[STM8] - External clock + internal clock switching

STM8S103K3: External clock + internal clock switching.

1. First, take a look at the clock structure diagram of STM8S103K3, which can help you understand it well. 
    
   There are several clocks here, which are the clocks marked in the figure, and you need to clarify them:

• fHSE: External high-speed crystal oscillator clock, which is generated by an external crystal oscillator and its size is determined by the size of the external crystal oscillator. The external crystal oscillator range of STM8S is: 1-24M, see "HSE OSC 1-24M" in the figure.

• fHSI: internal RC high-speed clock, which is generated by the internal RC oscillator circuit, and its value is 16M. However, it can be divided by the frequency divider behind it, and four frequency division coefficients are available (1, 2, 4, 8). Note: The accuracy is slightly worse than that of the external crystal oscillator.

• fMASTER: Master clock, which is provided by HSE or HSI. Its main function is to provide clock for peripheral devices (such as I2C, SPI, ADC, etc.) and provide clock source for CPU.

• fCPU: CPU clock, which is obtained by dividing fMASTER. Its function is to provide clock to the CPU. One mechanical cycle is one fCPU clock cycle.

• Using high-speed internal clock (register version)

• Using high-speed internal clock (library function version)

• Using an external clock (register version)

• Using an external clock (library version)

The source code is as follows, you should be able to read the comments.

/*******************************************************************************

* Function Name  : InitCpuClock.

* Description    : Initial CPU clock, .

* Input          : None.

* Output         : None.

* Return         : None.

*******************************************************************************/

static void InitCpuClock(void)

{

#if 1

    // Register version - uses high speed internal clock 

    //Use HSI @8MHZ, div = 2; 8=16/2

    CLK->ECKR &= ~CLK_ECKR_HSEEN; // Disable external clock

    CLK->CKDIVR &= (uint8_t)(~CLK_CKDIVR_HSIDIV); // Clear internal clock prescaler

    CLK->CKDIVR |= CLK_PRESCALER_HSIDIV2; // Set the internal clock prescaler to 2; For details, please refer to the datasheet (reference manual)

    CLK->ICKR |= CLK_ICKR_HSIEN; // Enable internal high-speed clock

    while(!(CLK->ICKR&CLK_ICKR_HSIRDY)); // Wait for the internal high-speed clock to stabilize. After stabilization, the internal clock has started running. Clock = 16/2 M

#endif

#if 0

    // Library version - use internal high speed clock

    //Use HSI @8MHZ, div = 2; 8=16/2

    CLK_HSECmd(DISABLE); // Disable external clock

    CLK_HSIPrescalerConfig(CLK_PRESCALER_HSIDIV2); // Set the internal clock prescaler to 2; For details, please refer to the data manual (reference manual)

    CLK_HSICmd(ENABLE); // Enable internal high-speed clock

    while(!(CLK->ICKR&CLK_ICKR_HSIRDY)); // Wait for the internal high-speed clock to stabilize. After stabilization, the internal clock has started running. Clock = 16/2 M

#endif

#if 0

// Register version - using external clock

    CLK->CKDIVR |= CLK_PRESCALER_CPUDIV1; // CPU clock divided by 1, CPU clock = external clock (that is, external crystal frequency)

    CLK->ECKR |= CLK_ECKR_HSEEN; // Allow external high speed oscillator to work

    while(!(CLK->ECKR & CLK_ECKR_HSERDY)); // Wait for the external high-speed oscillator to be ready

    CLK->SWCR |= CLK_SWCR_SWEN; // Enable switching

    CLK->SWR = CLK_SOURCE_HSE; // Select the high-speed oscillator outside the chip as the main clock

    while(!(CLK->SWCR&CLK_SWCR_SWIF)); // Wait for the switch to succeed

    CLK->SWCR &= ~(CLK_SWCR_SWEN|CLK_SWCR_SWIF); // Clear the switch flag

#endif

#if 0

// Library version - using external clock

    CLK->CKDIVR |= CLK_PRESCALER_CPUDIV1; // CPU clock divided by 1, CPU clock = external clock (that is, external crystal frequency)

    CLK_ClockSwitchConfig(CLK_SWITCHMODE_AUTO, // Clock automatic switching mode, note: the parameter is automatic switching, otherwise only using this function will not switch successfully

                          CLK_SOURCE_HSE, // Clock to switch (here is the external clock)

                          DISABLE, // Whether to enable the switching completion interrupt (disabled here)

                          CLK_CURRENTCLOCKSTATE_ENABLE); // Whether to enable the current clock (here choose to turn off the current clock HSI)

    CLK->SWCR &= ~(CLK_SWCR_SWEN|CLK_SWCR_SWIF); // Clear the switch flag

#endif

}