The Period register value (TCA.PER) and the Compare n register values (TCA.CMPn) are all double buffered (TCA.PERBUF and TCA.CMPnBUF).
Each buffer register has a Buffer Valid flag (PERBV, CMPnBV) in the Control F register (TCA.CTRLF), which indicates that the buffer register contains a valid, i.e. new, value that can be copied into the corresponding Period or Compare register. When the Period register and Compare n registers are used for a Compare operation, the BV flag is set when data is written to the buffer register, and cleared on an UPDATE condition. This is shown for a Compare register in below.
Both the TCA.CMPn and TCA.CMPnBUF registers are available as I/O registers. This allows initialization and bypassing of the buffer register and the double buffering function.