RE: [IBIS-Users] How do simulators handle C_Comp?

From: Abril, Giovanni \(Space Technology\) <giovanni.abril_at_.....> Date: Wed Mar 02 2005 - 20:33:19 PST · This archive was generated by hypermail 2.1.8 : Wed Mar 02 2005 - 20:35:57 PST

Sanjay,

Here is a response to the same question from a few years ago, with respect to the ICX simulator, also from Mentor:

The effect of C_comp is only double counted if you are using a substandard simulator.  All the major simulators that I know of expect C_comp to be specified with the same (static approximation) value as what is included in the waveforms.  Somewhere in the simulator this is reconciled.  Usually it is done is such a way that the capacitive effect is calculated out of the waveforms and then the lumped capacitor is present at the pin during the transient simulation..

As Arpad noted, the capacitive effect is usually impossible to remove from the SPICE models used to derive IBIS data.  It is part of the transistor models.  Arpad and others have proposed methods of finding the best static value of the die capacitance to use for C_comp and this value needs to be there.  If you put the value of 0 for C_comp then the simulator will try to back out the capacitance and put no capacitance at the pin.  At first this seems OK because the waveforms still have all the capacitive effect.  The problem appears when a reflected signal hits this pin or if this pin is hi-Z.  There would be no capacitive loading and you would then have invalid simulations.

I assume Hyperlynx is not a "substandard simulator" so it shouldn't double-count either.

-----Original Message-----
From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org]On
Behalf Of Sanjay Mehta
Sent: Wednesday, March 02, 2005 11:36 AM
To: ibis-users@eda.org
Cc: Muranyi, Arpad
Subject: Re: [IBIS-Users] How do simulators handle C_Comp?

In practice, I would like to know how simulators such as Hyperlynx account
for this C_comp? Does it exclude double-counting of the C_comp when
performing rise/fall time domain simulations?

Can someone from the tool vendor answer this question?

Thanks,

Sanjay Mehta
Xilinx

"Muranyi, Arpad" wrote:

> Lee,
>
> The IBIS model's IV curve which determines its impedance
> does not include any C_comp effects.  The Vt curves which
> are used to turn on and off the IV curves do include the
> C_comp effect in their shape, but they do not effect the
> DC nature of the IV curves.  Therefore we do need to add
> a capacitor to account for the capacitive nature of the
> drivers (or receivers).  As stated in another response,
> this is necessary for getting the correct reflections
> in the simulations when a wave hits the model.
>
> Arpad Muranyi
> Intel Corporation
> ------------------------------------------------------------
>
> -----Original Message-----
> From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
> Behalf Of lee yang
> Sent: Tuesday, March 01, 2005 8:58 PM
> To: ibis-users@eda.org
> Subject: [IBIS-Users] C_Comp
>
> Hi,
> Since the C_Comp effect is already embedded in the Spice model, hence,
> the generation of IBIS model automatically includes the effect of it. I
> am wondering, since the C_Comp effect is already included in the IBIS
> model, why do we still need to put it in the IBIS file? Why it is so
> important?
> Thanks a lot.
> -Lee
>
> _________________________________________________________________
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