[SI-LIST] Re: DDR traces across different power plane

Steve/Shikha,
Thanks for your reply. It may cause problems as you mentioned. But I'm still 
confused. Even if the high speed traces (on layer 1) cross different VCC planes 
on layer 2, the ground return current of these traces will still flow on the 
GND plane which is intact on layer 3. The problem that I'm worried is the 
impedance discontinuous for these traces. Could you help to explain why you 
think this kind of routing will cause SI issue except impedance broken? Thanks 
a lot. I tried to run some simulation with SPEED2000 and didn't found serious 
problems.

I thought of another routing scheme, parts of address & control traces of DDR 
are routed on VCC plane. In this way, most of traces won't cross power plane. 
However, there are some address trace segments which will be located above the 
routing area on VCC plane. I don't know whether this kind of routing will be 
better than previous one.

The bottom layer is full of DDR traces as well so it's useless to exchange VCC 
and GND layers. It's a challenge that MCLK frequency need to achieve 250MHz 
(DQS 500MHz).

Thanks for your help!
Best Regards,
Peter Pang

----- Original Message ----- 
  From: steve weir 
  To: pangning2000@xxxxxxxxx ; si-list@xxxxxxxxxxxxx 
  Sent: Tuesday, April 19, 2005 8:34 PM
  Subject: [SI-LIST] Re: DDR traces across different power plane


  Peter, if you mean a 250MHz clock and 500Mt/s data rate, then you likely 
  have some serious problems.  If you mean a 125MHz clock and 250Mt/s, then 
  with some care, you should be able to manage getting the address and 
  control lines across a split.

  Steve
  At 06:37 PM 4/19/2005 +0800, Pang Ning wrote:
  >Dear experts,
  >I am currently working on a 4-layer PCB layout. Due to the complicated 
  >power scheme, some of the DDR traces have to be routed across different 
  >power planes. In details, DDR traces are mainly on the layer 1, and VCC is 
  >layer 2. Many address & control traces must be routed above the gap 
  >between different VCC planes. I understood the impedance will be 
  >discontinuous for these traces, but it's unavoidable in order to reduce 
  >cost. I want to know whether there is any other problems for this kind of 
  >routing? The GND plane is intact on layer 3. The return path of these 
  >traces should be good since GND plane is good, right?
  >
  >The reflection may not be serious because these signal has relatively slow 
  >switching rate. Did anybody try this kind of routing experience for DDR 
  >before? Our target frequency will be 250MHz. Thanks for your comments.
  >
  >Best Regards,
  >Peter Pang
  >04/19/2005
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