STM8S datasheet(101/467 Pages) STMICROELECTRONICS

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Power management

104

Power management

By default, after a system or power reset, the microcontroller is in Run mode. In this mode

the CPU is clocked by fCPU and executes the program code, the system clocks are

distributed to the active peripherals and the microcontroller is drawing full power.

While in Run mode, still keeping the CPU running and executing code, the application has

several ways to reduce power consumption, such as:

•

Slowing down the system clocks

•

Gating the clocks to individual peripherals when they are unused

•

Switching off any unused analog functions

However, when the CPU does not need to be kept running, three dedicated low power

modes can be used:

•

Wait

•

Active-halt (configurable for slow or fast wakeup)

•

Halt (configurable for slow or fast wakeup)

You can select one of these three modes and configure them to obtain the best compromise

between lowest power consumption, fastest start-up time and available wakeup sources.

10.1

General considerations

Low power consumption features are generally very important for all types of application for

energy saving. Ultra low power features are especially important for mobile applications to

ensure long battery lifetimes. This is also crucial for environmental protection.

In a silicon chip there are two kind of consumption:

•

Static power consumption which is due to analog polarization and leakages. This so

small, it is only significant in Halt and Active-halt modes (refer to Section 10.2: Low

power modes).

•

Dynamic power consumption which comes from running the digital parts of the chip.

It depends on VDD, clock frequency and load capacitors.

In a microcontroller device the consumption depends on:

•

VDD supply voltage

•

Analog performance

•

MCU size or number of digital gates (leakages and load capacitors)

•

Clock frequency

•

Number of active peripherals

•

Available low power modes and low power levels

Device processing performance is also very important, as this allows the application to

minimize the time spent in Run mode and maximize the time in low power mode.

Using the MCU’s flexible power management features, you can obtain a range of significant

power savings while the system is running or able to resume operations quickly.

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