Re: XGMII electricals
In an effort to get us all on the same page, here are links to
the standard XGMII interface proposals, SSTL-2 and HSTL Class 1
on the JEDEC site under "Free Standards":
HSTL Class 1
http://www.jedec.org/download/search/jesd8-6.pdf
SSTL_2 Class 1 (per page 9, http://grouper.ieee.org/groups/802/3/ae/public/jul00/frazier_1_0700.pdf)
http://www.jedec.org/download/search/jesd8-9.pdf
First, I also discourage the development of a new 1.8V interface definition
for XGMII for many of the reasons already on the reflector.
I regret not making a larger issue in New Orleans about the fact
that HSTL is a 1.5V specification. I thought there was consensus on
the idea of saving power by going to HSTL, and was (too) willing to
go along with voting on HSTL and 1.8V at the same time based on
claims that there was another standard out there, and assuming that
lacking a standard, we would still go to real (1.5V) HSTL.
The very valid point has been made that interface variations outside
of the IEEE standard often become popular, and that may also
become true with XGMII. So the question is where the standard
should point, what guidance should we give implementers? Since HSTL is
available, standardized, and lower power, this makes a better *standard*
interface than SSTL_2 (similar attributes, but higher power).
That is, guide implementers toward a lower power solution.
It has been stated that 2.5V SSTL_2 interfaces are implemented on
early XGMII interfaces. There was discussion at New Orleans that at
least some of these interfaces also work down to 1.8V. Even if we select HSTL
(i.e. 1.5V), as a practical matter, many 0.18um HSTL interfaces may also work
up to 1.8V, which may be more convenient in systems at first than a 1.5V supply.
If XGMII lives long enough for some reason, the market might go to even lower
"1.5V tolerant" interface (e.g. 0.9-1.6V range specified in jesd8-11.pdf,
October 2000, but with 50 ohm drive level).
Perhaps a bigger question is, what about MDC/MDIO levels?
Jeff
"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