Default digital/analog pin state - input or output? (do unused pins need to be grounded to prevent floating inputs)

[LarryD]

@chrisknightley

Floating unused inputs of CMOS ICs (not Arduino) with built-in protection diodes is not allowed many cases in this forum, but allowed floating unused inputs of microcontrollers with built-in protection diodes.

Yes, generally, all unused CMOS inputs should be tied to either VCC or GND .