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Tord,
First
for some historical background. Differential skew and has been included since
early XAUI proposals. There was a proposal to replace it with an AC common
mode voltage limit in Tampa last November. It was pointed out then that the skew
measurement definition is complex for the XAUI waveform (particulalry with
pre-emphasis), that an AC common mode voltage specification would better serve
the purpose for EMI control and that jitter is already spec'ed
anyhow. The group decided at that time that no action was
required on differential skew. Two comments during the
last balloting cycle specifically asked that skew information be make MORE
explicit. (These were both approved: #1067 and #965.)
Finally, one new participant raised a concern with
measuring skew last month in Irvine, but no specific proposal was made at that
time so no action could be taken.
So
where do we go from here? I can see several options:
1. Do
nothing and keep the text as is. People apply reasonable engineering practices
to define their own test methods to verify the skew of the driver and skew
tolerance of the receiver. The reasoning here is that the standard does not have
to define test methods (although it does so in cases such as jitter where
considerable confusion might otherwise exist). This is the default decision
unless another option gets a 75% vote.
2.
Define a detailed skew measurement method and document it in section 47.4
(Electrical measurement requirements). Tom's text from FC looks like a good
start and you point out several other details that would need to be considered.
If anyone is interested in doing this, please submit a comment by tomorrow or at
least let me know you'd like to make a proposal at the Plenary. I suspect this
would be supported by a 75% majority, but someone has to sign up to do
it.
3.
Remove all references to differential skew as you suggest. I doubt this will
gain the 75% majority needed, but there is only one way to find
out.
-Dawson
Dawson,
Clause 47 has specifications on
differential skew both for the transmitter and receiver. There is however no
definition for differential skew, or method given for measuring it. In
paragraph 47.3.4.1 the differential skew is said to include the effects of
intersymbol interference, ISI. I would think the main effect of ISI is
deterministic jitter, which is part of the jitter specifications. To define
and measure skew, we would probably have to relate it to rise and fall times
and define a common mode load and load balance with tolerances. The
transmitter skew and possible crosstalk along the line is filtered by the
channel common mode transmission magnitude and group delay, which falls
outside the channel model work done to date.
In the receiver compliance test
some additional sinusoidal jitter is applied to ensure some margin for effects
otherwise not accounted for. How about lumping skew into this, to avoid going
into specifying enough parameters and conditions to take measurements on skew
and related parameters? That would mean the skew limits could be removed from
the spec. Comments?
Tord.
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