[IBIS] Comments on BIRD145

Dear BIRD 145 Authors,

I took some time to study BIRD 145 and have the following
comments.

#1) Please use page number references to the PDF file
version of the specification instead of line numbers. Most
BIRDs are written that way, and not everyone has text editors
with line number capabilities readily available.

#2) The Statement of the Issue section says the following:

"In this BIRD, we show how an IBIS I/O model can be included
in an [External Circuit] using the new [Model Call] keyword."

Two comments. BIRDs are not to "show" or "prove things" as
in a scientific paper which explains / proves a new discovery
or theory. A BIRD contains a proposal for the specification
for correction, or improvement. Please change the wording to
reflect that.

Second, this BIRD doesn't seem to propose the inclusion
(or encapsulation) of IBIS [Model]s into [External Circuit]s.
My reading of this BIRD gives me the impression that the
proposal describes a syntax for cascading IBIS [Model]s and
[External Circuit]s (side by side). Please try to say what
you mean, otherwise people will get confused...

#3) The following statement is strictly speaking not true:

| Description: This keyword is used to instantiate IBIS buffer [Model]s
| to connect their reserved ports to the die nodes or die pads
| of an [External Circuit]

because [External Circuit]s don't have die pads. They
are connected to the die pads, but don't have their own
pads. Please correct the wording.

#4) This is not clear in the Usage Rules section:

" When a [Model]
| is called in a circuit,"

What do you mean by circuit? What circuit is this
referring to?

Also, "T-V" should be spelled "V-T" throughout the document
because it is voltage vs. time, not time vs. voltage.

#5) The following sentence is inconsistent with the rest
of the concept in this BIRD:

" The A_signal
| port is connected to the die pad, to drive or receive an analog signal."

because the whole point of instantiating a [Model] from
[Circuit Call] is to cascade it with an [External Circuit],
and not to connect it directly to a die pad.

#6) The [Pin] section in the example is incorrect, because
the model_name of those pins which have [External Circuit]
has to be "CIRCUITCALL" according to pg. 133 in the specification:

| When a [Circuit Call] keyword defines any connections that
| involve one or more die pads (and consequently pins), the
| corresponding pins on the [Pin] list must use the reserved
| word "CIRCUITCALL" in the third column instead of a model
| name.

So the model name for pins 5 and 9 should show "CIRCUITCALL"
(or perhaps an equivalent to it called "MODELCALL"). If you
decide to use "MODELCALL" you will have to add modifications
of the corresponding places in the specification into this
BIRD.

#7) The existing specification has a few subparameters for
[Circuit Call] which should be also considered for [Model Call]:

Signal_pin,
Diff_signal_pins,
Series_pins

These have the purpose that the tool can find which buffer
to stimulate when the user picks a net they want to simulate.
I would highly recommend that we add these subparameters to
[Model Call] also. This would also make the two keywords
more consistent with each other. However, as opposed to the
prohibition of pseudo-differential buffers with [External
Circuit] calls (see pg. 129), I would also allow
pseudo-differential [Model]s or [External Model]s to be
placed behind an [External Circuit]. You will also have to
make provisions for having true differential models behind
[External Circuit], since [External Model]s can do that too.

#8) In Figure 1, the GND pin is #11, but inside the drawing
you show #12. Later in the examples you are using 12. I
think they should all be 11 or 12 but not mixed.

That's all for now.

Thanks,

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

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