Atmega328p clock and safe operation zone
Just let CKDIV8 set (it just sets system clock prescaler to /8). And you can set different system clock prescaler later.

For changing system clock prescaler you need:

#include &lt;avr/power.h&gt;


And:

clock_prescale_set(clock_div_2); // possible values are clock_div_1 ... clock_div_256


Just don't forget that millis, delays and basically everything based on F_CPU might be slower/faster when prescaler gets changed.