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RE: [10GBASE-CX4] Signal Detect - Proposed Text & Figures



Title:

Hi Ze'ev;

Answers/Comments below...

-----Original Message-----
From: Ze'ev Roth [
mailto:zeevr@mysticom.com]
Sent: Tuesday, February 04, 2003 9:36 AM
To: 'DOVE,DANIEL J (HP-Roseville,ex1)'; 10GBASE-CX4 (E-mail)
Subject: RE: [10GBASE-CX4] Signal Detect - Proposed Text & Figures


Hi Dan,
A few points I'd like to clarify.
1. Missing definitions
        -On 15m IB cable, the signal will be sometimes well below 125mvpp
(instantaneously) hence there should be a low pass definition, in terms of
poles/zeroes or multipliers. My understanding is that the figure describes
the signal envelope rather than the signal itself -so one way to
        implement it is using an envelope detector.
        - There should be some tolerance given for the analog voltages -
analog designers may have a fit with this definition
.

The spec indicates that once the input voltage has exceeded VSDA, it must
drop below VSDD for longer than a minimum amount of time. This is in essence
a mixed signal peak-detector function. Imagine two comparators, one with a
positive offset of (VSDA+VSDD)/2 and another with a negative offset of the
same amount. When either comparator triggers, two things happen. First,
Signal_Detect is asserted. Second, a counter is reset and starts
to count down. When that counter expires, Signal detect is lost. If you
have IDLE occurring at intervals during those few uSeconds (you will), there
will be instances where the voltage exceeds your threshold, the counter
is reset, and Signal_Detect remains asserted.

2. Questions
        Supposing that there is a CX4 signal at 50mvp2p envelope and the
receiver correctly decodes it
        (but it doesn't pass the proposed signal detect) - whould you want
the receiver to disregard its input ?

Only if SIGNAL_DETECT=FAIL. If, during the last 100uS, the voltage had
already exceeded SDAT, then the CX4 signal should be passed through. If
during the last 250us-500us, the signal voltage remained below VSDD, it
should be ignored.

        -The IEEE XAUI standard does not define / use the signal detect
function - why should CX4 have it?

XAUI is an internal interface. You don't typically have cables that are
connected, dis-connected. From a system standpoint, I know when a blade is
inserted into a chassis by other means. I have no way to tell if a cable
has been connected, whether the other end of the cable is attached, whether
that box is powered up, or sending proper signals.

        - You can add the comparators either directly to the input therby
     increasing  the input capacitance and  degrading performance,
        or they can be added after the equalizer which probably will have a
      gain and cause alot of headache to the designers.
         As a result, not all XAUI chips will be CX4 compatible and will
      require

It is not an objective of the group to make all XAUI chips compatible with
CX4. It is an objective to leverage the XAUI work. As for input capacitance,
I would recommend putting some resistance in series with the comparator
input to isolate the capacitance.

     
3. Is the proposed Signal Detector really needed ?
        -The XAUI standard provides couple of data validity guard bands:
                -PCS SYNC machine checks for a certain pattern
                -Alignment machine checks for Alignment validity
                -PCS layer checks validity of framing and XAUI protocol
                -PCS produces IDLES as long as there is not a valid XAUI
        frame
        These can be combined to yield an alternative "logical"  signal
       detect.
        In this case we'll need to define in the conformance tests a
      proceedure for ensuring that the receiver sensitivity is sufficient.

Signal_Detect is very useful in box-box interconnects to isolate
problems down to the "Is everything properly attached?" state. Also
I believe that testing the proposed spec will be pretty straightforward
as I can measure the voltage on the wire and evaluate the status of the
SIGNAL_DETECT function via MDI registers.

I hope I have responded adequately to your concerns.

Regards,

Dan