1.背景

利用stm32编程的时候，会涉及到时钟树的配置，根据硬件设计中使用的外部晶振频率来配置，GD32的时钟配置只能通过配置代码实现。

2.GD32F4时钟树

其中红圈部分是需要关注的。

3.代码部分

3.1 外部时钟频率

进入gd43f4xx.h，找到外部晶振定义代码

/* define value of high speed crystal oscillator (HXTAL) in Hz */
#if !defined  (HXTAL_VALUE)
#define HXTAL_VALUE    ((uint32_t)25000000)
#endif /* high speed crystal oscillator value */

因为我参考开发板电路设计，所以这里是25MHz。

3.2 设置外部晶振作为系统时钟来源

/* system frequency define */
#define __IRC16M          (IRC16M_VALUE)            /* internal 16 MHz RC oscillator frequency */
#define __HXTAL           (HXTAL_VALUE)             /* high speed crystal oscillator frequency */
#define __SYS_OSC_CLK     (__HXTAL)                /* main oscillator frequency */

将__SYS_OSC_CLK修改为外部晶振。

3.3 选择合适的主频，最高支持240MHZ，因为以前stm32用的是168,这里我继续用168.

/* select a system clock by uncommenting the following line */
//#define __SYSTEM_CLOCK_IRC16M                   (uint32_t)(__IRC16M)
//#define __SYSTEM_CLOCK_HXTAL                    (uint32_t)(__HXTAL)
//#define __SYSTEM_CLOCK_120M_PLL_IRC16M          (uint32_t)(120000000)
//#define __SYSTEM_CLOCK_120M_PLL_8M_HXTAL        (uint32_t)(120000000)
//#define __SYSTEM_CLOCK_120M_PLL_25M_HXTAL       (uint32_t)(120000000)
//#define __SYSTEM_CLOCK_168M_PLL_IRC16M          (uint32_t)(168000000)
//#define __SYSTEM_CLOCK_168M_PLL_8M_HXTAL        (uint32_t)(168000000)
#define __SYSTEM_CLOCK_168M_PLL_25M_HXTAL       (uint32_t)(168000000)
//#define __SYSTEM_CLOCK_200M_PLL_IRC16M          (uint32_t)(200000000)
//#define __SYSTEM_CLOCK_200M_PLL_8M_HXTAL        (uint32_t)(200000000)
//#define __SYSTEM_CLOCK_200M_PLL_25M_HXTAL       (uint32_t)(200000000)
//#define __SYSTEM_CLOCK_240M_PLL_IRC16M          (uint32_t)(240000000)
//#define __SYSTEM_CLOCK_240M_PLL_8M_HXTAL        (uint32_t)(240000000)
//#define __SYSTEM_CLOCK_240M_PLL_25M_HXTAL       (uint32_t)(240000000)

3.4 通过预分频器配置AHB,APB2,APB1域的时频率。

/*!
    \brief      configure the system clock to 168M by PLL which selects HXTAL(25M) as its clock source
    \param[in]  none
    \param[out] none
    \retval     none
*/
static void system_clock_168m_25m_hxtal(void)
{
    uint32_t timeout = 0U;
    uint32_t stab_flag = 0U;
    
    /* enable HXTAL */
    RCU_CTL |= RCU_CTL_HXTALEN;

    /* wait until HXTAL is stable or the startup time is longer than HXTAL_STARTUP_TIMEOUT */
    do{
        timeout++;
        stab_flag = (RCU_CTL & RCU_CTL_HXTALSTB);
    }while((0U == stab_flag) && (HXTAL_STARTUP_TIMEOUT != timeout));

    /* if fail */
    if(0U == (RCU_CTL & RCU_CTL_HXTALSTB)){
        while(1){
        }
    }
         
    RCU_APB1EN |= RCU_APB1EN_PMUEN;
    PMU_CTL |= PMU_CTL_LDOVS;

**    /* HXTAL is stable */**
**    /* AHB = SYSCLK */**
**    RCU_CFG0 |= RCU_AHB_CKSYS_DIV1;**
**    /* APB2 = AHB */**
**    RCU_CFG0 |= RCU_APB2_CKAHB_DIV2;**
**    /* APB1 = AHB */**
**    RCU_CFG0 |= RCU_APB1_CKAHB_DIV4;**

    /* Configure the main PLL, PSC = 25, PLL_N = 336, PLL_P = 2, PLL_Q = 7 */ 
    RCU_PLL = (25U | (336U << 6U) | (((2U >> 1U) - 1U) << 16U) |
                   (RCU_PLLSRC_HXTAL) | (7U << 24U));

    /* enable PLL */
    RCU_CTL |= RCU_CTL_PLLEN;

    /* wait until PLL is stable */
    while(0U == (RCU_CTL & RCU_CTL_PLLSTB)){
    }
    
    /* Enable the high-drive to extend the clock frequency to 168 Mhz */
    PMU_CTL |= PMU_CTL_HDEN;
    while(0U == (PMU_CS & PMU_CS_HDRF)){
    }
    
    /* select the high-drive mode */
    PMU_CTL |= PMU_CTL_HDS;
    while(0U == (PMU_CS & PMU_CS_HDSRF)){
    } 
    
    /* select PLL as system clock */
    RCU_CFG0 &= ~RCU_CFG0_SCS;
    RCU_CFG0 |= RCU_CKSYSSRC_PLLP;

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

如上述加粗部分所示，分别是不分频，2分频，和4分频。
关于这部分代码的详细解释可以参考连接：> https://blog.csdn.net/weixin_40913720/article/details/118251911
配置时钟的几个要点是：
*** 修改使用的外部晶振频率**
*** 将外部晶振作为系统时钟**
*** 取消注释，选择合适的系统时钟频率，取消注释后会有对应的函数进行配置处理，不需要自己操心，详细过程见上面的链接。**