(九)STM32——Systemlnit初始化函数讲解

发布时间:2022-11-25 12:00

        首先,我们需要了解一个东西,就是Systemlnit初始化函数会在main函数调用之前被执行,可以简单理解为,Systemlnit初始化函数在启动程序时在main函数之前,如下图所示:

(除此之外,本章还需要之前时钟树的基础,不清楚的可以去上篇文章看看)

(九)STM32——Systemlnit初始化函数讲解_第1张图片

        接下来我们就正式开始介绍Systemlnit初始化函数了,先上代码,我们后面来慢慢讲解这个函数:

void SystemInit(void)
{
  /* FPU settings 浮点运算的配置,此处不讲解------------------------------------------------------------*/
  #if (__FPU_PRESENT == 1) && (__FPU_USED == 1)
    SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2));  /* set CP10 and CP11 Full Access */
  #endif
  /* Reset the RCC clock configuration to the default reset state ------------*/
  /* Set HSION bit */
  RCC->CR |= (uint32_t)0x00000001;

  /* Reset CFGR register */
  RCC->CFGR = 0x00000000;

  /* Reset HSEON, CSSON and PLLON bits */
  RCC->CR &= (uint32_t)0xFEF6FFFF;

  /* Reset PLLCFGR register */
  RCC->PLLCFGR = 0x24003010;

  /* Reset HSEBYP bit */
  RCC->CR &= (uint32_t)0xFFFBFFFF;

  /* Disable all interrupts */
  RCC->CIR = 0x00000000;

#if defined (DATA_IN_ExtSRAM) || defined (DATA_IN_ExtSDRAM)
  SystemInit_ExtMemCtl(); 
#endif /* DATA_IN_ExtSRAM || DATA_IN_ExtSDRAM */
         
  /* Configure the System clock source, PLL Multiplier and Divider factors, 
     AHB/APBx prescalers and Flash settings ----------------------------------*/
  SetSysClock();

  /* Configure the Vector Table location add offset address ------------------*/
#ifdef VECT_TAB_SRAM
  SCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal SRAM */
#else
  SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH */
#endif
}

        我们是一行一行代码详细的讲解,不过只要理解了第一行代码,后面的都是类似的,所以我们现在来讲解第一行代码:

// 第一行代码
  /* Set HSION bit (打开HSI)*/
  RCC->CR |= (uint32_t)0x00000001;

        对于这样的代码,我们首先要去RCC_CR 寄存器查看一下相应的引脚,因为这是操控了RCC_CR 寄存器的第0位,我们就去看看这一位的含义吧!

(九)STM32——Systemlnit初始化函数讲解_第2张图片

        所以我们能清楚的看到,这一行代码作用就是打开了HSI(不懂的可以康康上篇文章)。 

/* Reset CFGR register */
  RCC->CFGR = 0x00000000;

  /* Reset HSEON, CSSON and PLLON bits */
  RCC->CR &= (uint32_t)0xFEF6FFFF;

  /* Reset PLLCFGR register */
  RCC->PLLCFGR = 0x24003010;

  /* Reset HSEBYP bit */
  RCC->CR &= (uint32_t)0xFFFBFFFF;

        这几行代码作用都是类似的,都是关闭某个引脚,我们举一个例子(第二个)来讲解,后面的都是一样的。 首先 0xFEF6FFFF = 1111 1110 1111 0110 1111 1111 1111 1111 。相当于把第16位和19、24位设置为0。

(九)STM32——Systemlnit初始化函数讲解_第3张图片

(九)STM32——Systemlnit初始化函数讲解_第4张图片        所以就是关闭了HSE和CSS(用来保障系统时钟,当HSE出了问题的时候,就会由CSS切换到HSI),以及主PLL。

/* Configure the System clock source, PLL Multiplier and Divider factors, 
     AHB/APBx prescalers and Flash settings ----------------------------------*/
  SetSysClock();

        SetSysClock()函数是特别重要的函数,接下来我们会来详细讲解一下,也是会一行一行进行讲解,先上代码:

static void SetSysClock(void)
{
#if defined (STM32F40_41xxx) || defined (STM32F427_437xx) || defined (STM32F429_439xx) || defined (STM32F401xx)
/******************************************************************************/
/*            PLL (clocked by HSE) used as System clock source                */
/******************************************************************************/
  __IO uint32_t StartUpCounter = 0, HSEStatus = 0;
  
  /* Enable HSE */
  RCC->CR |= ((uint32_t)RCC_CR_HSEON);
 
  /* Wait till HSE is ready and if Time out is reached exit */
  do
  {
    HSEStatus = RCC->CR & RCC_CR_HSERDY;
    StartUpCounter++;
  } while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));

  if ((RCC->CR & RCC_CR_HSERDY) != RESET)
  {
    HSEStatus = (uint32_t)0x01;
  }
  else
  {
    HSEStatus = (uint32_t)0x00;
  }

  if (HSEStatus == (uint32_t)0x01)
  {
    /* Select regulator voltage output Scale 1 mode */
    RCC->APB1ENR |= RCC_APB1ENR_PWREN;
    PWR->CR |= PWR_CR_VOS;

