Hey Dennis,

I read a bit of the datasheet for this chip. I can't find an answer to your question. Moreover, I don't find any detail on how to communicate with the bootloader through the USB. I see the standard ATMEL methods for launching the bootloader, through software or hardware. And I see that in this case the hardware condition is that ~ISP is held low. But when does the USB go through setup and configuration? What drivers have to be loaded? I know with the UART bootloader all you have to do is send a "U" character (alternating 1's and 0's) for the autobaud routine. Is there some sort of default bootloader configuration for the USB? What is the equivalent action to sending that "U" over the port. When does it negotiate with the host?

USB is a great protocol. It is extremely user friendly. Unfortunately, we ain't the users!