[SI-LIST] Re: SATA II Electrical Specification

Hi Cortex,

I checked over the numbers and methodology and they are correct.  At first
glance it appears wrong to have minimum transmit amplitude the same for a
long cable as a short one.  Let me explain.

The frequencies are in the microwave region and so the amplitudes are not
directly observable.  That is, the signals when connected in system are not
the same (mismatch) as when a transmitter is connected to a matched load,
and when a probe is connected the system is significantly changed (loading).
This situation is a consequence of the physics.

The next question to ask is: what do these specifications mean?  Some
specifications refer to amplitude that exists while connected in system.
Although it is possible to infer amplitude while connected in system, it
requires characterization of the paths and complex calculations.  Amplitude
while connected in system places a great burden on the test engineer,
especially if he is required to show and quantify accuracy.

The SATA spec covers amplitude seen while driving a matched load.  So it is
directly observable.  The accuracy of this type of measurement can be
quantified by knowing instrument accuracy and match, and the test adapter
characteristics.  Thus, systematic errors (or bias) in the measurements can
be removed.

The Gen1i/2i specs have transmitters driving a matched load; the SATA cable
is not part of this measurement.  The interconnect can be a 1 meter cable,
or no cable at all.  Comparing the Gen1i/2i transmit and receive amplitudes
should be consistent with this.

The Gen1x/2x specs have transmitters driving a matched load (the zero length
case) and driving through a TCTF (compliance channel).  The requirement for
transmitters is the amplitude should be within spec with zero length and
with a TCTF connected without any changes to the transmitter settings.  The
Gen2x spec is setup to be compatible with SAS.  In real cases, the output
match from the interconnect is an important parameter to the amplitude at a
receiver.  The absence of output match from the TCTF definition and the lack
of dealing with the impedance match issue in system are weaknesses in this
spec.

So if a Gen2x transmitter and TCTF delivers 400mVdpp into a matched load,
AND if the same transmitter without TCTF delivers greater than 400mVdpp and
less than 1600mVdpp, then it is within spec if both conditions are met.

Specifications aren't of much use unless they can be tested.  The harder it
is to test against a spec, the fewer devices will be tested, and the
effectiveness in ensuring quality suffers.  That's the advantage in specing
quantities observable in test setups.  It's practical.


Regards,
Chuck Hill
SATAIO Phy group chair

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of
Cortex.Chen@xxxxxxxxxxxx
Sent: Wednesday, February 22, 2006 0:08 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] SATA II Electrical Specification


Dear Experts,


Recently I read the SATA spec, the min differential output voltage is
defined 400mV for all Gen1 & Gen2 types (i/m/x) that shocks me.

It means that trace length should be as short as possible for current
project and long backplane is not long (interconnect loss budget is changed
from 9.275dB to 3.255 dB for 2x).

Who can tell me the story/history of the electrical specification?



Note 1:

<Serial ATA II: Electrical Specification, rev. 1.0, page 44 & 47>

          1i     1m   1x     2i    2m    2x

Min TX   400   500   800   400   500   800   mV

Min RX   325   240   275   275   240   275   mV



<Serial ATA Revision 2.5, page 135 & 138>

          1i     1m   1x     2i    2m    2x

Min TX   400   400   400   400   400   400   mV

Min RX   325   240   275   275   240   275   mV



Note 2:

i: PC motherboard to device application

m: Short backplane and external desktop application

x: Long backplane and system-to-system application



Sincerely



    Cortex Chen    ³¯¥ÃªN



SI Section, ESBU

Quanta Computer Inc.

No. 211, Wen Hwa 2nd., Kuei Shan Hsiang,

Tao Yuan Shien, Taiwan.

TEL: 886-3-3272345 ext. 11710

FAX: 886-3-3973614




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