During my current project and after ordering the PCB I red one think that I never red before. In the datasheet of ATmega2560, SPI unit it says that in Master SPI mode if the SS' pin is used as a digital input, then this must be held in high level.
In my design I am using the SS pin as a digital input with internal pull up enable. In this input there is a button connected. So when the button is pressed, low level appears in the SS' pin.
My question is if I have to switch to another unused digital pin or I will not have problems using the SS' pin as a digital input and the SPI in master mode. ????