RE: [IBIS-Users] IBIS Simulation algorithm

Andy,

A couple of comments (not arguments)...

You are right, that loading plays a big role on how good the
waveforms will be with [Ramp] vs. waveform tables. But the
issue is not necessarily how "heavy" the load is, but more
what kind it is. If, for example, there are lots of reflections
which come back to the driver while it is still in the transition,
you may need the waveforms to get more accurate results, because
the equivalent circuit of the reflections may look like one or
the other of those resistors I talked about in my previous post.
On the other hand, if the lines are really long, and the
transition starts from a quite state, the equivalent circuit
my look like the loading of the [Ramp] data.

Regarding crow bar currents, designers may actually deliberately
create overlapping pullup and pulldown transistors for slew
rate control. (I would call this a poor man's slew rate control,
because it comes relatively cheap, but has a lot of drawbacks)...

So much for now...

Arpad
===================================================================

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf Of Andrew Ingraham
Sent: Sunday, August 29, 2010 10:09 AM
To: ibis-users@eda.org
Subject: Re: [IBIS-Users] IBIS Simulation algorithm

Arpad,

I don't disagree with what you wrote. I was just attempting to shed
light on the question, how does (and did) IBIS work without V-T
tables, which is where IBIS got its start.

For some devices, especially ones used in slower circuits or with
heavy output loading, one might get by OK with only the simpler model
in the simulator where the buffer changes from one state to the other
and you have no knowledge about exactly how or when one device turns
off and the other turns on. And maybe that was good enough ten or
twenty years ago. But with today's modern components and bus speeds
it may be important to know more than that about how the buffer
switches, with V-T tables as your guide.

My recollection is that, after the V-T tables were added to IBIS,
Hspice continued to not use them for quite a while. They were ignored
if present in the IBIS data sheet.

I would guess there are still quite a number of parts where there was
little attempt to avoid the momentary crowbar current when switching.
I think that anyone designing a CPU or other large pin count IC today
would be foolish to do that; but for small pin count jelly-bean parts
(yes, some people still use them), it was unnecessary. There were a
few parts with notoriously bad switching noise, but others weren't so
bad.

Regards,
Andy

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