Power consumption factors

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Power consumption factors

The STM8L and STM8AL microcontrollers are digital logic devices using the complementary

metal oxide semiconductor (CMOS) technology. In these type of devices, power

consumption is a sum of:



Static power (mainly caused by transistor polarization and leakage)



Dynamic power which depends on the supply voltage and the clock frequency. It is

calculated from the formula: Dynamic power = C x V² x f, where:

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C is the CMOS load capacitance

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V is the supply voltage

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f is the clock frequency.

Static consumption is negligible compared to dynamic consumption when the clock is

running. In some low power modes, when no clock is running, static consumption is the

main consumption source.



Total consumption is a sum of static and dynamic consumption

Consequently, power consumption depends on:



The microcontroller unit (MCU) chip size: technology used, number of transistors,

analog features/peripherals embedded and used in the application.



The MCU supply voltage: the amount of current used in CMOS logic is directly

proportional to the squared power supply voltage (V²). Thus, power consumption may

be reduced by lowering the MCU supply voltage. This is less critical for STM8L and

STM8AL devices than for other microcontrollers, as an internal voltage regulator is

used but this could have an influence for the remaining components on the board.



The clock frequency: Power consumption may be reduced by decreasing the clock

frequency when fast processing is not required by the application.



The number of active peripherals or used MCU features (CSS, BOR, PVD,...): the

greater the number of active peripherals or features, the greater the amount of

consumed power.



The operating mode: Power consumption depends on which mode a particular

application is running (CPU on/off, oscillator on/off,...). For an application powered by a

battery, the consumption is very important. Usually, the average consumption should

be below a certain target value to ensure an optimum battery lifetime. This means that

an application can consume more for short periods of time and keep its average current

consumption below the target value.

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