本文以GD32F303型号为基础，依照标准库GD32F30x_Firmware_Library_V2.1.5为例，作为笔记简单记录个人对其时钟配置的理解，后续会持续更新本篇笔记内容。
因为某些原因，国外半导体供应供应短缺，以STM32为例的系列芯片紧缺，价格跳水严重。技术群的部分码友们所在公司纷纷寻找替代产品，将头转向兆易创新的GD32，在将以前的STM32项目移植到国产GD32过程中，踩坑是在所难免，我也重新梳理了对GD32的理解。作为笔记加深理解。本篇主要是对时钟树的配置的一些理解和问题,欢迎各位码友指点!
时钟控制单元提供有五个时钟源，见时钟树标记，包括：
1：内部8M RC振荡器时钟(IRC8M)：高速内部8MHz时钟，拥有8MHz的固定频率，设备上电后CPU默认选择的时钟源就是IRC8M时钟
2：内部48M RC 振荡器时钟(IRC48): 有一个固定的频率48MHz，可用作USB时钟或者PLL时钟源
3：外部高速晶体振荡器时钟(HXTAL：4到32MHz的外部振荡器，可为系统提供精确的主时钟。带有特定频率的晶体必须靠近两个HXTAL的引脚。和晶体连接的外部电阻和电容必须根据所选择的振荡器来调整
4：内部40K RC振荡器时钟(IRC40K)：它的时钟频率大约40 kHz，为独立看门狗定时器和实时时钟电路提供时钟。
5：外部低速晶体振荡器时钟(LXTA）：一个32.768kHz的低速外部晶体或陶瓷谐振器。它为实时时钟电路提供一个低功耗且精确的时钟源。
内部锁相环（PLL）：对输入参考频率为4到32MHz时钟进行分频操作，输出一个8-120 MHz的时钟输出
剩下就是HXTAL时钟监视器、时钟预分频器、时钟多路复用器和时钟门控电路等。
AHB、 APB和Cortex®-M4时钟都源自系统时钟（CK_SYS），系统时钟的时钟源可以选择IRC8M、HXTAL或PLL。系统时钟的最大运行时钟频率可以达到120MHz。
严格来说锁相环不算时钟源，它是其他时钟源头配置而来
此函数中主要操作为依内部8MHz时钟为时钟源，驱动时钟配置，线路为红色线路所示，步骤如下图：
代码如下（示例）：
void SystemInit (void)
{
/* FPU settings */
#if (__FPU_PRESENT == 1) && (__FPU_USED == 1)
SCB->CPACR |= ((3UL << 10*2)|(3UL << 11*2)); /* set CP10 and CP11 Full Access */
#endif
/* reset the RCU clock configuration to the default reset state */
/* Set IRC8MEN bit */
RCU_CTL |= RCU_CTL_IRC8MEN; //打开内部8MHz RC振荡器
while(0U == (RCU_CTL & RCU_CTL_IRC8MSTB)){ //等待内部8MHz RC振荡器稳定
}
RCU_MODIFY(0x50); //AHB时钟分频设置 先二分频再四分频
RCU_CFG0 &= ~RCU_CFG0_SCS; //00:选择 CK_IRC8M 时钟作为 CK_SYS 时钟源
#if (defined(GD32F30X_HD) || defined(GD32F30X_XD))
/* reset HXTALEN, CKMEN and PLLEN bits */
/*外部高速时钟使能位、HXTAL 时钟监视器使能位、PLL使能位 复位，方便后续操作*/
RCU_CTL &= ~(RCU_CTL_PLLEN | RCU_CTL_CKMEN | RCU_CTL_HXTALEN);
/* disable all interrupts */
/*关闭中断位*/
RCU_INT = 0x009f0000U;
#elif defined(GD32F30X_CL)
/* Reset HXTALEN, CKMEN, PLLEN, PLL1EN and PLL2EN bits */
RCU_CTL &= ~(RCU_CTL_PLLEN |RCU_CTL_PLL1EN | RCU_CTL_PLL2EN | RCU_CTL_CKMEN | RCU_CTL_HXTALEN);
/* disable all interrupts */
RCU_INT = 0x00ff0000U;
#endif
/* reset HXTALBPS bit */
RCU_CTL &= ~(RCU_CTL_HXTALBPS); //旁路模式位复位
/* Reset CFG0 and CFG1 registers */
RCU_CFG0 = 0x00000000U;
RCU_CFG1 = 0x00000000U;
/* configure the system clock source, PLL Multiplier, AHB/APBx prescalers and Flash settings */
system_clock_config();
}
根据宏定义，选择相关的时钟频率配置函数，我的是高速外部时钟源配置108MHz系统时钟
配置函数为;system_clock_108m_hxtal();
代码如下（示例）：
static void system_clock_config(void)
{
#ifdef __SYSTEM_CLOCK_IRC8M
system_clock_8m_irc8m();
#elif defined (__SYSTEM_CLOCK_48M_PLL_IRC8M)
system_clock_48m_irc8m();
#elif defined (__SYSTEM_CLOCK_72M_PLL_IRC8M)
system_clock_72m_irc8m();
#elif defined (__SYSTEM_CLOCK_108M_PLL_IRC8M)
system_clock_108m_irc8m();
#elif defined (__SYSTEM_CLOCK_120M_PLL_IRC8M)
system_clock_120m_irc8m();
