See & thou shall find (though I had sought but hadn't found earlier!), & found this tucked away in an app note...

Use the Watchdog Timer (WDT) as the time base while in Sleep mode and the CSM is
able to continue oscillating during Sleep mode.

Timer1 will continue to count until Watchdog Timer overflows and wakes the MCU.

Once the device wakes from Sleep and detects a change in capacitance, due to a finger
or hand in close proximity to the sensor, then the program can be set to another time
base to detect the actual button pressed and perform the desired function.


guess I'll just have to see how long I can extend the WDT as timebase to without timer1 overflowing