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 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 (see 20.5.1 Reading and Writing Timer1 in Asynchronous Counter Mode).