Good thoughts. Yes, it's using SPI to access the SD card. The native SD interface (4-wire) is supposed to be faster, assuming large data transfers to sequential addresses. But it requires money to license, and the protocol is reportedly pretty complex. I don't know of any low-end microcontrollers that support it in hardware. Unfortunately, I don't think it's a realistic option for hobbyists. But I think the SPI interface can be made fast enough to work.
The floppy emulator is using hardware SPI support in the AVR, but it doesn't help so much. There's no buffer, so the mcu has to do work after each byte sent or received. There are only 16 mcu clock cyles between each byte, which isn't enough to do anything else using an interrupt-driven approach. The real benefit of the hardware SPI support is that you don't have to manually toggle the SPI clock and data lines under program control, so SPI transfer speeds can be faster.
SPI is not used for the emulator-to-Mac communication-- that's handled by the CPLD. In theory it might be possible to do it from the mcu with SPI, assuming a mcu with multiple SPI interfaces, but there are some oddities involving synchronization that might make it difficult or impossible. Since I already needed the CPLD for the disk register read/writes, it was easiest to have it handle the Mac-side serial communication too.
If this sort of thing interests you, then you might enjoy reading some more detailed discussions of the Floppy Emu development from my blog: http://www.bigmessowires.com/category/macintosh-floppy-emu/
The floppy emulator is using hardware SPI support in the AVR, but it doesn't help so much. There's no buffer, so the mcu has to do work after each byte sent or received. There are only 16 mcu clock cyles between each byte, which isn't enough to do anything else using an interrupt-driven approach. The real benefit of the hardware SPI support is that you don't have to manually toggle the SPI clock and data lines under program control, so SPI transfer speeds can be faster.
SPI is not used for the emulator-to-Mac communication-- that's handled by the CPLD. In theory it might be possible to do it from the mcu with SPI, assuming a mcu with multiple SPI interfaces, but there are some oddities involving synchronization that might make it difficult or impossible. Since I already needed the CPLD for the disk register read/writes, it was easiest to have it handle the Mac-side serial communication too.
If this sort of thing interests you, then you might enjoy reading some more detailed discussions of the Floppy Emu development from my blog: http://www.bigmessowires.com/category/macintosh-floppy-emu/