RE: XGMII Clocks
It may be little out of place here, but I would suggest that now onwards we
make the timing w.r.t. center
of the clock rather than edge. In other words we should have symmetric
setup, hold requirements.
At 10:16 AM 9/6/00 -0700, Curt Berg wrote:
>One of the big problem with XGMII is the large voltage swing
>of SSTL. It would be of great benefit to use a different interface
>standard that is more compatible with today's ASIC technology.
>(Moving forward most ASICs will use 1.2-1.8 V core, and
>having 2.5V I/O is painful.) If we reduced the swing, power consumption
>will drop, timing gets easier, and EMI should be cut significantly.
>We have done significant SPICE simulations of SSO, ISI, of DDR interfaces
>like XGMII, and lowering the swing definitely gets much better eye pattern.
>I would be willing to help writing a proposal for standard that use
>HSTL, with extended VDD, so you could also use 1.8 V LVTTL/HSTL.
>Anyone else that would support this ?
>Having two clock make little impact on the timing budget, and you would
>have to match two clock paths instead of one. Asymmetry comes mainly
>from level shifting and that your internal threshold is different form
>Vref. Much better again would be to minimize level shifting be using
>a lower external swing. Hopefully it helps you both on the MAC and
>From: Howard Frazier [mailto:email@example.com]
>Sent: Wednesday, September 06, 2000 9:30 AM
>Subject: XGMII Clocks
>In a previous email thread, we debated the merits of using
>a single clock in each direction on the XGMII, versus using
>4 (frequency locked, but phase independent) clocks in each direction,
>with a clock dedicated to each of the four "lanes".
>Without repeating the discussion, it is safe to summarize that
>the majority opinion (from among those who expressed an opinion)
>was to stay with one clock in each direction.
>So, I would like to toss out another question for your consideration.
>Should we use a two phase clock? Clock and ClockBar?
>Some designers have suggested that this will make the ASIC and
>system timing more managable, because it is difficult to get
>symetric drive strengths from the clock output buffers, and
>the asymetry degrades the timing. With a two phase clock, you
>would still have asymetry on the data signals, but at least
>you won't have to account for the asymetry on the clock.
>At first blush, this seems like a modest addition. One more pin
>in each direction.
>Any opinions out there?
>Cisco Systems, Inc.
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