FEDEVEL
Platform forum

ElHeim , 06-12-2025, 04:58 PM

I'm designing an STM32MP1 based SOM board, along with a baseboard for peripherals. For the PCB edge connector I'll be using DDR3 SODIMM. Could have been the ubiquitous M.2, but I need more pins.

Now, I've been looking around for advice on taking care of SI in this kind of situation and there's *some* variation. Of course most sources I've checked (e.g. Phil's Lab, Ben Jordan from Altium, ...) want you to isolate signals between GND pins for an improved and predictable return path.

Termination, though, hasn't been mentioned that much, except for example here: https://www.youtube.com/watch?v=mIk1vYbdtQM&t=490s, where he suggests to add termination at either board (or both), but this is not something I've seen much (or at least I don't believe so) in the wild, so I'll see case by case.

But that's for signals in general. Now, what to do with lines that **require** termination and will be crossing PCBs? In my case the RAM will be on the SOM board, so I just need to follow the usual guidelines; but one of the interfaces exported to the baseboard is RGMII, which includes termination for most lines.

How does the connector affect those specific cases? Is there any general advice?

Thanks!

QDrives , 06-12-2025, 08:50 PM

What are your trace lengths and what is the rise time of your signals?

ElHeim , 06-12-2025, 10:19 PM

Still not 100% sure, but I expect no more than 30mm from MPU to PCB edge, and about the same from the connector to the transceiver

Robert Feranec , 06-13-2025, 05:02 AM

many SODIMM computer on modules exists and I don't think I have every seen any mention of taking super extra care of termination just because the signals are travelling through the connector. Even memories have the same termination as if they would be placed on the board directly. Unless you are going above 2.5GHz, I don't think SODIMM would need to be a big concern. But that is only my opinion. If you want, you can always simulate it e.g. I used hyperlynx for multiboard simulation

ElHeim , 06-13-2025, 01:34 PM

Thanks @Robert Feranec, that's what I suspected. My fastest interface is going to be RGMII. so I think I'm good.