I suspect that the reason they're considering external-memory-mapped interface to their CAN controller is that they don't know the 805x architecture and don't know the details of the T51 core, and perhaps not even the HDL in which it's written.

Mapping their peripheral into SFR-space should be dirt-simple. Mapping it into external memory should, too. They've not come up with reasons for what they're doing, nor have they mentioned why they're having trouble. In fact, they've not even come up with reasons for choosing the T51. I suspect it's because they don't know how to program, hence aren't willing to wander away from the ubiquitous 'C' language. Clearly they don't know the details of using external memory in Keil, though there's nothing particularly complicated about that.

I believe this is a "bass-ackwards" project design process gone awry.

RE