RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;    //HSE表示使用外部时钟，HSI表示使用的是内部时钟

  RCC_OscInitStruct.HSEState = RCC_HSE_ON; //  打开外部时钟

  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON; 

  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;  //时钟源设置

  RCC_OscInitStruct.PLL.PLLMUL = RCC_PLL_MUL6;  //表示6倍频率，如果外部时钟源是8MHZ，那么6倍的倍频就是48MHZ

 
  RCC_OscInitStruct.PLL.PREDIV = RCC_PREDIV_DIV1;  // 对时钟源的分频  如果是48MHZ,，2分频就是24MHZ

  
  HAL_RCC_OscConfig(&RCC_OscInitStruct);

#define RCC_OSCILLATORTYPE_NONE            ((uint32_t)0x00000000)

#define RCC_OSCILLATORTYPE_HSE             ((uint32_t)0x00000001)

#define RCC_OSCILLATORTYPE_HSI             ((uint32_t)0x00000002) 

#define RCC_OSCILLATORTYPE_LSE             ((uint32_t)0x00000004)  //低频外部时钟

#define RCC_OSCILLATORTYPE_LSI             ((uint32_t)0x00000008)//低频内部时钟

#define RCC_OSCILLATORTYPE_HSI14           ((uint32_t)0x00000010)

低频时钟只是用于RTC，RTC主要用来做低功耗

void IAP_Set(void)
{
   uint32_t i = 0;

/* Relocate by software the vector table to the internal SRAM at 0x20000000 ***/ 

  /* Copy the vector table from the Flash (mapped at the base of the application
     load address 0x08003000) to the base address of the SRAM at 0x20000000. */       
  for(i = 0; i < 48; i++)
  {
    *((uint32_t*)(0x20000000 + (i << 2)))=*(__IO uint32_t*)(APPLICATION_ADDRESS + (i<<2));
 }
  /* Enable the SYSCFG peripheral clock*/ 
 RCC->APB2ENR |= RCC_APB2ENR_SYSCFGCOMPEN;
 
 /* Remap SRAM at 0x00000000 */
  Memory_Remap(SYSCFG_MemoryRemap_SRAM); 
} 

#define APPLICATION_ADDRESS                  ((uint32_t)0x08004000)     函数运行的地址，有boot的时候函数是从自己设置的地址开始运行的