PM0223 datasheet(34/110 Pages) STMICROELECTRONICS

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The STM32L0 Cortex-M0+ Processor

PM0223

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DocID025763 Rev 1

2.5.1

Entering sleep mode

This section describes the mechanisms software can use to put the processor into sleep

mode.

The system can generate spurious wakeup events, for example a debug operation wakes

up the processor. For this reason, software must be able to put the processor back into

sleep mode after such an event. A program might have an idle loop to put the processor

back in to sleep mode.

Wait for interrupt

The Wait For Interrupt instruction, WFI, causes immediate entry to sleep mode. When the

processor executes a WFI instruction it stops executing instructions and enters sleep mode.

See 3.7.12: WFI on page 80 for more information.

Wait for event

The Wait For Event instruction, WFE, causes entry to sleep mode conditional on the value of

a one-bit event register. When the processor executes a WFE instruction, it checks the value

of the event register:

See 3.7.11: WFE on page 79 for more information.

If the event register is 1, this indicates that the processor must not enter sleep mode on exe-

cution of a WFE instruction. Typically, this is because of the assertion of an external event, or

because another processor in the system has executed a SEV instruction, see 3.7.9: SEV

on page 77. Software cannot access this register directly.

Sleep-on-exit

If the SLEEPONEXIT bit of the SCR is set to 1, when the processor completes the execution

of an exception handler and returns to Thread mode it immediately enters sleep mode. Use

this mechanism in applications that only require the processor to run when an interrupt

occurs.

2.5.2

Wakeup from sleep mode

The conditions for the processor to wakeup depend on the mechanism that caused it to

enter sleep mode.

Wakeup from WFI or sleep-on-exit

Normally, the processor wakes up only when it detects an exception with sufficient priority to

cause exception entry.

Some embedded systems might have to execute system restore tasks after the processor

wakes up, and before it executes an interrupt handler. To achieve this set the PRIMASK.PM

bit to 1. If an interrupt arrives that is enabled and has a higher priority than current exception

priority, the processor wakes up but does not execute the interrupt handler until the proces-

sor sets PRIMASK.PM to zero. For more information about PRIMASK, see Exception mask

The processor stops executing instructions and enters sleep mode.

The processor sets the register to zero and continues executing instructions

without entering sleep mode.

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