AHB：advanced high-performance bus；APB： advanced peripherals bus。

static void SetSysClockTo72(void)
{
__IO uint32_t StartUpCounter = 0, HSEStatus = 0;

/*

SYSCLK, HCLK, PCLK2 and PCLK1 configuration ---------------------------

SYSCLK 由PLL时钟，外部高速时钟，内部高速时钟获得，最大为72MHz

HCLK 由HCLK经过AHB预分频器得到，最大72MHz，至AHB总线，核心存储器和DMA

PCLK2 最大72MHz ,至APB2 外设

PCLK1 最大36MHz，至APB1外设

*/
/* 使能高速外部晶振*/
RCC->CR |= ((uint32_t)RCC_CR_HSEON);

/* 等待外部高速时钟稳定*/
do
{
HSEStatus = RCC->CR & RCC_CR_HSERDY;
StartUpCounter++;
} while((HSEStatus == 0) && (StartUpCounter != HSEStartUp_TimeOut));

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

if (HSEStatus == (uint32_t)0x01)
{
/* FLASH 存取控制寄存器配置 使能预取，两个等待周期（36MHz --72MHz为2个等待周期）*/
FLASH->ACR |= FLASH_ACR_PRFTBE;

/* Flash 2 wait state */
FLASH->ACR &= (uint32_t)((uint32_t)~FLASH_ACR_LATENCY);
FLASH->ACR |= (uint32_t)FLASH_ACR_LATENCY_2;

/* 先对HCLK PCLK1 PCLK2初始化

时钟配置寄存器 上电初始化初值应为为 0 如下配置的结果是：

8M的高速内部时钟为系统时钟

HCLK = 8M

PCLK2 = 8M

PCLK1 = 4M

*/
/* HCLK = SYSCLK */
RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;

/* PCLK2 = HCLK */
RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE2_DIV1;

/* PCLK1 = HCLK */
RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE1_DIV2;

#ifdef STM32F10X_CL
/* Configure PLLs ------------------------------------------------------*/

/* 如果定义了STM32F10x_CL 则进行如下 配置 该宏定义在编译选项中定义 */
/* PLL2 configuration: PLL2CLK = (HSE / 5) * 8 = 40 MHz */
/* PREDIV1 configuration: PREDIV1CLK = PLL2 / 5 = 8 MHz */
/*

PLL2 配置： 时钟信号输入源为PREDIV1, 8倍频后5分频
*/

/*清零相关控制域*/

RCC->CFGR2 &= (uint32_t)~(RCC_CFGR2_PREDIV2 | RCC_CFGR2_PLL2MUL |
RCC_CFGR2_PREDIV1 | RCC_CFGR2_PREDIV1SRC);

/* 配置CFGR2寄存器*/
RCC->CFGR2 |= (uint32_t)(RCC_CFGR2_PREDIV2_DIV5 | RCC_CFGR2_PLL2MUL8 |
RCC_CFGR2_PREDIV1SRC_PLL2 | RCC_CFGR2_PREDIV1_DIV5);

/* Enable PLL2 */
RCC->CR |= RCC_CR_PLL2ON;
/* Wait till PLL2 is ready */
while((RCC->CR & RCC_CR_PLL2RDY) == 0)
{
}


/* PLL configuration: PLLCLK = PREDIV1 * 9 = 72 MHz */
RCC->CFGR &= (uint32_t)~(RCC_CFGR_PLLXTPRE | RCC_CFGR_PLLSRC | RCC_CFGR_PLLMULL);
RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLXTPRE_PREDIV1 | RCC_CFGR_PLLSRC_PREDIV1 |
RCC_CFGR_PLLMULL9);
#else
/* PLL configuration: PLLCLK = HSE * 9 = 72 MHz */
RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLXTPRE |
RCC_CFGR_PLLMULL));
RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSE | RCC_CFGR_PLLMULL9);
#endif /* STM32F10X_CL */

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

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

/*

Select PLL as system clock source

选择PLL时钟作为系统时钟，并等待稳定

*/
RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));
RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL;

/* Wait till PLL is used as system clock source */
while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)0x08)
{
}
}
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 */
}
}

HSE(25MHZ)->PREDIV2(5分频)——5MHZ——>PLL2MUL(8倍频)——40MHZ——>PREDIV1SCR(选择PLL2)——40MHZ——>PREDIV1(5分频)——8MHZ——>PLLSCR(PREDIV1输入)——8MHZ——>PLLMUL(9倍频)——72MHZ——>SW(选择PLL)——SYSCLK(72MHZ).