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

Sanjay,

I'm sorry I failed to reply to this promptly.  To confirm statements 
implied, Hyperlynx avoids double counting C_comp when performing 
rise/fall simulations, just like ICX.  It is straightforward for the 
driver to supply the extra current necessary to charge the C_comp 
capacitance so that the simulation continues to reproduce V-t table 
waveforms when the buffer is driven into V-t table loads.

Best regards,

John Angulo
Software Development Engineer
Hyperlynx Products
Mentor Graphics Corp.

> -----Original Message-----
> From: owner-ibis-users@eda.org 
> [mailto:owner-ibis-users@eda.org] On Behalf Of Abril, 
> Giovanni (Space Technology)
> Sent: Wednesday, March 02, 2005 8:33 PM
> To: Sanjay Mehta; ibis-users@eda.org
> Cc: Muranyi, Arpad
> Subject: RE: [IBIS-Users] How do simulators handle C_Comp?
> 
> 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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