RE: [IBIS-Users] Ramp calculation in O/P, Tri-state, I/O, buffer ibis model

Suraj,
 
In simple complementary CMOS buffers they usually turn off
one side of the buffer quickly before they turn on the other
side. When you do a falling edge, the pullup turns off quickly
and then the pulldown starts turning on (usually slower). If
you connect a resistor to the supply rail for the falling
edge, then the output is not going to do anything while the
pullup is being turned off. The resistor just keeps it at
the supply rail when it was to begin with. When the pulldown
starts turning on, the pulldown will pull the voltage down
while fighting the resistor, forming a voltage divider. This
way you get the turn on characteristics of the pulldown.
 
If the resistor was connected to GND, the pullup would be holding
the voltage high fighting that resistor. As soon as the falling
edge's transition begins, the pullup will let go and the waveform
will go to GND. This way you get the characteristics of the pullup
turning off.
 
The [Ramp] contains how the devices turn on. The [Ramp] doesn't
tell you how the devices turn off. This was a major deficiency
in IBIS 1.1 and this is why we added the waveform tables in IBIS
2.1 rapidly after IBIS 1.1 was released, and this is why we
recommend very strongly to use a minimum of four waveform tables
for each buffer model.
 
Arpad
=================================================================

________________________________

From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf Of suraj.sukumarannair@wipro.com
Sent: Monday, August 02, 2010 8:24 AM
To: ibis-users@eda.org; ibis@eda-stds.org
Subject: [IBIS-Users] Ramp calculation in O/P, Tri-state, I/O, buffer
ibis model

Hi All,

 I had a query regarding the ramp calculation done in ibis file. Why is
it that we consider the falling waveform w.r.t Vfix=vdd and rising
waveform w.r.t Vfix=0 tables instead of the other two tables while
calculating Ramp?

Regards

Suraj

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