Changing the Period

The Counter period is changed by writing a new TOP value to the Period register (TCA.PER).

No Buffering: If double buffering is not used, any period update is immediate.

Figure 1. Changing the Period without Buffering
A counter wraparound can occur in any mode of operation when up-counting without buffering. This is due to the fact that the registers TCA.CNT and TCA.PER are continuously compared: if a new TOP value is written to TCA.PER that is lower than current TCA.CNT, the counter will wrap first before a compare match happened.
Figure 2. Unbuffered Dual-slope Operation
With Buffering: When double buffering is used, the buffer can be written at any time and still maintain correct operation. The TCA.PER is always updated on the UPDATE condition, as shown for dual-slope operation in the figure below. This prevents wraparound and the generation of odd waveforms.
Figure 3. Changing the Period Using Buffering