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Title: XAUI channel group delay
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
-----Original Message-----
From: Tord Haulin [mailto:Tord.Haulin@optillion.com]
Sent: Wednesday, February 21, 2001 2:38 PM
To: Kesling, Dawson W; Serial PMD reflector (E-mail)
Subject: XAUI skew

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.