20.5 Timer1 Operation in Asynchronous Counter Mode
When the SYNC Control bit is set, the external clock input is not synchronized. The timer increments asynchronously to the internal phase clocks. If the external clock source is selected, then the timer will continue to run during Sleep and can generate an interrupt on overflow, which will wake up the processor. However, special precautions in software are needed to read/write the timer.
Important: When switching from synchronous to
asynchronous operation, it is possible to skip an increment. When switching from
asynchronous to synchronous operation, it is possible to produce an additional
increment.