HATs and other add-ons • Re: Stacking multiple official HATs on Raspberry Pi

A month late to the party and posting poor advice too.

There are lots of different issues that may come into play when stacking multiple HATs - it depends.

First, let's assume you have appropriate mechanical fasteners / spacers to hold the stack together, mechanically. That's the first thing to sort out.

And, I assume you have "stacking headers".
Not all HAT boards will have a "passthrough" female header, so they might be constrained to only go at the top-most position on the HAT "stack".
One way or another, you will need to figure out to have headers connecting all the boards in the stack.

Stacking headers and hardware are not a hard requirement. There are third party boards that allow use of multiple HAT[+] and HAT like boards without either. For example: https://shop.pimoroni.com/products/phat ... 8973392906

If you have multiple HATs with ID EEPROMs with the same ID then that will be problematic.
In that case, you'll need to desolder the EEPROM from one of the boards (or otherwise figure out how to completely disable the IC) and manually load any overlays relevant for that board.

Nope. No need to remove or disable the EEPROM. As I said above:

[O]lder spec HATs can be stacked if you disable the boot time probe for the HAT EEPROM and PoE fan control and load the necessary config and drivers manually.

Then, you'll need to check if any GPIO pins are used and they're clashing between multiple boards.

Again, as I said above:

Provided there are no pin conflicts [...]

[...]
You won't be able to use two different PCIe devices, as there is only one PCIe port - that's a different issue, not a HAT issue.

Correct in that it isn't strictly a HAT[+] issue but incorrect in that multiple PCIe devices cannot be used. That can be achieved by putting a suitable PCIe packet switch between Pi and devices. For example: https://shop.pimoroni.com/products/nvme ... 4434895955

Statistics: Posted by thagrol — Wed Jul 31, 2024 2:18 pm

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