1. 系统滴答时钟

1.1 简介

SysTick：系统滴答时钟，属于Cortex-M4内核中的一个外设，24bit向下递减计数。
Systick定时器常用来做延时，或者实时系统的心跳时钟。这样可以节省MCU资源，不用浪费一个定时器。比如UCOS中，分时复用，需要一个最小的时间戳，一般在STM32+UCOS系统中，都采用Systick做UCOS心跳时钟

1.2 配置

用STM32CubeMX配置时钟树时，可以看到Cortex system timer就是滴答时钟

1.3 启动和更新

在STM32CubeMX生成的工程中，滴答时钟启动过程如下：

1. HAL_Init() 初始化时:SystemCoreClock =16000000,
   • HAL_InitTick（）
     • HAL_SYSTICK_Config(SystemCoreClock / (1000U / uwTickFreq))
       • SysTick_Config()

---

SystemCoreClock 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

//main()函数启动
/* Reset of all peripherals, Initializes the Flash interface and the Systick. */HAL_Init();//stm32f4xx_hal.c
HAL_StatusTypeDef HAL_Init(void)
{/* Configure Flash prefetch, Instruction cache, Data cache */ 
#if (INSTRUCTION_CACHE_ENABLE != 0U)__HAL_FLASH_INSTRUCTION_CACHE_ENABLE();
#endif /* INSTRUCTION_CACHE_ENABLE */#if (DATA_CACHE_ENABLE != 0U)__HAL_FLASH_DATA_CACHE_ENABLE();
#endif /* DATA_CACHE_ENABLE */#if (PREFETCH_ENABLE != 0U)__HAL_FLASH_PREFETCH_BUFFER_ENABLE();
#endif /* PREFETCH_ENABLE *//* Set Interrupt Group Priority */HAL_NVIC_SetPriorityGrouping(NVIC_PRIORITYGROUP_4);/* Use systick as time base source and configure 1ms tick (default clock after Reset is HSI) */HAL_InitTick(TICK_INT_PRIORITY);/* Init the low level hardware */HAL_MspInit();/* Return function status */return HAL_OK;
}//stm32f4xx_hal.c
__weak HAL_StatusTypeDef HAL_InitTick(uint32_t TickPriority)
{/* Configure the SysTick to have interrupt in 1ms time basis*/if (HAL_SYSTICK_Config(SystemCoreClock / (1000U / uwTickFreq)) > 0U){return HAL_ERROR;}/* Configure the SysTick IRQ priority */if (TickPriority < (1UL << __NVIC_PRIO_BITS)){HAL_NVIC_SetPriority(SysTick_IRQn, TickPriority, 0U);uwTickPrio = TickPriority;}else{return HAL_ERROR;}/* Return function status */return HAL_OK;
}// stm32f4xx_hal_cortex.c
uint32_t HAL_SYSTICK_Config(uint32_t TicksNumb)
{return SysTick_Config(TicksNumb);
}// core_cm4.h
__STATIC_INLINE uint32_t SysTick_Config(uint32_t ticks)
{if ((ticks - 1UL) > SysTick_LOAD_RELOAD_Msk){return (1UL);                                                   /* Reload value impossible */}SysTick->LOAD  = (uint32_t)(ticks - 1UL);                         /* set reload register */NVIC_SetPriority (SysTick_IRQn, (1UL << __NVIC_PRIO_BITS) - 1UL); /* set Priority for Systick Interrupt */SysTick->VAL   = 0UL;                                             /* Load the SysTick Counter Value */SysTick->CTRL  = SysTick_CTRL_CLKSOURCE_Msk |SysTick_CTRL_TICKINT_Msk   |SysTick_CTRL_ENABLE_Msk;                         /* Enable SysTick IRQ and SysTick Timer */return (0UL);                                                     /* Function successful */
}

2. SystemClock_Config() 时，调用HAL_RCC_ClockConfig()对系统SystemCoreClock更新

• SystemClock_Config();
  • SystemCoreClock = HAL_RCC_GetSysClockFreq() >> AHBPrescTable[(RCC->CFGR & RCC_CFGR_HPRE)>> RCC_CFGR_HPRE_Pos];
  • HAL_InitTick (uwTickPrio);

// main()
/* Configure the system clock */SystemClock_Config();void SystemClock_Config(void)
{RCC_OscInitTypeDef RCC_OscInitStruct = {0};RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};/** Configure the main internal regulator output voltage*/__HAL_RCC_PWR_CLK_ENABLE();__HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE1);/** Initializes the RCC Oscillators according to the specified parameters* in the RCC_OscInitTypeDef structure.*/RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;RCC_OscInitStruct.HSEState = RCC_HSE_ON;RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;RCC_OscInitStruct.PLL.PLLM = 8;RCC_OscInitStruct.PLL.PLLN = 336;RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;RCC_OscInitStruct.PLL.PLLQ = 4;if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK){Error_Handler();}/** Initializes the CPU, AHB and APB buses clocks*/RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK|RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV4;RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV2;if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_5) != HAL_OK){Error_Handler();}
}// stm32f4xx_hal_rcc.c
HAL_RCC_ClockConfig()
{
HAL_InitTick (uwTickPrio);
}

3. 在中断中更新时钟计数

// stm32f4xx_it.c
void SysTick_Handler(void)
{/* USER CODE BEGIN SysTick_IRQn 0 *//* USER CODE END SysTick_IRQn 0 */HAL_IncTick();/* USER CODE BEGIN SysTick_IRQn 1 *//* USER CODE END SysTick_IRQn 1 */
}// stm32f4xx_hal.c
__weak void HAL_IncTick(void)
{uwTick += uwTickFreq;
}

参考：

1. STM32入门：Systick（嘀嗒定时器）学习
2. stm32f4xx-systick系统滴答时钟