Re: Differential pins in IBIS

Stephen, D. C., etc.:

All that I can add to this is that I believe BIRD62.6
additions will provide additional detail for this
technology. The data input pins should be treated as
single-ended inputs based on the function table of
the SN74SSTL16857.

Bob Ross
Mentor Graphics

"D. C. Sessions" wrote:
>
> Christian Klein wrote:
> >
> > DC, Stephen,
> >
> > What if you created additional "virtual" Vref pins, and referenced each
> > differential input to a different pin? I don't think this would be a problem, as
> > long as they share the same POWER and GND busses.
>
> I talked about this with Bob Ross (Please chime in, Bob) and it seems
> that this is one of those cases where Mr. Nolan's IBIS files are
> within the letter of the specification but violate tool requirements.
>
> Arguably, we don't need to make any changes in IBIS to accomodate this
> application, except maybe some comments to the effect that this is a
> supported use. The tool companies just need a nudge to get them to
> support this.
>
> In the meantime, your proposed workaround might trick them into doing
> what they Should Have In the First Place.
>
> > "D. C. Sessions" wrote:
> >
> > > Weston Beal wrote:
> > > >
> > > > Stephen,
> > > >
> > > > Although this receiver is implemented as a differential receiver, from the
> > > > outside it looks more like a single ended receiver because the system really
> > > > only feeds in one data signal. Vref is held constant at 1.25V. You can
> > > > simply use the parameters Vinl and Vinh to specify the thresholds Vref -
> > > > 350mV (0.9V) and Vref + 350mv (1.6V).
> > > >
> > > > Now, since Vref should follow Vcc, it changes for min and max cases. For
> > > > this we use Vinl and Vinh under the [Model_Spec] keyword. They work the
> > > > same but now have typ, min, and max values.
> > >
> > > Not good enough. Vref and Vtt are related to Vddq, but there are both
> > > tolerances and noise terms involved which don't track the simplistic
> > > typ/min/max axis. In particular, for the devices Mr. Nolan is discussing,
> > > the common-mode rejection of the receivers is an issue. Differential is
> > > a must.
> > >
> > > For instance, right now there is a JC-42.3 task group dealing with the
> > > impact of SSI noise on Vref. Crosstalk analysis where is can make or
> > > break standard technology.
> > >
> > > > The IBIS committee is still working on the AC thresholds vs. DC thresholds
> > > > problem.
> > >
> > > One of the reasons that Mr. Nolan now represents TI on the IBIS committee.
> > >
> > > --
> > > D. C. Sessions
> > > dc.sessions@vlsi.com
>
> --
> D. C. Sessions
> dc.sessions@vlsi.com
Received on Wed Mar 22 15:22:11 2000