RE: usefullness of V/T curves

Betty,

Can't comment specifically on the HSPICE implementation, but the intended
approach is that the V/T waveforms be used to determine the dynamic thevenin
equivalent(s) of the driver (not just current or voltage which sounds like
what you're thinking). This dynamic thevenin equivalent (source impedance
and voltage) is assumed not to change significantly even as the actual load
used in practice will. Four V/T curves are required in order to cover the
all loads that will be seen on the board as well as distribute currents
properly between GND and PWR.

Your IBIS model (assuming standard technology) with the 4 recommended V/T
curves should get the right answer for all practical loads.

Chris Rokusek
Innoveda

> -----Original Message-----
> From: Betty Luk [mailto:Betty@genesis-microchip.com]
> Sent: Tuesday, September 19, 2000 2:28 PM
> To: ibis-users@eda.org
> Subject: usefullness of V/T curves
>
>
> Hello IBIS users,
>
> I just started trying to create an IBIS model for a CMOS 3-State
> output pad. I am not an expert on modelling or signal integrity,
> so I would greatly appreciate any help and feedback.
>
> I have a question regarding the usefullness of an IBIS model:
>
> In the IBIS Cookbook, it recommends that 4 V/T curves be
> generated for a standard CMOS buffer, using a 50 Ohm load
> connected to GND or Vcc. When using the IBIS model in HSPICE,
> there is an optional parameter that can be specified (ramp_rwf)
> to indicate that the [Rising waveform] and/or [Falling waveform]
> data is to be used instead of the [Ramp] ratio.
>
> Specifying the [Rising waveform] and [Falling waveform] should
> provide better accuracy for the IBIS model because in reality the
> output switching of the device is not a linear ramp.
>
> My confusion is, how is this data useful for the customer, when
> the load is not a 50 Ohm resistor, but, say capacitive? How can
> the data provided by the [Rising waveform] and [Falling waveform]
> in the IBIS model give accurate simulation results, when the
> loading on the device has been changed?
>
> Thank you very much,
>
> Betty Luk
>
> IC Technology
> Genesis Microchip Inc.
> 165 Commerce Valley Dr. W.
> Thornhill, Ont. L3T 7V8
> (905) 889-5400 x2256
>
>

 
Received on Tue Sep 19 15:07:25 2000