[SI-LIST] Re: PDS analysis?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: Chris Cheng <Chris.Cheng@xxxxxxxx>
- Date: Mon, 31 Mar 2008 21:17:01 -0700
Chris,
Chris:
Remember Chris Rule B
, providing enough The signal consist of the signal trace AND its return
path. While your =
driver may not be switching, the return path that is most likely shared =
between multiple drivers may not.=20
A classic example is your processor FSB or DDR1/2/3 during a write =
cycle. The DQS is switching between the DQ data packet. While your DQS =
may be quiet and not switching, the entire DQ bus can be switching and =
raise or drop your return path high or low or ring like a gong. Your DQS =
will not be in a steady state of voltage.
Steve:
We agree that a contiguous return path is a good practice, ( Chris' Rule
B ). However, while a contiguous return path removes the PDN impedance
/ noise from the signal path, it does not guarantee any particular limit
to SSO.
Chris:
Agree and the decoupling will be done by either I/O decoupling cap on =
die or the plane capacitance of the reference plane that sandwich the =
signal combined with the proper via that switch together with the signal =
when it entires or leave a reference plane.
Steve:
I am OK with you up to the point that you assume that the PDN sandwiches
the signals. The power delivery is separate from the signal
propagation. In the case of a 4/6 layer DDR we would reference DQ lnes
to Vss and address and control to Vcc without running any inside a Vss /
Vcc cavity. At a minimum the signals that reference the far plane
penetrate the cavity at least once, and excite it.
For signals that penetrate a cavity, maintaining continuity through the
cavity reduces the amount of energy injected. As soon as we drop a
signal via down we lose TEM and all manner of interesting things happen.
Even when we map return vias 1:1 w/ signal vias we do not eliminate the
discontinuity. We just make it smaller. Until the day that the
mythical coaxial via and matching lossless transitions are invented the
best that we can do is to reduce the energy loss to acceptable levels
through good path design which includes the signal and return via
designs. Certainly no nearby vias at all will result in far more energy
injection into the cavity than some number of vias. Scott spends lots
of time optimizing such structures for very high performance.
Best Regards,
Steve
Chris Cheng wrote:
thread way too long and getting hard to follow, snip
--
Steve Weir
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