上接：/otaganyuki/p/10310979.html
前面一些宏定义略过，但有一个后面用到提下
这里定义了系统时钟频率为72mhz
从SystemInit开始讲，寄存器和时钟树参照stm32参考手册即可
void SystemInit (void){/* Reset the RCC clock configuration to the default reset state(for debug purpose) *//* Set HSION bit */RCC->CR |= (uint32_t)0x00000001;//打开内部8MHz振荡器/* Reset SW, HPRE, PPRE1, PPRE2, ADCPRE and MCO bits
SW 00：HSI设为系统时钟
ADCPRE 00:ADC的时钟预分频设置为2
PPRE2 000:APB2预分频器设置为1
PPRE1 000:APB1预分频设置为1
HPRE 000：AHB预分频设置为1
MCO 00：MCO时钟输出设为无
*/#ifndef STM32F10X_CL //CL就是类似HD的单片机型号选择RCC->CFGR &= (uint32_t)0xF8FF0000;#elseRCC->CFGR &= (uint32_t)0xF0FF0000;#endif /* STM32F10X_CL */ /* Reset HSEON, CSSON and PLLON bits
PLLON 0:关闭PLL
CSSON 0:Clock detector OFF
HSEON 0:HSE oscillator OFF
*/RCC->CR &= (uint32_t)0xFEF6FFFF;/* Reset HSEBYP bit
0: external 4-16 MHz oscillator not bypassed使用外部晶振
*/RCC->CR &= (uint32_t)0xFFFBFFFF;/* Reset PLLSRC, PLLXTPRE, PLLMUL and USBPRE/OTGFSPRE bits
PLLSRC 0:HSI oscillator clock / 2 selected as PLL input clock
PLLXTPRE 0: HSE clock not divided
PLLMUL 0000: PLL input clock x 2
*/RCC->CFGR &= (uint32_t)0xFF80FFFF;
/*下面一大段由于一般没有定义（我没用到），所以忽略即可*/#ifdef STM32F10X_CL/* Reset PLL2ON and PLL3ON bits */RCC->CR &= (uint32_t)0xEBFFFFFF;/* Disable all interrupts and clear pending bits */RCC->CIR = 0x00FF0000;/* Reset CFGR2 register */RCC->CFGR2 = 0x00000000;#elif defined (STM32F10X_LD_VL) || defined (STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)/* Disable all interrupts and clear pending bits */RCC->CIR = 0x009F0000;/* Reset CFGR2 register */RCC->CFGR2 = 0x00000000;#else/* Disable all interrupts and clear pending bits */RCC->CIR = 0x009F0000;#endif /* STM32F10X_CL */#if defined (STM32F10X_HD) || (defined STM32F10X_XL) || (defined STM32F10X_HD_VL)#ifdef DATA_IN_ExtSRAMSystemInit_ExtMemCtl(); #endif /* DATA_IN_ExtSRAM */#endif
/*最开始那段只是归零寄存器，实际设置在下面这个函数里执行*//* Configure the System clock frequency, HCLK, PCLK2 and PCLK1 prescalers *//* Configure the Flash Latency cycles and enable prefetch buffer */SetSysClock();#ifdef VECT_TAB_SRAMSCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal SRAM. */#elseSCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH. */#endif }
SetSysClock函数 我这边时默认执行SetSysClockTo72子函数（之前开头宏定义过了），如下
static void SetSysClockTo72(void){__IO uint32_t StartUpCounter = 0, HSEStatus = 0;/* SYSCLK, HCLK, PCLK2 and PCLK1 configuration ---------------------------*/ /* Enable HSE */ RCC->CR |= ((uint32_t)RCC_CR_HSEON);/* Wait till HSE is ready and if Time out is reached exit
HSE_STARTUP_TIMEOUT就是之前在stm32f10x.h里提到的等待时钟稳定的时间
下面就是在等HSERDY被置高，若超过设定的TIMEOUT，则跳出
*/do{HSEStatus = RCC->CR & RCC_CR_HSERDY;StartUpCounter++; } while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT));if ((RCC->CR & RCC_CR_HSERDY) != RESET){HSEStatus = (uint32_t)0x01;}else{HSEStatus = (uint32_t)0x00;} if (HSEStatus == (uint32_t)0x01){/* Enable Prefetch Buffer
此寄存器在手册3.3节描述
*/FLASH->ACR |= FLASH_ACR_PRFTBE;/* Flash 2 wait state
这个等待时间貌似改小也是有可能没问题的，因为芯片的实际质量一般高于手册的标准
*/FLASH->ACR &= (uint32_t)((uint32_t)~FLASH_ACR_LATENCY);FLASH->ACR |= (uint32_t)FLASH_ACR_LATENCY_2; /* HCLK = SYSCLK */RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;/* PCLK2 = HCLK */RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE2_DIV1;/* PCLK1 = HCLK */RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE1_DIV2;#ifdef STM32F10X_CL/* Configure PLLs ------------------------------------------------------*//* PLL2 configuration: PLL2CLK = (HSE / 5) * 8 = 40 MHz *//* PREDIV1 configuration: PREDIV1CLK = PLL2 / 5 = 8 MHz */RCC->CFGR2 &= (uint32_t)~(RCC_CFGR2_PREDIV2 | RCC_CFGR2_PLL2MUL |RCC_CFGR2_PREDIV1 | RCC_CFGR2_PREDIV1SRC);RCC->CFGR2 |= (uint32_t)(RCC_CFGR2_PREDIV2_DIV5 | RCC_CFGR2_PLL2MUL8 |RCC_CFGR2_PREDIV1SRC_PLL2 | RCC_CFGR2_PREDIV1_DIV5);/* Enable PLL2 */RCC->CR |= RCC_CR_PLL2ON;/* Wait till PLL2 is ready */while((RCC->CR & RCC_CR_PLL2RDY) == 0){}/* PLL configuration: PLLCLK = PREDIV1 * 9 = 72 MHz */ RCC->CFGR &= (uint32_t)~(RCC_CFGR_PLLXTPRE | RCC_CFGR_PLLSRC | RCC_CFGR_PLLMULL);RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLXTPRE_PREDIV1 | RCC_CFGR_PLLSRC_PREDIV1 | RCC_CFGR_PLLMULL9); #else /* PLL configuration: PLLCLK = HSE * 9 = 72 MHz */RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_PLLSRC | RCC_CFGR_PLLXTPRE |RCC_CFGR_PLLMULL));RCC->CFGR |= (uint32_t)(RCC_CFGR_PLLSRC_HSE | RCC_CFGR_PLLMULL9);#endif /* STM32F10X_CL *//* Enable PLL */RCC->CR |= RCC_CR_PLLON;/* Wait till PLL is ready */while((RCC->CR & RCC_CR_PLLRDY) == 0){}/* Select PLL as system clock source */RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL; /* Wait till PLL is used as system clock source */while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS) != (uint32_t)0x08){}}else{ /* If HSE fails to start-up, the application will have wrong clock configuration. User can add here some code to deal with this error */}}
这样，这个文件剩下的只有SystemCoreClockUpdate没提了，其他函数基本类似或者没有用到，这个具体内容就不复制上来了，功能为：使SystemCoreClock变量保存当前的系统时钟频率，我稍微做了下测试，如果默认启动后调用此函数，SystemCoreClock自然就是72000000，如果把系统时钟切为HSI，即执行如下代码
RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));RCC->CFGR |= (uint32_t)RCC_CFGR_SW_HSI;
再调用此函数，SystemCoreClock的值就是8000000，当然，单片机的执行速度也慢了许多。
