Initialization

By default the TCB is in Periodic Interrupt mode. Follow these steps to start using it:

Write a TOP value to the Compare/Capture register (TCB.CCMP).
Enable the counter by writing a '1' to the ENABLE bit in the Control A register (TCB.CTRLA).
The counter will start counting clock ticks according to the prescaler setting in the Clock Select bit field (CLKSEL in TCB.CTRLA).
The counter value can be read from the Count register (TCB.CNT). The peripheral will generate an interrupt when the CNT value reaches TOP.