It seems not possible to let digital peripherie running in the power-down mode.
I solve this with the 32bit rtc timer. Keep in mind, that the RTC has 2 internal timers:
1x 16bit one-shot timer, counting down to zero, 1kHz input
1x 32bit rtc, counting up endless, 1Hz input
I set a rtc alarm, after wakeup I update my 32kHz fast internal timer with 32000*rtc_time_passed.
