Author Topic: CM4 carrier board differential pairs swapping  (Read 267 times)

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Offline Deku Tree

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CM4 carrier board differential pairs swapping
« on: February 16, 2021, 10:09:48 pm »
Hi all,

I'm working on a small breakout board for the raspberry pi 4 compute module. This is my first time routing differential pairs and I keep running into the same issue. I've moved some of the connectors to the bottom of the board and this has caused many of the differential pairs to be both flipped within the pair, and crossed over other pairs. Here are a few screenshots to clarify:




My only thought is to simply run the last part of the pair through two vias with one trace on the top layer and one on the bottom. I typically work with much lower speed layout though and don't have a good idea of how vias will affect impedance matching.

Thanks for any help you can offer
 

Offline SiliconWizard

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Re: CM4 carrier board differential pairs swapping
« Reply #1 on: February 18, 2021, 05:03:00 pm »
Note that there's very little room for vias here. Maybe you'll need to push back the HD1 pair a little. Or maybe the smallest vias you can afford will fit.

See on this picture how to usually swap the pair: https://www.orcad.com/sites/orcad/files/image2.png

(On this they use via in pad, but this is usually a costly manufacturing option. Of course you can use standard vias instead here in the space between HD1_P and the two CAM0_C pins.)

You'll need four vias as shown to keep it symmetrical.
 
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Offline Deku Tree

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Re: CM4 carrier board differential pairs swapping
« Reply #2 on: February 19, 2021, 04:51:48 am »
Thanks for the tips! When moving the signals through a via does it matter what direction they come out on the other plane? I'm wondering if I can double back after a via on some of my signals without introducing too many reflections.
 

Offline SiliconWizard

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Re: CM4 carrier board differential pairs swapping
« Reply #3 on: February 21, 2021, 03:57:56 pm »
I don't think there's much to worry about here as long as you keep it symmetrical. Also, I don't know what kind of PCB stack you're working with. I can only see two layers on your capture? Is it a 2-layer PCB? I  would suggest a 4-layer arrangement for this kind of stuff...
 
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Offline Deku Tree

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Re: CM4 carrier board differential pairs swapping
« Reply #4 on: February 23, 2021, 01:49:41 am »
The board itself is 4 layers (internal power and ground layers) but the internal planes are hidden.

This is kind of what I'm talking about with the vias;



It just doesn't look right to me, but I don't see a better way to do it..
 


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