AN2857 datasheet(14/31 Pages) STMICROELECTRONICS

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Run and low-power modes

AN2857

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Doc ID 15241 Rev 3

4.2.3

Activation level/low-power mode control

In a very low-power application, the MCU spends most of the time in WFI/halt mode and is

woken up (through interrupts) at specific moments to execute particular tasks. Some of

these tasks are recurring and short enough to be treated directly in an interrupt service

routine (ISR), rather than returning to the main program. In this case, setting the AL bit

before going to wait mode, ensures the run time/ISR execution is reduced because the

‘context’ (core register content) is not saved/restored each time.

In very simple applications, all operations can be executed in ISR only. In more complex

applications, an interrupt routine may relaunch the main program by resetting the AL bit.

The activation level/low-power mode control feature works only with wait mode. It is not

available for active halt or halt mode.

4.3

Active halt mode

Active halt mode can be defined as a ‘hybrid’ mode between run and halt mode. It is

composed of two phases:

●

Halt phase: In this phase, the MCU is in halt mode except that the AWU unit and the

LSI clock are kept running if they are used as the AWU clock source.

●

Active phase: In this phase, the MCU is in run mode.

When entering active halt mode, the AWU counters start to run. The AWU interrupt wakes

up the CPU at regular programmed intervals. Once the device is in run mode the AWU

counters are stopped.

Figure 3.

Active halt diagram

In active halt mode during halt phase, the user can choose the regulator (either MVR or

LPVR) and the Flash state (either operating or power-down mode). Using the LPVR and

putting the Flash in power-down mode reduces power consumption, but, increases wakeup

time.

Active halt mode is very useful for reducing average consumption of a battery based

application.

Run

Halt phase

AWU time interval

AWU interrupt

MCU state

Run

Go to halt mode

ai15046

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