Re: [IBIS] Question regarding V-t tables

Arpad:

The brief response is that the standard recommendations
such as setting V_fixture to ground and Vcc give excellent
modeling data and USUALLY avoid computational pit falls
for real buffers.

However, different EDA vendors may have different algorithms
with different levels of protection around their algorithms.
So it is difficult to generalize that some setups are not
solvable.

Consider for example, the algorithm on pages 6 and 7 of:

   http://www.eda.org/pub/ibis/summits/jun03a/muranyi1.pdf

Some R_fixture/V_fixture/(V-T) combinatons will most
likely be singular at some time t during the transition
when the denominator passes through 0. Your suggestion
of two different V-T tables for the same R_fixture/V_fixture
is an extreme case where the starting and ending points
and perhaps other time points where voltages are equal would
produce a singular (unsolvable) situation - if both sets of
data are to be used in this algorithm. (I am assuming your
distinct V-T tables satisfy the end-point conditions
consistent with the I-V tables and fixture loads, and
therefore have equal voltages and load currents. You
cannot get a solution for two k(t) table values from
two identical equations.) Different vendors may deal
differently with this type of stiuation.

So the response is to make positive recommendations, which
produce accurate models that simulate well in most tools
- rather than try to come up combination situations that
are likely to fail in some EDA tools.

Bob

Muranyi, Arpad wrote:

> Bob,
>
> This is an interesting "response", because according to the
> IBIS specification, a model like that is still perfectly legal...
> So to cover this, do we need to write a BIRD that says we should
> not make any models that way? Right now there is nothing that
> would prohibit something like this.
>
> Arpad
> ================================================================
>
> -----Original Message-----
> From: Bob Ross [mailto:bob@teraspeed.com]
> Sent: Wednesday, September 08, 2004 5:30 PM
> To: Muranyi, Arpad
> Cc: ibis@eda.org
> Subject: Re: [IBIS] Question regarding V-t tables
>
> Arpad:
>
> I would expect some EDA tools to reject V-T tables with the same
> R_fixture/V_fixture values because of processing equation singularities.
> To avoid some EDA tool dependencies (both positive and negative), the
> best approach is to use [Model Selector] to set up different data
> pattern situation of interest.
>
> Bob
>
> Muranyi, Arpad wrote:
>
>
>>All,
>>
>>I just realized something regarding the V-t tables in IBIS that raises
>
>
>>a question in my mind.
>>
>>The spec says that we can have up to 100 V-t tables. It doesn't say
>>anything on how Vfixture and Rfixture should be selected for these to
>>make a good model.
>>
>>Many simulators can only support the first four tables, but I am aware
>
>
>>of some which can make use of all of them. As far as I know, the
>>algorithms which make use of all of them assume that the additional
>>tables generated with the various Rfixture and Vfixture values provide
>
>
>>a better understanding for how the buffer reacts to various loading
>>conditions.
>>
>>What I would like to find out is this: What would happen if an IBIS
>>file had multiple V-t tables with repeated Rfixture and Vfixture
>>parameters, but different waveform shapes? (This ***CAN*** happen
>>nowadays with advanced buffers which exhibit data pattern dependent
>>behaviors). Obviously, given the current spec, the tool wouldn't know
>
>
>>when to use which table. Would this be a good reason to write another
>
>
>>BIRD for some additional subparameters for the V-t tables?
>>
>>Arpad
>
>
> --
> Bob Ross
> Teraspeed Consulting Group LLC Teraspeed Labs
> 121 North River Drive 13610 SW Harness Lane
> Narragansett, RI 02882 Beaverton, OR 97008
> 503-750-6481 503-430-1065
> http://www.teraspeed.com 503-246-8048 Direct
> bob@teraspeed.com
>
>
>
> -----------------------------------------------------------------
> |For help or to subscribe/unsubscribe, email majordomo@eda.org
> |with the appropriate command message(s) in the body:
> |
> | help
> | subscribe ibis <optional e-mail address, if different>
> | subscribe ibis-users <optional e-mail address, if different>
> | unsubscribe ibis <optional e-mail address, if different>
> | unsubscribe ibis-users <optional e-mail address, if different>
> |
> |or email a request to ibis-request@eda.org.
> |
> |IBIS reflector archives exist under:
> |
> | http://www.eda.org/pub/ibis/email_archive/ Recent
> | http://www.eda.org/pub/ibis/users_archive/ Recent
> | http://www.eda.org/pub/ibis/email/ E-mail since 1993
>

-- 
Bob Ross
Teraspeed Consulting Group LLC     Teraspeed Labs
121 North River Drive              13610 SW Harness Lane
Narragansett, RI 02882             Beaverton, OR 97008
503-750-6481                       503-430-1065
http://www.teraspeed.com           503-246-8048 Direct
bob@teraspeed.com
-----------------------------------------------------------------
|For help or to subscribe/unsubscribe, email majordomo@eda.org
|with the appropriate command message(s) in the body:
|
|  help
|  subscribe   ibis       <optional e-mail address, if different>
|  subscribe   ibis-users <optional e-mail address, if different>
|  unsubscribe ibis       <optional e-mail address, if different>
|  unsubscribe ibis-users <optional e-mail address, if different>
|
|or email a request to ibis-request@eda.org.
|
|IBIS reflector archives exist under:
|
|  http://www.eda.org/pub/ibis/email_archive/  Recent
|  http://www.eda.org/pub/ibis/users_archive/  Recent
|  http://www.eda.org/pub/ibis/email/          E-mail since 1993