I can't find it now, but I recall reading somewhere that USART needed to be re-initialized.
Is there a general list somewhere of all subsystems that need attention after power-down?
Note that I'm not including subsystems that are explicitly put to sleep here (though
normally many of them would be shut down).
The datasheet for atmega328P also says regarding explicitly shutdown systems:
Resources used by the peripheral when stopping the clock will remain occupied, hence the
peripheral should in most cases be disabled before stopping the clock.
Is there any general description of what it takes to do this?