By default, all Arduino (ATmega328P) pins are configured for input without the internal pull-up enabled. This default seems to come from the AVR microcontroller itself.
When it comes to saving power - is this an optimal configuration for unused (and thus unconnected) pins?
Section 14.2.6 Unconnected Pins of the ATmega328P datasheet (page 88) recommends to configure pull-ups in order to save power:
If some pins are unused, it is recommended to ensure that these pins have a defined level. Even though most of the digital inputs are disabled in the deep sleep modes as described above, 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, it is recommended to use an external pull-up or pull-down.
So I'm tempted to call something like
for (uint8_t i = first_unused_pin; i < 20; ++i)
Although, I don't understand where an increased power-consumption would come from when the INPUT pin is unconnected and no pull-up is active. I mean, since the pull-up is connected to VCC, there definitely is some current flowing, whereas without the pull-up I don't see where current would flow, since the pin is unconnected ...
However, Nick Gammon reports in his extensive article on power saving techniques (Section Configuring pins as inputs/outputs, Sketch D) an increased power consumption when 6 unused pins are configured as inputs with internal pull-ups attached:
All pins as inputs, and LOW (in other words, internal pull-ups disabled): 0.35 µA (same as before).
All pins as inputs, and HIGH (in other words, internal pull-ups enabled): 1.25 µA.
So the datasheet's statements seem to conflict with these results.
Is this an error in the datasheet are am I missing something?