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

I think ³By definition all time after a VT table has ended should equal the
last point in the VT table until a new transition occurs.³ raises an
interesting question. Since many V-T curves don¹t ever settle and the
calculated endpoint currents don¹t match the I-V curves perfectly, how many
simulators freeze the K factors when V-T data runs out as Tom suggested, and
how many allow the K factors go all the way to 0 or 100% after V-T data runs
out? The exact high and low state voltages would be different depending on
which approach was used.

Mike

On 5/25/11 12:18 PM, "Tom Dagostino" <tom@teraspeed.com> wrote:

> I guess I¹m surprised this causes an issue. I don¹t 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
> I¹ve seen many more normal totem pole outputs exhibit the same
> characteristics. And I¹ve 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
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> tom@teraspeed.com
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> 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>
> 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
> Web: http://www.siconsultant.com <http://www.siconsultant.com/>
> SharkSim <http://www.sharksim.com/> - Signal Integrity IBIS Modeling Software
>

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