expects to see.

  4) A Shunt C=50pF may prove informative also.

>regular old VT table (w/o package) in IBIS?

I claim that a regular old VT table **IS** a golden waveform.

It doesn't seem appropriate to place waveforms that use a particular package
"within the [Model] block" using IBIS since a particular [Model] block may
be used in with different packages.  The [Model] block is meant to represent
the output stage w/o (independent of) the package parasitics.

If waveforms with particular package parameters are generated then they
would have to be spelled out in some parseable format at the location of the
waveform.  Otherwise, there is the danger that someone will edit the package
information at the top of the .ibs file and not re-generate the waveforms.

Regards,

Chris Rokusek
Viewlogic Systems

---

From: "Chris Rokusek" <[email-masked]>
Date: Mon, 15 Mar 1999 12:23:16 -0800

Greg,

I understand why you want to distinguish between the a waveform with and
without packaging parasitics.  However, there is a another problem that I
think the "Golden Waveforms" specification you are coming up with can solve
with little impact.

The problem is that many model makers do NOT read the cookbook.  Often the
most important waveforms (w/o the parasitics) are not present.  If there is
going to yet another document that model makers will refer to when building
models, that document may suffer this same fate as the IBIS Cookbook.  A
model maker may read one document and not the other.

If both documents are referring to the construction of VT waveforms, they
should probably be rolled into ONE document so that the "publicity" of one
is leveraged by the other.  It creates one-stop-shopping for the model
maker.  We want to make it easy for the model maker.

Stephen, do you approve/dissaprove of combining the documents?

If you must create a seperate document, then I think the "Golden Waveforms"
should INCLUDE the specification for waveforms found in the cookbook (w/o
parasitics) as the most important waveforms.  So the LEVEL I would be w/o
parasitics, LEVEL II be with.  At a minimum the new spec must make an
"imperative reference" to the cookbook for 3.2.2.

The bottom line is if a user only reads this new Accuracy specification and
not the Cookbook then all is lost because the maker wouldn't include the
most important waveforms and your goals would not be attainable (nor would
ours).

Thanks for your consideration,

Chris Rokusek
Viewlogic Systems

-----Original Message-----
From: [email-masked] <[email-masked]>
To: [email-masked] <[email-masked]>
Date: Monday, March 15, 1999 11:39 AM
Subject: Re: accuracy BIRD

>Stephen,
>
>Thanks for the insight.  This clears up my question, and I believe Syed's,
>too.  Actually, I went back and re-read section 3.2.2 of the Cookbook, and
>you said it quite well there.  The simulator NEEDS the no-load VT tables to
>generate a realistic stimulus, if you will.  The VT tables with a complex
>load are icing on the cake and useful for verifying simulator results.
>
>What the Accuracy Subcommittee is proposing is some new complex loads that
>involve transmission lines.  (And we'll probably come up with more when we
>start looking at more advanced drivers.)  We can use these extra loads to
>validate both lab results AND simulator results against the "golden
>waveforms" in the IBIS datasheet - a cool thing.  The real question is,
>should we add some new keywords and/or subparameters to describe these new
>loads, thereby facilitating automated simulation and correlation, or should
>we count on comments to describe the load.  I like the automated approach
>myself, but I agree with Bob Ross:  let's not make the IBIS Accuracy Spec
>dependent on which of the two approaches we choose to use.  That way we can
>proceed with getting it approved.
>
>Thank you all for your input.
>
>Greg Edlund
>---------------------- Forwarded by Gregory R Edlund/Rochester/IBM on
>03/15/99 12:06 PM ---------------------------
>
>
>Stephen Peters <[email-masked]> on 03/15/99 11:54:07 AM
>
>To:   Syed Huq <[email-masked]>
>cc:   [email-masked], Gregory R Edlund/Rochester/IBM
>Subject:  Re: accuracy BIRD
>
>
>
>
>
>
>Hello Syed, Greg:
>
>  Sorry for the delayed response -- I was out of the office back week and
>am just now getting back to my mail.  To elaborate:  I was trying to make
>a distinction between at V/T table that is created by an output driving a
>pure, non-reactive impedance, and a V/T table created with the output
>driving
>a more complex load.  The *former* V/T table contains the data the
>simulator
>needs/uses when building the output model.  Specifically, it describes how
>the pullup and pulldown transistor turn on/off, and does not -- indeed,
>should
>not -- contain any extra time constants due to reactive elements.   In the
>latter case (complex, reactive impedences) the inclusion of these waveforms
>is a check.  Given the model that was build with the first V/T tables, the
>question is can the simulator produce the waveform of the second case.
>
>Hope this helps clarify things.
>
> Regards,
> Stephen Peters
> Intel Corp.
>
>
>> Hi,
>>
>> Since the V/T tables allow keywords such as C_dut, R_dut, L_dut, doesn't
>> that imply that it DOES use the package numbers. I am a bit confused
>about
>> Stephen's statement below regarding old VT tables..
>>
>> Syed.
>> Cisco Systems, Inc
>>
>> >
>> > On the question of "golden waveforms" vs. "regular old VT tables,"
>Stephen
>> > Peters mentioned that there is a difference between the two.  Golden
>> > waveforms have the package while regular old VT tables do not.  Is
>there
>> > any syntactical construct within IBIS to differentiate between the two?
>> > Care to embellish here, Stephen?
>> >
>> > Thanks all.
>> >
>> > Greg
>> >
>> >
>> >
>
>
>
>
>
>

---

From: "Muranyi, Arpad" <[email-masked]>
Date: Mon, 15 Mar 1999 15:27:00 -0800

Fred,

There is really no such thing as "no load answer".  The 50 Ohm resistor was 
invented because the transmission lines look like a resistor for the time 
duration while the wave is traveling down the line and back.  So the driver 
really never drives a "no load"...

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

Actually there is always a load on the IBIS models I have seen. The load
is typically 50 ohms to gnd or vcc. Because of this the simulation
vendors must extrapolate the no load answer. The DC loads are presumably
put there to determine turn off and turn on currents. One other comment.
The load is always a complex impedance. This is because c_comp is
required.

Best Regards,

--
Fred Balistreri
[email-masked]

http://www.apsimtech.com