Re: sweep range overlay in clamp curves ??

Text item:

Sasha,

Your observation is correct. If you have an always present pullup or pulldown
"resistor" (usually implemented as a MOSFET transistor) in a device, the GND
and Power clamp curves would both show the same effect in the measurement. And
you are also right, if nothing is done, this causes double counting.

For this reason, I developed an algorythm in my IBIS model maker tool that will
take the appropriate steps to avoid double counting. I am going to describe the
case here which applies to a situation when you have a pullup "resistor" in the
device. This example can also be applied to the opposite case when the device
has a pulldown "R", you just have to invert the whole thing symmetrically.

The following two drawings show the I-V curve of a device with a pullup "R",
first as it is measured GND relative for the GNC clamp curve, and second as it
is measured Vcc relative for the power clamp curve.

GND relative: | /
                 | /
                 | /
              GND| /
       ----------+-------------------+-----------
                 | /Vcc
               /-|-----------------/
              / |
             / |
            / |

Vcc relative:\ |
              \ |
               \ |
                \| GND
       ----------+-------------------+-----------
              Vcc|\
                 | \-------------------\
                 | \
                 | \
                 | \

Obviously, if both of these are put into an IBIS model, you will double count
the effects of the "resistor", because the clamp curves are to be added together
in an IBIS simulator. To avoid this double counting, you need to eliminate one
of these curves. But how do you do it correctly?

Since the device has a pullup "resistor" it makes sence to use the Vcc relative
curve to preserve its Vcc relativeness. Since the slope on the right is coming
from the GND clamp, I cut that part off. The question is what do you do with
the GNC clamp then? The simple answer is to use the left side slope from the
GND relative data, but there is a problem. This data is shifted down by the
amount of saturation current from the pullup "resistor". To avoid double
counting, you need to take the left side slope in the GND relative measurement
and shift it up by the amount of that current so that this curve goes into the
origin, and not Isat at 0 volts. To satisfy the -Vcc to Vcc range, I just add
another point at Vcc with 0 mA.

Comment: It is true that IBIS only requires a range of -Vcc to 0 V for the
power clamp curve. When I brought this issue up in one of the open forum
meetings, we all decided that the spec doesn't say that you cannot provide data
outside that range, so it was legal to do what I described above for the power
clamp curves. This might be recorded in one of the meeting minutes, just don't
ask me which one...

I hope this explaination will help you making correct IBIS models for these kind
of devices.

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

> From sascha Wed Feb 19 15:54:53 1997
> To: sabineh
> Subject: bitte abschicken, danke
> Content-Length: 992
> X-Lines: 24
>
> To:ibis-users@vhdl.org
> Subject: sweep range overlay in clamp curves ??
>
>
> Hello all,
>
> I tried to model an input structure with constantly connected pullup
> resistor using GND_clamp and POWER_clamp keywords. That causes these
> tables to contain values of some mA for instance for input voltage =
> 5V (Vcc = 5V) as opposed to some nA current for inputs without pullup
> resistor. I hope simulators will handle this correctly. (has anyone
> experiance?) The problem, anyway, is: The POWER_clamp table for 0V
> (that means 0V above Vcc) and the GND_clamp table for 5V describe the
> same point, for the input characteristic follows the sum of both tables.
> That ends up with double counting of this current value at Vout = Vcc.
> I'm not quite sure I'm right here at all since I saw a published IBIS
> file having two differend currents for POWER_clamp at 0V and GND_clamp
> at 5V and so two differend currents for the same Vout = 5V.
>
> Any comments are greatly appreciated,
>
> Sascha Pawel
>
> email: sabineh@thesys.de
>

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Cc: sabineh@s01.thesys.de
Subject: sweep range overlay in clamp curves ??
To: ibis-users@vhdl.org
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From: sabineh@s01.thesys.de (Sabine Herre)
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