STM32F4第一课：时钟配置之SystemInit

　　OK，学习stm32f4开始的第一个笔记。好吧，先打开第一个，led和按键的程序。从main函数开始，部分代码如下

int main(void)
{
    uint8_t ucKeyCode;        /* 按键代码 */

        bsp_Init();        /* 硬件初始化 */
    PrintfLogo();    /* 打印例程信息到串口1 */

    printf("LED4正在闪烁(闪烁频率 = 1Hz)\r\n");
    printf("按下K1键点亮LED1\r\n");
    printf("按下K2键点亮LED2\r\n");
    printf("按下K3键点亮LED3\r\n");
    printf("操作按键和摇杆会打印按键事件\r\n");

    bsp_StartAutoTimer(0, 500);    /* 启动1个500ms的自动重装的定时器 */
    //··························
    //略
}

 　　疑问来了，配置时钟的函数哪里去咧？原来由于ST固件库的启动文件已经执行了SystemInit() 函数，该函数CPU系统时钟的初始化，所以不必再次重复配置系统时钟。启动文件配置了CPU主时钟频率、内部Flash访问速度和可选的外部SRAM FSMC初始化。系统时钟缺省配置为168MHz，如果需要更改，可以修改 system_stm32f4xx.c 文件。

　　那么接下来，我们仔细看看SystemInit()这个函数。这个函数是在抚慰以后在调用main函数前被调用的，这个调用在startup_stm32f4xx.s文件里(我都觉得自己罗嗦了)。

　　言归正传，如下是SystemInit的代码(点开查看哦)。

/**
  * @brief  Setup the microcontroller system
  *         Initialize the Embedded Flash Interface, the PLL and update the 
  *         SystemFrequency variable.
  * @param  None
  * @retval None
  */
void SystemInit(void)
{
  /* FPU settings ------------------------------------------------------------*/
  #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;

#ifdef DATA_IN_ExtSRAM
  SystemInit_ExtMemCtl(); 
#endif /* DATA_IN_ExtSRAM */
         
  /* 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
}

 

 

 

　　注释说的很清楚，这个函数大体分为 4 部分。

1.设置FPU，这玩意F4新加上的，浮点运算的玩意，不懂，略过·

2.重置RCC时钟配置默认的复位状态（拗口··就是恢复出厂设置 的  设置，不过这个设置你不爽可以自己修改）ps：红配绿冒傻气

　　NOTE①:需要注意一下，我们重启机器后，都应该使用默认时钟 HSI（F4的内置16MHz RC振荡器），让它先起振.而后如果我们需要如果需要使用 外部时钟 作为时钟源，也不是马上就能切换，需要等待对应的时钟源 是否READY 了。为啥？因为你用人家的频率，起码等待它稳定吧。这一点 就是 CR寄存器，就绪标志位存在的意义！！！

　　NOTE②：再看看这个小函数，假如有外部ram，就需要注意一下喽，配置FSMC用于外部SRAM，如下　

#ifdef DATA_IN_ExtSRAM
  SystemInit_ExtMemCtl(); 
#endif 

 

3.配置 System Clock 的时钟源（3个哦，HSI,HSE,PLLCLK）、PLL的倍频和分频因子、APB和AHBx的预分频系数、FLASH的设置，时序啦，延时周期和使能预取缓冲期····

　　好吧，这个功能是由一个函数完成的。代码如下：

static void SetSysClock(void)
{
/******************************************************************************/
/*            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, System frequency up to 168 MHz */
    RCC->APB1ENR |= RCC_APB1ENR_PWREN;
    PWR->CR |= PWR_CR_VOS;

    /* HCLK = SYSCLK / 1*/
    RCC->CFGR |= RCC_CFGR_HPRE_DIV1;
      
    /* PCLK2 = HCLK / 2*/
    RCC->CFGR |= RCC_CFGR_PPRE2_DIV2;
    
    /* PCLK1 = HCLK / 4*/
    RCC->CFGR |= RCC_CFGR_PPRE1_DIV4;

    /* 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)
    {
    }
   
    /* 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;

    /* 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 */
  }

}

 

　作为一个新时代四有青年，注释说的是太清楚了。不解释了

4.设置 Vector(中断向量表？)的位置，就是重定向。关于重定向的问题，是 CORTEX-M4内核的知识。

 

#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

 

　　需要注意的是。 system_stm32f4xx.c是可以修改的。这在开篇就说过了。所以我们需要什么功能需要按照我们的意愿修改。

下面的几点是 在 stm32f4 的 1.01库里面的。无论是上面说的还是没说的，不想翻译了。留个记号。以后看

1. This file provides two functions and one global variable to be called from
* user application:
* - SystemInit(): 
* - SystemCoreClock variable: Contains the core clock (HCLK), it can be used
* by the user application to setup the SysTick
* timer or configure other parameters.
*
* - SystemCoreClockUpdate(): Updates the variable SystemCoreClock and must
* be called whenever the core clock is changed
* during program execution.
*
* 2. After each device reset the HSI (16 MHz) is used as system clock source.
* Then SystemInit() function is called, in "startup_stm32f4xx.s" file, to
* configure the system clock before to branch to main program.
*
* 3. If the system clock source selected by user fails to startup, the SystemInit()
* function will do nothing and HSI still used as system clock source. User can
* add some code to deal with this issue inside the SetSysClock() function.
*
* 4. The default value of HSE crystal is set to 25MHz, refer to "HSE_VALUE" define
* in "stm32f4xx.h" file. When HSE is used as system clock source, directly or
* through PLL, and you are using different crystal you have to adapt the HSE
* value to your own configuration.
*

完工，好累。明天写stm32f4的 GPIO····