STM32F系列ARM Cortex_M3核微控制器之系统时钟三
问题一：若未对系统进行时钟配置，那么系统上电后时钟系统将是怎样配置的？
在STM32启动文件startup_stm32f10x_hd.s中有下面一段汇编：
Reset_Handler PROC
EXPORT Reset_Handler [WEAK]
IMPORT __main
IMPORT SystemInit
LDR R0, =SystemInit
BLX R0
LDR R0, =__main
BX R0
ENDP我们来看看system_stm32f10x.c文件中的SystemInit()函数对系统时钟的配置情况
/**
* @brief Setup the microcontroller system
* Initialize the Embedded Flash Interface, the PLL and update the
* SystemCoreClock variable. 初始化内部Flash，PLL，更新系统时钟变量
* @note This function should be used only after reset. 仅能在复位后使用
* @param None
* @retval None
*/
void SystemInit (void)
{
/* Reset the RCC clock configuration to the default reset state(for debug purpose) */
//复位RCC时钟配置为默认复位状态（为了调试目的）
/* Set HSION bit 置位内部高速时钟使能位*/
//内部8MHz振荡器开启
RCC->CR |= (uint32_t)0x00000001;
/* Reset SW, HPRE, PPRE1, PPRE2, ADCPRE and MCO bits */
//复位系统时钟切换，AHB预分频，低速APB预分频，高速APB预分频，ADC预分频，微控制器时钟输出位
//HSI作为系统时钟，SYSCLK不分频作为HCLK，HCLK不分频做为PCLK1,PCLK2，PCLK2 2分频后作为ADC时钟，MCO引脚没有微控制器时钟输出
#ifndef STM32F10X_CL
RCC->CFGR &= (uint32_t)0xF8FF0000;
#else
RCC->CFGR &= (uint32_t)0xF0FF0000;
#endif /* STM32F10X_CL */
/* Reset HSEON, CSSON and PLLON bits */
//复位外部高速时钟使能，时钟安全系统使能，PLL使能
//外部高速时钟关闭，时钟监测器关闭，PLL关闭
RCC->CR &= (uint32_t)0xFEF6FFFF;
/* Reset HSEBYP bit */
//复位外部高速时钟旁路
//外部3-25MHz时钟没有旁路
RCC->CR &= (uint32_t)0xFFFBFFFF;
/* Reset PLLSRC, PLLXTPRE, PLLMUL and USBPRE/OTGFSPRE bits */
//复位PLL输入时钟源，HSE分频作为PLL输入，PLL倍频系数，USB预分频位
//HSI振荡器时钟经2分频后作为PLL输入时钟，HSE不分频作为PLL输入时钟，PLL2倍频输出，PLL时钟1.5倍分频作为USB时钟
RCC->CFGR &= (uint32_t)0xFF80FFFF;
#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 */
//失能所有时钟中断，清除挂起位
RCC->CIR = 0x009F0000;
#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 */
//配置系统时钟频率，HCLK,PCLK2，PCLK1预分频
/* Configure the Flash Latency cycles and enable prefetch buffer */
//配置Flash延时周期，使能预取缓存
SetSysClock();
#ifdef VECT_TAB_SRAM
SCB->VTOR = SRAM_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal SRAM. */
#else
//向量表放置于内部FLASH
SCB->VTOR = FLASH_BASE | VECT_TAB_OFFSET; /* Vector Table Relocation in Internal FLASH. */
#endif
}/**
* @brief Configures the System clock frequency, HCLK, PCLK2 and PCLK1 prescalers.
配置系统时钟频率，HCLK,PCLK2，PCLK1预分频
* @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
SetSysClockTo72();
#endif
/* If none of the define above is enabled, the HSI is used as System clock
source (default after reset) */ //如果上面的宏定义都没定义，HSI用作系统时钟 }#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
#endif/**
* @brief Sets System clock frequency to 72MHz and configure HCLK, PCLK2
* and PCLK1 prescalers.
设置系统时钟为72MHz，配置HCLK,PCLK2,PCLK1预分频
* @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 ---------------------------*/
//配置SYSCLK, HCLK, PCLK2 and PCLK1
/* Enable HSE */
//使能高速外部振荡器
RCC->CR |= ((uint32_t)RCC_CR_HSEON);
/* Wait till HSE is ready and if Time out is reached exit */
//等到HSE稳定，时间到则退出循环
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 */
//使能预取缓冲
FLASH->ACR |= FLASH_ACR_PRFTBE;
/* Flash 2 wait state */
//FLASH时序延迟2周期
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/2 */
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 */
//PLL配置 HSE时钟作为PLL输入时钟 PLL9倍频
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 */
//使能PLL
RCC->CR |= RCC_CR_PLLON;
/* Wait till PLL is ready */
while((RCC->CR & RCC_CR_PLLRDY) == 0)
{
}
/* Select PLL as system clock source */
//选择PLL作为系统时钟源
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 */
//等到PLL作为系统时钟源
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 */
//如果HSE启动失败，应用会产生错误时钟配置，在这里对错误情况进行处理
}
}初始化之后可以通过变量SystemCoreClock获取系统核心时钟频率(HCLK)。SystemCoreClock变量：包含核心时钟(HCLK),可以用于建立SysTick或配置其他参数
/*******************************************************************************
* 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) */
#endifz再看看该文件下的另一个函数SystemCoreClockUpdate()， 由于SystemCoreClock变量中存储的是核心频率HCLK，每次系统时钟改变时，都需要使用该函数对该变量进行更新。
/**
* @brief Update SystemCoreClock variable according to Clock Register Values.
* The SystemCoreClock variable contains the core clock (HCLK), it can
* be used by the user application to setup the SysTick timer or configure
* other parameters.
