I'm curious about this statement in the ATtinyx1x documentation:
The peripheral clock (CLK_PER) is required to be at least four times faster than the RTC clock (CLK_RTC) for reading the counter value, regardless of the prescaler setting.
Not that I'd normally do this, but what would happen if I ran both the CPU and the RTC off of OSCULP32K? Or ran them both off EXTCLK? I'm guessing the 4x requirement has to do with synchronization, but as I said, I'm just curious about this restriction.