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Re: XAUI Driver Spec


I've discussed this issue internally and agree with you. Three
modifications to XAUI electricals were floated in Austin and two were
tentatively approved, the remaining one was not approved. The proposals

1) Raise minimum receive amplitude to 200 mV from 175 mV. A lot of
shrugging of the shoulders led to it's approval.

2) Raise minimum transmit amplitude from 800 mV from 500 mV. This was
rejected. The rationale was that this change would require 60% higher
power XAUI I/O where the current performance specifications can be met
by using relatively simple and low-power techniques such as transmit

3) Raise maximum transmit amplitude from 800 mV from 1600 mV. This was
approved. However, my notes say that no one seemed to know whether the
InfiniBand specs were there to drive copper cables. Also, I was supposed
to check in at the ranch to see how much the receiver would be affected. 

I'm uncomfortable with the sheer magnitude of the change in (3). I am
worried about the impact on link performance, especially jitter in
designing a receiver to support these large swings. Please note that the
original Hari requirements from Ali Ghiasi from November '99 "is based
on a low swing swing AC coupled differential interface". Any news from
the InfiniBand camp on driving chips vs. cables.

I have to agree with Robbie and will likely oppose the change in (3).

Best Regards,

"Kesling, Dawson W" wrote:
> Thank you for brigning this up, Robbie. The 1.6V idea was floated in Austin
> as it was at the XAUI breakout in New Orleans and there were no objections
> in either case. I expect that it will come up again and be officially moved
> in Tampa, so now is a good time to express concerns. I encourage other
> receiver designers to consider the implications and come prepared with an
> opinion.
> -Dawson
> -----Original Message-----
> From: Robbie Shergill [mailto:Robbie.Shergill@xxxxxxx]
> Sent: Friday, November 03, 2000 6:57 AM
> To: stds-802-3-hssg@xxxxxxxx
> Subject: XAUI Driver Spec
> Hello all,
> Last week in Austin the XAUI group decided to change Max Diff. Amplitude to
> 1.6 volts. As I understood it, the rationale for increasing the max
> amplitude
> was to achieve commonality with the Infiniband spec. Although I agree with
> the spirit of this rationale, I checked the Infiniband spec and found
> that they are trying to drive a cable as well as a backplane with one
> electrical specification; thus the reason for the 1.6v spec. In this
> case, I'm not sure if it is worth matching the Infiniband spec in this
> one area. My feeling is that a driver circuit can be made to drive
> up to either 1.0 volt or 1.6 volt relatively easily; but it would be
> much more troublesome for a (XAUI) receiver to tolerate 1.6 volts
> *needlessley*.
> So, if the above reasoning is agreed to by others, I would propose that we
> stay with 1.0 volt max. diff. amplitude that is in the current draft 1.0
> (page 119, line 22).
> -Robbie Shergill
>  National Semiconductor
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