RE: Subtracting clamp curves from pull-up/down curves

Stephen,

Here is how I do it.

When the buffer is 3-stated, I do two sweeps. One GND relative,
and another one Vcc relative. Notice that these two measurements
have the same information in them, they are just referenced
differently.

I also make two measurements when the buffer is driving (provided
it is complementary with both pullup and pulldown). The pulldown
curve is swept GND relative, and the pullup is swept Vcc relative.

I do all four of these sweeps from -Vcc to 2*Vcc. Because of this,
both GND and Vcc clamps will be present in all four of the curves.

When I subtract, I do

  pulldown_sweep - GND_rel_3state_sweep => this will be my [Pulldown]
  pulldup_sweep - Vcc_rel_3state_sweep => this will be my [Pullup]

Then I cut off the power clamp part from the GND_rel_3state_sweep and
the ground clamp part from the Vcc_rel_3state_sweep data to avoid double
counting the clamp currents.

That's all you need to do if you have a simple CMOS buffer.
I hope this helps,

Arpad Muranyi
Intel Corporation
=====================================================================

-----Original Message-----
From: Stephen Nolan [mailto:s-nolan1@ti.com]
Sent: Wednesday, July 05, 2000 9:16 AM
To: ibis-users@eda.org
Subject: Subtracting clamp curves from pull-up/down curves

Hello IBIS gurus,

When creating a model for a three-state device, it is necessary to subtract
the
clamp curves from the pull-up/down curves to avoid double counting, as the
EDA
tool adds the clamp-curves back in to the pull-up/down curves to obtain the
devices output charactersitics in the enabled state. Right so far?

The question is this, do you only subtract the corresponding clamp curve
from
the related pull-up/down curve, or do you subtract BOTH clamp curves from
each
pull-up/down curve?

For example, if I am creating the pull-down curve, do I subtract only the
ground-clamp curve, or do I subtract both the power-clamp and ground-clamp
curves? Does the EDA tool add only the ground-clamp data back in, or does it
add
both curves back in?

-- 
Regards,
Stephen M. Nolan