My Spartan 3E board has SDRAM for external memory.

I don't know much about the specifics of your "T51" 805x core implementation, but if you have the MOVX instructions implemented correctly, you have external memory bus access.

You should, in fact, have no trouble implementing your CAN controller if you use SFR space as a venue for its registers, viewing SFR-space as I/O-space. That means you have to assign otherwise unused SFR-space locations to that controller. That will relieve you of the burden of assigning external memory-bus space to the subject peripheral.

You can, of course, do as you've suggested, namely assign a block of external memory addresses to the controller, but there's really no need to do that. SFR space is in the directly addressable internal memory range from 0x80..0xFF. IF you have a working UART, and a working set of timers, then you already use a few locations within that space.

Good luck!

RE