    /* HCLK = SYSCLK / 1*/
    RCC->CFGR |= RCC_CFGR_HPRE_DIV1;

#if defined (STM32F40_41xxx) || defined (STM32F427_437xx) || defined (STM32F429_439xx)      
    /* PCLK2 = HCLK / 2*/
    RCC->CFGR |= RCC_CFGR_PPRE2_DIV2;
    
    /* PCLK1 = HCLK / 4*/
    RCC->CFGR |= RCC_CFGR_PPRE1_DIV4;
#endif /* STM32F40_41xxx || STM32F427_437x || STM32F429_439xx */

#if defined (STM32F401xx)
    /* PCLK2 = HCLK / 2*/
    RCC->CFGR |= RCC_CFGR_PPRE2_DIV1;
    
    /* PCLK1 = HCLK / 4*/
    RCC->CFGR |= RCC_CFGR_PPRE1_DIV2;
#endif /* STM32F401xx */
   
    /* Configure the main PLL */
    RCC->PLLCFGR = PLL_M | (PLL_N << 6) | (((PLL_P >> 1) -1) << 16) |
                   (RCC_PLLCFGR_PLLSRC_HSE) | (PLL_Q << 24);

    /* Enable the main PLL */
    RCC->CR |= RCC_CR_PLLON;

    /* Wait till the main PLL is ready */
    while((RCC->CR & RCC_CR_PLLRDY) == 0)
    {
    }
   
#if defined (STM32F427_437xx) || defined (STM32F429_439xx)
    /* Enable the Over-drive to extend the clock frequency to 180 Mhz */
    PWR->CR |= PWR_CR_ODEN;
    while((PWR->CSR & PWR_CSR_ODRDY) == 0)
    {
    }
    PWR->CR |= PWR_CR_ODSWEN;
    while((PWR->CSR & PWR_CSR_ODSWRDY) == 0)
    {
    }      
    /* Configure Flash prefetch, Instruction cache, Data cache and wait state */
    FLASH->ACR = FLASH_ACR_PRFTEN | FLASH_ACR_ICEN |FLASH_ACR_DCEN |FLASH_ACR_LATENCY_5WS;
#endif /* STM32F427_437x || STM32F429_439xx  */

#if defined (STM32F40_41xxx)     
    /* Configure Flash prefetch, Instruction cache, Data cache and wait state */
    FLASH->ACR = FLASH_ACR_PRFTEN | FLASH_ACR_ICEN |FLASH_ACR_DCEN |FLASH_ACR_LATENCY_5WS;
#endif /* STM32F40_41xxx  */

#if defined (STM32F401xx)
    /* Configure Flash prefetch, Instruction cache, Data cache and wait state */
    FLASH->ACR = FLASH_ACR_PRFTEN | FLASH_ACR_ICEN |FLASH_ACR_DCEN |FLASH_ACR_LATENCY_2WS;
#endif /* STM32F401xx */

    /* Select the main PLL as system clock source */
    RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));
    RCC->CFGR |= RCC_CFGR_SW_PLL;

    /* Wait till the main PLL is used as system clock source */
    while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS ) != RCC_CFGR_SWS_PLL);
    {
    }
  }
  else
  { /* If HSE fails to start-up, the application will have wrong clock
         configuration. User can add here some code to deal with this error */
  }
#elif defined (STM32F411xE)
#if defined (USE_HSE_BYPASS) 

        现在要介绍SetSysClock函数,首先,我们要清楚一个知识点,就是使能时钟之后,需要等待一段时间才能使用。其次,因为我使用的是F407系列的开发板,所以就只介绍407系列的部分。

  /* Enable HSE */
  RCC->CR |= ((uint32_t)RCC_CR_HSEON);
// 宏定义
#define  RCC_CR_HSEON                        ((uint32_t)0x00010000)

        这个其实就是我们之前介绍的代码,只不过使用了宏定义,我把宏定义也贴出来。

0x00010000 = 0000 0000 0000 0001 0000 0000 0000 0000,也就是使能了第16位。

(九)STM32——Systemlnit初始化函数讲解_第5张图片

 /* Wait till HSE is ready and if Time out is reached exit */
  do
  {
    HSEStatus = RCC->CR & RCC_CR_HSERDY;
    StartUpCounter++;
  } while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));
 if ((RCC->CR & RCC_CR_HSERDY) != RESET)// RESET为0
  {
    HSEStatus = (uint32_t)0x01;
  }
  else
  {
    HSEStatus = (uint32_t)0x00;
  }

#define  RCC_CR_HSERDY                       ((uint32_t)0x00020000)

        这个就是等待的代码,控制的是第17位,当稳定后,HSEStatus为1。

 if (HSEStatus == (uint32_t)0x01)
  {
    /* Select regulator voltage output Scale 1 mode (这个是电压的代码,暂时不讲解)*/
    RCC->APB1ENR |= RCC_APB1ENR_PWREN;
    PWR->CR |= PWR_CR_VOS;

