[ibis-macro] Re: [Model] and [external Model] Voltage Levels
- From: "Muranyi, Arpad" <Arpad_Muranyi@xxxxxxxxxx>
- To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx>
- Date: Mon, 29 Apr 2013 20:39:43 +0000
Walter,
Since you are quoting me, I feel I need to respond, although
it seems that you are soliciting a response from others too.
Let's go through some facts.
In the case of a [Model] (without [External Model]), we have
a collection of I-V curves, [Ramp] and optionally V-t tables
and a few more items. There is nothing in a [Model] that
serve as a voltage source in the sense of a circuit element.
The [Voltage Range] and related keywords give the information
to the EDA tool or the user about the supply voltage that the
[Model] data was extracted, so that they would know how to power
it up. The IBIS specification doesn't say how the power should
be connected to the [Model] or who is responsible to do so. The
EDA vendor can choose to make it easy for their user by providing
an automated way, or let the user put it on their schematics
manually (like the HSPICE B-element does it).
The package modeling keywords existed in IBIS way before the
days of [External ***], and the capability was there for the
user to run power integrity simulations (as far as the IBIS
specification was concerned). Not all tools implemented this
capability the same way, but if you were an HSPICE user, you
could put all kinds of parasitics around the B-element and
enjoy watching the bouncing power and GND nodes. In my Intel
days, I worked with the HSPICE guys pretty closely and they
implemented the I-V curve scaling feature on my request, which
later became BIRD 95/98 and the ISSO keywords. This feature
allows the user to generate more realistic simulation results
because the actual (bouncing) supply's deviation from the
IBIS (nominal) supply value is used as a scaling coefficient
on the I-V curves.
Of course if the user turns on the internal supply voltage
option of the B-element, all this power integrity capability
is gone, and if they are not careful in their netlist, they
can occasionally get inductor / voltage supply loop errors...
The point I am trying to make here is that this was all
happening before [External ***] existed or was used for
anything in this context, and that the question you are
raising about where the supply source is added to the
simulation is really a decision the user or the EDA tool
vendor has to make.
I don't see anything different in this picture when there
is an [External Model] in the [Model]. The only difference in
this case is that we don't have control over what a model maker
puts into an [External Model] and therefore it can be written
so that it includes an ideal DC source to power itself. In
this case the model's author should have enough brains to
either not connect these sources to the power terminals of
their model, or provide a switch to turn them on or off (i.e.
to disconnect them from the power terminals of [Model]),
because multiple such models are connected together, they
will most likely generate an inductor/voltage source loop
error. The model maker should also make it clear to the user
that this type of model is not suitable for power integrity
simulations, since it has its own DC power supply built in.
But note that while it is possible, it would be quite weird
for the model maker or user to rely on one such a [Model] to
power another (unpowered) [Model].
Going back to your questions, I don't think that there was any
intent or requirement in the IBIS specification for the model
to supply itself. What the B-element does is a nice feature,
but it is a shorthand feature for the user's benefit, and not
something that the IBIS specification requires or controls.
Regarding:
"Does this mean that the subckt generate A_puref, or the EDA tool must supply a
DC voltage source with value [Voltage Range] ([Pullup Reference]). In a power
aware simulation in only makes sense that the EDA Tool supply a signal to
A_puref with a value ~[Voltage Range]."
No, the subcircuit should not generate a voltage. When did you
see a buffer on silicon that acts as a power plant? :) The supply
related keywords provide the information to the user or tool so
that they know what the nominal value is for the supply. And even
if the power delivery network contains a bunch of losses and bounces,
there is somewhere at the end of the chain some source that will need
these values, right?
Regarding:
"This implies that the EDA tool supplies the Pullup Voltage in the range of typ
min max. But of course this says than a Max (Fast) simulation cannot supply
more than max, and a Min simulation cannot supply less than min, but this make
no sense if there is fluctuations of the reference voltage."
No, it doesn't mean that. The meaning is that the I-V and V-t data
was extracted at these values, and if your tool's [Model] algorithm
is smart enough to account for the scaling of the I-V curves due to
voltage variations, the user or simulator can apply any voltage to
the [Model] and still get more or less accurate results by modulating
the [Model]'s drive strength based on the actual voltage at its terminals.
BIRD 95/98 was added to the IBIS specification to help in doing this
based on data in the .ibs file instead of guesses.
Regarding:
"So we clearly cannot use [External Model] to do power simulations since the
PuRef must be fixed."
None of the supply references must be fixed. There is nothing in
the IBIS specification that says so, as far as I can tell. OK, now
we can start arguing over what those red highlighted words mean in
the spec. What I am telling you above is the intent I remember
when we were working on the earlier versions of the specification.
Thanks,
Arpad
====================================================================
From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx]
On Behalf Of Walter Katz
Sent: Saturday, April 27, 2013 1:23 PM
To: IBIS-ATM
Subject: [ibis-macro] [Model] and [external Model] Voltage Levels
All,
IBIS 5.1 says (p 95):
The rules for pad connections with [External Model] are identical to those for
[Model]. The [Pin Mapping] keyword may be used with [External Model]s but is
not required. If used, the [External Model] specific voltage supply
ports-A_puref, A_pdref, A_gcref, A_pcref, and A_extref-are connected as defined
under the [Pin Mapping] keyword. In all cases, the voltage levels connected on
the reserved supply ports are defined by the [Power Clamp Reference], [GND
Clamp Reference], [Pullup Reference], [Pulldown Reference], and/or [Voltage
Range] keywords, as in the case of [Model].
Does this mean that the subckt generate A_puref, or the EDA tool must supply a
DC voltage source with value [Voltage Range] ([Pullup Reference]). In a power
aware simulation in only makes sense that the EDA Tool supply a signal to
A_puref with a value ~[Voltage Range]. Arpad claims that the [External Model]
subckt can either consume A_puref or generate A_puref.
On page 45 it says:
Keyword: [Voltage Range]
Required: Yes, if [Pullup Reference], [Pulldown Reference], [POWER Clamp
Reference], and [GND Clamp Reference] are not present
Description: Defines the power supply voltage tolerance over which the
model is intended to operate. It also specifies the default voltage rail to
which the [Pullup] and [POWER Clamp] I-V data is referenced.
This implies that the EDA tool supplies the Pullup Voltage in the range of typ
min max. But of course this says than a Max (Fast) simulation cannot supply
more than max, and a Min simulation cannot supply less than min, but this make
no sense if there is fluctuations of the reference voltage.
So we clearly cannot use [External Model] to do power simulations since the
PuRef must be fixed.
Arpad's statement in the Interconnect meeting that an External Model can either
consume or generate the PuRef without an indication to the EDA tool of what it
is going to do. The HSPICE B element has a switch to do it either way. External
Model does not define such a switch.
Walter
Walter Katz
wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>
Phone 303.449-2308
Mobile 303.335-6156
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