* 根据时钟寄存器值来更新SystemCoreClock变量，SystemCoreClock保存的是系统核心时钟(HCLK)
* @note Each time the core clock (HCLK) changes, this function must be called
* to update SystemCoreClock variable value. Otherwise, any configuration
* based on this variable will be incorrect.
* 当核心时钟变化时，必须调用该函数来更新SystemCoreClock变量。
* @note - The system frequency computed by this function is not the real
* frequency in the chip. It is calculated based on the predefined
* constant and the selected clock source:
* 该函数计算的系统频率是基于预定义的频率和选择的时钟源，不一定是芯片真正的频率
* - If SYSCLK source is HSI, SystemCoreClock will contain the HSI_VALUE(*)
*
* - If SYSCLK source is HSE, SystemCoreClock will contain the HSE_VALUE(**)
*
* - If SYSCLK source is PLL, SystemCoreClock will contain the HSE_VALUE(**)
* or HSI_VALUE(*) multiplied by the PLL factors.
*
* (*) HSI_VALUE is a constant defined in stm32f1xx.h file (default value
* 8 MHz) but the real value may vary depending on the variations
* in voltage and temperature.
*
* (**) HSE_VALUE is a constant defined in stm32f1xx.h file (default value
* 8 MHz or 25 MHz, depedning on the product used), user has to ensure
* that HSE_VALUE is same as the real frequency of the crystal used.
* Otherwise, this function may have wrong result.
*
* - The result of this function could be not correct when using fractional
* value for HSE crystal.
* @param None
* @retval None
*/
void SystemCoreClockUpdate (void)
{
uint32_t tmp = 0, pllmull = 0, pllsource = 0;
#ifdef STM32F10X_CL
uint32_t prediv1source = 0, prediv1factor = 0, prediv2factor = 0, pll2mull = 0;
#endif /* STM32F10X_CL */
#if defined (STM32F10X_LD_VL) || defined (STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)
uint32_t prediv1factor = 0;
#endif /* STM32F10X_LD_VL or STM32F10X_MD_VL or STM32F10X_HD_VL */
/* Get SYSCLK source -------------------------------------------------------*/
//得到SYSCLK所选时钟源
tmp = RCC->CFGR & RCC_CFGR_SWS;
switch (tmp)
{
case 0x00: /* HSI used as system clock */
SystemCoreClock = HSI_VALUE;
break;
case 0x04: /* HSE used as system clock */
SystemCoreClock = HSE_VALUE;
break;
case 0x08: /* PLL used as system clock */
/* Get PLL clock source and multiplication factor ----------------------*/
//得到PLL时钟源和倍频系数
pllmull = RCC->CFGR & RCC_CFGR_PLLMULL;
pllsource = RCC->CFGR & RCC_CFGR_PLLSRC;
#ifndef STM32F10X_CL
pllmull = ( pllmull >> 18) + 2;
if (pllsource == 0x00)
{
/* HSI oscillator clock divided by 2 selected as PLL clock entry */
//HSI 2分频后作为PLL时钟源
SystemCoreClock = (HSI_VALUE >> 1) * pllmull;
}
else
{
#if defined (STM32F10X_LD_VL) || defined (STM32F10X_MD_VL) || (defined STM32F10X_HD_VL)
prediv1factor = (RCC->CFGR2 & RCC_CFGR2_PREDIV1) + 1;
/* HSE oscillator clock selected as PREDIV1 clock entry */
SystemCoreClock = (HSE_VALUE / prediv1factor) * pllmull;
#else
/* HSE selected as PLL clock entry */
if ((RCC->CFGR & RCC_CFGR_PLLXTPRE) != (uint32_t)RESET)
{/* HSE oscillator clock divided by 2 */
//HSE时钟2分频
SystemCoreClock = (HSE_VALUE >> 1) * pllmull;
}
else
{
//HSE不分频
SystemCoreClock = HSE_VALUE * pllmull;
}
#endif
}
#else
pllmull = pllmull >> 18;
if (pllmull != 0x0D)
{
pllmull += 2;
}
else
{ /* PLL multiplication factor = PLL input clock * 6.5 */
pllmull = 13 / 2;
}
if (pllsource == 0x00)
{
/* HSI oscillator clock divided by 2 selected as PLL clock entry */
SystemCoreClock = (HSI_VALUE >> 1) * pllmull;
}
else
{/* PREDIV1 selected as PLL clock entry */
/* Get PREDIV1 clock source and division factor */
prediv1source = RCC->CFGR2 & RCC_CFGR2_PREDIV1SRC;
prediv1factor = (RCC->CFGR2 & RCC_CFGR2_PREDIV1) + 1;
if (prediv1source == 0)
{
/* HSE oscillator clock selected as PREDIV1 clock entry */
SystemCoreClock = (HSE_VALUE / prediv1factor) * pllmull;
}
else
{/* PLL2 clock selected as PREDIV1 clock entry */
/* Get PREDIV2 division factor and PLL2 multiplication factor */
prediv2factor = ((RCC->CFGR2 & RCC_CFGR2_PREDIV2) >> 4) + 1;
pll2mull = ((RCC->CFGR2 & RCC_CFGR2_PLL2MUL) >> 8 ) + 2;
SystemCoreClock = (((HSE_VALUE / prediv2factor) * pll2mull) / prediv1factor) * pllmull;
}
}
#endif /* STM32F10X_CL */
break;
default:
SystemCoreClock = HSI_VALUE;
break;
}
/* Compute HCLK clock frequency ----------------*/
//计算
/* Get HCLK prescaler */
tmp = AHBPrescTable[((RCC->CFGR & RCC_CFGR_HPRE) >> 4)];
/* HCLK clock frequency */
SystemCoreClock >>= tmp;
}