    /* HCLK = SYSCLK / 1*/
    RCC->CFGR |= RCC_CFGR_HPRE_DIV1;

#if defined (STM32F40_41xxx) || defined (STM32F427_437xx) || defined (STM32F429_439xx)      
    /* PCLK2 = HCLK / 2*/
    RCC->CFGR |= RCC_CFGR_PPRE2_DIV2;
    
    /* PCLK1 = HCLK / 4*/
    RCC->CFGR |= RCC_CFGR_PPRE1_DIV4;
#endif /* STM32F40_41xxx || STM32F427_437x || STM32F429_439xx */

#if defined (STM32F401xx)
    /* PCLK2 = HCLK / 2*/
    RCC->CFGR |= RCC_CFGR_PPRE2_DIV1;
    
    /* PCLK1 = HCLK / 4*/
    RCC->CFGR |= RCC_CFGR_PPRE1_DIV2;
#endif /* STM32F401xx */
   
    /* Configure the main PLL */
    RCC->PLLCFGR = PLL_M | (PLL_N << 6) | (((PLL_P >> 1) -1) << 16) |
                   (RCC_PLLCFGR_PLLSRC_HSE) | (PLL_Q << 24);

    /* Enable the main PLL */
    RCC->CR |= RCC_CR_PLLON;

    /* Wait till the main PLL is ready */
    while((RCC->CR & RCC_CR_PLLRDY) == 0)
    {
    }

这部分是稳定下来后做的事,我们还是来一一讲解。

 /* HCLK = SYSCLK / 1*/
    RCC->CFGR |= RCC_CFGR_HPRE_DIV1;

#define  RCC_CFGR_HPRE_DIV1                  ((uint32_t)0x00000000)        /*!< SYSCLK not divided */

        HCLK需要通过AHB分频器(系统时钟部分讲过,看一下时钟树就好了),这样设置,就是选择不分频。

(九)STM32——Systemlnit初始化函数讲解_第6张图片

 

#if defined (STM32F40_41xxx) || defined (STM32F427_437xx) || defined (STM32F429_439xx)      
    /* PCLK2 = HCLK / 2*/
    RCC->CFGR |= RCC_CFGR_PPRE2_DIV2;
    
    /* PCLK1 = HCLK / 4*/
    RCC->CFGR |= RCC_CFGR_PPRE1_DIV4;
#endif /* STM32F40_41xxx || STM32F427_437x || STM32F429_439xx */

        控制的是这两个PCLK,过程一样。

(九)STM32——Systemlnit初始化函数讲解_第7张图片

 

 /* Configure the main PLL */
    RCC->PLLCFGR = PLL_M | (PLL_N << 6) | (((PLL_P >> 1) -1) << 16) |
                   (RCC_PLLCFGR_PLLSRC_HSE) | (PLL_Q << 24);

        这部分是用来设置PLL频率的,这个是系统启动文件直接设置好了的,如下所示,就不需要我们去管了,出来就直接是168MHz。

#if defined (STM32F40_41xxx) || defined (STM32F427_437xx) || defined (STM32F429_439xx) || defined (STM32F401xx)
/* PLL_VCO = (HSE_VALUE or HSI_VALUE / PLL_M) * PLL_N */
#define PLL_M      8
#else /* STM32F411xE */
#if defined (USE_HSE_BYPASS)
#define PLL_M      8    
#else /* STM32F411xE */   
#define PLL_M      16
#endif /* USE_HSE_BYPASS */
#endif /* STM32F40_41xxx || STM32F427_437xx || STM32F429_439xx || STM32F401xx */  

/* USB OTG FS, SDIO and RNG Clock =  PLL_VCO / PLLQ */
#define PLL_Q      7

#if defined (STM32F40_41xxx)
#define PLL_N      336
/* SYSCLK = PLL_VCO / PLL_P */
#define PLL_P      2
#endif /* STM32F40_41xxx */

#if defined (STM32F427_437xx) || defined (STM32F429_439xx)
#define PLL_N      360
/* SYSCLK = PLL_VCO / PLL_P */
#define PLL_P      2
#endif /* STM32F427_437x || STM32F429_439xx */
    /* Enable the main PLL */
    RCC->CR |= RCC_CR_PLLON;
  /* Wait till the main PLL is ready */
    while((RCC->CR & RCC_CR_PLLRDY) == 0)
    {
    }

        打开主PLL,和等待,现在应该不难理解了。

 /* Select the main PLL as system clock source */
    RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));
    RCC->CFGR |= RCC_CFGR_SW_PLL;

#define  RCC_CFGR_SW_PLL                     ((uint32_t)0x00000002)        /*!< PLL selected as system clock */

        这是最后一步,选择主PLL为时钟源。

(九)STM32——Systemlnit初始化函数讲解_第8张图片

 

总结: Systemlnit初始化函数做的事情如下所示:

  1. 关闭所有的引脚
  2. 打开HSE
  3. 打开AHB以及HCLK
  4. 配置好PLL
  5. 选择PLL为系统时钟源

好了,Systemlnit初始化函数就介绍到这,希望能对大家有所帮助。 

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