Subject: RE: True or False?

From: "Muranyi, Arpad" <[email-masked]>
Date: Fri, 9 Oct 1998 09:31:00 -0700

Chris,

Thanks for the clarification.  I reread the section you pointed out to me 
(several more times) and I believe I am starting to understand the problem.

It seems to me that the problem is not that I am using the series model
together
with a terminator model, but that these were "coupled" as a differential
pair.

You made a statement that "the Diff Pins had to be NON-terminators" (I guess

Input, I/O, etc...).  Looking at the [Diff Pin] section of the spec,
however, I 
could not find anything that suggests that differential pins had to be a
certain
type of a model.  Also, I don't see anything in the [Diff Pin] section that 
calls a [Model](s) for the pins it defines as differential.  The [Model] 
keywords are still referenced only by the pin list section (some times
through 
the [Model Selector] keyword) even if we have a [Diff Pin] definition for
them. 
In my opinion then, it would be perfectly legal (even if it makes no sense)
to 
have two pins in the pin list defined as "NC" while defining them as 
differential pairs in the [Diff Pin] section.  If this is legal then calling

Terminator type models in the pin list and defining them as differential
pairs 
should also be legal.

Using this argument I would summarize that the model we made is NOT
violating 
the spec. because

1)  The pin list calls a model that is type Terminator
2)  Terminator model types are legal with Series models
3)  Applying the [Diff Pin] keyword does not change the model type
    already defined on the pin list to anything else
3)  Applying the [Diff Pin] keyword does not add models of any type
    to the models already defined on the pin list
4)  There are no restrictions for what model types can be associated
    as differential pair in the [Diff Pin] section

Please let me know whether this argument is acceptable or not.

Sincerely,

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

Arpad,

> 1)  Could you please point it out to me where the spec says that it is NOT

> allowed to have anything, except terminators on either side of the series 
> device?  I don't recall reading that, but might have missed it.

Yes, from ver3_2.a.ibs, see 2nd paragraph under Other Notes, sentence #2 
below.

<snip>
|     Keyword:  [Series Pin Mapping] 
<snip>
| Other Notes:  If the model_name is for a non-symmetrical series model, 
|               then the order of the pins is important.  The [Series 
Pin
|               Mapping] and pin_2 entries must be in the columns that
|               correspond with Pin 1 and Pin 2 of the referenced model. 
|
|               This mapping covers only the series paths between pins. 
The
|               package parasitics and any other elements such as 
additional
|               capacitance or clamping circuitry are defined by the
|               model_name that is referenced in the [Pin] keyword.  The 
|               model_names under the [Pin] keyword that are also 
referenced
|               by the [Series Pin Mapping] keyword must be either 'NC' 
or
|               reference a [Model] whose Model_type is 'Terminator'. 
Thus.
|               for example, a Series_switch model may contain 
Terminator
|               models on EACH of the pins to describe both the 
capacitance
|               on each pin and some clamping circuitry that may exist 
on
|               each pin.
<snip>

> 2)  Our device which has this situation actually does sense the signal
> differentially, but since it never drives and the clamps are so far out
from 
> the
> signal swing, we didn't include a receiver model (even though I was 
> considering
> to do it).

We've never had to model "paths through active devices" so that is where 
the problem comes in where a single pin is defined as both a recevier 
(customarily terminating the signal) and defined as a passive series 
device to another pin.

However using only the [Series mapping] only will work A.O.K for us as 
you state in #3 below assuming another component on the board joins 
these two nets differentially.

> 3)  It may be sufficient to define [Series mapping] only if we didn't have
a 
>
> receiver in the device.
>
> Since we just made an IBIS model like that, you got my curiosity up, is it

> really illegal to do this?  By the way, it passed the parser, so if it is 
> illegal, should the parser check for that?

The part I thought was illegal was the tying of a single pin together 
with other pins using BOTH [Series Mapping] and [Diff Pin] since the 
Series pins could only be terminators but the Diff Pins had to be 
NON-terminators.

The parser should check.

It sounds like you may wish to propose a BIRD to allow this type of 
thing in the future in which case we will enhance the capabilities of 
our software to handle.  Its just never come up before.  There's always 
been a buffer between Input and Output allowing us to decoupling the two 
sides of the analysis.

Chris

---

From: Chris Rokusek <[email-masked]>
Date: Fri, 09 Oct 1998 10:42:21 -0700

Arpad,

I wonder if this is what it feels like to be an attourney?...

> to have two pins in the pin list defined as "NC" while defining them as 
> differential pairs in the [Diff Pin] section. 

"EVEN IT MAKES NO SENSE" -- Abusing the spec in a insensible manner
promotes confusion and indicates that the spec should be revised to
either:

      A) eliminate the insensible use

          - don't use [Diff Pin] to assign a vdiff to a pair
            of "non-sensing" (excuse the pun) terminators.

      B) make the use sensible
      
          - Allow [Series Mapping] to be applied to a receiver
            as well as a terminator.

I don't think you should release a model that makes no sense even if
it's debatably "legal."

Chris