#elif defined (__SYSTEM_CLOCK_HXTAL)
system_clock_hxtal();
#elif defined (__SYSTEM_CLOCK_48M_PLL_HXTAL)
system_clock_48m_hxtal();
#elif defined (__SYSTEM_CLOCK_72M_PLL_HXTAL)
system_clock_72m_hxtal();
#elif defined (__SYSTEM_CLOCK_108M_PLL_HXTAL)
system_clock_108m_hxtal();
#elif defined (__SYSTEM_CLOCK_120M_PLL_HXTAL)
system_clock_120m_hxtal();
#endif /* __SYSTEM_CLOCK_IRC8M */
}
该函数以外部时钟源配置108兆系统时钟步骤如图，时钟线路为绿色线路所示：
代码如下（示例）：
static void system_clock_108m_hxtal(void)
{
uint32_t timeout = 0U;
uint32_t stab_flag = 0U;
/* enable HXTAL */
RCU_CTL |= RCU_CTL_HXTALEN; //使能外部晶振
/* 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 时钟使能
PMU_CTL |= PMU_CTL_LDOVS; //11： LDO 输出高电压模式
/* HXTAL is stable */
/* AHB = SYSCLK */
RCU_CFG0 |= RCU_AHB_CKSYS_DIV1;
/* APB2 = AHB/1 */
RCU_CFG0 |= RCU_APB2_CKAHB_DIV1;
/* APB1 = AHB/2 */
RCU_CFG0 |= RCU_APB1_CKAHB_DIV2;
#if (defined(GD32F30X_HD) || defined(GD32F30X_XD))
/* select HXTAL/2 as clock source */
RCU_CFG0 &= ~(RCU_CFG0_PLLSEL | RCU_CFG0_PREDV0);
RCU_CFG0 |= (RCU_PLLSRC_HXTAL_IRC48M | RCU_CFG0_PREDV0);//HXTAL 时钟或者 IRC48M 时钟（寄存器 RCU_CFG1 位 PLLPRESEL 决定）被选择为 PLL 时钟的时钟源
/*CK_PLL = (CK_HXTAL/2) * 27 = 108 MHz*/
RCU_CFG0 &= ~(RCU_CFG0_PLLMF | RCU_CFG0_PLLMF_4 | RCU_CFG0_PLLMF_5);
RCU_CFG0 |= RCU_PLL_MUL27; //30 27 21 20 19 18：011010
#elif defined(GD32F30X_CL)
/* CK_PLL = (CK_PREDIV0) * 27 = 108 MHz */
RCU_CFG0 &= ~(RCU_CFG0_PLLMF | RCU_CFG0_PLLMF_4 | RCU_CFG0_PLLMF_5);
RCU_CFG0 |= (RCU_PLLSRC_HXTAL_IRC48M | RCU_PLL_MUL27);
/* CK_PREDIV0 = (CK_HXTAL)/5 *8 /10 = 4 MHz */
RCU_CFG1 &= ~(RCU_CFG1_PLLPRESEL | RCU_CFG1_PREDV0SEL | RCU_CFG1_PLL1MF | RCU_CFG1_PREDV1 | RCU_CFG1_PREDV0);
RCU_CFG1 |= (RCU_PLLPRESRC_HXTAL | RCU_PREDV0SRC_CKPLL1 | RCU_PLL1_MUL8 | RCU_PREDV1_DIV5 | RCU_PREDV0_DIV10);
/* enable PLL1 */
RCU_CTL |= RCU_CTL_PLL1EN;
/* wait till PLL1 is ready */
while((RCU_CTL & RCU_CTL_PLL1STB) == 0){
}
#endif /* GD32F30X_HD and GD32F30X_XD */
/* enable PLL */
RCU_CTL |= RCU_CTL_PLLEN; //PLL 被打开
/* wait until PLL is stable */
while(0U == (RCU_CTL & RCU_CTL_PLLSTB)){
}
/* enable the high-drive to extend the clock frequency to 120 MHz */
PMU_CTL |= PMU_CTL_HDEN; //使能高驱动模式
while(0U == (PMU_CS & PMU_CS_HDRF)){
}
/* select the high-drive mode */
PMU_CTL |= PMU_CTL_HDS; //有高驱动模式切换器
while(0U == (PMU_CS & PMU_CS_HDSRF)){
}
/* select PLL as system clock */
RCU_CFG0 &= ~RCU_CFG0_SCS;
RCU_CFG0 |= RCU_CKSYSSRC_PLL; //10：选择 CK_PLL 时钟作为 CK_SYS 时钟源
/* wait until PLL is selected as system clock */
while(0U == (RCU_CFG0 & RCU_SCSS_PLL)){
}
}
1.外部晶振时钟频率，在gd32f30x.h文件中，根据实际外部晶振频率修改参数，我的板子是8兆晶振，选择：
#define HXTAL_VALUE ((uint32_t)8000000)
