Subject: [IBIS] BIRD 75 comments
From: Muranyi, Arpad (arpad.muranyi@intel.com)
Date: Fri Aug 09 2002 - 12:20:44 PDT
All,
This is a public posting of my message to Bob Ross from last night
regarding BIRD 75. Based on some private discussions and the Open
Forum meeting this morning, I added the last item (#20). Item #5
was my misreading of the BIRD, and it was clarified to me by Bob,
I just left it in there for completeness, and to see if anyone
else has a problem understanding it.
I would like to encourage everyone to comment and express their
opinions so we could come up with a robust solution for our needs.
Thanks,
Arpad
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Bob,
Here are my comments on BIRD 75.2.
Some of the wording is still not entirely to my liking, but I can
live with that, or we can do an editorial session once again on
section 6b. I think this is important, because we have to make
sure that the reader who has never hear of this can understand
what is being said here.
Technical issues:
1) The 3rd and 4th drawings do not label the core side of the nodes.
I think they should also be labeled.
[External Model] section:
2) In general, I wonder whether this is the right way of doing it,
I mean to have this inside the [Model] keyword. Our last discussion
on true differential buffers and the associate rules bothers me a lot.
So I wonder what is the reason to want to have this [External Mode]
keyword inside the [Model] keyword. Is it for its subparameters
(C_comp, Model_type, Vinh, Vinl, etc)? For one, only two of these
are required, all the other ones are optional. Second, if we had
the [External Model] as an alternate to the [Model] then the [External
Model] keyword could have its own set of (duplicate) subparameters
analogous to the [Model] keyword. I know this is kind of duplication
but this way some of those ugly rules would go away, and the two
model keywords would be independent. (Similar to the [Nodal Path
Description] and the [Tree Path Description] keywords in the
ICM spec. Doing it this way would also make me happier because
we could perhaps combine the [External Circuit] and [External Model]
keywords as I commented the first time. This would also make BIRD 77
unnecessary, because those subparameters could be added for the
[External Model] keyword. We could also combine everything that
is under the [Model Spec] keyword and do it right for once all
under the [External Model] keyword.
3) Regarding the typ., min., max corners, Lynne asked for capability
of having more than just three.
4) It is still not clear to me how the .LIB statement would work with
the subcircuit calls as you show it. Process files are usually
separate from the netlist, and here we only have netlist references
only.
5) Regarding Ports, it is not clear to me whether these include the
internal nodes labeled (a) - (j) as well as the pad nodes which
correspond to the inside of the package.
*** This was my misunderstanding of the text, please ignore.****
6) Regarding D_to_A and A_to_D it is not clear from this writing
whether these are an "element" or parameters to an element.
If they are elements, then I wonder whether they should be
lifted out from being subparameters and make them keywords.
If they are just parameters to an element that resides inside
the [External Model] itself, then this needs to be explained more
clearly.
7) In the port description under A_to_D and D_to_A you describe
port1 and port2 as 1 referenced to 2. In that case isn't this
just one port having a + and a - node (two terminals).
8) Also, I believe that you mistakenly used A_receive twice in this
section instead of D_receive.
9) In the "true differential" buffers section you also say "two
differential ports", which is really ONE differential port.
10) The last section before the examples talks about the regular
keywords under [Model] being useful for things if this is still
not supported, etc. It would be nice to spell out what is
required and what is optional from the old stuff if this new
stuff is there.
11) The paragraph before the [External Circuit] is misleading,
because it makes me believe that the [External Model] keyword
can be placed anywhere in the document as it is with the
[External Circuit].
12) Regarding corners typ., min., max, comment same as above,
Lynne wants more then three.
13) [Node Declaration] keyword is required for internal nodes.
What is an internal node? Are the die pads considered internal
nodes, since they are not listed on the pin list?
14) The third [Circuit Call] example with BUS_SPI is confusing
because the port names are not identified on the 4th drawing
up front. Without those showing on the drawing I have a hard time
to identify the connection between the pin to circuit, and then
circuit to internal node, and then internal node to buffer port
name.
15) There are a couple of "}" in this section.
16) The last paragraph before the true differential example uses
200m for Vlow etc. It should be spelled 200mV. Otherwise it
means meters.
17) In the example(s) you mention C_comp. This raises the question:
With external models is it still required to have C_comp under the
[Model] keyword? This is not mentioned anywhere (at least I couldn't
find it). This doesn't make sense, because the external
model could probably do a much better job in describing C_comp.
On the other hand, if yes, than we need to have at least a C_comp_diff
that is a capacitance between the two signal pins... Not to speak
about the new ideas Luca proposed. I just found it now at the
interaction limits. I wonder if it should be mentioned earlier.
18) It should be mentioned somewhere in the "Reference Voltages"
section that the an external model using SPICE should not have
any global nodes (most often Vdd and Vss) because that will short
them together across all of them, and thereby making the external
circuit useless. (Global scaling factors could also be mentioned
for similar reasons).
19) In the interaction limits you mention "series switch" models
using the [External Circuit] needing the [Series Pin Mapping].
I think this is also true for [Series Current] and [Series MOSFET]
and I would mention them here too.
20) SPICE and AMS models can be parameterized very well. It would
be a pity not utilizing this capability. However, I don't see any
mention and provisions for bringing out the parameters from an
[External Model] or [External Circuit] call to the user interface
of the EDA tool. I think this could be done easily, we just need
to define how.
Arpad
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