SAS-2 Spread spectrum clock
Alvin.Cox at seagate.com
Alvin.Cox at seagate.com
Wed Jul 5 08:29:06 PDT 2006
Formatted message: <A HREF="r0607052_f.htm">HTML-formatted message</A>
In this statement, "switching off" refers to the frequency modulation
applied to the transmitter clock (spread spectrum clocking) to reduce EMI.
Some SATA devices already have SSC enabled and receiver devices of
expanders and initiators that allow attachment of SATA devices are
required in SAS 1.1 to accommodate SSC. On last week's PHY teleconference,
three makers of expanders/initiators indicated that SSC applied to SAS 1.1
rates would be accepted by their receivers without issue. So far there has
been no case identified where SSC applied to the signal transmitted by an
end device transmitting at SAS 1.1 rates will cause an issue.
I hope this clarifies the situation.
Seagate Technology, LLC
E-Mail alvin.cox at seagate.com
Ralph Weber <roweber at ieee.org>
Sent by: owner-t10 at t10.org
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07/05/2006 08:42 AM
t10 at t10.org
Re: SAS-2 Spread spectrum clock
* From the T10 Reflector (t10 at t10.org), posted by:
* Ralph Weber <roweber at ieee.org>
To be honest, I do not fully follow everything being said in
this thread, but one suggestion sticks out like a sore thumb.
"Why don't we switch off the SSC of the SAS device, if it is
connected to a SAS-1.1 expander?"
Fibre Channel includes features where valid physical connections
are automatically disabled because of protocol interoperability
These features have generated deep anger in the user community.
They are a black stain in Fibre Channel products for some very
"Switching off" anything that is correctly connected is a very
bad idea. This is not a path to be followed without grave
Rob, you will have to twist my are really hard to really hard
to get my vote for this kind of proposal and I suspect you
prefer things that way.
All the best,
Massimo POZZONI wrote:
>* From the T10 Reflector (t10 at t10.org), posted by:
>* Massimo POZZONI <massimo.pozzoni at st.com>
>The SAS-II device could transmit to a SAS-II expander using +/-2500ppm.
>In this case we have the same SSC for TX and RX and the RX is not
>>SAS-1.1 expanders/HBAs are supposed to support receiving +0/-5000 ppm
>>(I hope they were more compliant than SATA HBAs), but were not
>>expected to tolerate +2500/-2500 ppm. SAS disk drive vendors would
>>prefer to implement just one behavior regardless of the expander/HBA
>>to which they are attached, so transmitting +0/-5000 ppm is the
>>only safe choice.
>Why don't we switch off the SSC of the SAS device, if it is connected
>to a SAS-1.1 expander?
>As you mention, the SAS-1.1 expander is not supposed to receive
>SSC from a SAS device.
>>Disk drive vendors would prefer not to include separate SSC controls
>>per phy. Since there are generally more disk drives than HBAs or
>>expanders, it makes sense to keep them as simple as possible.
>I completely agree on this request. Switching off the SSC in
>case separate SSC controls would be required, can always solve this
>Date: Tue, 4 Jul 2006 15:04:22 -0500
>From: "Elliott, Robert (Server Storage)" <Elliott at hp.com> Add To Address
>Subject: RE: SAS-2 Spread spectrum clock -
>To: <t10 at t10.org>
>>From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf
>>Of Massimo POZZONI
>>Sent: Tuesday, July 04, 2006 2:13 AM
>>To: t10 at t10.org
>>Subject: SAS-2 Spread spectrum clock -
>>* 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
>> 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.
>What problems? I haven't seen any errata to the SATA specification
>trying to reduce the ppm range.
>There are many accounts on the Internet about problems arising from
>HBAs not supporting SSC as required. Disk drive vendors that tried
>turning on SSC have had to provide consumers special programs or
>jumpers to turn SSC off so they will work in non-compliant systems.
>> Now we are proposing to stress it at 7500 ppm !!!.
>> My proposal:
>> 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
>SAS-1.1 expanders/HBAs are supposed to support receiving +0/-5000 ppm
>(I hope they were more compliant than SATA HBAs), but were not
>expected to tolerate +2500/-2500 ppm. SAS disk drive vendors would
>prefer to implement just one behavior regardless of the expander/HBA
>to which they are attached, so transmitting +0/-5000 ppm is the
>only safe choice.
>HBAs would like to have as few choices (e.g. clock trees) as possible.
>They must use +0/-0 when attached to a SAS-1.1 phy, and can only
>use +0/-5000 ppm when attached to a SATA phy, so +2500/-2500 ppm
>would require a third clock tree that is not really necessary.
>Nothing should break if they did transmit +2500/-2500 ppm to a
>SAS-2 phy, since all SAS-2 receivers will be required to
>> 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.
>Disk drive vendors would prefer not to include separate SSC controls
>per phy. Since there are generally more disk drives than HBAs or
>expanders, it makes sense to keep them as simple as possible.
>> In summary: is it possible to have the same SSC range for both the
>> transmitter and the receiver of a SAS device?
>I don't think so.
>Rob Elliott, elliott at hp.com
>Hewlett-Packard Industry Standard Server Storage Advanced Technology
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