The PHY layer and the ACC layer can operate on different clock domains. The PHY layer clock is derived from the dedicated internal oscillator, and the ACC layer clock is the same as the peripheral clock. There is a synchronization boundary between the PHY and the ACC layer, which ensures correct operation between the clock domains. The UPDI clock output frequency is selected through the ASI, and the default UPDI clock start-up frequency is 4 MHz after enabling or resetting the UPDI. The UPDI clock frequency can be changed by writing to the UPDI Clock Divider Select (UPDICLKSEL) bit field in the ASI Control A (UPDI.ASI_CTRLA) register.

Figure 1. UPDI Clock Domains