stm8s clock source switching

The STM8 microcontroller has a rich clock source. The chip has both a 16MHZ high-speed RC oscillator and a 128KHZ low-speed RC oscillator. It can also connect a high-speed crystal oscillator externally. During system operation, it can be switched freely as needed. After the microcontroller is reset, the internal high-speed RC oscillator is used first, and the frequency division factor is 8, so the clock frequency of the CPU power-on operation is 2MHZ.

Switching the clock source mainly involves the following registers: main clock switching register CLK_SWR and switching control register CLK_SWCR.

The reset value of the master clock switch register is 0xe1, which means switching to the internal high-speed RC oscillator. When 0xb4 is written to the register, it means switching to the external high-speed crystal oscillator.

In the actual switching process, the SWEN (bit 1) in the switching control register should be set to 1 first, then the value of CLK_SWCR should be set, and finally, it should be determined whether the SWIF flag in the switching control register is switched successfully.

The following experimental program first switches the main clock source to an external crystal oscillator with an oscillation frequency of 8MHZ, and then quickly flashes the LED indicator. Next, the main clock source is switched to the internal oscillator with an oscillation frequency of 2MHZ, and then the LED indicator flashes slowly. By observing the flashing frequency of the LED indicator, it can be seen that the same loop code changes the flashing frequency and duration due to the change of the main clock source.

Still using ST's development tools, generate a C language program framework, and then modify main.c. The modified code is as follows.

 

// Program description: Change the CPU running speed by switching the CPU's main clock source

 

#include "STM8S207C_S.h"

 

// Function: Delay function

// Input parameter: ms -- the number of milliseconds to delay, assuming the CPU frequency is 2MHZ

// Output parameters: None

// Return value: None

// Note: None

void DelayMS(unsigned int ms)

{

  unsigned char i;

  while(ms != 0)

  {

    for(i=0;i<250;i++)

    {

    }

    for(i=0;i<75;i++)

    {

    }

    ms--; 

  }    

}

 

main()

{

  int i;

 

  // Set PD3 to push-pull output to drive the LED

  PD_DDR = 0x08;

  PD_CR1 = 0x08;             

  PD_CR2 = 0x00;

 

  // Start the external high speed crystal oscillator

  CLK_ECKR = 0x01; // Allow external high speed oscillator to work

  while((CLK_ECKR & 0x02) == 0x00); // Wait for the external high-speed oscillator to be ready

              

  // Note that after reset, the CPU clock source comes from the internal RC oscillator

 

  for(;;) // Enter an infinite loop                             

  {

 

    // Next, switch the CPU clock source to an external high-speed crystal oscillator. The frequency on the development board is 8MHZ.

    // Through the LED, we can see that the speed of program running has indeed been significantly improved

    CLK_SWCR = CLK_SWCR | 0x02; // SWEN <- 1

    CLK_SWR = 0xB4; // Select the high-speed oscillator outside the chip as the main clock

    while((CLK_SWCR & 0x08) == 0); // Wait for the switch to succeed

         

    CLK_SWCR = CLK_SWCR & 0xFD; // Clear the switch flag

 

    for(i=0;i<10;i++) //LED flashes 10 times at high speed

    {

      PD_ODR = 0x08;

      DelayMS(100);

      PD_ODR = 0x00;

      DelayMS(100);

    }

  

    // Next, switch the CPU clock source to the internal RC oscillator. Since the reset value of CLK_CKDIVR is 0x18

    // Therefore, the 16MHZ RC oscillator must be divided by 8 before it can be used as the main clock, so the frequency is 2MHZ

    // Through the LED, it can be seen that the speed of program running has indeed dropped significantly

    CLK_SWCR = CLK_SWCR | 0x02; // SWEN <- 1

    CLK_SWR = 0xE1; // Select HSI as the main clock source

    while((CLK_SWCR & 0x08) == 0); // Wait for the switch to succeed

 

    CLK_SWCR = CLK_SWCR & 0xFD; // Clear the switch flag

 

    for(i=0;i<10;i++) //LED flashes 10 times at low speed

    {

      PD_ODR = 0x08;

      DelayMS(100);

      PD_ODR = 0x00;

      DelayMS(100);

    }

  }

}