Differential pairs!

[Simon]

Right. what do I do? the written documentation is not the most enlightening. the video is second to useless and I'm left wondering what to do.

So I finally worked out how to declare two nets as pairs, I sort of worked out how to route differential pairs but it looks like i must set pins up to be on the right side as the differential router won't automatically swap a layer so that the tracks can cross ?

[voltsandjolts]

What bandwidth are you hoping to run your diff pair at?
e.g. for USB full speed you would probably OK with via crossover. For high speed, I dunno, but I'm a pessimist.

[Joel_l]

I don't think the question was, should you it, as much as it was, how do you get CS to do it.

I played with this a little today, no progress.

[voltsandjolts]

The question is first of all 'should you' and then maybe 'how you' cross a diff pair. I typically do 4 layer boards that are not controlled impedance and do not have high bandwdth diff pairs. I would guess that Simon is in a similar situation. So, this method works for me:

Once you have decided what pattern you want to use for the crossover (one or two pairs of vias & position), place vias and set their nets appropriately. Use vias in both traces to match impedance as best you can and to provide a via pair that the diff route tool can 'snap' to.

Manually route the (very short) crossover section. Manually matching the trace lengths in this short section is not really a chore and not an issue for a low bandwidth diff pair.

Now use the diff pair routing tool to route the long sections of the pair - they will 'snap' to the via pairs of your crossover because you set the nets for those.

[voltsandjolts]

For example, matched length / matched via crossovers with layer change and without layer change: