需了解系统时钟源相关知识:STM32时钟系统
本文以HSE外接8MHz晶振,PLL倍频9倍得到72MHz的系统时钟为讲解背景
一，系统初始化函数SystemInit:
文件路径：USER->system_stm32f10x.c->头文件system_stm32f10x.h中SystemInit(void)
在system_stm32f10x.c文件中找到SystemInit(void)源码:
/**
* @brief Setup the microcontroller system
* Initialize the Embedded Flash Interface, the PLL and update the
* SystemCoreClock variable.
* @note This function should be used only after reset.
* @param None
* @retval None
*/
void SystemInit (void)
{
/* Reset the RCC clock configuration to the default reset state(for debug purpose) */
/* Set HSION bit */
RCC->CR |= (uint32_t)0x00000001; //RCC_CR寄存器最低位置1:打开HSI(内部高速时钟8M)
/* Reset SW, HPRE, PPRE1, PPRE2, ADCPRE and MCO bits */
#ifndef STM32F10X_CL
RCC->CFGR &= (uint32_t)0xF8FF0000;
#else //stm32f103ZET6为大容量芯片HD
RCC->CFGR &= (uint32_t)0xF0FF0000; //RCC_CFGR寄存器初始化
#endif /* STM32F10X_CL */
/* Reset HSEON, CSSON and PLLON bits */
RCC->CR &= (uint32_t)0xFEF6FFFF; //将RCC_CR寄存器HSEON,CSSON,PLLON位置0
/* Reset HSEBYP bit */
RCC->CR &= (uint32_t)0xFFFBFFFF; //将RCC_CR寄存器HSEBYP位置0
/* Reset PLLSRC, PLLXTPRE, PLLMUL and USBPRE/OTGFSPRE bits */
RCC->CFGR &= (uint32_t)0xFF80FFFF; //将RCC_CFGR寄存器PLLSRC, PLLXTPRE, //PLLMUL,USBPRE/OTGFSPRE位置0
#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 //stm32f103ZET6为大容量芯片HD
/* Disable all interrupts and clear pending bits */
RCC->CIR = 0x009F0000; //关闭所有的中断和对应的位(初始化中断)
#endif /* STM32F10X_CL */
#if defined (STM32F10X_HD) || (defined STM32F10X_XL) || (defined STM32F10X_HD_VL)
#ifdef DATA_IN_ExtSRAM
SystemInit_ExtMemCtl();
#endif /* DATA_IN_ExtSRAM */
#endif
/* Configure the System clock frequency, HCLK, PCLK2 and PCLK1 prescalers */
/* Configure the Flash Latency cycles and enable prefetch buffer */
SetSysClock();
#ifdef VECT_TAB_SRAM
SCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal SRAM. */
#else
SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH. */
#endif
}
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二，默认定义系统时钟:
#if defined (STM32F10X_LD_VL) || (defined STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)
/* #define SYSCLK_FREQ_HSE HSE_VALUE */
#define SYSCLK_FREQ_24MHz 24000000
#else
/* #define SYSCLK_FREQ_HSE HSE_VALUE */
/* #define SYSCLK_FREQ_24MHz 24000000 */
/* #define SYSCLK_FREQ_36MHz 36000000 */
/* #define SYSCLK_FREQ_48MHz 48000000 */
/* #define SYSCLK_FREQ_56MHz 56000000 */
#define SYSCLK_FREQ_72MHz 72000000 //默认定义系统是中文为72MHz
#endif
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在这里可以修改系统时钟值
三，SetSysClock(Void)
/**
* @brief Configures the System clock frequency, HCLK, PCLK2 and PCLK1 prescalers.
* @param None
* @retval None
*/
static void SetSysClock(void)
{
#ifdef SYSCLK_FREQ_HSE
SetSysClockToHSE();
#elif defined SYSCLK_FREQ_24MHz
SetSysClockTo24();
#elif defined SYSCLK_FREQ_36MHz
SetSysClockTo36();
#elif defined SYSCLK_FREQ_48MHz
SetSysClockTo48();
#elif defined SYSCLK_FREQ_56MHz
SetSysClockTo56();
#elif defined SYSCLK_FREQ_72MHz //默认定义的系统时钟是72MHz
SetSysClockTo72();
#endif
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四，SetSysClockTo72(Void)
/**
* @brief Sets System clock frequency to 72MHz and configure HCLK, PCLK2
* and PCLK1 prescalers.
* @note This function should be used only after reset.
