Re: XAUI AC coupling
Hi Pat
pat_thaler@xxxxxxxxxxx wrote:
>
> 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.
Assuming you only use a single capacitor on one side of the line card,
then you are correct
about being forced to use two caps. In many instances you add AC
coupling Caps on both
line cards. If you are on a single line card obviously you have total
control.
>
> 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.
I also worked at System company and I know there is a need for DC
coupled links.
The DC coupled link are often controlled by single company. It is about
having
parts which can be connected back to back in those demanding
applications without 1000's
caps.
The other standard which you are part of it as well has already chosen
the 0.75 V
common mode. I proposed 0.75 V common mode here to be compatible with
the other standard.
Even if we decide to chuck the DC coupling here due to the other
standard
effort we will get some parts capable of DC coupling in 10 Gig Ethernet.
Thanks,
Ali
>
> 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
> > >