Expansion Bay Pinout

Hi Again Everyone,

Another question I have is regarding the expansion bay connectivity/pinout.
I see the 8 PCIE lanes but also noticed that Signal Interface Pin 74 GPIO3_EC connects to the EC controller.

Forgive me because I am not an EE but is there sufficient connectivity here to allow, for example, a peripheral SOC or microcontroller to interact with the EC, and more over, with other EC connected devices?

For example, could I connect an Input Module (which I believe is EC controlled) to such another device?

Not sure what Pin 74 is for, it doesn’t say.

Input modules, as in the keyboard, numpad, macropad, led matrix spacer, (everything except touchpad)? Those are just usb.

What are you looking to do?

I’m thinking of putting an out of band system on chip in the external bay (basically where the expansion NVME would go) and am trying to think if it is feasible to leverage the input module expansions to somehow display information from it and/or interface with it.

Ideally this would be when the OS is powered down and I am assuming the BIOS isn’t modifiable enough to drive this interaction with the main chipset (hence my leaning toward the EC which I had read is effectively Open Source now)

Edit: For context this is for a specialty technical testing environment and we have been considering designing something from scratch but the semi-open nature of FW was appealing as a potential base to dramatically cut down our development cycle

I was afraid you wanted it to work without going through the OS. I don’t know if that’s possible. Since input module communication is (primarily) done over usb you need a running usb stack to process normal communications.

Now, the input modules do also have sleep and board-id pins, that should go to the EC. If you really wanted to, it might be feasible to abuse that. Repurpose it to carry communications. But if you don’t absolutely need it to work while the OS is sleep, then it would be much easier to not.

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Thank you @MJ1 , that’s exactly the thought validation I needed!

Trying to map out use cases and conditions now to see what’s the least dramatic / KISS approach to design here!