Subject: [IBIS] Comments on BIRD 75.5, part II [LONG]
From: Mirmak, Michael (michael.mirmak@intel.com)
Date: Fri Oct 11 2002 - 11:25:32 PDT
All,
Please find enclosed additional comments on BIRD 75.5.
(PART II)
USAGE ISSUES:
1) There is little distinction made in the document between [External Model]
and [External Circuit]. Most of the descriptive language for the two
keywords appears to be exactly the same. The only minor difference I can
perceive is that [External Model] is only permitted within the [Model]
keyword scope, while [External Circuit] can appear almost anywhere (see
below). Why have two distinct keywords with mostly common subparameters for
similar functions?
2) [External Circuit] is explicitly described as being at the same level in
the spec "tree" as the [Model] keyword; in other words, [External Circuit]
data sections can appear anywhere that [Model] sections can. However, the
allowable placement of [External Model] data is not so well-defined. The
examples and language imply that [External Model] must appear as part of a
[Model] section. However, this is not explicitly stated anywhere as a
requirement. Must [External Model] data appear only within a [Model]
section?
"SPECSMANSHIP":
1) Part of the [External Model] description includes a summary of the
required keywords and subparameters from previous versions of the IBIS
specification. There is no obvious reason why a 4.x-compliant model which
uses the new BIRD 75.5 keywords should also be *required* to contain legacy
parameters like c_comp; these legacy tokens should be legal but not required
and therefore ignored if 75.5 extensions are used.
A parser should be able to easily distinguish between the various cases
which might therefore appear (assuming "IBIS 4.x" includes BIRD 75.5
support):
- IBIS ver. = 3.2 or below
* BIRD 75.5 keywords are used = ILLEGAL
* no BIRD 75.5 keywords are used = LEGAL
AND c_comp, etc. appear
- IBIS ver. = 4.x
* no BIRD 75.5 keywords are used = LEGAL
if c_comp, etc. appear
* BIRD 75.5 keywords used, no c_comp, etc. = LEGAL
* BIRD 75.5 keywords AND c_comp appear = LEGAL
(c_comp, etc. ignored)
The requirement for legacy keywords in combination with BIRD 75.5 extensions
should be removed.
2) There appears to be no reason, from a user perspective, for the [Node
Declarations] keyword. A listing of the internal nodes may be convenient
for the parser or tool to allocate memory space, etc., but this listing
could also be generated by a more rigorous analysis of the data under the
other keywords.
DOCUMENT ORGANIZATION AND CLARITY:
1) Overall, the document is not structured to be optimally clear to the
reader. The best approach would be to introduce the reader to the how the
keywords "fit together" in an overview section with some brief examples
before becoming involved in the implementation details.
More specifically, having the summary of the new keywords added at the
beginning of the document is a great idea. However, instead of leading
almost immediately into the reference diagrams, the document should continue
with a few short, conversational paragraphs which explain how [External
Model], etc. are to be used and what kinds of data they should include. An
example *should* be shown, but only with a brief text description of the
parts of the example system and explanations of how the new keyword concepts
apply ("You would then describe the individual buffers of the FPGA using the
[External Model] keyword and data, with the on-die interconnect and
packaging details covered in an [External Circuit] section."). A flowchart
or conceptual diagram would help the reader much more than specific
connection diagrams.
2) The [Circuit Call] example uses extremely generic parameter values which
could confuse the reader (the [Circuit Call] model name is "A," the
Signal_pin is "1," etc.). More descriptive names should be used and the
document should specifically refer the reader to the earlier diagrams.
SUMMARY
BIRD 75.5 appears to include the details and structures necessary to
incorporate other modeling standards within the general IBIS format.
Unfortunately, the document is not yet clear enough to allow the average
reader or model author to create a multi-lingual model. Personally, even
after several readings, I still do not have an unambiguous idea of how to
combine [External Model], [External Circuit], [Circuit Call], etc. to create
a syntactically-correct model. I cannot imagine how someone new to IBIS
modeling would interpret the BIRD 75.5 language before attempting to create
a compliant model. Rather than make more recommendations on improving
specific areas of the BIRD, I would simply suggest that the structure of the
document be re-examined with the capabilities and experience of the likely
model author in mind.
- Michael Mirmak
Intel Corp.
michael.mirmak@intel.com
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