从自己的板子STM32F407入手,参考官方的SystemInit()函数:
核心在SetSysClock()这个函数,官方默认是采用HSE(设定为8MHz)作为PLL锁相环的输入输出168MHz的SYSCLK;
/** * @brief Setup the microcontroller system * Initialize the Embedded Flash Interface, the PLL and update the * SystemFrequency variable. * @param None * @retval None */ void SystemInit(void) { /* Reset the RCC clock configuration to the default reset state ------------*/ /* Set HSION bit */ RCC->CR |= (uint32_t)0x00000001; /* Reset CFGR register */ RCC->CFGR = 0x00000000; /* Reset HSEON, CSSON and PLLON bits */ RCC->CR &= (uint32_t)0xFEF6FFFF; /* Reset PLLCFGR register */ RCC->PLLCFGR = 0x24003010; /* Reset HSEBYP bit */ RCC->CR &= (uint32_t)0xFFFBFFFF; /* Disable all interrupts */ RCC->CIR = 0x00000000; /* Configure the System clock source, PLL Multiplier and Divider factors, AHB/APBx prescalers and Flash settings ----------------------------------*/ SetSysClock(); }
这里大致分析一下官方默认的SetSysClock()配置:
由于我个人采用的是STM32F407型号的芯片,因此精简一下函数;
总体思路的话:
使能HSE;
等待HSE初始化完毕,进行下一步设置;
设置HCLK、PCLK1、PCLK2的分频系数;
配置PLL,使能PLL,等待PLL初始化完毕;
选择PLL作为SYSCLK,等待SYSCLK时钟设置完毕;
/** * @brief Configures the System clock source, PLL Multiplier and Divider factors, * AHB/APBx prescalers and Flash settings * @Note This function should be called only once the RCC clock configuration * is reset to the default reset state (done in SystemInit() function). * @param None * @retval None */ static void SetSysClock(void) { /******************************************************************************/ /* PLL (clocked by HSE) used as System clock source */ /******************************************************************************/ __IO uint32_t StartUpCounter = 0, HSEStatus = 0; /* Enable HSE */ RCC->CR |= ((uint32_t)RCC_CR_HSEON); /* Wait till HSE is ready and if Time out is reached exit */ 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) { /* Select regulator voltage output Scale 1 mode */ RCC->APB1ENR |= RCC_APB1ENR_PWREN; PWR->CR |= PWR_CR_VOS; /* HCLK = SYSCLK / 1*/ RCC->CFGR |= RCC_CFGR_HPRE_DIV1; /* PCLK2 = HCLK / 2*/ RCC->CFGR |= RCC_CFGR_PPRE2_DIV2; /* PCLK1 = HCLK / 4*/ RCC->CFGR |= RCC_CFGR_PPRE1_DIV4; /* PCLK2 = HCLK / 1*/ RCC->CFGR |= RCC_CFGR_PPRE2_DIV1; /* PCLK1 = HCLK / 2*/ RCC->CFGR |= RCC_CFGR_PPRE1_DIV2; /* Configure the main PLL */ RCC->PLLCFGR = PLL_M | (PLL_N << 6) | (((PLL_P >> 1) -1) << 16) | (RCC_PLLCFGR_PLLSRC_HSE) | (PLL_Q << 24); /* Enable the main PLL */ RCC->CR |= RCC_CR_PLLON; /* Wait till the main PLL is ready */ while((RCC->CR & RCC_CR_PLLRDY) == 0) { } /* Configure Flash prefetch, Instruction cache, Data cache and wait state */ FLASH->ACR = FLASH_ACR_PRFTEN | FLASH_ACR_ICEN |FLASH_ACR_DCEN |FLASH_ACR_LATENCY_5WS; /* Configure Flash prefetch, Instruction cache, Data cache and wait state */ FLASH->ACR = FLASH_ACR_PRFTEN | FLASH_ACR_ICEN |FLASH_ACR_DCEN |FLASH_ACR_LATENCY_2WS; /* Select the main PLL as system clock source */ RCC->CFGR &= (uint32_t)((uint32_t)~(RCC_CFGR_SW)); RCC->CFGR |= RCC_CFGR_SW_PLL; /* Wait till the main PLL is used as system clock source */ while ((RCC->CFGR & (uint32_t)RCC_CFGR_SWS ) != RCC_CFGR_SWS_PLL); { } } 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 */ } }
