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Re: 16-bit 625Mbaud XGMII


I do not really want to open another front, but here
are my two cents ...

From the point of view of moving 10 Gbps from point A
to point B on a PCB or backplane - I prefer to use
the 4-lane architecture. I think it is better.

With respect to the definition of the XGMII (number of
I/Os and electrical characteristics) I think that it
would be worthwhile to take an additional look at this
issue. The 32-bit wide I/O at 312.5 Mbaud (as specified
electrically by H. Frazier in his "10Gig MII update",
Kauai, Nov 99) is very nice, simple and easy to implement
if one so desires.

However I think that one aspect of this 32-bit I/O has
been overlooked and could pose a headache to the designers
of the PCS/PMA chip: 32 output drivers in the receiver
side of the PCS/PMA chip switching at 312.5 Mbaud create
a lot of noise that could propagate to the serial outputs of
the transmitter and create a lot of jitter on the 3.125 Gbaud
(or 2.578 or 1.25) waveforms.

I had this problem six years ago when I designed a 266 MHz
transceiver in CMOS for Fiber Channel, with the Tx and Rx
integrated in the same chip: the 10 output drivers in the
receiver side switching each at 26.6 Mbaud created a lot of
jitter on the serial output at 266 Mbaud in the transmitter
side (that was far away on the opposite side of the chip).
Not so much in the loopback mode but specially in the normal
operating mode when the Rx and Tx PLLs had slightly different

I had used the normal precautions that everyone does: separate
power supply for the output drivers, heavy isolation rings, and
so on. I used initially the standard I/Os buffers provided by the
library. These standard drivers were well designed and had
slew-controlled slopes. However, I had to redesign these
drivers and slow the rise and fall time as much as I could
until I finally got the jitter on the serial 266 Mbaud line
at the transmitter side under control.

Now we have 32 output drivers in the receiver side pointing
to the MAC, each of them switching at 312 Mbaud. How are
we going to control the jitter on the four serial waveforms at
the transmitter on the other side of the PCS/PMA chip ?
From this point of view, fully-differential low-swing drivers
would be preferable.


Jaime E. Kardontchik
Micro Linear
San Jose, CA 95131

"Mittal, Millind" wrote:

>      Curt, Jaime - But in this discussion, pin count is not the only
> consideration. Other important consideration is the length of the wire that
> you can drive. Isn't LVDS 16 bit interface is better than half speed single
> ended interface?
> Can someone refer me to some discussion where we have comparison of
> different options done in one place
>     for example something like this ..
>                                          XAUI             LVDS
> singled ended
> speed                                3 GHz          625 MHz        312 MHz
> pin count                           20+              70+                70+
> distance drive ability           20"               ??                  2"
> coding scheme requried      yes               no                 no
> design complexity
> etc...
> also did any proposal consider something in between 4 wide and 16 wide, for
> example 8 wide differential signaling interface?
> Millind Mittal
> Level One Communications/An Intel Company
> mmittal@xxxxxxxxxx