CCMP and CNT Registers Operate as 16-Bit Registers in 8-Bit PWM Mode
When the TCB is operating in 8-bit PWM mode (CNTMODE in TCBn.CTRLB is ‘0x7’), the low and high bytes for the CNT and CCMP registers operate as 16-bit registers for read and write. They cannot be read or written independently.
Work Around
Use 16-bit register access. Refer to the data sheet for further information.