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

From: Muranyi, Arpad <arpad.muranyi@intel.com>
Date: Thu Sep 09 2004 - 12:16:47 PDT

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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