21.3.3.2 Double Buffering
The Period register value (TCAn.PER) and the Compare n register values (TCAn.CMPn) are all double buffered (TCAn.PERBUF and TCAn.CMPnBUF).
Each buffer register has a Buffer Valid flag (PERBV, CMPnBV) in the Control F register (TCAn.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 (CMPn) below.
Both the TCAn.CMPn and TCAn.CMPnBUF registers are available as I/O registers. This allows initialization and bypassing of the buffer register and the double buffering function.