After any Reset, CLK_MAIN is provided by the 16/20 MHz oscillator (OSC20M) and with a prescaler division factor of 6. The actual frequency of the OSC20M is determined by the Frequency Select (FREQSEL) bits of the Oscillator Configuration (FUSE.OSCCFG) fuse. Refer to the description of FUSE.OSCCFG for details of the possible frequencies after Reset.