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Re: Serial PMA reference clock?

In a message dated 5/16/01 7:49:11 AM Pacific Daylight Time,
jgaither@xxxxxxxxxxxxxxx writes:

   What is the rate of the PMA reference clock?  Where does it come from?
Is it locked to the same source as the MAC clock?  

I am assumeing the reference for the PMA is 644 or 322 Mhz, and that it
is locked to the same source as the MAC XGMII TX clock.  So this would
mean a frequency synthesizing PLL is required in the 10GBase-R PCS.
Shouldnt we have specifications associated with this PLL?  


Hello Justin,

The reference clock has been purposely left undefined in clause 51. Reason
that the XSBI is a spec of only the interface between PMA and its clients. As
long as the clocks specs between the PMA and its clients are met (as spec'd
in the standard) one should be able to use any ref clock they want. The serial
side also has an impact of course but the specs related to this resides more
w/ the PMD specs on jitter and such. Complicated. The choices for the ref
clock are many ... 6xx, 3xx or 1xx for either WAN or LAN PHY's ... it gets
bloated very fast if the standard has to spec these ref clocks for all these

It is not necessary to have the ref clock for the PMA to come from another
system clock src. The fundamental use of the ref clock is for quality
transmission onto the serial side to meet the PMA/PMD serial link
This is a separate time domain than the rest of the system. The PMC_TXCLK_SRC
clock is provided back to the PMA client to help in the transition to the time
domain between the system and the PMA.

Justin Chang
Quake Technologies, Inc.
2880 Zanker Road, Suite 104
San Jose, CA 95134
Tel: 408-922-6888 x108
Fax: 408-922-6827
email: justin@xxxxxxxxxxxxx