18.6.3.3.1 IDLE Mode

IDLE mode allows power optimization with the fastest wake-up time.

The CPU is stopped, and peripherals are still working. As in Active mode, the AHBx and APBx clocks for peripheral are still provided if requested. As the main clock source is still running, wake-up time is very fast.

• Entering Idle mode: The Idle mode is entered by executing the WFI instruction. Additionally, if the SLEEPONEXIT bit in the Cortex System Control register (SCR) is set, the Idle mode will be entered when the CPU exits the lowest priority ISR (Interrupt Service Routine, refer to the ARM Cortex documentation for details). This mechanism can be useful for applications that only require the processor to run when an interrupt occurs. Before entering the Idle mode, the user must select the Idle Sleep mode in the Sleep Configuration register (SLEEPCFG.SLEEPMODE=IDLE).
• Exiting Idle mode: The processor wakes the system up when it detects any non-masked interrupt with sufficient priority to cause exception entry. The system goes back to the Active mode. The CPU and affected modules are restarted.

GCLK clocks, regulators and RAM are not affected by the Idle Sleep mode and operate in normal mode.