在官方的基础上,直接设定HSE作为SYSCLK时钟:
初始化HSE;
等待HSE初始化成功后再继续;
设置调压器电压输出级别为1以便使器件在最大频率工作;
设置HCLK、PCLK1、PCLK2分频系数;
设置HSE作为系统时钟;
void HSE_SetSysClock(void) { __IO uint32_t HSEStartUpStatus = 0; /* 开启HSE时钟 */ /* 此函数从stm32f0xx_rcc.c获取,用于配置外部时钟HSE: * 有三个配置:RCC_HSE_OFF关闭外部HSE时钟 * RCC_HSE_ON开始外部HSE晶振 * RCC_HSE_Bypass开始HSE旁路设置 */ RCC_HSEConfig(RCC_HSE_ON); /* 等待HSE开启设置成功,并获取到时钟配置状态 */ HSEStartUpStatus = RCC_WaitForHSEStartUp(); /* 若时钟配置成功 */ if(HSEStartUpStatus == SUCCESS) { RCC->APB1ENR |= RCC_APB1ENR_PWREN; PWR->CR |= PWR_CR_VOS; RCC_HCLKConfig(RCC_SYSCLK_Div1); RCC_PCLK2Config(RCC_HCLK_Div1); RCC_PCLK1Config(RCC_HCLK_Div1); /* 将SYSCLK系统时钟设置为HSE */ RCC_SYSCLKConfig(RCC_SYSCLKSource_HSE); /* 等待SYSCLK系统时钟设置成功 */ while(RCC_GetSYSCLKSource() != 0x04) { } }else { while(1); } }
使用HSI经过PLL配置系统时钟:
使能HSI时钟;
获取HSI状态并等待HSI稳定;
设置调节器电压输出级别配置为1;
设置HCLK、PCLK1/2分频系数;
设置PLL时钟分频系数;
使能PLL并等待PLL稳定后配置PLL状态;
设置PLL作为SYSCLK时钟并等待设置完成;
void HSI_SetSysClock(uint32_t m, uint32_t n, uint32_t p, uint32_t q) { __IO uint32_t HSIStartUpStatus = 0; /* 去初始化RCC */ RCC_DeInit(); /* 使能HSI时钟 */ RCC_HSICmd(ENABLE); /* 从RCC的CR寄存器中获取HSI配置状态 */ HSIStartUpStatus = RCC->CR & RCC_CR_HSIRDY; /* 若HSI配置成功 */ if(HSIStartUpStatus == RCC_CR_HSIRDY) { /* 配置调节器电压输出级别为1 */ RCC->APB1ENR |= RCC_APB1ENR_PWREN; PWR->CR |= PWR_CR_VOS; /* 配置SYSCLK到HCLK的分频系数为1 */ RCC_HCLKConfig(RCC_SYSCLK_Div1); /* 配置HCLK到PCLK1/2的分频系数为2/4 */ RCC_PCLK2Config(RCC_HCLK_Div2); RCC_PCLK1Config(RCC_HCLK_Div4); /* 配置PLL参数,选用HSI作为PLL参数,同时使能PLL */ RCC_PLLConfig(RCC_PLLSource_HSI, m, n, p, q); RCC_PLLCmd(ENABLE); /* 等待PLL设置完成 */ while(RCC_GetFlagStatus(RCC_FLAG_PLLRDY) == RESET); FLASH->ACR = FLASH_ACR_PRFTEN | FLASH_ACR_ICEN | FLASH_ACR_DCEN | FLASH_ACR_LATENCY_5WS; RCC_SYSCLKConfig(RCC_SYSCLKSource_PLLCLK); while(RCC_GetSYSCLKSource() != 0x08); }else { while(1); } }
HAL时钟配置分析:
与STM32标准外设库不同,HAL库来实现时钟配置需要重新适应配置方式,但是本质的寄存器调动是类似不变的,且配置的过程也和STM32标准外设库相似;
参考使用STMCube生成的代码,时钟树如图所示:
在STM32Cube中设置:
HSE设置为Crystal/Ceramic Resonator,Input Frequency设置为16MHz;
在工程中要配置的参数:
第一个HSE_VALUE参数位于stm32f0xx_hal_conf.h中,此参数与在STMCube时钟树上定义的一致,需要手动设置为实际的参数值;
第二个HSE_VALUE参数位于system_stm32f0xx.c中,此参数默认为8MHz,可以通过用户程序来提供和调整;
第三个SystemCoreClock参数位于system_stm32f0xx.c中,其默认值也是8MHz,可以根据以下三种方式来更新:
调用CMSIS函数SystemCoreClockUpdate()、
调用HAL API函数HAL_RCC_GetHCLKFreq()、
调用HAL_RCC_ClockConfig();
