I took a scalpel to my Orthrus prototype today to swap the wiring for MOSI and SCK so that I could try out USART0 in SPI Master mode. It took quite a bit of swearing to get the kludge wires to work, but they finally do, and at least with the code I've written, USART0 in SPI master mode is around 5% slower than straight-up SPI.

I'm surprised by this, but I've stared at the code and experimented with it for a while now and I can't see any improvements to be made. The USART code works, it just doesn't work any better.

So with that, the final performance numbers I'm getting on a small variety of different SD card makes is around 225 KB/sec.

It's possible that a future version of LUFA might bring improvements in performance - in particular the ability to use ping-pong buffers might be a big boost (if it's the USB performance that's throttling the system). To test that out, I replace the disk block read method with one that skips all of the SPI stuff and just reads zeros. That achieved a throughput of ~270 KB/sec - only 20% faster than actually doing the I/O properly.

So with that, I'm going to declare that v2.0.1 is ready for prime time. v2.0.2 just swaps SCK and MOSI. I will keep that change going forward just in case there's some sort of epiphany down the road that makes USART SPI mode work, but there's no reason not to release the current design now.