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Power management

RM0016

 

 

Main voltage regulator (MVR) auto power-off

By default the main voltage regulator is kept on Active-halt mode. Keeping it active ensures fast wakeup from Active-halt mode. However, the current consumption of the MVR is nonnegligible.

To further reduce current consumption, the MVR regulator can be powered off automatically when the MCU enters Active-halt mode. To configure this feature, set the REGAH bit in the

Internal clock register (CLK_ICKR) register. In this mode:

The MCU core is powered only by the LPVR regulator (same as in Halt mode).

Only the LSI clock source can be used, as the HSE clock current consumption is too high for the LPVR.

The Main voltage regulator is powered on again at wakeup and it requires a longer wakeup time (refer to the datasheet electrical characteristics section for wakeup timing and current consumption data).

Fast clock wakeup

As described for Halt mode, in order to get the shortest wakeup time, it is recommended to

select HSI as the fMASTER clock source. The FHWU bit is also available to save switching time.

A fast wakeup time is very important in Active-halt mode. It supplements the effect of CPU processing performance by helping to minimize the time the MCU stays in Run mode between two periods in low power mode, and thus reduces the overall average power consumption.

Since the clock is not automatically switched to the original master clock, the application must restore the clock source before entering Halt/Active-halt mode as soon as it is ready.

10.3Additional analog power controls

10.3.1Fast Flash wakeup from Halt mode

By default the Flash is in power-down state when the microcontroller enters Halt mode. The current leakage is negligible, resulting in very low consumption in Halt mode. However the Flash wakeup time is relatively slow (several µs).

If you need the application to wakeup quickly from Halt mode, set the HALT bit in

Section 4.8.1: Flash control register 1 (FLASH_CR1). This ensures that the Flash is in Standby mode when the microcontroller enters in Halt mode. Its wakeup time is reduced to a few ns. However, in this case the consumption is increased up to several µAs.

Refer to the electrical characteristics section of the datasheet for more details.

10.3.2Very low Flash consumption in Active-halt mode

By default, in Active-halt mode, the Flash remains in operating mode to ensure the fastest wakeup time, however in this case the power consumption is not optimized.

To optimize the power consumption you can set the AHALT bit in Flash control register 1 (FLASH_CR1). This will switch the Flash to power-down state when entering Active-halt mode. The consumption decreases but the wakeup time increases up to a few µs.

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Doc ID 14587 Rev 9

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