Re: Source centered vs source sync
Boaz,
I'm assuming that you "usually don't" have a 2X internal clock in the
XGMII, XAUI/XGXS aside since it's optional. The only reason for having a
2X internal and 1X external (i.e. DDR) XGMII clock would be if EMI were
the primary concern. I believe that EMI is actually a secondary concern.
Therefore, a source-centered clocking scheme requires delay elements at
the transmitter and source-synchronous requires delay elements at the
receiver. PVT dependence is applicable to the respective delay elements,
further complicating the problem. This is why I view the recent change
as six of one, half dozen of the other. The change only moves the
problem and forces everyone to change both their Rx and Tx I/O buffers.
P.S. The PCB trace can be a delay element as well as Curt Berg suggests.
Boaz Shahar wrote:
>
> Hi,
> I absolutely agree.
>
> I would like to add that you do not need to use buffers/delay elements in
> order to create the Source Centered clocking scheme, because in the source
> you usually have 2x internal clock which allows you to create the
> Delay-Shift in an accurate and process independent manner.
>
> Boaz
>
> > -----Original Message-----
> > From: Devendra Tripathi [mailto:tripathi@xxxxxxxxxxx]
> > Sent: Tuesday, September 26, 2000 7:51 PM
> > To: stds-802-3-hssg@xxxxxxxx
> > Subject: Source centered vs source sync
> >
> >
> >
> > Hi,
> >
> > One problem I found in implementing source synchronous design
> > is that we need
> > to adjust timing by inserting buffers. When worst case to
> > best case ratio
> > of delays
> > are high, this becomes a tough problem to solve (especially
> > when available
> > time is
> > a few ns). In centered clock case, this does not become a
> > problem. There may be
> > still buffers in clock/data paths but they track each other
> > under various
> > conditions.
> > To take example of FC vs. GMII timings, I have found GMII
> > timings (which are
> > source sync.) much difficult to implement compared to FC
> > (which is source
> > centered).
> >
> > Regards,
> > Tripathi.
> >
> >
> > At 01:25 AM 9/26/00 -0700, you wrote:
> > >stds-802-3-hssg@xxxxxxxx
> >
> > Best Regards,
> >
> > Devendra Tripathi
> > Vitesse Semoconductor Corporation
> > 3100 De La Cruz Boulevard
> > Santa Clara, CA 95054
> > Phone: (408) 986-4380 Ext 103
> > Fax: (408) 986-6050
> >
--
Best Regards,
Rich
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