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RE: XGMII Clocks


There are two ways I could interpret your message regarding Clock and Clock
1. A two phase clock
2. A differential clock

In some circles, these might be considered the same.

My preference would be differential. Yes, this makes sense.


> -----Original Message-----
> From: Howard Frazier [mailto:hfrazier@xxxxxxxxx]
> Sent: Wednesday, September 06, 2000 9:30 AM
> To: stds-802-3-hssg@xxxxxxxx
> Subject: XGMII Clocks
> In a previous email thread, we debated the merits of using
> a single clock in each direction on the XGMII, versus using
> 4 (frequency locked, but phase independent) clocks in each direction, 
> with a clock dedicated to each of the four "lanes".
> Without repeating the discussion, it is safe to summarize that
> the majority opinion (from among those who expressed an opinion)
> was to stay with one clock in each direction.
> So, I would like to toss out another question for your consideration.
> Should we use a two phase clock? Clock and ClockBar?
> Some designers have suggested that this will make the ASIC and
> system timing more managable, because it is difficult to get
> symetric drive strengths from the clock output buffers, and
> the asymetry degrades the timing.  With a two phase clock, you
> would still have asymetry on the data signals, but at least
> you won't have to account for the asymetry on the clock.
> At first blush, this seems like a modest addition. One more pin
> in each direction.
> Any opinions out there?
> Howard Frazier
> Cisco Systems, Inc.