4.16.5.4 Bypassing the Main Crystal Oscillator

Prior to bypassing the main crystal oscillator, the XOUT pin must be grounded and the external clock frequency provided on the XIN pin must be stable and within the values specified in the XIN Clock characteristics in the section “Electrical Characteristics”. Then the main crystal oscillator must be enabled by setting the CKGR_MOR.MOSCXTEN bit to 1.