In an AT90USB1287, what is the distinction between Transmit Complete and Data Register Empty?
I've written three USB to SPI versions of Dean Camera's USBtoSerial demo.
They all use MSPIM.
The one using Data Register Empty interrupts was by far the slowest,
also the least reliable.
The one using Transmit Complete was reliable and faster,
but nowhere near as fast as the one that didn't use interrupts or the ring buffer.