No different than any other pin. Leaving it floating is generally not advised (especially if you care about consumption).
It's best to ground the unused pins through a 10K pull-down resistor and set them as analog inputs to disable the Schmitt triggers on the pins (which are required when the pin is in the default digital input state).
AN3430 ( https://www.st.com/content/ccc/resource/technical/document/application_note/77/74/4f/da/9b/9c/43/e7/DM00033348.pdf/files/DM00033348.pdf/jcr:content/translations/en.DM00033348.pdf ) recommends this:
- At the startup of the application:
- Enable all GPIO clocks
- Configure all I/Os in analog input mode
- Disable all GPIO clocks
- Enable the required GPIO clock
- Configure the required I/Os on the application
- If the user application just needs the I/Os to configure it as a peripheral alternate function in order to output or input signals (no read or write from/to GPIO registers), we recommend disabling the associated GPIO clock of the used I/Os after configuration. In this case I/O signals are still available and we optimize the GPIO current consumption by disabling its clock.
- When configuring the I/O, pay specific attention to the I/O speed value configured in OSPEEDR (2 MHz, 10 MHz, 25 MHz, 50 MHz and 100 MHz). Current consumption is as high as the I/O toggling frequency, so, to optimize current consumption, OSPEEDR should be configured with the lowest possible value.
