28.3.2.1 SOF - Start-of-Frame

When a Start-of-Frame (SOF) token is detected, the Start-of-Frame Interrupt Flag (SOFIF) in the Interrupt Flags A (USB.INTFLAGSA) register is set.

If the Store Frame Number Enable (STRFNUM) bit in the Control A (USB.CTRLA) register is ‘1’, the frame number from the token is stored in the Frame Number (USB.FRAMENUM) register in the endpoint configuration table. If a Cyclic Redundancy Check (CRC) or bit-stuff error occurs, the Frame Error (FRAMEERR) bit in the FRAMENUM register is set.