[SI-LIST] Re: SATA eye diagram - how much is too much
- From: "Charles Hill" <chuck@xxxxxxxxxxx>
- To: <Charles.Grasso@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 5 Oct 2004 11:01:10 -0600
Charles,
I'll try to answer each of your questions.
First to set the context of compliance testing. In SATA II (Serial ATA II
Electrical Specification revision 1.0 26 May 2004), eye diagrams are not a
compliance measurement. From the specification page 101 section 6.4
Measurements: "Additional measurement methods for several parameters are
aimed at providing quick No-Go testing. These use any valid data pattern
and a Laboratory Load to quickly produce a visual ?picture? of the
performance of the unit under test. For example, one measurement method
uses Data Eyes to quickly understand jitter. Another uses a mode measurement
for identifying a potentially complex signal with a single amplitude value.
However, none of these No-Go measurement methods may be used for testing
compliance to electrical specifications. These measurement methods are
valuable for gaining useful information about the performance of the unit
under test which goes beyond specification compliance issues.
Both methods produce measurements of electrical performance, however, the
parametric method shall be used for validation of the unit under test to the
Serial ATA requirements 6.2 while other methods may be used as general No-Go
tests."
Question: "How many violations of the eye does it take before a design is
brought?" SATA II defines both jitter and amplitude statistically. The
measurement of maximum amplitude is described in section 6.4.2.2 page 114.
Essentially, an equivalent time sampling oscilloscope is used and a
histogram is setup in the amplitude direction. The number of samples that
fall beyond the allowable threshold is counted as well as the total number
of samples taken. The relative frequency is computed and if low enough the
test is passed (see step 7 page 115). This statistical definition allows
for unbounded amplitude noise to be present in the test setup and still have
a meaningful test. The test is also structured to encourage low noise test
setups. Further, the impedance match of the unit under test and test setup
is also described.
The minimum amplitude test is described in section 6.4.2.1 page 106. Once
again, the histogram function in the scope is used, placing the histogram
limits in time at 0.45UI and 0.55UI (this is where minimum amplitude
matters) and a test equation defines the requirement based on the mean,
standard deviation, and number of the voltage samples. This is based on
meeting a minimum amplitude with a confidence level of 95%.
The jitter test is more complicated due to the industry problem of
reproducible jitter measurements. This is dealt with in section 6.4.7.1
page 125. Jitter measurement tools intend to predict BER by extrapolation
from sample sizes which are not large enough to be statistically significant
at BER=1E-12. The accuracy of jitter measurement tools is left to the
instrument manufacturers but a standard for comparison is defined by SATA II
as a direct measure of total jitter (TJ) at BER=1E-12: "Methods do exist to
extrapolate TJ on a BERT from time scan values at higher rates. While such
methods can be used to predict TJ at a desired BER, only a direct measure
all the way to the desired BER shall be used when using the BERT as a jitter
standard for comparison to extrapolation methods." This is a statistical
measure of jitter and the required confidence interval is specified (page
40) and an explanation is given in section 6.4.1.4.2 page 104 as 4 errors
are allowed for a BER target of 1E-12 with a sample size of 1E13.
I'm not sure whether 1 minute of data is enough since I don't know how fast
your instrument is.
"Can eye violations be graded in terms of severity based on the location
of the violation (that is : at the outer bounds, or the internal eye
trapezoid )?" NO. That is not in SATA II.
Regards,
Chuck Hill
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Grasso, Charles
Sent: Tuesday, October 05, 2004 8:28 AM
To: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] SATA eye diagram - how much is too much
Greetings!
I have a question regarding the interpretation of the eye diagram data vis a
vis
SATA.
How many violations of the eye does it take before a design is brought to a
screeching halt? A follow up question:
Can eye violations be graded in terms of severity based on the location
of the violation (that is : at the outer bounds, or the internal eye
trapezoid )?
The basis of this question (naturally) is that I have SATA data that have
exactly
one 1 violation at the outer bound of the eye at 1 minute of data capture.
Typical go/no go specification will determine that to be a fail. I am not so
sure.
Any insight would be greatly appreciated.
Thanks
Best Regards
Charles Grasso
Senior Compliance Engineer
Echostar Communications Corp.
Tel: 303-706-5467
Fax: 303-799-6222
Cell: 303-204-2974
Email: charles.grasso@xxxxxxxxxxxx; <mailto:charles.grasso@xxxxxxxxxxxx; >
Email Alternate: chasgrasso@xxxxxxxx <mailto:chasgrasso@xxxxxxxx>
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