RE: [IBIS] *-AMS solves all of IBIS problems?

All,

There is an aspect of VHDL-AMS that hasn't been brought up yet. The
models are compiled into executable modules so they actually run very
fast, even for complex constructs like look-up tables, or complex
equations. Actually, a good programmer can make those run fast anyway.
So the compilation takes a few seconds the first time the model is used,
but then the executable module is faster. 

Maybe this was discussed a bit when we talked about protecting IP. The
model is essentially encrypted by making it an executable binary file.
Does anyone know if Verilog-AMS is compiled before running?

Later,
Weston


-----Original Message-----
From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Mike
LaBonte
Sent: Monday, April 04, 2005 1:47 PM
To: 'ibis'
Subject: RE: [IBIS] *-AMS solves all of IBIS problems?

I think I see where Scott is coming from. The lines of AMS code that
call hard-coded primitives will simulate very quickly. The lines that
call interpreted instructions, table lookups and equations will simulate
exceedingly slowly. The overall performance will depend on the mix of
these.
It can potentially cause an AMS model with relatively few lines to
simulate more slowly than a complex HSPICE model with more lines.

For illustration imagine an HSPICE circuit with a good number of
.measure statements and many equation-style controlled sources that use
the .measure outputs. My experience is that this can be horribly slow.
An AMS model might look like that. Analog simulators are able to
optimize convergence for circuits built only from primitives. The
rational approximations used to iteratively home in on the matrix values
for the next time step are well known. Simulators are less able to do
this with more flexible language elements.

The key to AMS performance may lie in using as few as possible of the
general purpose language constructs that make it so flexible. But for
many people those constructs might be the easiest to use. To make a fast
AMS model one may have to "be the simulator". But no one said it would
be easy.

Mike

________________________________

From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Scott
McMorrow
Sent: Monday, April 04, 2005 3:27 PM
To: Muranyi, Arpad
Cc: ibis
Subject: Re: [IBIS] *-AMS solves all of IBIS problems?


Arpad,

See my comments in blue:

scott


Muranyi, Arpad wrote: 

	Syed, Scott,
	
	I renamed that ugly subject line of mine... and would like to
	respond to this quick quote from Scott:
	
	  

		...that the implied assumption that AMS modeling will
somehow solve
		all of our IBIS modeling problems is sorely mistaken.
		    

	
	I will take that blame on myself, because I feel quite
optimistic
	about AMS these days, and may have said things that come across
	that way.
	
	I would like to ask Scott, please give some examples for why
not.
	Or can I take your next sentence as an example?
	
	  

Thus far, all of the "correlations" between *-AMS and either HSPICE or
measurements have been rather simplified models.  Quite frankly, when
you take a look at the Altera correlation, it ain't that close to
measurement either, and it's not clear whether it is the interconnect
modeling and simulation that is at fault, the power delivery modeling,
or the device modeling.  We've found not just a few SerDes drivers which
operate quite a bit differently than even their HSPICE models of just
the driver would predict, when transmitting long differential data
streams.  We suspect that the differences between modeling and
measurement occurs because of lack of package power delivery modeling,
and integration of this  into the driver model.  Once you add the full
power delivery path and a quad-SerDes into the model, will the results
be that encouraging?  I hope so. However, I am on record as having
misgivings. Personally, I think that correlations of *-AMS should start
at this level of complexity, so that a fully correlated solution is
shown, before delving into model order reduction techniques to speed  up
performance.
.


			Interaction between the silicon and that pesky
passive
		interconnect must be dealt with simultaneously, if we
are to believe
		that our simulation results have true high-fidelity to
measured
		results.
		    

	
	Why would this not be solvable by writing models with *-AMS?
	The way I see it, this is really just up to the model writer
	what they characterize and how it is written up in the *-AMS
	model.  It doesn't seem to be a language limitation to me.
	Am I missing something?
	  

You are not missing a thing. I believe that *-AMS can be used to model
anything.  The question is whether it will be competitive with HSPICE in
performance once all of the imporatant details (gate modulation,
feedback, power starvation, resonances, frequency dependent power
delivery, etc ...) are modeled.  This is the unresolved question.  My
gut tells me no, since *-AMS is designed to be a more general language.
I will be happy to learn that I am wrong.  


	Arpad
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