/* define value of high speed crystal oscillator (HXTAL) in Hz */
#if !defined HXTAL_VALUE
#ifdef GD32F30X_CL
#define HXTAL_VALUE ((uint32_t)25000000) /*!< value of the external oscillator in Hz */
#else
#define HXTAL_VALUE ((uint32_t)8000000) /* !< from 4M to 32M *!< value of the external oscillator in Hz*/
#endif /* HXTAL_VALUE */
#endif /* high speed crystal oscillator value */
2.外部晶振时钟频率，在system_gd32f30x.c文件中，_SYS_OSC_CLK为系统时钟主频率选择宏定义，使用外部晶振选择__HXTAL
我需要的系统时钟配置函数为：__SYSTEM_CLOCK_108M_PLL_HXTAL()
/* system frequency define */
#define __IRC8M (IRC8M_VALUE) /* internal 8 MHz RC oscillator frequency */
#define __HXTAL (HXTAL_VALUE) /* high speed crystal oscillator frequency */
#define __SYS_OSC_CLK (__HXTAL) /* main oscillator frequency */
/* select a system clock by uncommenting the following line */
/* use IRC8M */
//#define __SYSTEM_CLOCK_IRC8M (uint32_t)(__IRC8M)
//#define __SYSTEM_CLOCK_48M_PLL_IRC8M (uint32_t)(48000000)
//#define __SYSTEM_CLOCK_72M_PLL_IRC8M (uint32_t)(72000000)
//#define __SYSTEM_CLOCK_108M_PLL_IRC8M (uint32_t)(108000000)
//#define __SYSTEM_CLOCK_120M_PLL_IRC8M (uint32_t)(120000000)
/* use HXTAL(XD series CK_HXTAL = 8M, CL series CK_HXTAL = 25M) */
//#define __SYSTEM_CLOCK_HXTAL (uint32_t)(__HXTAL)
//#define __SYSTEM_CLOCK_48M_PLL_HXTAL (uint32_t)(48000000)
//#define __SYSTEM_CLOCK_72M_PLL_HXTAL (uint32_t)(72000000)
#define __SYSTEM_CLOCK_108M_PLL_HXTAL (uint32_t)(108000000)
//#define __SYSTEM_CLOCK_120M_PLL_HXTAL (uint32_t)(120000000)
#if (defined(GD32F30X_HD) || defined(GD32F30X_XD))
/* select HXTAL/2 as clock source */
RCU_CFG0 &= ~(RCU_CFG0_PLLSEL | RCU_CFG0_PREDV0);
RCU_CFG0 |= (RCU_PLLSRC_HXTAL_IRC48M | RCU_CFG0_PREDV0);//HXTAL 时钟或者 IRC48M 时钟（寄存器 RCU_CFG1 位 PLLPRESEL 决定）被选择为 PLL 时钟的时钟源
/*CK_PLL = (CK_HXTAL/2) * 27 = 108 MHz*/
RCU_CFG0 &= ~(RCU_CFG0_PLLMF | RCU_CFG0_PLLMF_4 | RCU_CFG0_PLLMF_5);
RCU_CFG0 |= RCU_PLL_MUL27;
系统时钟准确与否关系到整个工程的功能能否正常运行，拿到祖传代码或者移植新项目一定记得先测时钟，别问为什么，问就是教训！！！
1.通过系统函数获取时钟配置,uint32_t rcu_clock_freq_get(rcu_clock_freq_enum clock)函数可用来获取时钟参数，
获取APB1时钟：uclk = rcu_clock_freq_get(CK_APB1);
获取APB2时钟：uclk = rcu_clock_freq_get(CK_APB2);
获取CK_AHB时钟：uclk = rcu_clock_freq_get(CK_AHB);
获取CK_SYS时钟：uclk = rcu_clock_freq_get(CK_SYS);
STM32中函数测试方法位：
RCC_ClocksTypeDef RCC_Clocks;
RCC_GetClocksFreq(&RCC_Clocks);
2.上述方法是配置参数，结果是跟着参数走的，有时候与实际情况不一样，一般情况是写一个简单的定时器或延时函数，周期性改变某个GPIO电平，通过示波器进行检测是最准的
3.如图，通过配置CK_OUTO时钟输出引脚,配置引脚位复用输出模式，使用void rcu_ckout0_config(uint32_t ckout0_src)进行配置，然后也是通过示波器看波形
1.#define HXTAL_VALUE ((uint32_t)8000000)外部晶振时钟频率，这个时钟频率与实际晶振频率不一样可能导致串口输出异常
2.如果没有外部晶振，被迫使用内部时钟，记得一定要通过配置的到并测试，不要不管配置等待系统默认选择内部时钟。
3.后续跟进
系统时钟非常重要，各个板子的支持时钟可能不同，但是配置方法大同小异
如有错误，欢迎指正，原创不易，转载留名！