External Pull Up Resistance ATMEGA328P

Using Arduino Microcontrollers
1

Hello all,

I am using 4 I/Os of an Arduino UNO to communicate with a GPS and a GSM modul but because it`s to big for my application I decided to create my own circuit board where I just use the ATMEGA328P.

The datasheet of ATMEGA328p says: "If some pins are unused, it is recommended to ensure that these pins have a defined level" & "If low power consumption during reset is important, it is recommended
to use an external pull-up or pull-down."

So I want to use a pull up resistor for that.

My question now is what is the usual resistance which is used in this case and how could I caltulate it?
I think I also have to define the unused I/Os as Inputs because of the 5V- Is it possible to define this in the IDE or where shoul this be done?

2

HannesM:
So I want to use a pull up resistor for that.

Why ?

Whats the problem with just defining all the unused pins as 'INPUT_PULLUP' if your worried about it ?