7 4 extra key(consumer/system)
8 5 nkro keyboard(supported only on ATmega32U4/16U4 and AT90USB64/128)
11 ATmega32U4/16U4, AT90USB64/128
12 • Endpoint 0:programmable size FIFO up to 64 bytes, default control endpoint
13 • Endpoints 1 programmable size FIFO up to 256 bytes in ping-pong mode.
14 • Endpoints 2 to 6: programmable size FIFO up to 64 bytes in ping-pong mode.
16 AT90USB82/162, ATmega8U2/16U2/32U2
17 • Endpoint 0:programmable size FIFO up to 64 bytes, default control endpoint
18 • Endpoints 1 and 2: programmable size FIFO up to 64 bytes.
19 • Endpoints 3 and 4: programmable size FIFO up to 64 bytes with ping-pong mode.
21 ping-pong mode means double buffer feature.
24 NOTE: ATmega8U2/16U2/32U2 is not supported with PJRC stack at this time.
25 TODO: Macro definition for ATmega8U2/16U2/32U2 in usb.h