Re: Subtracting clamp curves from pull-up/down curves

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

At 11:07 AM 07/07/2000 -0400, Ian Dodd wrote:
>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
>
>
>
>
>"Haller, Robert" wrote:
>>
>> 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: rhaller@cereva.com
>>
>> -----Original Message-----
>> From: Mike LaBonte [mailto:mikelabonte@cadence.com]
>> Sent: Friday, July 07, 2000 8:28 AM
>> To: ibis-users@eda.org
>> 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
>
>
Donald Telian
PCB Systems Division
Cadence Design Systems
phone: 408-944-7791
donaldt@cadence.com
Received on Fri Jul 7 12:57:08 2000