Other considerations

AN4635

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4

Other considerations

For a complete overview of low power advantages please refer to the already mentioned

AN4445. Following recommendations are specific to our case and example.

4.1

GPIO configuration

Some GPIO settings may have great influence on power consumption while others do not.

The pins used for the UART communication lines should be configured to their alternate

function mode. It is not recommended to activate pull-ups if lowest possible power

consumption is the most important goal, however, in some applications it may be necessary

to improve the communication reliability. The speed setting has no consequences regarding

the power consumption.

Half-duplex mode, where applicable, may also lead to further power savings, however this

configuration is not addressed in this document.

Other pins, unused by the application, should be configured as analog inputs. The

developer should not forget to put the debug lines also to analog, once the application is

ready for deployment.

4.2

Clock configuration

For LPUART peripheral clock, LSE source is the choice to reach 9600 baud with wakeup

from Stop mode.

For system clock, in order to keep the power consumption low, try to avoid using the PLL.

Refer to the product datasheet for typical power consumptions for different clock

configurations.

4.2.1

Clock prescalers

The RCC module offers the possibility to tune down peripheral bus clocks, the AHB and

APB frequency (RCC_CFGR register).

Slowing the AHB brings significant power savings but in most cases it is not worth using.

The memory interface is also clocked with AHB. While power consumption with prescaler of

8 drops by approximately 50%, the processing capability of the system drops even more

Table 3. Clock options

Source

Advantages

Limitations

HSI

Ready to use high speed clock

Worst MHz / W ratio option

MSI

Easy and fast clock throttling from 65 kHz

to 4.2 MHz

Lowest overall power consumption when

clocked below 1 MHz

Clock is relatively unstable and imprecise

HSE

Potentially most power efficient at 1 MHz

and above.

Needs additional external component