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RM0016

Clock control (CLK)

104

The LSI RC can be switched on and off using the LSIEN bit in the Internal clock register

(CLK_ICKR).

The LSIRDY flag in the Internal clock register (CLK_ICKR) indicates if the low-speed

internal oscillator is stable or not. At startup, the clock is not released until this bit is set by

hardware.

Calibration

Like the HSI RC, the LSI RC device is factory calibrated by ST. However, it is not possible to

perform further trimming.

Note:

When using the independent watchdog with the LSI as clock source, in order to guarantee

that the CPU will never run on the same clock in case of corruption, the LSI clock cannot be

the master clock if LSI_EN option bit is reset. Refer to the option bytes section in the

datasheet.

9.2

Master clock switching

The clock switching feature provides an easy to use, fast and secure way for the application

to switch from one master clock source to another.

9.2.1

System startup

For fast system startup, after a reset the clock controller configures the master clock source

as HSI RC clock output divided by 8 (HSI/8). This is to take advantage of the short

stabilization time of the HSI oscillator. The /8 divider is to ensure safe start-up in case of

poor VDD conditions.

Once the master clock is released, the user program can switch the master clock to another

clock source.

9.2.2

Master clock switching procedures

To switch clock sources, you can proceed in one of two ways:

•

Automatic switching

•

Manual switching

Automatic switching

The automatic switching enables, the user to launch a clock switch with a minimum number

of instructions. The software can continue doing other operations without taking care of the

switch event exact time.

To enable automatic switching, follow the sequence below (refer to the flowchart in

Figure 22):

Enable the switching mechanism by setting the SWEN bit in the Switch control register

(CLK_SWCR).

Write the 8-bit value used to select the target clock source in the Clock master switch

and the target source oscillator starts. The old clock source continues to drive the CPU

and peripherals.

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