Clock management

AN3147

10/25

DocID17035 Rev 3

The default clock source after reset is HSI/8. The user can then switch the clock to different

frequencies and sources by using the following procedure:



On the STM8L101xx devices:

–

Choose the prescaler (/1, /2, /4 or /8) for an internal RC 16 MHz (HSI) clock

through the HSIDIV[1:0] bits in the CLK_CKDIVR register.



On the STM8L05xxx, STM8L15xxx, STM8L162xx, STM8AL31xx and STM8AL3Lxx

devices:

–

Choose the prescaler (/1, /2 ... /128) for an internal RC 16 MHz (HSI) clock

through the CKM[2:0] bits in the CLK_CKDIVR register.

–

Or, change the system clock (to HSE, LSE or LSI). Refer to the STM8L05xx,

STM8L15xx, STM8L162x, STM8AL31xx and STM8AL3Lxx microcontroller family

reference manual (RM0031) for more details about the clock switching

mechanism.

Before switching off a previous clock source when using automatic switching mode, the user

must ensure that the core is no longer running on the current clock. This means that the

previous clock must be turned off after the SWIF flag has been set. If the user tries to switch

off the clock and the SWIF flag has not been set by hardware, the current clock is not

switched off as the microcontroller is still running using this clock source.

Such clock switching can be combined with Wait mode (for example, if the HSE crystal is

the new clock) as clock switch execution interrupt can wake up the MCU from wait mode.

This enables the synchronization of the MCU wakeup with the new clock availability. The

clock switching feature can also be used at the beginning and the end of a regular or

interrupt routine to speed it up (for instance, if the clock master is LSI RC, but some parts of

the code have to be executed quickly with HSI RC).

Table 2. Clock sources

HSE oscillator

external HSE

HSI

external LSE

LSE crystal

LSI

Cost

Low to medium

Low to high

Free

Low to medium

Low to

medium

Free

Accuracy

~ ppm / %

depending on

external clock

~ %

depending on

external clock

~ ppm

~ %

Consumption

0.7 mA max

High/medium

100 µA

1.4 µA max

100 - 400 nA

Start-up time

1 ms

depending on

external clock

4µs

depending on

external clock

1s

200µs max

Other

information

Crystal or

ceramic

resonator

Clock already

existing in the

system

-

Clock already

existing in the

system

Watch crystal

-