Re: ibis VT curves

Betty,

The IV and VT curves must provide a consistent description of the dynamic
i.e. transient behavior of an IBIS driver.

The IV curves describe the starting and ending conditions for transitions
under all possible loading conditions. The VT curves describe the switching
behavior under specific loading conditions. For accurate simulation, it
is required that the VT data start before the start of the transition and
end after the transition has completed. At these times, the voltage and current
into the test fixture should be consistent with the IV data. In fact, the IBIS
golden
parser will validate this condition and report if there is a significant
deviation.

Presently it is left to the simulator vendor to create an algorithm that will
use the
VT data for the device driving a specified loads and predict the behavior into
the load that is actually in the circuit being simulated.

For simulation purposes most tool vendors will ignore the IBIS edge rate data if
VT data
has been supplied. It is however important that the IBIS edge rate be reasonably
accurate, even when VT tables are available, because some tool vendors use the
data
to do some quick evaluations e.g. which nets should be simulated for crosstalk.

I hope this helps

Ian Dodd
idodd@cadence.com

Betty Luk wrote:
>
> Hi,
>
> With regards to:
>
> 1) Actually, with the question I had earlier, i found that after restarting
> my computer, I did see the ramp data affect my simulation in HyperLynx, as
> expected. Sorry about the confusion.
>
> 3) To my understanding, VI curves describe DC behaviour, and VT curves
> describe transient behaviour.
>
> Regards,
> Betty Luk
>
> > -----Original Message-----
> > From: Simon Schaeffer [mailto:simon.schaeffer@cern.ch]
> > Sent: Thursday, November 16, 2000 8:13 AM
> > To: ibis
> > Subject: ibis VT curves
> >
> >
> > Hi,
> >
> > I'm working on ibis models and I met the same problem as
> > Betty Luk, it's
> > to say that in playing around with my IBIS model (changing
> > the ramp data
> > manually), I didn't see any changes with Sigxplorer (Specctraquest
> > signal explorer expert). However, in making the same changes
> > in the ibis
> > model used in pspice I saw differences.
> > I have done this with an ibis model with and without the VT curves.
> > I have used "ibis2spice" from Intusoft but this program
> > doesn't convert the VT curves, it just uses the [Ramp] parameters.
> > So, I am trying to build a new ibis2spice program using the VT curves.
> >
> > My questions are:
> >
> > 1) Why doesn't the [Ramp] modifications influence the results (Betty
> > Luk's question)
> >
> > 2) Why use the VT curves instead of the [Ramp] parameters?
> > Is it "only"
> > to get a more accurate curve?
> >
> > 3) I am puzzled by the apparent contradiction that might arise if both
> > the
> > VT and VI curves are available. For example, a VT curve has been
> > specified with the following load:
> >
> >
> > out -----/\/\/\/\/-----|
> > 50 |
> > + ___|___ 1.25V
> > ---
> > |
> > |
> > |
> > GND
> >
> > Going from low to high, the VT curve switches from 400mV to 2V.
> >
> > How can I use this information for other loads?
> > For example:
> >
> > - 50 Ohm load only
> > (i.e. the dc offset has been removed. My simulated output
> > will now want
> > to be something defined by the VI curve. How do I sensibly use the VT
> > curve in this case?)
> >
> > - 100 Ohm load only
> >
> > 4) Do I have to take into account the Rfixture and Vfixture
> > values used
> > while specifying the VT curves?
> >
> >
> >
> > Thanks for your feedback
> > Simon Schaeffer
> >
 
Received on Thu Nov 16 07:58:14 2000