STM8S datasheet(84/467 Pages) STMICROELECTRONICS

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Clock control (CLK)

RM0016

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DocID14587 Rev 14

As soon as the target clock source is ready (stabilized), the content of the CLK_SWR

The SWBSY bit is cleared and the new clock source replaces the old one. The SWIF flag in

the CLK_SWCR is set and an interrupt is generated if the SWIEN bit is set.

Manual switching

The manual switching is not as immediate as the automatic switching but it offers to the user

a precise control of the switch event time.

To enable manual switching, follow the sequence below (refer to the flowchart in Figure 23):

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

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

The software has to wait until the target clock source is ready (stabilized). This is

indicated by the SWIF flag in the CLK_SWCR register and by an interrupt if the SWIEN

bit is set.

The final software action is to set, at the chosen time, the SWEN bit in the CLK_SWCR

In both manual and automatic switching modes, the old master clock source will not be

powered off automatically in case it is required by other blocks (the LSI RC may be used to

drive the independent watchdog for example). The clock source can be powered off using

the bits in the Internal clock register (CLK_ICKR) and External clock register (CLK_ECKR).

If the clock switch does not work for any reason, software can reset the current switch

operation by clearing the SWBSY flag. This will restore the CLK_SWR register to its

previous content (old master clock).

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