32.5.3 Clocks
A generic clock (GCLK_SERCOMx_CORE) is required to clock the SERCOMx_CORE. This clock must be configured and enabled in the CRU registers before using it for SERCOMx_CORE. See Clock and Reset Unit (CRU) and Overview from Related Links.
This generic clock is asynchronous to the bus clock (PB1_CLK). Therefore, writing to certain registers requires synchronization to the clock domains.