Subject: Re: [IBIS-Users] C_comp value in IBIS model
From: Kim Helliwell (kimgh@apple.com)
Date: Fri Aug 01 2003 - 15:52:42 PDT
OK, I sit corrected. Sorry to Giovanni for the misinformation...
Kim
On Friday, August 1, 2003, at 03:05 PM, Muranyi, Arpad wrote:
> Kim,
>
> I hate to argue with you, but your response is quite wrong.
>
> It is not possible, or the least very hard to remove the effects
> of die capacitance from a SPICE model. So the waveforms that
> were generated from a SPICE simulation will always include the
> effects of C_comp. Also, putting such a waveform and a C_comp
> value into an IBIS model will not double count C_comp. This
> is actually how IBIS simulators expect to get the data, because
> they are the ones who do the compensation to avoid double counting.
>
> So Giovanni did exactly the right thing.
>
> The only thing he needs to watch out for is that if he does a
> SPICE vs. IBIS correlation, he should have the value for C_comp
> in the IBIS model that was measured on the SPICE model, and not
> the data sheet number, because the two may not be the same value.
> For correlation work. However, if the IBIS model needs to be
> according to the data book, using the data book values is the right
> thing to do. (Remember, the SPICE model may not have all the
> layout related extra capacitances in the circuit, which is the
> case most often when someone has a pre layout model).
>
> Arpad Muranyi
> Intel Corporation
> =================================================================
>
> -----Original Message-----
> From: Kim Helliwell [mailto:kimgh@apple.com]
> Sent: Friday, August 01, 2003 2:45 PM
> To: Abril, Giovanni
> Cc: ibis-users@eda.org
> Subject: Re: [IBIS-Users] C_comp value in IBIS model
>
> If the waveforms already include the effects of C_comp, putting
> the C_comp value in the IBIS model will double-count the effect
> of C_comp, which is probably not what you want.
>
> I feel certain that the right thing to do is re-run the spice sims
> with C_comp removed from the simulations, and put the correct
> typ/min/max values of C_comp directly in the IBIS model
>
> If I were your customer trying to use this model, I'd be really unhappy
> to find that C_comp was (silently) being double counted. And I'd
> be suspicious of your model if C_comp were 0 with no explanation.
>
> At the very least, if you don't want to re-run things, you should set
> C_comp to 0 but put in a comment that the reason it's zero is that
> the effects of C_comp are included in the rising/falling waveforms.
>
> But this is definitely a distant second-best, in my opinion.
>
> Kim
>
> On Friday, August 1, 2003, at 02:13 PM, Abril, Giovanni wrote:
>
> Hello IBIS gurus,
>
> I've created an IBIS model using a spice netlist from the vendor, and
> I'm wondering what to do with C_comp. My rising and falling waveforms
> include the effect of C_comp, but what should I put in the IBIS
> model? I have a spec sheet that says min C_comp is 2.8pF, max is
> 10pF. Currently I have 5.0pF for typ, 2.8pF min and 10pF max.
>
> Thanks,
>
> Giovanni
>
>
> Kim Helliwell
> Apple Computer
> kimgh@apple.com
> 408 974 9936
>
>
Kim Helliwell
Apple Computer
kimgh@apple.com
408 974 9936
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