Subject: Question regarding the [POWER_clamp] curve

From: Prasad Rau <[email-masked]>
Date: Wed, 11 Jul 2001 12:15:06 -0700

Hi,

I am modelling onchip termination for certain IO standards and unfortunately the [POWER_clamp] curve
puts out information from -VDD to 0v. The region of interest( termination effects) is 0v to +VDD. Is there a way to input information
( without modifying the source code of the simulator ) such that the IBIS file spits out -VDD to +VDD in the
[POWER_clamp] curve. By the way, the termination is modelled as a split termination ie 50 Ohms to VDD and
50 Ohms to GND. These are not passive devices but active( transistors) devices.

Thanks in advance,

Regards,

Prasad Rau
Xilinx

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From: Mike LaBonte <[email-masked]>
Date: Wed, 11 Jul 2001 16:00:03 -0400

Prasad,

The numbers in an IBIS POWER_clamp curve will go from -VDD to 0V,
but this is because they are relative to VDD and inverted. They
are not absolute voltages. Basically, positive voltage values
in the POWER_clamp curve correspond to actual voltages below VDD,
and negative curve voltage values correspond to actual voltages
above VDD.

You can extend the data into the positive region to model a
terminator, and I believe any IBIS simulator will simulate the
entire curve as expected.

Mike

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From: Bob Ross <[email-masked]>
Date: Wed, 11 Jul 2001 15:54:40 -0700

Prasad:

To add to Mike LaBonte's response, it is permissible
to extend both [Gnd Clamp] and [Power Clamp] tables over
the full -vdd to 2*Vdd range.  For internal terminators,
this would be a good idea.

If you have ideal 50 ohm resistors to Gnd and also to
Vdd, the [Gnd Clamp] and [Power Clamp] tables might
look like this for Vdd = 3.3 V:

[Gnd Clamp]
-3.3 -66ma  -66ma  -66ma
0      0      0      0
6.6  132ma  132ma  132ma

[Power Clamp]
-3.3  66ma   66ma   66ma
0      0      0      0
6.6 -132ma -132ma -132ma

For your active devices, you would need to find a
method to extract the actual (transistor) table
information and to properly decompose the extracted
information into the two tables.

A presentation that proposes improvements to
s2ibis2 for internal termination situations was
given under:

  http://www.eda.org/pub/ibis/summits/oct99/nasef.zip

Bob Ross
Mentor Graphics

> puts out information from -VDD to 0v. The region of interest( termination
> effects) is 0v to +VDD. Is there a way to input information
>

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From: Mike LaBonte <[email-masked]>
Date: Wed, 11 Jul 2001 19:34:20 -0400

Bob,

I found out that Prasad's issue is indeed how to get s2ibis2
to produce an IBIS file where drivers with internal termination.
My proposed workaround is run s2ibis2 twice, once with model type
Terminator, and steal the clamp curves to use in the other IBIS
file. But I tried it and found that s2ibis2 seems to double count
the resistance, because it appears in both the POWER_clamp and
GND_clamp curves. Does it seem reasonable to steal only one of
the curves from the Terminator run to include in the IBIS file?

Mike

P.S. I find that the Nasef presentation contains unresolved
     references to image files.

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From: Bob Ross <[email-masked]>
Date: Wed, 11 Jul 2001 17:09:05 -0700

Mike:

The proper way is to improve the s2ibis2 utility or
the surrounding processes as you attempted to do.

I was able to download and read the nasef.zip file.
I can send you privately a copy if you have trouble.

More responses are in your response.

Bob
Mentor Graphics

> Mike LaBonte wrote:
> GND_clamp curves.

This may be a flaw in the s2ibis2 algorithms.  Also, s2ibis2
never was designed to handle internal terminators.

> Does it seem reasonable to steal only one of

No.  The proper method is to figure out a way to partition
the currents to the proper rails.