I am confused about how to configure an ATmega16A microcontroller to use an external clock instead of the internal one. I do not know whether I should set up the last bit as 0 or 1 for CKSEL in crystal mode. Why are there three different values given for this last one? Is there any reason or I can choose any one?
Image source: Microchip (Atmel) ATmega16A datasheet