Unused I/O pins should be configured as I/O function, output direction, and left unconnected on the PC
board, to prevent a floating input and reduce power consumption. The value of the PxOUT bit is irrelevant,
since the pin is unconnected. Alternatively, the integrated pullup/pulldown resistor can be enabled by
setting the PxREN bit of the unused pin to prevent the floating input. See the System Resets, Interrupts,
and Operating Modes chapter for termination of unused pins.

最省电的还是置为三态的高阻态

接地也是会产生电流的，消耗资源的

模拟输入的状态的，这样IO的电流就降到最低了。

都设置成模拟输入状态的吧。