RE: XAUI Related Issues
Ali,
Thank you for your inputs. I'm sorry I was too swamped to reply last week. I
was also hoping that some other participants would jump in and fill the gap
with their thoughts! My thoughts are embedded in your message below. I'm
still eager to hear what others think as well.
-Dawson
-----Original Message-----
From: Ali Ghiasi
Sent: Tuesday, February 27, 2001
Subject: XAUI Related Issues
Hi Dawson
I was looking at D2.1 and I found several potential issue:
- In Tampa meeting we agreed the RX amplitude will be fixed to
1600 mV and TX need to be determined. Here you need some margin
and as allowing some reflection. Probably maximum TX need to be
lowered.
Dawson's comment: This is a good discussion point for Hilton Head if you or
someone else can do a quick analysis and come with a proposal. (Or send it
to the reflector anytime.)
- Return loss originally was intended only for receiver (10 dB
diff, and 5 dB common mode). It would very difficult to meet
to meet transmit return loss. In the IB we added Zdiff,
ZSingle_Ended, 10% matching, but not the return loss.
Dawson's comment: I think you are right, but we've voted it in and need to
vote in order to make a change. Again, a specific proposal is needed from at
least one participant to have 75% of the group to make a change.
- Differential Skew can't be measured separately from DJ even if
you are sending 1010, you still may have DCD. The total DJ
already include the Diff_Skew so I would propose removing it
since it can't be measured.
Dawson's comment: A good discussion point for Hilton Head. I've heard one
voice agreeing with you and none in opposition so far.
- We need to have better definition of how much skew you can
have lane to lane. Chapter 48 says you can have 1 UI allocated
to the PMA, but we need to allocated like 0.5 UI to the electrical
XAUI portion.
Dawson's comment: We should coordinate this with clause 48. The plenary is a
good time since clauses 47 and 48 will be sharing a room. I agree that 0.5UI
is reasonable for the interconnect. But since we haven't spec'ed the
interconnect so far, one thought is to spec the transmit lane-to-lane skew
and the the receive tolerance for lane-to-lane skew.
- It seems to me the connector loss is lumped as part of
interconnect
loss resulting in 7.5 dB of loss. Under some cases where the 7.5 dB
is just ISI some implementation may not work. We should try to
separate
ISI loss from connector+misc+croasstalk loss.
Dawson's comment: I'm not sure I understand. (I would not expect an
interconnect with 7.5 dB of ISI loss to work very well with a minimum
amplitude non-equalized driver and marginal receiver. The compliance channel
only budgets 4 dB of ISI loss.) Can you expand on this concern and propose
some numbers?
- Fig 47-4 assuming measured with an ideal load does not include
4% cross talk which equates to 63 mV under worst case scenario of
1600 mV and 200 mV. Loss allocation in the table 47-6 specifies 4.5
dB
for others but according to eye diagram Fig 47-4 some one may
introduce
more loss. The 4% crosstalk results in 31% additional eye closure
with
an actual XAUI receiver. Return loss penalty associated with a XAUI
receiver gets absorbed in to the SerDes effective sensitivity, but
the
crosstalk may not. As the biggest source of the crosstalk is the
connector.
Dawson's comment: Thank you for noticing this. Do you think it needs fixing
given that the 200mV of Fig 47-4 is a minimum driver requirement and we
don't spec the interconnect? If someone wants to design a driver and
receiver that just barely meet the minimum requirements, then shouldn't it
be expected that they will require a cleaner channel? Perhaps the
informative loss budget needs modification if we want to be purists.
Thanks,
--
Ali Ghiasi
Broadcom Corp.