5
\$\begingroup\$

Doing my first DDR2 layout and I'm hitting some conflicting requirements.

I have dogbones to an internal ground-referenced layer, and then short top layer traces at the other end going from the other via to the DDR2 pins.

I my second data lane (DQ15-8,DQS1, DM1) is looking really good except for two of the data lines I can't seem to figure out how to keep the top trace on the DDR2 side <2mm. I have to make it longer to allow the other traces to pass through with the proper spacing.

So my question is: How much of this data lane do I need to keep on the same layer? The bottom layer is reference a different ground plane (same ground but different plane), so could I put those traces there because they are all referencing a ground plane?

Picture attached. Length matching hasn't been done yet.

DDR2 Data Lane 1.

\$\endgroup\$
1
\$\begingroup\$

Provided each signal has single layer routing, you do not normally need to route the entire lane on the same layer.

There may be some small timing mismatch but usually not something to be too concerned about.

| improve this answer | |
\$\endgroup\$
0
\$\begingroup\$

I'm not 100% clear from your question what you are saying but I think you are saying you want to start routing a trace on the top layer with one reference plane, then via down to another reference plane and finish the route?

In your case you are saying both these reference planes are at the same DC potential (0V or GND).

This shouldn't be a problem, keep in mind an extra via will be another discontinuity that your signal has to pass through but at DDR2 it shouldn't be much of a problem. Remember that all current flows in a loop so your high speed return current is going to want to find a path from that bottom plane to the top plane right in the region around your via.

I would make it easy on him by adding a stitching via near the trace via, otherwise he'll find his own path and you may not like the results :)

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Yes, the red and brown traces are being referenced to the same electrical ground, but different ground planes. I just wasn't sure about how grouped the traces had to be on the same plane. DQ9 and DQ11, the most bottom two traces could actually just go on the top layer quite easily. It sounds like what I have in the picture is fine, and that putting those two traces on the top layer should be okay too. \$\endgroup\$ – Rudolf Feb 24 '15 at 22:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.