修改system_stm32Fxx.c文件，启动内部晶振代码

从启动文件上分析，该函数在reset复位之后，进入main主函数之前，芯片调用该函数进行系统时钟的初始化和设置，对这段代码进行修改，就能实现启动内部晶体了。

void SystemInit (void)

{

/* Set HSION bit */

RCC->CR |= (uint32_t)0x00000001;

#if defined (STM32F031) || defined (STM32F072) || defined (STM32F042)

/* Reset SW[1:0], HPRE[3:0], PPRE[2:0], ADCPRE and MCOSEL[2:0] bits /

RCC->CFGR &= (uint32_t)0xF8FFB80C;

#else

/ Reset SW[1:0], HPRE[3:0], PPRE[2:0], ADCPRE, MCOSEL[2:0], MCOPRE[2:0] and PLLNODIV bits /

RCC->CFGR &= (uint32_t)0x08FFB80C;

#endif / STM32F031*/

/* Reset HSEON, CSSON and PLLON bits */

RCC->CR &= (uint32_t)0xFEF6FFFF;

/* Reset HSEBYP bit */

RCC->CR &= (uint32_t)0xFFFBFFFF;

/* Reset PLLSRC, PLLXTPRE and PLLMUL[3:0] bits */

RCC->CFGR &= (uint32_t)0xFFC0FFFF;

/* Reset PREDIV1[3:0] bits */

RCC->CFGR2 &= (uint32_t)0xFFFFFFF0;

/* Reset USARTSW[1:0], I2CSW, CECSW and ADCSW bits */

RCC->CFGR3 &= (uint32_t)0xFFFFFEAC;

/* Reset HSI14 bit */

RCC->CR2 &= (uint32_t)0xFFFFFFFE;

/* Disable all interrupts */

RCC->CIR = 0x00000000;

/* Configure the System clock frequency, AHB/APBx prescalers and Flash settings */

SetSysClock();

}