Re: XGMII electricals
Two issue primary issue with going to HSTL are:
- The HSTL JDEC specification do not fully document the interface for
operating at high speed 300-600 Mb/s. Some additional parameters
- Interface to the optics is chicken and egg issue. The SerDes
have now migrated to SSTL2 the optics is at LVTTL. I have not been
find any single or dual channel LVTTL to SSTL2 or HSTL.
Rich Taborek wrote:
> I can't agree with you more. I took issue with the same decisions over
> this reflector soon after the New Orleans meeting. We need a resolution
> to this issue very soon. You've already listed most of the possible
> resolutions in your original note. I feel very uncomfortable with (3)
> which in essence requires the development of a brand new interface spec
> for an interface which will likely be integrated very quickly. If I may,
> I'd like to suggest an alternative resolution, call it #4:
> 4. Allow XGMII I/O to be either SSTL or HSTL per the appropriate EIA/
> specs and selection of options thereof.
> Best Regards,
> "Grow, Bob" wrote:
> > Implementing the XGMII concensus of the Task Force expressed through straw
> > polls in New Orleans is a problem. In fact, I would characterize the actions
> > we took in New Orleans to be an example of group think gone wild. We had a
> > comprehensive SSTL specification in the draft, but made the straw poll votes
> > to change on concepts, not proposed specifications.
> > There is no standard for HSTL at 1.8 volts (the preferred voltage per straw
> > poll), nor did the TF select any other parameters of the electrical
> > specifications. (Class I, 1.5 volt HSTL as specified in EIA/JESD8-6 is the
> > closest standardized alternative that the team working on clause 46 could
> > find). Because we couldn't find a standard to reference and the Task Force
> > didn't endorse a complete set of 1.8 volt specifications, there was no way
> > an HSTL electrical specification could be inserted into the draft without
> > adding a lot of technical material that hadn't been endorsed by the
> > committee. Therefore, all you will find in Draft 1.1 on HSTL is an editor's
> > note describing the situation.
> > Most discussion supports the idea that the XGMII electrical interface is for
> > near term usage (with continued use as an module to module logic interface
> > within a chip). Implemeters expect the electrical interface to be supported
> > by I/O devices in quick turn silicon libraries. Some participants in the
> > editorial session thought ASIC vendors might have a 1.8 volt HSTL derived
> > from the above referenced specification, but weren't sure of any vendors
> > supporting it (for inclusion in the standard it should be supported by many
> > vendors).
> > We have a similar problem with the clock alignment were the straw poll
> > endorsed a change without any specifications to implement the change (e.g.,
> > skew specifications).
> > As it now stands, I would vote against going to Task Force ballot. It would
> > be a shame for TF ballot to be delayed because of the absence of XGMII
> > electricals. I see three alternatives that would allow us to go forward to
> > TF ballot.
> > 1. Return to the SSTL specifications of Draft 1.0
> > 2. Reference HSTL at 1.5 volts per EIA/JESD8-6 and select from the options
> > within that specification.
> > 3. Someone presents a detailed proposal including all appropriate
> > specifications (timing, thresholds, AC and DC characteristics, termination,
> > etc.)
> > As the clause editor, I will be proposing alternative 1 in Tampa unless
> > participants come through with presentations (sufficiently detailed to go to
> > TF ballot), and the Task Force endorses the specifications presented.
> > Bob Grow
> > Editor Clause 46
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