[IBIS-Users] Port type with [External Circuit]

Hello IBIS fans,

We discovered a potential problem with the port types when using
[External Circuit]s.

The "Ports under [Model]s" section of the spec discusses the naming
convention of the reserved port names used with [External Model].

| The first letter of the port name designates it as either digital ("D") or
| analog ("A").  Reserved ports 1 through 13 listed above are assumed or 
| implied under the native IBIS [Model] keyword.

Later, the "Ports under [External Model]s" section describes the port name
rules for [External Circuit]s:

| The [External Circuit] keyword allows the user to define any number of ports
| and port functions on a circuit.  The [Circuit Call] keyword instantiates
| [External Circuit]s and connects their ports to specific die nodes (this can
| include pads).  In this way, the ports of an [External Circuit] declaration
| become specific component die nodes.  Note that, if reserved digital port
| names are used with an [External Circuit], those ports will be connected
| automatically as defined in the port list above (under [External Circuit],
| reserved analog port names do not retain particular meanings).

From these sections of the spec we know how to deal with the ports of
[External Model]s, i.e. it is known what their type is, but there is a
problem with [External Circuit]s.

Other than the usage of the reserved digital port names, there is no way
of knowing what the port type is for a user defined port name, because we
do not have a mechanism to declare port types in IBIS.  Unfortunately the
[Node Declaration] keyword doesn't do more than list the die nodes and/or
pads, so this keyword would not be useful to find out what the type of a
port may be that is connected to it.

This is a problem, because some tools need to know the type of a port in
order to know how to connect things.  Looking into the [External Circuit]
to find out the port type may not be as easy as it sounds either if only
compiled (or encrypted) models are available.

I would like to make two suggestions:

1)  If the "first letter rule" was only intended to apply to the reserved
port names (to be honest, this is not 100 % clear to me from reading the
spec.) we need to write a clarification BIRD to clear this up.

2)  If the "first letter rule" doesn't apply to user defined port names,
I would propose that we invent a port type declaration mechanism to declare
the user defined port types.

Any comments or questions are appreciated.

Thanks,

Arpad
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