SAS-2 Spread spectrum clock -
massimo.pozzoni at st.com
Tue Jul 4 00:13:05 PDT 2006
* From the T10 Reflector (t10 at t10.org), posted by:
* Massimo POZZONI <massimo.pozzoni at st.com>
The 06-263r2 (SAS-2 Spread spectrum clock) specifies that:
- the device trasmitter SSC is 0/-5100 ppm (both SAS and SAS)
On the other hand,
- expander transmitter is 0/-5100 ppm if talking to a SATA device
- expander transmitter is + 2500/-2500 ppm if talking to a SAS device
Let's consider the receiver Clock Recovery (CDR) of the device.
Let's assume that the receiver Clock Recovery makes use of the same
clock reference used by its transmitter (to save power and area
in the PHY).
As a consequence, this clock reference, used by the receiver CDR
to recover the incoming data, will be SSC modulated as the
We see now that the receiver CDR is much more stressed than in SATA.
In fact its reference clock can be at -5000 ppm, while the data
it is receiving can be at +2500 ppm. Total 7500 ppm.
My concern is for the Clock Recovery. It has been proven that
many problems are already existing in sata devices, where the
clock recovery is stressed up to 5000 ppm.
Now we are proposing to stress it at 7500 ppm !!!.
If an expander is transmitting to a SATA device: TX @ 0/-5000
If an expander is transmitting to a SAS device: TX @ +2500/-2500
(This is not changed)
If a SAS device is transmitting to a SAS expander: TX @ +2500/-2500
(This is the new !!)
If a SATA device is transmitting to a SAS expander: TX @ 0/-5000 ppm
If the SAS expander cannot tolerate the +2500/-2500 from the
SAS device, the SSC is turned off in all the ports of the device.
In summary: is it possible to have the same SSC range for both the
transmitter and the receiver of a SAS device?
* For T10 Reflector information, send a message with
* 'info t10' (no quotes) in the message body to majordomo at t10.org
More information about the T10