切换到system_gd32fxxx.c(如：system_gd32f403.h),然后将以下程序
/* 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 (__IRC8M) /* 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)
//#define __SYSTEM_CLOCK_168M_PLL_IRC8M (uint32_t)(168000000)
/* use IRC48M */
//#define __SYSTEM_CLOCK_168M_PLL_IRC48M (uint32_t)(168000000)
/* 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)
#define __SYSTEM_CLOCK_168M_PLL_HXTAL (uint32_t)(168000000)
更换成
/* 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 (__IRC8M) /* 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)
//#define __SYSTEM_CLOCK_168M_PLL_IRC8M (uint32_t)(168000000)
/* use IRC48M */
//#define __SYSTEM_CLOCK_168M_PLL_IRC48M (uint32_t)(168000000)
/* 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)
//#define __SYSTEM_CLOCK_168M_PLL_HXTAL (uint32_t)(168000000)
即，将#define __SYSTEM_CLOCK_168M_PLL_HXTAL 注释掉，更换成#define __SYSTEM_CLOCK_IRC8M即可将系统时钟源更换成内部的8M时钟。
- 目前只知道这些，待补齐。
