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

[chrisknightley]

FYI, the microchip that manufactures AVR has announced as follows.

https://microchipdeveloper.com/8avr:ioports

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Unconnected Pins

If some pins are unused, we recommend that you ensure that these pins have a defined level, even though most of the digital inputs are disabled in the deep sleep modes. Floating inputs should be avoided to reduce current consumption in all other modes where the digital inputs are enabled (Reset, Active Mode and Idle Mode).

The simplest method to ensure a defined level of an unused pin is to enable the internal pull-up. In this case, the pull-up will be disabled during reset. If low power consumption during reset is important, we recommend you use an external pull-up or pull-down. Connecting unused pins directly to VCC or GND is NOT recommended, since this may cause excessive currents if the pin is accidentally configured as an output.