RM0312 datasheet(70/275 Pages) STMICROELECTRONICS

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Power management

RM0312

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DocID022352 Rev 3

9.4.1

Wait mode

Wait mode is entered from run mode by executing a WFI (Wait For Interrupt) or WFE (Wait

For Event) instruction: this stops the CPU but allows the other peripherals and interrupt

controller to continue running. The consumption decreases accordingly. Wait mode can be

combined with PCG to further reduce power consumption of the device. Refer to

Section 8.1.1: Peripheral clock gating (PCG) on page 62.

In Wait mode, all the registers and RAM contents are preserved and the clock configuration

selected through the Clock divider register (CLK_CKDIVR) remains unchanged.

Wait for interrupt (WFI) mode

When an internal or external interrupt request occurs, the CPU wakes up from WFI mode,

services the interrupt and resumes processing.

Note:

In an interrupt based application, where most of the processing is done through the interrupt

routines, the main program may be suspended by setting the activation level bit (AL) in the

CFG_GCR register. Setting this bit causes the CPU to return to WFI mode without restoring

the main execution context. This saves power by removing both the save/restore context

activity and the need for a main software loop execution for power management (in order to

return to WFI mode).

Wait for event (WFE) mode

There are two ways to wake up the CPU from WFE mode (see Table 17.: Low power mode

management on page 69):

•

When an interrupt occurs:

–

the CPU wakes up from WFE mode and services the interrupt. After processing

the interrupt, the processor goes back to WFE mode.

•

When a WFE event occurs:

–

the CPU wakes up and resumes processing. As the processing resumes directly

after the WFE instruction, there is no context save/restore activity (this saves time

and power consumption).

Note:

In WFE mode, the interrupt sources are configured as external interrupts only if the

corresponding status flags are cleared in the WFE_CR1 or WFE_CR2 register. Otherwise,

they generate WFE events (no interrupt serviced and the user has to properly clear the

corresponding status flags in the EXTI_SR1 or EXTI_SR2 register). Refer to Section 9.5:

WFE registers on page 72, External interrupt status register 1 (EXTI_SR1) on page 53 and

External interrupt status register 2 (EXTI_SR2) on page 54.

Further power consumption reduction may be achieved using this mode together with

execution from RAM. In some very low power applications, when the main software routine

is short and has a low execution time, this routine can be moved to RAM and executed from

RAM. As the Flash program memory is not used at wakeup, the power consumption is then

reduced during run time.

At any moment, another routine (stored in the Flash program memory) can be executed by

software by simply calling/jumping to this routine.

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