How, if at all, do I use ping-pong mode with MyUSB?
I've modified the USBtoSerial demo to do USB to SPI.
Moving the out endpoint to 1 let me use 128 bytes,
but to use 256 bytes, the 1287 wants ping-pong mode.
BTW the At90USBKey is running at 5 volts,
so it can do SPI with an STK500 and other 5 volt things.