Subject: Clarifications on IBIS standard

From: Stephen Peters <[email-masked]>
Date: Thu, 02 Feb 1995 13:39:05 -0800

Hello Raj:
     I can answer some of your questions, my comments are prefixed by >>

                     Regards,
                     Stephen Peters
                     Intel, Corp.
                     (503) 696-4108

Clarifications on IBIS standard

We are in the process of making IBIS models and there were a few items
in the standard and in the discussions which were not clear to us. I hope
somebody can clear these issues.

1. When is the Golden Parser for version 2.0 likely to be ready ?

2. It is rightly pointed out that the pulldown and pullup
characteristic for tristate outputs may be non-monotonic. The standard
says that this can happen in at most one place. The i-v characteristic
may then locally have a negative resistance. Will this not pose a
problem to simulators ?

     >>  The IBIS standard specifies that the pullup and pulldown curves
     >>  contain pullup and pulldown data ONLY, i.e. in the region where
     >>  the clamp diodes are active the current due to the clamp diodes must
     >>  be subtracted from the pullup/pulldown current.  This is where the
     >>  non-monotonic curves come from -- at the extremes of the I/V curves
     >>  where you are subtracting a large diode current from the combined
     >>  diode/on-state-IV curve.  In practice, a simulator sums the two
     >>  currents together (power clamp and pullup or gnd clamp and pulldown)
     >>  thereby making the result monotonic.

3. During a transition, it is not clear whether the pullup or pulldown
characteristics should be used by a simulator. This is not a problem
for making the IBIS models, but only for using them in a simulator.

     >>  Which curve to use and how they transistion is a characteristic
     >>  of the simulator.  In general, the purpose of the IBIS standard
     >>  is to present consistant data in a consistant format and not to
     >>  specify how simulators are to use the data.

4. Is there a provision for specifying a pad or die resistance i.e
R_comp in addition to C_comp?

    >>  No.  The effects of a pad or die resistance are 
    >>  accounted for in the I/V curves.

5. The standard does not explicity specify the nature of the input
ramp in obtaining ramp rates. What should be the input rise time as
well as the high and low values of the input pulse? 

Perhaps many of these issues have been thrashed out at different
forums. I would be greatly obliged if somebody would educate me on
this.

	Raj Raghuram
	National Semiconductor
	2900 Semiconductor Drive
	Mail Stop D3-677
	Santa Clara, CA-95052
	email: [email-masked]            
	Phone: (408) 721-6220 (O)
	Phone: (408) 252-1285 (H)

---

From: Will Hobbs <[email-masked]>
Date: Thu, 2 Feb 95 15:42:44 PST

Text item: 

Fred,

Please post your questions and comments to this reflector.  We have a good group
of people here who are more than willing to help.  We are interested in 
problems, observations or questions you have.

The 2.1 Spec does offer new data in the models, but retains 1.1 backward 
compatibility.  Backward compatibility is one of our guiding principles, to 
avoid scaring people off with a spec that is evolving to better meet the needs 
of the engineers using the models.

Regards,

Will Hobbs
Intel Corp.

Raj,

Items 3 through 5 are also of interest to me. I too have been wondering if 
[email-masked] is the proper forum for those of us who are interested in the 
application of IBIS data rather than the creation of IBIS data and standards.

It seems to me from the regular IBIS participation that there are mostly 
representatives of IC and EDA tools vendors. I am sure that the tool 
vendors will
eventually offer means of using IBIS data with their tools. In the 
meantime, some
of the tool users (like me) must be struggling along trying to use 
IBIS data as it
becomes available (and changes form).

I recently subscribed to the SI reflector, [email-masked] 
(subscribe to
[email-masked]), but in the past two weeks, I've seen very little 
discussion, an nothing relating to IBIS use.

If you want, I could let you know what I've been doing with IBIS data, 
but maybe I
should take it off-line.

