RE: [IBIS-Users] input stimulus for extracting IBIS VT curves

Andy,

Regarding your comments on the transistor models, while it is
a reasonable concept, it usually doesn't matter. I verified
this by sweeping the stimulus edge rate from very fast to very
slow, and overlaid the output waveforms, and didn't see any
difference, other than a time shift due to the input stimulus
reaching the threshold levels increasingly later with the slower
slopes. This can be explained by the fact that most transistor
buffer models have multiple stages in them, and the final stage
is so far removed from the first stage that its waveform is
only determined by the transistor sizing relationships between
the stages, but not by the stimulus waveform.

Your reasoning would only be true when the transistor model
doesn't have any pre-driver stage(s), or very few of them. But
I would strongly advise against using such transistor models
for IBIS data extractions, because if the output waveform is
a factor of the stimulus, then the model maker can end up making
all kinds of incorrect IBIS models if they don't know what the
waveform looks like on the core side of this model.

Regarding the stimulus to the B-element (or the like), it is true
that in terms of its operation it doesn't matter what the edge
rate is, the output waveform will still be the same. But you
need to consider that due to the slow stimulus waveforms you will
get a certain delay in the transition. This can cause complications
if the user forgets that the stimulus supposed to be a 0-1 volt
waveform, and the thresholds are set to 0.2 and 0.8 if I remember
correctly. I have seen many times people applying a source to the
input of the B-element with a voltage swing that used the supply
voltage value of the buffer, for example 0 to 3.3 volts. With a
slow stimulus edge rate, the rising edge will trigger sooner than
the falling edge very noticeably. (In other words you get duty
cycle distortions from the IBIS model while the transistor model
didn't do that).

Even if this was handled properly by the user, similar shifting
could occur due to the different stimulus edge rates between the
transistor and IBIS buffer stimuli. I had people often show me
"discrepancies" between the transistor waveforms and the IBIS
waveforms which were due to that, and they had no clue why the
IBIS and transistor waveforms were shifted apart in time.

These are the reasons I am suggesting the use of fast edge rates...

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

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf Of Andrew Ingraham
Sent: Wednesday, August 25, 2010 11:14 AM
To: ibis-users@eda.org
Subject: Re: [IBIS-Users] input stimulus for extracting IBIS VT curves

I disagree that the edge should be "as fast as possible".

For the IBIS models used in HSPICE, my recollection is that HSPICE
treats it as a switch, waiting until the (digital) input waveform
crosses a threshold, then switching the IBIS model. The edge rate of
that digital stimulus waveform therefore shouldn't matter ... except
that very slow edge rate causes a known time delay before the IBIS
modeled device switches, and extremely fast edge rates might make the
simulation run slow or lead to convergence problems. Theoretically
it's good to make it fast so that it works like an instantaneous
binary switch, but there could be practical problems if you try to do
that.

For the transistor (SPICE) models, ideally you should attempt to match
the actual waveform (edge rate and amplitude) that is at that point
inside the IC, NOT the "fastest possible edge rate". Hopefully that
is what the IBIS modeler did too. Using a faster edge rate than what
those transistors normally see, makes it unrealistic.

Andy

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