Re: XAUI AC coupling
Tom, Ed,
I agree with this direction and side with Ed on the issue of spec'ing
DC-coupling for the reasons that Ed provides. Thanks for your support on
this issue.
Best Regards,
Rich
--
Ed Grivna wrote:
>
> Hi Tom,
>
> I agree with the consideration, that IF we are to standardize a
> DC coupled interface, then it is required to also spec the levels.
> I think my main concern is that as soon as you do this, you will
> open a Pandora's box of technologies that do not interoperate
> at a DC level, but work fine when AC-coupled. For my own part,
> would love to specify DC lvels, just so long as they match up
> with the DC-levels _I_ want. If anything else goes in, escpecially
> this late in the game, I'll be one of the first to scream restraint
> of trade.
>
> As stated earlier, I have no problem with allowing DC-coupling, but
> I don't want it to be mandatory. However, once you spec the levels
> in the standard, you now have requirements that must be met.
>
> -Ed Grivna
>
> > All,
> > I concur that it would be advantageous to support both AC and DC
> > coupling. Maybe we have a section with AC coupling specs and a separate
> > table for DC coupling specs. This would allow a vendor to be just AC
> > compliant or just DC compliant or both. If we leave out the DC specs all
> > together (respectfully disagreeing with my fellow Minnesotan) than I
> > don't think we have standardized anything for the DC coupled interfaces
> > and we will not have interoperability.
> >
> > Tom
> >
> >
> > Ed Grivna wrote:
> > >
> > > 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
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