以GD32E230为例。

启动文件：startup_gd32e230x.s 中的代码：

 

 

 

系统初始化代码：

void SystemInit (void)
{
    /* enable IRC8M */
    RCU_CTL0 |= RCU_CTL0_IRC8MEN;
    while(0U == (RCU_CTL0 & RCU_CTL0_IRC8MSTB)){
    }
    /* reset RCU */
    RCU_CFG0 &= ~(RCU_CFG0_SCS | RCU_CFG0_AHBPSC | RCU_CFG0_APB1PSC | RCU_CFG0_APB2PSC |\
                  RCU_CFG0_ADCPSC | RCU_CFG0_CKOUTSEL | RCU_CFG0_CKOUTDIV | RCU_CFG0_PLLDV);
    RCU_CFG0 &= ~(RCU_CFG0_PLLSEL | RCU_CFG0_PLLMF | RCU_CFG0_PLLMF4 | RCU_CFG0_PLLDV);
    RCU_CTL0 &= ~(RCU_CTL0_HXTALEN | RCU_CTL0_CKMEN | RCU_CTL0_PLLEN | RCU_CTL0_HXTALBPS);
    RCU_CFG1 &= ~(RCU_CFG1_PREDV);
    RCU_CFG2 &= ~(RCU_CFG2_USART0SEL | RCU_CFG2_ADCSEL);
    RCU_CFG2 &= ~RCU_CFG2_IRC28MDIV;
    RCU_CFG2 &= ~RCU_CFG2_ADCPSC2;
    RCU_CTL1 &= ~RCU_CTL1_IRC28MEN;
    RCU_INT = 0x00000000U;

    /* configure system clock */
    system_clock_config();
    
#ifdef VECT_TAB_SRAM
    nvic_vector_table_set(NVIC_VECTTAB_RAM,VECT_TAB_OFFSET);
#else
    nvic_vector_table_set(NVIC_VECTTAB_FLASH,VECT_TAB_OFFSET);
#endif
}

系统时钟配置函数：

static void system_clock_config(void)
{
#ifdef __SYSTEM_CLOCK_8M_HXTAL
    system_clock_8m_hxtal();
#elif defined (__SYSTEM_CLOCK_72M_PLL_HXTAL)
    system_clock_72m_hxtal();
#elif defined (__SYSTEM_CLOCK_72M_PLL_IRC8M_DIV2)
    system_clock_72m_irc8m();
#elif defined (__SYSTEM_CLOCK_72M_PLL_IRC48M_DIV2)
    system_clock_72m_irc48m();
#else
    system_clock_8m_irc8m();
#endif /* __SYSTEM_CLOCK_8M_HXTAL */
}

　　根据不同的时钟源宏定义（选择不同的时钟源及频率），调用不同的函数，以选择system_clock_72m_hxtal()为例：

static void system_clock_72m_hxtal(void)
{
    uint32_t timeout = 0U;
    uint32_t stab_flag = 0U;

    /* enable HXTAL */
    RCU_CTL0 |= RCU_CTL0_HXTALEN;

    /* wait until HXTAL is stable or the startup time is longer than HXTAL_STARTUP_TIMEOUT */
    do{
        timeout++;
        stab_flag = (RCU_CTL0 & RCU_CTL0_HXTALSTB);
    }
    while((0U == stab_flag) && (HXTAL_STARTUP_TIMEOUT != timeout));
    /* if fail */
    if(0U == (RCU_CTL0 & RCU_CTL0_HXTALSTB)){
        while(1){
        }
    }
    
    FMC_WS = (FMC_WS & (~FMC_WS_WSCNT)) | WS_WSCNT_2;   // WFY增加2个等待状态
    
    /* HXTAL is stable */
    /* AHB = SYSCLK */
    RCU_CFG0 |= RCU_AHB_CKSYS_DIV1;
    /* APB2 = AHB */
    RCU_CFG0 |= RCU_APB2_CKAHB_DIV1;
    /* APB1 = AHB */
    RCU_CFG0 |= RCU_APB1_CKAHB_DIV1;

    /* PLL = HXTAL * 9 = 72 MHz */
    RCU_CFG0 &= ~(RCU_CFG0_PLLSEL | RCU_CFG0_PLLMF | RCU_CFG0_PLLDV);
    RCU_CFG0 |= (RCU_PLLSRC_HXTAL | RCU_PLL_MUL9);
    
//    /* PLL = HXTAL * 6 = 72 MHz   AAA */
//    RCU_CFG0 &= ~(RCU_CFG0_PLLSEL | RCU_CFG0_PLLMF | RCU_CFG0_PLLDV);
//    RCU_CFG0 |= (RCU_PLLSRC_HXTAL | RCU_PLL_MUL6);
    

    /* enable PLL */
    RCU_CTL0 |= RCU_CTL0_PLLEN;

    /* wait until PLL is stable */
    while(0U == (RCU_CTL0 & RCU_CTL0_PLLSTB)){
    }

    /* select PLL as system clock */
    RCU_CFG0 &= ~RCU_CFG0_SCS;
    RCU_CFG0 |= RCU_CKSYSSRC_PLL;

    /* wait until PLL is selected as system clock */
    while(0U == (RCU_CFG0 & RCU_SCSS_PLL)){
    }
}

　函数选择的是 :HXTAL =8MH在