IBIS-USERS - EIA IBIS Open Forum Users Activity: RE: [IBIS-Users

From: <birendra.rana@wipro.com>
Date: Thu Sep 09 2004 - 21:32:34 PDT

Arpad,

The s2ibis2 actually does what Ambarish had mentioned. It subtracts the
Power clamp current from the Pullup currents before placing the data in
the [Pullup] tables. So what can be noticed from the s2ibis2 generated
IBIS models is that from Vcc to 2Vcc the [Pullup] data has subtracted
Power_clamp, whereas the remaining (-Vcc to 0) the currents are same as
extracted from the enabled 3-state buffer.

Interestingly the I/O Buffer Modelling Cookbook, prepared by the IBIS
open forum, has the following description (IBIS Forum I/O Buffer
Modeling Cookbook, Rev 2.0.X, Page 19) :

Pulldown
Enter into the [Pulldown] table the I/V data gathered while the output
or I/O buffer was in the logic low
state. Pulldown data must cover the range of -Vcc to Vcc *2. If the
buffer is a 3-state device then first
subtract the ground clamp current from the pulldown current then enter
the result into the [Pulldown] table.

-----------(Similarly under the Pullup section)---------

Pullup
Enter into the [Pullup] table the data gathered while the output or I/O
buffer was in the logic high state.
Again, if the buffer is a 3-state device then first subtract the power
clamp current from the pullup current
then enter the result into the [Pullup] table.

Regards,

Birendra

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf Of Muranyi, Arpad
Sent: Friday, September 10, 2004 12:47 AM
To: ibis-users@eda.org
Subject: RE: [IBIS-Users] Need clarification

Ambrish,

Your description for the subtraction voltage range being different for
each curve doesn't seem to be completely right. Let me explain it
through an example for the pulldown GND clamp case. But first a
question: Over what range are you getting the IV curves for the clamps?
Are you doing it over -Vcc to 2Vcc or the reduced range the IBIS spec
defines: -Vcc to 0 for the GND clamp and -Vcc to Vcc (Vcc relative) for
the POWER clamp?

Now the example. When you sweep a 3-stated buffer from -Vcc to 0, you
get its GND clamp. When you sweep the same buffer driving low over -Vcc
to 2Vcc, you will get the channel current over the entire range, plus
the GND clamp current in the -Vcc to 0 range and the POWER clamp current
in the Vcc to 2Vcc (GND relative) range. If did your subtraction only
in the -Vcc to 0 range, the resulting curve will still include the POWER
clamp in the Vcc to 2Vcc range. You would still have to do something to
get rid of that.

You could say that you will use the Vcc relative sweep of the 3-stated
buffer over the range of -Vcc to Vcc which includes the POWER clamp,
turn it over to make it GND relative and subtract it from the curve in
the 0 to Vcc range. This should give you the channel current alone for
the pulldown.

You could, however, simplify this a lot by just sweeping the 3-stated
buffer the same range as the driving buffer. In this case a GND
relative sweep from -Vcc to 2Vcc will include the GND clamp on the left
side and the POWER clamp (in a GND relative way) on the right side. You
can then just take this entire 3-stated data and subtract it from the
entire driving data, and you will get the same difference curve as
above, but with a lot less work...

The same can be done for the pullup too. Just sweep the 3-stated buffer
Vcc relative over the entire range, sweep the buffer driving high over
the same range, subtract the curves over the whole range and you got the
pullup channel current for the entire range.

After this you can chop off the unwanted sections from the 3-stated
curves to eliminate the double counting of the clamps in them, i.e.
reduce the range of the GND clamp data to -Vcc to 0, and the POWER clamp
data to -Vcc to Vcc (Vcc relative).

While we are on this subject, I would also like to mention that there
are some issues with the ranges the IBIS spec specifies for the clamps
when there is on die termination in the buffer, especially when it is
connected to GND. The range of -Vcc to 0 in the GND clamp table will
not include the ODT in the GND clamp, but the range of -Vcc to Vcc (Vcc
relative) will put the ODT into the POWER clamp table. This is not
making sense, because if the ODT is connected to GND, than it should be
in the GND clamp table so that if someone has a non ideal GND around the
IBIS model and there is GND bounce, the GND clamp IV curve should also
bounce the ODT along. For this reason I have an algorithm that keeps
the entire range for both clamp tables, but numerically eliminates the
double counting of the clamps and ODT. You can read the algorithm in my
IBIS modeling class which is also posted on teh IBIS web site.

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

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf Of Ambrish Varma
Sent: Thursday, September 09, 2004 10:36 AM
To: lgreen22@mindspring.com; ibis-users@eda.org
Subject: RE: [IBIS-Users] Need clarification

Hi,
Here's how the VI curves are obtained:
If the driver has an enable input, a sweep is performed with the driver
disbled. (This gives the performance of whatever clamping structures may
be present.) This curve is then subtracted from the curve obtained with
the output enabled, resulting in a curve that models the performance of
only the driver (i.e. it excludes the contributions made by the clamp
structures).

The voltage range over which the subtraction is performed is different
for each curve: for a pullup curve, the subtraction is performed over
the voltage range defined by the power clamp curve, while for a pulldown
curve, the subtraction is performed over the voltage range defined by
the ground clamp curve.

My apologies for any confusion that my response generated. -Ambrish.

Ambrish K Varma
akvarma@ncsu.edu
North Carolina State University
ph(919)513-2015, fax(919)515-2285

-----Original Message-----
From: Lynne Green [mailto:lgreen22@mindspring.com]
Sent: Thursday, September 09, 2004 12:33 PM
To: akvarma@ncsu.edu; ibis-users@eda.org
Subject: RE: [IBIS-Users] Need clarification

Hi, Ambrish,

Did you mean to say:

Pullup = total_current - power_clamp - gnd_clamp
Pulldown = total_current - power_clamp - gnd_clamp

- 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-users@eda.org [mailto:owner-ibis-users@eda.org] On
Behalf Of akvarma@ncsu.edu
Sent: Wednesday, September 08, 2004 6:16 AM
To: ibis-users@eda.org
Subject: Re: [IBIS-Users] Need clarification

Hi,
Both the pull-up and pull-down VI curves subtract the power and ground
clamp curves respectively to obtain the VI table in s2ibis2 and s2ibis3.
This is done only for models with enable inputs. Hope that helps.
-Ambrish.

--Ambrish K Varma
North Carolina State University.

>
> Hi,
>
> I wanted to know how the VI table is generated in the IBIS file. Is

> the VI data generated by subtracting the both the power clamp and
> ground clamp currents from the pull up current? I wanted to have
> clarity on how exactly s2ibis script is generating the pull up data.
>
>
> Please answer the same
>
> Thanks and Regards,
> Radhika
>
>
>
>
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Received on Thu Sep 9 21:33:17 2004

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