Re: [IBIS-Users] VT Data Length Mismatch and Over Clocking

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Including [Test Data] ("golden waveforms") can
make it easier to detect things like this.

Lynne Green
Chair, IBIS Model Review Committee

On 5/25/2011 10:25 AM, Timothy Coyle wrote:
>
> Hi Tom,
>
> I don't think the issue is with the IBIS
> Specification but rather how different EDA tools
> interpret and use the VT data. So one could
> argue that you could make your VT tables the
> same duration to avoid possible EDA tool
> interpolation. Or likewise one could argue that
> you will have better resolution in your VT data
> if they have separate durations and let the EDA
> tool handle it.
>
> I don't think either approach is wrong but like
> anything will have its advantage/disadvantages
> depending on the data and usage case.
>
> I was interested to know if other people had run
> into this issue or were aware that it could be a
> potential problem. Another potential item to add
> to your IBIS checklist :)
>
> Best,
>
> Tim
>
> *From:*owner-ibis-users@eda.org
> [mailto:owner-ibis-users@eda.org] *On Behalf Of
> *Tom Dagostino
> *Sent:* Wednesday, May 25, 2011 12:19 PM
> *To:* 'Prabhat Ranjan'; 'Timothy Coyle'
> *Cc:* ibis-users@eda.org
> *Subject:* RE: [IBIS-Users] VT Data Length
> Mismatch and Over Clocking
>
> I guess I'm surprised this causes an issue. I
> don't recall seeing anything in the IBIS spec
> requiring equal length VT tables. By definition
> all time after a VT table has ended should equal
> the last point in the VT table until a new
> transition occurs.
>
> I also see a lot of parts that have different
> output amplitude specifications at low
> frequencies vs. high frequency. This implies
> the part was never designed to have finished
> transitions at the fastest toggle rate of the
> buffer. So overclocking is a reality in modern
> parts.
>
> There are many parts that have much faster
> falling transition than rising. A good example
> of this is an open sink. Others include RS232
> and the like. And I've seen many more normal
> totem pole outputs exhibit the same
> characteristics. And I've seen the reverse
> where the rise is much faster than the fall by a
> factor or 3 to 5. Why should the rising
> table(s) have the same time duration as the falling?
>
> Tom Dagostino
>
> Teraspeed Labs
>
> 9999 SW Wilshire St.
>
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>
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>
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>
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>
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>
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>
> tom@teraspeed.com <mailto:tom@teraspeed.com>
>
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>
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>
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>
> Narragansett, RI 02882
>
> 401-284-1827
>
> *From:*owner-ibis-users@eda.org
> [mailto:owner-ibis-users@eda.org] *On Behalf Of
> *Prabhat Ranjan
> *Sent:* Wednesday, May 25, 2011 8:22 AM
> *To:* Timothy Coyle
> *Cc:* ibis-users@eda.org
> *Subject:* Re: [IBIS-Users] VT Data Length
> Mismatch and Over Clocking
>
> Hello Timothy,
>
> I have also came across the same problem with
> one simulator where different time lengths of
> V-T waveform gave me completely wrong results
> and just making all tables equal by adding same
> last point has solved the problem. Simulator
> vendor has accepted it as bug in tool.
>
> Various simulators take care this issue
> internally by extending the last points to make
> them equal.Thats why, somewhere it is
> recommended to have same two last points in VT
> curve. I have checked the same model with 2-3
> another simulators and all of them give correct
> results with different time length.
>
>
> Regards
> Prabhat
>
>
> On Wed, May 25, 2011 at 8:14 PM, Timothy Coyle
> <tim.coyle@siconsultant.com
> <mailto:tim.coyle@siconsultant.com>> wrote:
>
> Hi,
>
> I'm starting to run across quite a few IBIS
> models that have different time lengths in the
> V-T waveforms even for corresponding loads. For
> example, a Rising Waveform with 50 Ohms to
> Ground may have a total time of 5ns but the
> equivalent Falling Waveform with 50 Ohms to
> Ground may have a total time of 10ns. So between
> the four different V-T waveforms they can all
> have a different total time length. All of the
> waveforms transition to a "steady state" but I'm
> wondering if this will cause potential problems
> in IBIS simulators?
>
> I know a lot of IBIS simulators say they can
> handle an over-clocked IBIS model but will this
> cause even more issues? I've also actually
> started to see a few models that say they must
> be over clocked to run properly (apparently
> model developer had issues getting VT waveform
> to reach steady state within bit time)
>
> I would think that the VT waveform total lengths
> should be matched based upon that they are used
> to define rise/fall transitions but clearly it
> all depends on how EDA IBIS simulator processes
> this data. It's difficult to do this type of
> correlation in multiple tools and without access
> to the original SPICE model but wanted to see if
> anyone else is running into this issue or can
> see potential issues with this type of model?
>
> Best,
>
> Timothy Coyle
>
> President
>
> Signal Consulting Group LLC
>
> 405 Western Ave #430
>
> South Portland, ME 04106
>
> Tel: 617.297.2566
>
> Email: tim.coyle@siconsultant.com
> <mailto:tim.coyle@siconsultant.com>
>
> Web: http://www.siconsultant.com
> <http://www.siconsultant.com/>
>
> SharkSim <http://www.sharksim.com/>- Signal
> Integrity IBIS Modeling Software
>
>

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