I would like to hear opionions regarding the proper forum for IBIS 
use from all
you IBISians and SI-listians out there.

Fred
  [email-masked]
  FirePower Systems, Inc.
  190 Independence Dr.
  Menlo Park, CA 94025
  (415) 462-3055

Begin forwarded message:

Date: Thu, 2 Feb 95 10:30:08 PST
From: [email-masked] (Raj Raghuram) 
To: [email-masked]
Subject: Clarifications on IBIS standard 
Cc: [email-masked]

	Raj Raghuram
	National Semiconductor
	2900 Semiconductor Drive
	Mail Stop D3-677
	Santa Clara, CA-95052

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Subject: Re: Clarifications on IBIS standard
To: [email-masked], [email-masked]
Date: Thu, 2 Feb 95 12:12:33 -0800
From: Fred Vance <[email-masked]>

---

From: [email-masked] ( Bob Ross)
Date: Thu, 2 Feb 95 18:19 PST

Fred and Raj:

As Will Hobbs, Chairman of IBIS Forum responded, the [email-masked] reflector is
the appropriate place to ask questions.  Several questions have resulted in
good discussions and refinements to IBIS.

Here are some of my thoughts on your questions.

(1) The Golden Parser is in the final stages of review, and the final version
is expected shortly (weeks).  We cannot commit to an exact date.

(2) Monotonicity has been a big concern with the committee and has generated
a lot of discussion.  Some simulators are more sensitive to non-monotonic
data than others.  Without getting totally bogged down in this discussion
here, I will just respond that some [Pulldown] tables are non-monotonic
in the negative voltage region.  When the data is summed with the [GND Clamp]
table, the TOTAL table is monotonic.  Even simulators that are sensitve to 
non-monotonic data will usually handle this case.  The Golden Parser gives
a non-monotonic data warning.  So, if necessary, you can adjust the data.
The overall philosophy in allowing non-monotonic tables is that IBIS presents
raw data extractions without appling any particular data modification
strategy which would corrupt the data.

(3) The transition algorithms regarding the [Pulldown] and [Pullup] tables
will probably be different between simulator companies.  Also, some companies
convert the IBIS data into their internal representations, so their existing
transition algorithms are used.  Generally, the transition algorithms are
not public.  The Waveform table extension in IBIS Version 2.0 captures actual
reponses and can be used to check the accuracy and/or to shape the transitions.

(4) Adjustments to the [GND Clamp] table can be made to create an effective
R_comp if it is parallel to C_comp.  If the resistance is in series, it possibly
can be lumped with R_pin or the default R_pkg sub-parameters.  We need
to know about the specific concerns to provide more detail.

(5) You are right, IBIS does not provide an Input pulse specification for
deriving Ramp rates (and Waveform Tables).  A reasonable guideline is to
mimic actual conditions for which the device would be used.  Therefore it is 
probably better not to mandate a specific condition.  The voltage swing should
be appropriate for the technology, e.g., 0 to 5V for CMOS and about 0 to 3V
for TTL.  A signal faster than the expected Ramp rates is my preference,
although a case could be made to provide a response that mimics the
data book input ramps specified for timing tests.   Possibly a 50 Ohm series
resistance approximating the pulser source impedance and trace environment
to the device input should be included.  However, since the actual thresholds
are narrow (several 100 mV), the Ramp rates and Waveform tables should not
differ significantly for any reasonable, appropriate Input.

Bob Ross,
Interconnectix, Inc.

> representatives of IC and EDA tools vendors. I am sure that the tool vendors will  
> eventually offer means of using IBIS data with their tools. In the meantime, some  
> of the tool users (like me) must be struggling along trying to use IBIS data as it  

> I recently subscribed to the SI reflector, [email-masked] (subscribe to  

> If you want, I could let you know what I've been doing with IBIS data, but maybe I  
> should take it off-line.

> I would like to hear opionions regarding the proper forum for IBIS use from all