random notes on the AT90USB...

= Bootloader =
I have an AT90USBkey with the DFU bootloader.

 * http://sourceforge.net/projects/dfu-programmer/files/ is the right tool to flash it (Debian package available).
 * Run on 5V power supply : http://www.newae.com/tiki-view_blog_post.php?postId=9 . Good to know : the bootloader will detect the supply voltage with the voltage sense resistor, and enable the USB regulator as needed.

= Reserved pins =
On the dev board :
 * PF0 = tempertature sensor. May or may not be a problem.
 * PF3 = resistors to sense power supply.
 * PE0/PE1 = on board flash chip select
 * PE2 = HWB button.
 * PD7..4 = LEDs
 * PB7..5 and PE5/4 = Joystick

Leaves us with :
 * PA 0..7
 * PB 0..4
 * PC 0..7
 * PD 0..3
 * PE 6..7
 * PF 1,2,4..7

33 free I/O pins.
Some more can be used in some cases.