GD32 的时钟控制单元叫做 RCU
GD32 系统时钟修改和 STM32 不一样，STM32是调用库函数，GD32需要手动改文件。
在 system_xxx.c 文件中，它是使用宏定义来决定时钟的配置，一般都是使用外部晶振和PLL
/* select a system clock by uncommenting the following line */
//#define __SYSTEM_CLOCK_8M_HXTAL (__HXTAL)
//#define __SYSTEM_CLOCK_8M_IRC8M (__IRC8M)
#define __SYSTEM_CLOCK_72M_PLL_HXTAL (uint32_t)(72000000)
//#define __SYSTEM_CLOCK_72M_PLL_IRC8M_DIV2 (uint32_t)(72000000)1、如果是想改时钟频率，直接在对应的函数中改就行了：
#elif defined (__SYSTEM_CLOCK_72M_PLL_HXTAL)
/*!
\brief configure the system clock to 72M by PLL which selects HXTAL as its clock source
\param[in] none
\param[out] none
\retval none
*/
static void system_clock_72m_hxtal(void)最好的办法还是重新搞个宏定义和函数，这样更加清晰。
2、如果想改外部晶振频率，打开 gd32____.h 这个头文件
/* define value of high speed crystal oscillator (HXTAL) in Hz */
#if !defined (HXTAL_VALUE)
#define HXTAL_VALUE ((uint32_t)8000000) // 在这里修改
#endif /* high speed crystal oscillator value */获取当前系统时钟频率：
SystemCoreClockUpdate();
printf("Current System Clock: %d", SystemCoreClock);设置 systick 时钟：
systick_clksource_set(SYSTICK_CLKSOURCE_HCLK_DIV8); // 8 分频
systick_clksource_set(SYSTICK_CLKSOURCE_HCLK); // 1 分频开启外设时钟：
void rcu_periph_clock_enable(rcu_periph_enum periph)
// 参数
typedef enum
{
/* AHB peripherals */
RCU_DMA = RCU_REGIDX_BIT(IDX_AHBEN, 0U), /*!< DMA clock */
RCU_CRC = RCU_REGIDX_BIT(IDX_AHBEN, 6U), /*!< CRC clock */
RCU_GPIOA = RCU_REGIDX_BIT(IDX_AHBEN, 17U), /*!< GPIOA clock */
RCU_GPIOB = RCU_REGIDX_BIT(IDX_AHBEN, 18U), /*!< GPIOB clock */
RCU_GPIOC = RCU_REGIDX_BIT(IDX_AHBEN, 19U), /*!< GPIOC clock */
RCU_GPIOF = RCU_REGIDX_BIT(IDX_AHBEN, 22U), /*!< GPIOF clock */
/* APB2 peripherals */
RCU_CFGCMP = RCU_REGIDX_BIT(IDX_APB2EN, 0U), /*!< CFGCMP clock 比较器、exti外部中断、引脚重映射时需要开启 */
RCU_ADC = RCU_REGIDX_BIT(IDX_APB2EN, 9U), /*!< ADC clock */
RCU_TIMER0 = RCU_REGIDX_BIT(IDX_APB2EN, 11U), /*!< TIMER0 clock */
RCU_SPI0 = RCU_REGIDX_BIT(IDX_APB2EN, 12U), /*!< SPI0 clock */
RCU_USART0 = RCU_REGIDX_BIT(IDX_APB2EN, 14U), /*!< USART0 clock */
RCU_TIMER14 = RCU_REGIDX_BIT(IDX_APB2EN, 16U), /*!< TIMER14 clock */
RCU_TIMER15 = RCU_REGIDX_BIT(IDX_APB2EN, 17U), /*!< TIMER15 clock */
RCU_TIMER16 = RCU_REGIDX_BIT(IDX_APB2EN, 18U), /*!< TIMER16 clock */
RCU_DBGMCU = RCU_REGIDX_BIT(IDX_APB2EN, 22U), /*!< DBGMCU clock */
/* APB1 peripherals */
RCU_TIMER2 = RCU_REGIDX_BIT(IDX_APB1EN, 1U), /*!< TIMER2 clock */
RCU_TIMER5 = RCU_REGIDX_BIT(IDX_APB1EN, 4U), /*!< TIMER5 clock */
RCU_TIMER13 = RCU_REGIDX_BIT(IDX_APB1EN, 8U), /*!< TIMER13 clock */
RCU_WWDGT = RCU_REGIDX_BIT(IDX_APB1EN, 11U), /*!< WWDGT clock */
RCU_SPI1 = RCU_REGIDX_BIT(IDX_APB1EN, 14U), /*!< SPI1 clock */
RCU_USART1 = RCU_REGIDX_BIT(IDX_APB1EN, 17U), /*!< USART1 clock */
RCU_I2C0 = RCU_REGIDX_BIT(IDX_APB1EN, 21U), /*!< I2C0 clock */
RCU_I2C1 = RCU_REGIDX_BIT(IDX_APB1EN, 22U), /*!< I2C1 clock */
RCU_PMU = RCU_REGIDX_BIT(IDX_APB1EN, 28U), /*!< PMU clock */
/* Backup domain control(BDCTL) */
RCU_RTC = RCU_REGIDX_BIT(IDX_BDCTL, 15U) /*!< RTC clock */
}rcu_periph_enum;时钟树
