13.5.6 Sleep Mode Operation
In all IDLE Sleep modes, the power manager is still running on the selected main clock.
In STANDDBY Sleep mode, the power manager is frozen and is able to go back to ACTIVE mode upon any asynchronous interrupt.
Note: Do not make APB read access to read-synchronized registers when
APB clock is stopped and GCLK is running. This will cause the system to Freeze. Instead
power cycle the device or reset the device using the RESET pin.