/** * @brief Adjust the value of External High Speed oscillator (HSE) used in your application. * This value is used by the RCC HAL module to compute the system frequency * (when HSE is used as system clock source, directly or through the PLL). */ #if !defined (HSE_VALUE) #define HSE_VALUE ((uint32_t)16000000) /*!< Value of the External oscillator in Hz */ #endif /* HSE_VALUE */ #if !defined (HSE_VALUE) #define HSE_VALUE ((uint32_t)8000000) /*!< Default value of the External oscillator in Hz. This value can be provided and adapted by the user application. */ #endif /* HSE_VALUE */ /** @addtogroup STM32F0xx_System_Private_Variables * @{ */ /* This variable is updated in three ways: 1) by calling CMSIS function SystemCoreClockUpdate() 2) by calling HAL API function HAL_RCC_GetHCLKFreq() 3) each time HAL_RCC_ClockConfig() is called to configure the system clock frequency Note: If you use this function to configure the system clock there is no need to call the 2 first functions listed above, since SystemCoreClock variable is updated automatically. */ uint32_t SystemCoreClock = 8000000;
实际的时钟配置函数如下图:
使用了三个参数来配置:
RCC_OscInitStruct用来配置外部时钟参数,这里设置晶振类型为HSE、设置HSE的状态为开启状态、不使用PLL;
RCC_ClkInitStruct用来配置系统时钟内的参数(如Sys CLK、HCLK、PCLK1),这里设置要配置的时钟类型为HCLK、SYSCLK、PCLK1,选择HSE时钟作为SYSCLK的时钟源,并设置系统时钟SYSCLK分频系数为0、HCLK的分频系数为4;
PeriphClkInit用来配置外设时钟的时钟源,这里设置USART1/2的时钟源为PCLK1;
/** * @brief System Clock Configuration * @retval None */ void SystemClock_Config(void) { RCC_OscInitTypeDef RCC_OscInitStruct = {0}; RCC_ClkInitTypeDef RCC_ClkInitStruct = {0}; RCC_PeriphCLKInitTypeDef PeriphClkInit = {0}; /** Initializes the CPU, AHB and APB busses clocks */ RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE; RCC_OscInitStruct.HSEState = RCC_HSE_ON; RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE; if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) { Error_Handler(); } /** Initializes the CPU, AHB and APB busses clocks */ RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK |RCC_CLOCKTYPE_PCLK1; RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSE; RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1; RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV4; if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0) != HAL_OK) { Error_Handler(); } PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_USART1|RCC_PERIPHCLK_USART2; PeriphClkInit.Usart1ClockSelection = RCC_USART1CLKSOURCE_PCLK1; PeriphClkInit.Usart2ClockSelection = RCC_USART2CLKSOURCE_PCLK1; if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit) != HAL_OK) { Error_Handler(); } HAL_SYSTICK_Config(SystemCoreClock / 1000); HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK); HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0); }
这里加入了对SysTick的时钟配置,参考HAL库本身的设置:
HAL_SYSTICK_Config()用来配置使能和配置SysTick寄存器;
HAL_SYSTICK_CLKSourceConfig()选择AHB时钟(或AHB时钟除以8)作为SysTick时钟源;
HAL_NVIC_SetPriority()配置SysTick_IRQn的中断优先级为0,默认为最高;