RE: [10GBASE-CX4] Comments on latest revision
Other reasons for specifying the common mode voltage that Dan did not mention are the concern about the voltage on the series capacitors, and the spike on plugging in the connector. Although no-one appears to want to built CX4 drivers using a 12V supply, or the like, it is theoretically possible; in this case, a 6V capacitor at the receiver input (adequate normally) is being over-stressed, and this load should be shared by the driver (which can be AC coupled). Also, the voltage spike on plugging in a DC-coupled driver would be quite large, and there is some concern about the injected charge under 'hot plugging', so both the capacitor size and the voltage step are matters to be limited.
Peter Bradshaw
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> -----Original Message-----
> From: DOVE,DANIEL J (HP-Roseville,ex1) [mailto:dan.dove@hp.com]
> Sent: Wednesday, February 26, 2003 4:55 PM
> To: 'clarkf@mxim.com'; 'stds-802-3-10GBCX4@ieee.org'
> Subject: RE: [10GBASE-CX4] Comments on latest revision
>
>
>
> Hi Clark,
>
> Good comments. We will address them directly in the Dallas
> meeting. I have a few specific comments which I will note below.
>
> Please keep the comments coming in via the reflector. These will
> help us to nail down any final issues.
>
> BTW: Funny thing, while looking into your points, I found a typo
> in clause 45 that we carried over. Ooops! We will fix that and
> inform 802.3 for maintenance.
>
> Thanks,
>
> Dan
>
> -----Original Message-----
> From: Clark Foley [mailto:clarkf@mxim.com]
> Sent: Wednesday, February 26, 2003 2:45 PM
> To: 'stds-802-3-10GBCX4@ieee.org'
> Cc: Clark Foley (E-mail)
> Subject: [10GBASE-CX4] Comments on latest revision
>
>
>
> To the CX4 SG,
>
> Congratulations on a great job of pulling all of this
> together. Thank you
> for the opportunity to comment.
>
> sections
> 54.6.7 and 54.6.8 Global and Lane-by-lane transmit disable functions.
> ---------------------------------------------------------------------
> I think that the "Global" disable function should be
> mandatory and the
> "Lane-by-lane" disable should be optional. Because this is
> CX4 and not
> four asynchronous channels, the link should disabled with one
> bit. It
> doesn't really matter if you have MDIO, one bit is as easy as
> four bits to
> set. In designs without the optional MDIO, one bit becomes
> much easier to
> implement than four. The priority should be placed on
> disabling the link
> in as simple a manner as possible. One bit (a.k.a. one pin)
> does this.
> "Lane-by-lane" disable is helpful for debugging and
> evaluating designs,
> but it does not make sense at runtime for a XAUI link.
> Please make the
> "Lane-by-lane" disable the option for those who need to
> tinker off line.
>
> DD: I think we need to look this over one more time. I think
> I agree with
> you but would like to hear it discussed in the larger group.
>
> section
> 54.7.3.4 Amplitude and Swing (and Table 54-6)
> ----------------------------------------------
> Line 37 - Dc-reference logic level is meaningless because of the ac
> coupling. This makes sense.
>
> DD: Yep.
>
> Line 38 - If dc-reference logic level is meaningless because
> of the ac
> coupling, then the common mode voltage should also be
> meaningless. A dc
> common-mode voltage requirement does not make sense. I
> recommend deleting
> the common-mode voltage requirement.
>
> DD: The point of distinction is that line 37 is addressing
> *logic levels*
> while line 38 is addressing common-mode voltage. The reason
> for spec'ing
> the common-mode DC voltage is to provide direction for the AC
> coupling
> requirements of the receiver. We wanted to limit it to a
> number that allows
> either AC coupling at the transmitter (VDC_CM=0) or DC
> coupling with a
> source terminated up to 1.8V rails and anything in between.
>