FEDEVEL
Platform forum

ljo , 07-07-2016, 09:24 AM

Hello,

Can I mix high speed signals (DDR3 in my case, just few traces for simplifying the layout) with the power plane? If it is OK, wich clearance do I have to use?
Here is my layout :

robertferanec , 07-07-2016, 09:42 AM

I wouldn't route it like this. I always try to route all the signals from one bank the same way. In your case it looks to me like you have some signals of one bank routed on this layer and other signals routed on other layer(s). Also, consider impedance requirements (would you be able to keep the required impedance of the tracks on this layer?).

If needed, try to swap the signals, that may help you.

I keep saying that. Routing is hard

ljo , 07-07-2016, 03:44 PM

Thanks for answer.
Its hard i confirm.
But I saw that in beaglebone black and the reference design from ti the am335i starter kit, they use use outer and inner layers for routing ddr3 signals. I wanted to do the same and for the length matching i will base the matching on the time not on the length due to different speed. Do you think it is possible to do this way?

robertferanec , 07-07-2016, 04:08 PM

Yes, you can do that. BTW: you are not the first once pointing to beaglebone

I am pretty sure it may work ok in some cases and for some frequencies, but I do not do it. I have had a bad experience once, when we contracted PCB layout. The guy didn't follow Intel design guide recommendations to route the signals in the same groups the same topology and we ended up with a design where some of the boards were failing memory test at -20C degrees. It was then impossible to fix the problem. Also, be aware, when you change your PCB manufacturer or PCB stackup you may need to adjust your layout then .... and in some cases, you may see differences between different PCB batches.

On the other hand, in all designs, when I have done the memory layout the way as it should be, I have never seen problems - so, I always do it the way, that all the signals in same group are routed by the same way.

ljo , 07-07-2016, 04:18 PM

What do you mean the same way? Only one layer? I cant route all the signals on one layer. In my stackup i have only 3 layers for routing ddr3 signals l1 l4 and l6. I use 6 layers pcb. In my case i also use the l3 which is power layer for the routing as i show you in the screenshot. L2 ans l5 is gnd.

robertferanec , 07-07-2016, 04:24 PM

Same way, for example: All the signals from the group are routed on L1 then all the signals go to L3 and then all the signals go to L1. They dont have to be routed on one layer, but they need to be routed on same layers.

ljo , 07-07-2016, 04:26 PM

Ok thanks i will try.