Re: XAUI AC coupling
Hi Rich,
I agree with the Dawson implementation; i.e., where capability for
AC-coupling is mandatory, and you CAN DC-couple if both ends
will support it.
But in that regard, I don't believe it apporpriate to add ANY
DC-level numbers to the standard for XAUI signaling, not even as
a guideline. This places the responsibility on the implementer
(where it always belongs) to ensure that the devices they choose
will support DC-coupling (if they wish to implement it).
The other place I find a minor dissagrement is with your statement
that if both ends of the link are made by the same manufacturer
that a DC-coupled link will generally be possible. This is not
necessarily the case, especially when CML drivers are used. These
switch very close to the power rail, and are not generally positioned
in the middle of the common mode operating range of the receiver.
While a DC-coupled connection MAY work, it is not optimal.
Regards,
Ed Grivna
Cypress Semiconductor
>
> All,
>
> By my count, I have 4 votes for allowing XAUI DC-coupling against 0
> votes for requiring only AC-coupling. The 4 votes are:
>
> Ed Grivna - Cypress
> Dawson Kesling - Intel
> Jeff Porter - Motorola
> Rich Taborek - nSerial
>
> Any other opinions out there?
>
> Best Regards,
> Rich
>
> --
>
> "Jeff Porter (rgbn10)" wrote:
> >
> > Folks,
> >
> > I feel consensus emerging here.
> >
> > Rich writes
> >
> > > a) A XAUI implementer can always get away with AC-coupling and
> > > AC-coupling details for XAUI are readily available;"
> >
> > and
> >
> > > That said, I'd be happy to go with (1) or (2).
> >
> > Dawson writes
> >
> > > An alternative is to mandate CAPABILITY for AC coupling. This allows DC
> > > coupling where compatible implementations permit, but ensures that ALL
> > > implemenations will interoperate via AC coupling.
> >
> > I agree. Specify the differential signal. Require the receiver
> > to function *when* driven by ac coupled signals to provide a method
> > that insures interoperability. After all, we've increased baud rate, among
> > other reasons, to permit ac coupling as an approach to interoperability.
> > Do not require ac coupling since dc coupling will often work, and we've
> > left a way to interoperate.
> >
> > The remaining technical work is to include in an (informative) XAUI link
> > budget (if we choose to explain how this could work) the attenuation,
> > skew, and jitter, etc. budgeted for ac coupling.
> >
> > Proposals and justification for this budget item?
> >
> > Jeff
> >
> > Rich Taborek wrote:
> > >
> > > Dawson,
> > >
> > > In terms of specsmanship, I believe that we have two alternatives with
> > > regard to coupling for XAUI:
> > >
> > > 1) Leave coupling out altogether as an implementation detail;
> > > 2) Specify detail for both AC-coupling and DC-coupling.
> > >
> > > It sound like you're leaning towards (2) where I'm leaning towards (1).
> > > My argument is that (2) is a whole heck of a lot more work than (1) and
> > > may be more costly since compliance verification has some non zero cost.
> > > I believe that (1) works and is interoperable because:
> > >
> > > a) A XAUI implementer can always get away with AC-coupling and
> > > AC-coupling details for XAUI are readily available;
> > > b) A savvy XAUI implementer may save $$$, increase reliability (fewer
> > > components), increase signal fidelity (fewer vias), etc. by going with
> > > DC-coupling if possible given their component selection.
> > >
> > > The only other possibilities are not palatable to me:
> > >
> > > 3) Mandate AC-coupling;
> > > 4) Mandate DC-coupling.
> > >
> > > That said, I'd be happy to go with (1) or (2).
> > >
> > > Best Regards,
> > > Rich
> > >
> > > --
> > >
> > > "Kesling, Dawson W" wrote:
> > > >
> > > > Rich and all,
> > > >
> > > > I agree that it would be nice to avoid AC coupling if we can still
ensure
> > > > interoperability.
> > > >
> > > > If we remove reference to coupling altogether, we must add a common mode
> > > > specification or definite logic levels; we cannot only specify
peak-to-peak
> > > > swing as we are now doing and expect interoperability. (All chip-to-chip
> > > > interconnect spec's I know of specify either DC-referenced logic levels
or
> > > > common mode and differential mode levels. Is there an exception? We have
> > > > avoided this by mandating AC coupling up to this time.)
> > > >
> > > > An alternative is to mandate CAPABILITY for AC coupling. This allows DC
> > > > coupling where compatible implementations permit, but ensures that ALL
> > > > implemenations will interoperate via AC coupling.
> > > >
> > > > -Dawson Kesling
> > > > Intel Corporation, NCD
> > > > 916 855-5000 ext. 1273
> > >
> > > -------------------------------------------------------
> > > Richard Taborek Sr. Phone: 408-845-6102
> > > Chief Technology Officer Cell: 408-832-3957
> > > nSerial Corporation Fax: 408-845-6114
> > > 2500-5 Augustine Dr. mailto:rtaborek@nSerial.com
> > > Santa Clara, CA 95054 http://www.nSerial.com
>
> -------------------------------------------------------
> Richard Taborek Sr. Phone: 408-845-6102
> Chief Technology Officer Cell: 408-832-3957
> nSerial Corporation Fax: 408-845-6114
> 2500-5 Augustine Dr. mailto:rtaborek@nSerial.com
> Santa Clara, CA 95054 http://www.nSerial.com