[SI-LIST] Re: ground planes at top / bottom layer
- From: "Chris Cheng" <Chris.Cheng@xxxxxxxx>
- To: "Martin Euredjian" <martin_05@xxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Fri, 2 Jan 2009 23:56:12 -0800
XBOX is an impressive piece of hardware indeed but it is hardly a super
challenging SI design.
There is only two significant high speed buses with one running as differential
signal between the CPU and GPU a few inches away from each other.
The other is just a 700MHz PTP GDDR3 bus on the motherboard.
There is
a) no meter long multi-boards multi-connectors multi-terabyte per sec full mesh
super computer
b) nor there is a few GB of main memory on a multiple sockets multiple DIMMs
memory subsystem.
c) nor there is a few PCIe Gen II or III cards with connectors sitting in the
box.
It is meant to be a highly integrated high performance but low cost system with
simple I/O subsystem and highly embedded ASIC and specialized function CPU/GPU
to easy the external complexity (chips and board integration).
With the above in mind, I would think a 4 layer design with microstrip signals
will be good enough.
I would however extend the discussion to multi-layer boards beyond 4 layers
with multiple reference planes.
I like to have at least the two reference planes for the microstrip layers to
be ground planes so that I can stitch them together and form a nice faraday
cage for EMI.
Happy New Year.
________________________________
From: si-list-bounce@xxxxxxxxxxxxx on behalf of Martin Euredjian
Sent: Fri 1/2/2009 9:51 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: ground planes at top / bottom layer
> No, the Xbox did not start at 6 layers.
Wouldn't it be relevant to ask how much it cost to make it work with four
layers? How many board spins? How much longer did it take to achieve
this? What was the cost of the added design and testing efforts?
While I am the first to appreciate superb engineering...this is also a
business. If you are doing a low volume product and slapping on a few
more layers gets you to market faster (whether they are there to ease
routing or EMI/SI issues), cheaper and with fewer (or none) board
spins...what's wrong with that picture?
-Martin
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