Subject: RE: Subtracting clamp curves from pull-up/down curves

From: "Haller, Robert" <[email-masked]>
Date: Fri, 7 Jul 2000 10:16:56 -0400

Mike,
	Not trying to play devil's advocate, but I have always wondered 
why the model maker has to subtract the clamps out, then the software vendor

puts the clamps back in ?  Why not just have syntax (in IBIS) for a pullup 
and pulldown model WITH the clamp ? Thus reducing everyone's work ?

My old (internal) simulator and models were defined this way (IV curves
included clamps)
and it seemed a lot easier. We could feed bench data from a curve tracer (or
semiconductor analyzer)
directly into the model without any math, and when you looked model curves
they were intuitive.
I know its probably now water over the dam,  but I was just wondering....

regards,
Bob

Cereva Networks
100 Locke Drive
Marlboro MA. 01752
Phone: 508-486-9660 X 3365
FAX: 508-486-9661
Email: [email-masked]

-----Original Message-----
From: Mike LaBonte [mailto:[email-masked]]
Sent: Friday, July 07, 2000 8:28 AM
To: [email-masked]
Subject: Re: Subtracting clamp curves from pull-up/down curves

Stephen,

Arpad's reply indicates that both clamp curves are indeed subtracted
to produce pullup and pulldown curves. I can verify the other side of
your question; that simulators (at least Cadence SPECCTRAQuest)
re-combine the curves as follows:

   low state:  pulldown + powerclamp + groundclamp
  high state:  pullup   + powerclamp + groundclamp

So the clamps are never disabled (unless Submodel is used).

Mike LaBonte
Cadence

Stephen Nolan wrote:
> 
> Hello IBIS gurus,
> 
> When creating a model for a three-state device, it is necessary to
subtract the
> clamp curves from the pull-up/down curves to avoid double counting, as the
EDA
> tool adds the clamp-curves back in to the pull-up/down curves to obtain
the
> devices output charactersitics in the enabled state. Right so far?
> 
> The question is this, do you only subtract the corresponding clamp curve
from
> the related pull-up/down curve, or do you subtract BOTH clamp curves from
each
> pull-up/down curve?
> 
> For example, if I am creating the pull-down curve, do I subtract only the
> ground-clamp curve, or do I subtract both the power-clamp and ground-clamp
> curves? Does the EDA tool add only the ground-clamp data back in, or does
it add
> both curves back in?
> --
> Regards,
> Stephen M. Nolan

---

From: Ian Dodd <[email-masked]>
Date: Fri, 07 Jul 2000 11:07:31 -0400

Bob,

I think there are two obvious reasons for keeping the clamps separate:

1. It was intended that models be usable for a range of supply voltages
with the pull-up and power clamps tracking with the supply voltage
and the pull-down and ground clamps tracking with the ground rail.
This was also the rational for referencing the former to the supply voltage.

2. Separating the clamps from the pull-up/down allows modeling of
high/low/tristate
for devices that support these modes.

Recent feedback has indicated that the characteristics of many devices change
markedly with supply voltage, so using an IBIS model for a supply voltage
other than at which it was designed, can give inaccurate results

Ian Dodd
Cadence Design Systems

---

From: Todd Westerhoff <[email-masked]>
Date: Fri, 07 Jul 2000 15:31:52 -0400

My $0.02:

>high/low/tristate for devices that support these modes.

Is the right answer.

Thanks, Ian!

Todd.

    Todd Westerhoff
    Product Marketing Director | High Speed Systems Design | Performance Engineering
    Cadence Design Systems | 270 Billerica Road | Chelmsford, MA  01824
    
    ph: (978) 262-6327
    fx: (978) 446-6798
    email: [email-masked]
    internal information website: http://www-ma.cadence.com/~toddw

---

From: Donald Telian <[email-masked]>
Date: Fri, 07 Jul 2000 12:54:32 -0700

Bob,

Perhaps I can provide some background.

We knew we had to support models of devices in tri-state, so that required
the implementation of a standalone (not embedded in the output curves)
clamp section in IBIS.  If it was embedded, the EDA tool builder would not
have had enough data to figure out exactly how to remove the tri-state
clamp from the output characteristic.

Given that we had to have a clamp curve, the choice then was whether or not
to have the same curve embedded in the output curve too.  The decision on
this was based on our view at the time of how an IBIS "template" or "macro"
circuit would be built.  Our view was that the circuit would have separate
circuit elements for both "driver" and "clamp" primarily because one of
them had to have the ability to be turned "off" (for tri-state).  (And
actually, if you read closely, we did allow provision for an output only
device to have the embedding.)

So two curves were required, and we decided to NOT have overlap in the
clamp data.  That way the data could just be directly applied to the macro
circuit without anyone mucking with it.  Stepping back and looking at it,
either the model maker or the EDA tool would have to do the subtraction in
a typical macro implementation.  I guess we opted to put the burden on the
model maker.  In hindsight, this may have required 100 companies to have to
learn the process rather than 10.

What we have now is simply convention.  But this is the history, as I
recall it.

Donald T.
CADENCE

>> > curves? Does the EDA tool add only the ground-clamp data back in, or does
Donald Telian
PCB Systems Division
Cadence Design Systems
phone: 408-944-7791
[email-masked]