All oscillators and clock generation will be disabled in Power-down and Power-save sleep modes. The exception is if the Real-time Counter is enabled in Power-save mode, which leaves the RTC clock source running. If the RTC is not used, Power-down mode is recommended instead of Power-save mode. With these two sleep modes, start-up delays behave as if the oscillators were disabled.
In Standby and Extended Standby sleep modes, the oscillators are kept running, enabling the CPU to wake up without extensive delays.
In Idle mode, the oscillators are kept running and the CPU will wake up from sleep without any delay.
Refer to the device datasheet for details on sleep modes and wake-up conditions.