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Crappy Raspberry Pi Compute Module Breakout

Single layer breakout board for the RPI CM. Mostly a learning exercise for myself.

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This doesn't do anything more than the official dev board, actually a lot less than that.
It's an exercise for me to learn more about the compute module and how to route high speed signals.
Its not a good breakout in any shape or form but it might help others getting started with the CM. It only implements basic stuff to get some use out of the module.

Tested:
Booting from eMMC and SD-Card
GPIO
USB (ethernet dongle working, max speed of about ~820KB/s over SFTP).

Untested:
DSI Display connection

Those are all the features I want to get going right now. Maybe at some point HDMI but I highly doubt it will be possible with a milled PCB without any form of impedance control.

I calculated* all differential signals for a double sided 1.5mm FR1 board with 35um copper. The FR1 boards you get with the Othermill.

*As far as that is actually possible with a milled board. I tried to not be totally off but of course the tolerance will be absolute utter

sch - 208.89 kB - 05/12/2017 at 23:32

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brd - 117.06 kB - 05/12/2017 at 23:32

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Asher Gomez wrote 12/20/2018 at 18:09 point

Nice Project!

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Laetitia BEL wrote 03/22/2018 at 22:09 point

First time I see your project, and like it too much!

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mbt28 wrote 05/18/2017 at 14:16 point

I think HDMI and DSI will be quite tough to make it work with this technique. Do you plan to enhance it?

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Prof. Fartsparkle wrote 05/18/2017 at 16:47 point

HDMI most definitely impossible yes. DSI doesnt operate at such high speed, so I was hoping I could maybe get atleast something going even if its very flaky. So far I was even unable to get DSI working with the official dev board. Will have to investigate that.

I don't plan on making this a neat breakout. Its a playground to test out stuff for other projects. I may make a different version that breaks out different features but all thats left to try apart from HDMI and CSI (camera) is Audio and Analog Video out. The latter should be both doable.

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mbt28 wrote 05/19/2017 at 08:36 point

Thank you for your comments. 

What is your opinions about 2 layer pcb? Even if it is silk printed, is it possible to make differential pairs working properly?

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Prof. Fartsparkle wrote 05/19/2017 at 13:42 point

@mbt28 I'm really no expert on this topic but the differential pairs for USB and DSI on this board are calculated as micro strips (https://www.eeweb.com/toolbox/edge-coupled-stripline-impedance).  The board is technically a 2 layer board, I just use the bottom layer as a GND plane without any traces. This should in theory enable differential pairs, but I'm lacking the capability that a PCB manufacturer has to do real impedance control which is fairly complicated and dependent on the stack-up of the board and the materials used. So yes a 2 layer board, if manufactured with impedance control, should work fine. Personally I have never done it and I often read that its recommended to work with 4 Layer boards so you can have a dedicated GND plane on more crowded circuits like you would likely have with a fully featured CM breakout.

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Prof. Fartsparkle wrote 05/19/2017 at 13:44 point

I tried this is also without a GND plane on the bottom, at least USB worked the same, I dont know enough about the physics involved to know when the GND plane actually comes into play.

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