* @param None
* @retval None
*/
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); //使能HSE:RCC_CR_HSEON=0x00010000(修改的是第16位HSEON)
/* Wait till HSE is ready and if Time out is reached exit */
do
{
HSEStatus = RCC->CR & RCC_CR_HSERDY; //开启并就绪:RCC_CR_HSERDY=0x00020000(第17位HSERDY置1)
StartUpCounter++;
} while((HSEStatus == 0) && (StartUpCounter != HSE_STARTUP_TIMEOUT)); //循环直到HSE稳定
if ((RCC->CR & RCC_CR_HSERDY) != RESET)
{
HSEStatus = (uint32_t)0x01; //就绪后赋值标志位HSEStatus
}
else
{
HSEStatus = (uint32_t)0x00;
}
if (HSEStatus == (uint32_t)0x01) //判断就绪
{
/* Enable Prefetch Buffer */
FLASH->ACR |= FLASH_ACR_PRFTBE;
/* Flash 2 wait state */
FLASH->ACR &= (uint32_t)((uint32_t)~FLASH_ACR_LATENCY);
FLASH->ACR |= (uint32_t)FLASH_ACR_LATENCY_2; //cpu的速度比芯片速度快的多 设置FLASH等待:两个等待状态
/* HCLK = SYSCLK */
RCC->CFGR |= (uint32_t)RCC_CFGR_HPRE_DIV1;//配置AHB预分频器分频系数为1,使HCLK = SYSCLK=72M
//RCC_CFGR_HPRE_DIV1=0x00000000(见手册)
/* PCLK2 = HCLK */
RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE2_DIV1;//配置PCLK2预分频器分频系数为1,使PCLK2 = HCLK=72M
//RCC_CFGR_PPRE2_DIV1=0x00000000(见手册)
/* PCLK1 = HCLK/2 */
RCC->CFGR |= (uint32_t)RCC_CFGR_PPRE1_DIV2;//设置PCLK1预分频器分频系数为2,使PCLK1 = HCLK/2=36M
//RCC_CFGR_PPRE1_DIV2=0x00000400(见手册)
//RCC->CFGR第10,9,8位设置为100 (设置为2分频)
#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 //stm32f103ZET6为大容量芯片HD
/* 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);//HSE为时钟源且PLL倍频系数为9(8*9=72兆)
#endif /* STM32F10X_CL */
/* Enable PLL */
RCC->CR |= RCC_CR_PLLON; //使能PLL时钟,RCC->CR寄存器第24位置1
//RCC_CR_PLLON=0x01000000
/* Wait till PLL is ready */
while((RCC->CR & RCC_CR_PLLRDY) == 0) //等待PLL时钟源就绪
//RCC_CR_PLLRDYRCC->CR寄存器第25位
{
}
/* Select PLL as system clock source */
RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW));//配置PLL作为系统时钟来源
RCC->CFGR |= (uint32_t)RCC_CFGR_SW_PLL; //RCC_CFGR_SW_PLL=0x00000002(RCC->CFGR第1,0位设置为10,见手册)
/* 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 */
}
}
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代码中相关寄存器配置文档说明：
１.FLASH等待设置:
2.AHB预分频设置:
3.APB2预分频设置:
4.APB1预分频设置:
5.PLL倍频设置:
6.配置PLL为系统时钟源:
最低位设置为10 所以 0x00000002
五，总结系统初始化函数流程
1,打开HSE,等待就绪后,设置Flash等待操作
2,设置AHB,APB1,APB2分频系数,确定他们各自和系统时钟的关系
3,设置CFGR寄存器确定PLL的时钟来源和倍频系数(HSE外部8M*9倍=72MHz)
4,使能PLL,将系统时钟源切换到PLL
六，通过SystemCoreClock获取当前系统时钟频率
/*******************************************************************************
* Clock Definitions
*******************************************************************************/
#ifdef SYSCLK_FREQ_HSE
uint32_t SystemCoreClock = SYSCLK_FREQ_HSE; /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_24MHz
uint32_t SystemCoreClock = SYSCLK_FREQ_24MHz; /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_36MHz
uint32_t SystemCoreClock = SYSCLK_FREQ_36MHz; /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_48MHz
uint32_t SystemCoreClock = SYSCLK_FREQ_48MHz; /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_56MHz
uint32_t SystemCoreClock = SYSCLK_FREQ_56MHz; /*!< System Clock Frequency (Core Clock) */
#elif defined SYSCLK_FREQ_72MHz
uint32_t SystemCoreClock = SYSCLK_FREQ_72MHz; /*!< System Clock Frequency (Core Clock) */
#else /*!< HSI Selected as System Clock source */
uint32_t SystemCoreClock = HSI_VALUE; /*!< System Clock Frequency (Core Clock) */
#endif
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可以通过SystemCoreClock获取当前系统时钟频率
七，SystemInit的调用
在系统初始化调用main函数前SystemInit(void)
在CORE->startup_stm32f10x_hd.s启动文件中
; Reset handler
Reset_Handler PROC
EXPORT Reset_Handler [WEAK]
IMPORT __main
IMPORT SystemInit
LDR R0, =SystemInit
BLX R0
LDR R0, =__main
BX R0
ENDP
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