Sleep Mode Entry Sequence

Write the correct sequence for entering deep sleep on a Cortex-M part, ensuring no pending interrupt is lost in the race between the check and the WFI.

Constraints

• Set SLEEPDEEP in the System Control Register.
• Disable interrupts before checking for pending work.
• Use WFI with interrupts masked by PRIMASK so a pending IRQ still wakes the core.
• Re-enable interrupts after waking.
• Include the required barriers.
• Clear SLEEPDEEP on exit.