[SI-LIST] Re: DDR2 2-slot design preference...

Hello Monji,
From an SI point, signals routed with a consistent ground reference is 
perferable to signals that switch reference planes.  In addition, it appears 
that the butterfly design would have longer via transitions and more of them 
(to one of the DIMMS) which could be an area of SI concern.  Of course, with 
good design/simulation either of these designs will work.
-Jory
----- Original Message ----
From: "Jabori, Monji" <Monji.Jabori@xxxxxx>
To: si-list@xxxxxxxxxxxxx
Sent: Saturday, October 27, 2007 2:57:53 PM
Subject: [SI-LIST] DDR2 2-slot design preference...

Hi Experts,


I am looking at two designs for a 2-slot DDR2-800 memory system. One is
a butterfly design and the other has the slot on opposite sides of the
motherboard (top and bottom).

I am looking at the GND reference for both designs and have the
following question knowing that I have to use GND referenced layers for
each DDR2 channel.



For a butterfly design, my traces on an 8-layer design will flow from
the memory controller via 2 internal layers with 2 different GND layers,
i.e., one slot will have to change GND reference while the other will
not.



In an opposite 2-slot design, however, we can make the traces that go to
each internal layer have the same GND reference as the DIMM that is
closer to it. For example, the Top DIMM will have the same GND reference
(L2) as its channel traces coming on L3 while the Bottom DIMM will have
its GND reference (L7) as its channel traces coming on L6.



Having said the above, would you guys prefer one design over another
from an Signal Integrity point of view??



Thanks in advance.



Monji






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