AN2857

Run and low-power modes

Doc ID 15241 Rev 3

13/31

4.1

Run mode

Run mode is the default functioning mode of STM8S and STM8A devices. This mode is

used for normal operations and consumes the most power. Use of PCG and low speed

clock sources reduce power consumption of the device in run mode.

Clock frequency can be slowed down in several ways. For example, the LSI clock can be

must be set. Refer to the STM8S and STM8A datasheets (which are available on st.com) for

more details about the option byte configurations.

4.2

Wait mode

Wait mode, or more exactly wait for interrupt mode, is designed to reduce STM8S and

STM8A device power consumption by switching off the core when it is not used. Wait mode

is mainly used when the STM8S or STM8A device is waiting for an external or internal event

which allows the program to continue its execution. Instead of waiting for the event in run

mode, the device can be switched to wait mode. This mode can be used with PCG and with

a low speed clock source to further reduce power consumption of the device.

4.2.1

Entering wait mode

Wait mode is entered by executing the WFI assembly instruction. This stops the CPU, but,

allows other peripherals and the interrupt controller to continue to run.

When entering wait mode, the interrupts are automatically enabled.

4.2.2

Exiting wait mode

When an internal or external interrupt request occurs, the CPU wakes up from wait mode

and resumes processing. This mode offers the lowest wakeup time.

Examples of peripherals or features with interrupts having exit-from-wait capability include:

●

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USART

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SPI

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CAN

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ADC

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AWU

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External interrupt

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Timers

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Clock controller (clock switch execution)

Refer to the STM8S and STM8A reference manual for more details on the functioning of the

above peripherals and features. Refer to the STM8S and STM8A datasheets for availability

of above peripherals and features on particular devices.