What should I do with unused pins ? (hardware connection and software processing)

Microcomputer Frequently Asked Questions (FAQs) NO.EB9901001

Answer

[Dedicated input pins]
Unused input pins should be connected to the power supply or ground either directly or via an external pull-up/pull-down resistor (5 kΩ to 10 kΩ recommended).
If an unused pin is left open, the input gate becomes floating and a penetration current or latch-up may occur.

[Sink open drain pins]
Unused sink open drain pins should be set to output 1 by software and connected to an external pull-down resistor (5 kΩ to 10 kΩ recommended) or ground.
If you have no alternative but to leave an unused sink open drain pin open, be sure to set the pin to output 0 by software.