- Make all protocols mandatory (current situation). Safest option, but precludes making the boards simpler at a later stage.
- Define one of the protocols as compulsory (my preference: USB) and leave the others optional. All DIMMs and all backplanes must support the compulsory protocol and can fall back to that if no other protocol is available.
- If you have one compulsory protocol that must be supported, why do you need the others? So only support that one protocol. Very lean bus design, but forces all future DIMMs to have adapter logic to translate between the bus protocol and what is used on the DIMM.
In my mind, if you have an MCU on the board, you could reflash the MCU to adapt to a future motherboard design. This gives maximum flexibility. We should route as many IOs to the DIMM connector as possible, with at least a basic idea of what each one will serve as, but they can be reassigned (or disabled) by downloading new firmware for the MCU on the daughterboard. If we do this, we would need to make sure that reflashing can be easily done without extra hardware by including a USB bootloader or something similar.
[...] I have more experience with Atmel AVRs and I would prefer the
AT90USB82.
[...] My understanding was that it is possible to brick the Atmels. Can you confirm (or better: rebut) this?
What exactly would you define as bricked? If you had a USB bootloader and accidentally overwrote the firmware, you would need to reload the bootloader with a programmer (or have the motherboard do this). The only "bricking" I can think of with the AVRs is if you accidentally lock the flash memory or disable the RESET pin (needed to load new firmware). These can still be recovered from by using a special programmer.
The only real problem I can see with the AT90USB82 is that it doesn't support 2.5V operation.
Is 2.5V operation a requirement? Can USB run at 2.5V? I don't think so.