Setting STM32F4 SystemCoreClock to 100 MHz but can't get it (only get 57.6 MHz)

Question

I am trying to change the STM32F407 system core clock to be 100 MHz. To do this, I need to set the source of the PLL to be HSE, and configure the PLL coefficient so as to get the right value of SYSCLK.

Here is a screenshot of the STMCube that shows the right values of PLL M, N, P, Q:

Here is my function

// function to configure the rc clk for running on HSE : sysclk at 100 MHz
void rcc_clk_config()
{
    // sysclck to 100 MHz, systick to 12.5 MHz
    RCC_PLLConfig(RCC_PLLSource_HSE, 4, 200, 4, 4);

    RCC_HCLKConfig(RCC_SYSCLK_Div1);
    RCC_PCLK1Config(RCC_HCLK_Div4);
    RCC_PCLK2Config(RCC_HCLK_Div2);

    do {
        RCC_HSEConfig(RCC_HSE_ON);
    } while(RCC_GetFlagStatus(RCC_FLAG_HSERDY) != SET);

    do {
        RCC_PLLCmd(ENABLE);
    } while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) != SET);

    RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);
    SystemCoreClockUpdate();
}

In the debug session, I get SystemCoreClock = 57.600.000.

Answer 1

It works now, here is the new code:

{
    RCC_DeInit();//a must
    do{
       RCC_HSEConfig(RCC_HSE_ON);
      }
     while (!RCC_WaitForHSEStartUp());
     RCC_PLLConfig(RCC_PLLSource_HSE, 4, 200, 4, 4);//100MHZ

    do{
       RCC_PLLCmd(ENABLE);
      }

   while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY)!=SET);
   RCC_HCLKConfig(RCC_SYSCLK_Div1);
   RCC_PCLK1Config(RCC_HCLK_Div4);
   RCC_PCLK2Config(RCC_HCLK_Div2);

  RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK);
  SystemCoreClockUpdate();
}

With some experimentation and some more time reading the stm32f4xx_rcc.c, I discovered there was in fact a function: RCC_WaitForHSEStartUp(), which role is to wait for the HSE oscillator to start. I used it as follows:

do{
           RCC_HSEConfig(RCC_HSE_ON);
  }
         while (!RCC_WaitForHSEStartUp());

to replace in the previous version

do{
          RCC_HSEConfig(RCC_HSE_ON);;

  }

          while(RCC_GetFlagStatus(RCC_FLAG_HSERDY)!=SET);

I also "had" to start the code by initializing the clock system using the function:

 RCC_DeInit();