AN2857

Clock management

Doc ID 15241 Rev 3

9/31

3

Clock management

3.1

Clock system overview

Four different clock sources can be used to drive the master clock:

●

1-24 MHz high speed external clock from the crystal oscillator (HSE crystal)

●

Up to 24 MHz high speed external clock provided by the user (HSE user-external)

●

16 MHz high speed internal RC oscillator (HSI)

●

128 kHz low speed internal RC oscillator (LSI)

Each peripheral clock source can be switched on or off independently when it is not used, to

optimize power consumption. This is done by using the peripheral clock gating (PCG)

feature. See the ‘clock control’ sections of the STM8S and STM8A reference manual for

more details.

Table 1 summarizes the main features of each clock source. STM8S and STM8A devices

offer a complete range of clock sources to fit customer application requirements in terms of

cost, accuracy and consumption.

Table 1.

Clock source comparison

HSE crystal

HSE external

HSI

LSI

Cost

Low to medium

Free to high

Free

Free

Accuracy

Crystal

External source

High

Medium

Low

Very low

Other information

Resonator or

crystal

Existing clock to

very complex

clock systems

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