RE: XAUI AC coupling
Ali,
The solution you suggest would result in many XAUI links having
AC coupling at both ends. This doubles the amount of capacitors
required for many implementations in order to allow some implementations
to use no capacitors.
If we specify AC coupling as being present on one side of XAUI,
most implementations will have one set of capacitors. If an internal
interface is designed such that it does not need AC coupling, it
can be DC coupled because XAUI is optional. The internal interface
doesn't have to meet XAUI requirements.
In this forum, Larry Miller and Howard Frazier (both of whom have
a systems vendor perspective) have spoken of the compatability
problems experienced with DC coupling. This isn't as system vendor
vs. transceiver vendor issue. It is about having a robust interoperable
interface.
Pat
-----Original Message-----
From: Ali Ghiasi [mailto:aghiasi@xxxxxxxxxxxxx]
Sent: Tuesday, September 26, 2000 9:23 PM
To: richard_dugan@xxxxxxxxxxx
Cc: rtaborek@xxxxxxxxxxxxx; stds-802-3-hssg@xxxxxxxx;
aghiasi@xxxxxxxxxxxxx
Subject: Re: XAUI AC coupling
Hi
About 8 month ago we debated the same issue rigorously in another
standard. The XCVR mfg
wanted AC coupling and the system company wanted DC coupling. Can you
imagine if you
have a 64 ports 10 Gig switch how many capacitor you need.
In that group I proposed the following satisfying the need of both AC
and DC:
- every transmitter must provide 0.75+/- 0.25 volts of common mode
else it must be AC coupled.
- Every receiver must be capable of operating with 0.75 V of common
mode
else it must be AC coupled.
- All external link the receiver will be AC coupled.
I believe there is genuine need for DC coupled link in the backplane and
switch applications. The
above implementation may add two capacitors one for each blade, assuming
both serialize and
de-serializer do not operate with the 0.75 V common mode. But the
capacitors can be eliminated
from the blades if you operate with common mode of 0.75 V.
Why 0.75 V Common mode; it is half 1.5 Volts, support >500 mV on each
leg, supports
advance CMOS (Sub 0.18 um).
AC coupling will be used often, but lets not force high density
applications to use
1000's of caps. We need to define the common mode voltage, but many can
just ignore it.
Thanks,
Ali Ghiasi
Newport Communications
richard_dugan@xxxxxxxxxxx wrote:
>
> Rich,
>
> I would strongly oppose allowing DC-coupling in the spec since we would
then
> have to pick a DC level. This would certainly hamper some technologies
over
> others and lead to sub-optimum solutions, either now or in the future.
> Pluggable modules need to be AC coupled. Whether or not someone wants to
> use a proprietary chip-to-chip solution on a backplane is another issue,
one
> that should be outside the scope of the standard.
>
> So given your choices below I vote for only AC-coupling.
>
> Regards,
>
> - Richard
>
> > -----Original Message-----
> > From: Rich Taborek [mailto:rtaborek@xxxxxxxxxxxxx]
> > Sent: Friday, September 22, 2000 10:20 PM
> > To: HSSG
> > Subject: Re: XAUI AC coupling
> >
> >
> >
> > 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@xxxxxxxxxxx
> > > > 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@xxxxxxxxxxx
> > Santa Clara, CA 95054 http://www.nSerial.com
> >