RE: [IBIS] Open drain strange behaviour

Hello, Roberto,

One of the first questions to ask oneself is: how does the shape of the two
waveforms differ?  If your SI tool's waveform matches your SPICE waveform
with a time offset, your are probably seeing an effect of how the buffer
DATA (from chip core) is triggered.

In SPICE, waveforms show Time from Time=0 (which is start of simulation).
But any "step" input is actually a ramp between two simulation time steps.
This ramp rate can vary between simulations, since the time step depends on
the "stop time" and other .OPTIONS settings.

In many SI simulators, the buffer's DATA "trigger" is a fixed ramp.  The
ramp slope and trigger level for the buffer DATA are different between
simulators.

The result is that simulators, from SPICE to SI tool1 to SI tool2, etc,
will each produce a different Time offset,   but the waveform shape should
match (for the same R_fixture load).

If your waveforms do not match, then you have a different problem.  In that
case, you might wish to submit your model to the IBIS Model Review Committee
for feedback (http://www.eigroup.org/ibis/support.htm).

Best regards,
Lynne


"IBIS training when you need it, where you need it."

Dr. Lynne Green
Green Streak Programs
http://www.greenstreakprograms.com
425-788-0412
lgreen22@mindspring.com


-----Original Message-----
From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Roberto
IZZI
Sent: Monday, September 11, 2006 5:17 AM
To: ibis@eda-stds.org; ibis-users@eda.org
Subject: [IBIS] Open drain strange behaviour

Hello everybody

  I 'd like to have some information about a strange behaviour of Open drain
buffer
  Ibis model. I have noticed a mismatch between Transistor level and Ibis
model
  voltage output during a transient analysis. In fact in presence of the
same input voltage
  wave and the same value of load resistance (for example 70 ohm),
  we can observe a delay between TL output and Ibis output. This delay is
the same
  if we change the value of  load resistance. What is the cause of this
strange behaviour?

  Thanks and best regards

        Roberto Izzi


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