SetSysClockTo72（）寄存器写法

system_stm32f10x.c中函数，将系统时钟配置成72M

  1. static void SetSysClockTo72(void)
  2. {
  3.   __IO uint32_t StartUpCounter = 0, HSEStatus = 0;
  5.   /* SYSCLK, HCLK, PCLK2 and PCLK1 configuration ---------------------------*/    
  6.   /* Enable HSE */    
  7.   RCC->CR |= ((uint32_t)RCC_CR_HSEON);
  9.   /* Wait till HSE is ready and if Time out is reached exit */
  10.   do
  11.   {
  12.     HSEStatus = RCC->CR & RCC_CR_HSERDY;
  13.     StartUpCounter++;  
  14.   } while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));
  16.   if ((RCC->CR & RCC_CR_HSERDY) != RESET)
  17.   {
  18.     HSEStatus = (uint32_t)0x01;
  19.   }
  20.   else
  21.   {
  22.     HSEStatus = (uint32_t)0x00;
  23.   }  
  25.   if (HSEStatus == (uint32_t)0x01)
  26.   {
  27.     /* Enable Prefetch Buffer */
  28.     FLASH->ACR |= FLASH_ACR_PRFTBE;
  30.     /* Flash 2 wait state */
  31.     FLASH->ACR &= (uint32_t)((uint32_t)~FLASH_ACR_LATENCY);
  32.     FLASH->ACR |= (uint32_t)FLASH_ACR_LATENCY_2;    
  35.     /* HCLK = SYSCLK */
  36.     RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;
  38.     /* PCLK2 = HCLK */
  39.     RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE2_DIV1;
  41.     /* PCLK1 = HCLK */
  42.     RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE1_DIV2;
  45.     /*  PLL configuration: PLLCLK = HSE * 9 = 72 MHz */
  46.     RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLXTPRE |
  47.                                         RCC_CFGR_PLLMULL));
  48.     RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSE | RCC_CFGR_PLLMULL9);
  51.     /* Enable PLL */
  52.     RCC->CR |= RCC_CR_PLLON;
  54.     /* Wait till PLL is ready */
  55.     while((RCC->CR & RCC_CR_PLLRDY) == 0)
  56.     {
  57.     }
  59.     /* Select PLL as system clock source */
  60.     RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));
  61.     RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL;    
  63.     /* Wait till PLL is used as system clock source */
  64.     while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)0x08)
  65.     {
  66.     }
  67.   }
  68.   else
  69.   { /* If HSE fails to start-up, the application will have wrong clock 
  70.          configuration. User can add here some code to deal with this error */
  71.   }
  72. }

上图中，第7行代码 RCC->CR |= ((uint32_t)RCC_CR_HSEON);
查询系统定义，#define RCC_CR_HSEON ((uint32_t)0x00010000) ，发现RCC_CR_HSEON=0x00010000，RCC_CR初始值为0x0000 xx83 ， RCC->CR =RCC->CR |=RCC_CR_HSEON=0x0000 xx83 |= 0x0001 0000 = 0x0001 xx83， 将RCC->CR寄存器第16位置1，HSE振荡器开启。

接着，在do-while循环中，HSEStatus = RCC->CR & RCC_CR_HSERDY;
查询系统定义#define RCC_CR_HSERDY ((uint32_t)0x00020000)，又上一句将RCC->CR寄存器第16位置1，RCC->CR & RCC_CR_HSERDY=0x0001 xx83 & 0x0002 0000 =0x0000 0000. 则HSEStatus的值始终为0，无法结束while循环。

HSE配置系统时钟

新建.h、.c文件

在user文件夹中，新建rcc文件夹，rcc文件夹中新建.c、.h文件
image.png
打开程序，添加.c、.h文件
防止头文件重复引用，.h文件中添加条件编译
image.png

详解代码部分见玩转sTm32。