11.3.2 Main Clock After Reset

After any Reset, the Main Clock (CLK_MAIN) is provided either by the OSCHF, running at the default frequency of 4 MHz, or the OSC32K, depending on the Clock Select (CLKSEL) bit field configuration of the Oscillator Configuration (FUSE.OSCCFG) fuse. Refer to the description of the FUSE.OSCCFG fuse for details of the possible frequencies after Reset.