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Would it be possible to ran an AVR-USB device at a higher clock rate than the default 12MHZ, i.e. at 20MHz for the attiny2313 out of the box, or is it necessary that a bit is decoded at 8cycles@12MHz?
If it is, would it suffice to stuff in some NOPs to satisfy that requirement?

The currently shipping firmware works with 12 MHz only, it's required that one USB bit is exactly 8 CPU cycles.

In principle, it's possible to insert NOPs in the assembler module to accommodate higher clock rates. However, the clock rate must be divisible by 3 (i.e. 15, 18, 21, ... MHz).

The assembler module can also be adapted for other clock rates, but that's not as easy as adding a NOP here and there.