Systick简介

1. Systick是CM4内核的系统定时器，是一个24位的向下递减计数器，理论上可以计数$2^{24}$个值，当重装载寄存器的值减为0时就产生一次中断。
2. STM32CUBEMX生成的工程，会自动把Systick配置成1ms中断的定时器，并在中断函数里，在中断处理函数中将变量uwTick每1ms增加1。这样我们就可以知道芯片的走时情况，知道芯片到目前为止上电多长时间。

Systick是向下递减的，而51单片机是向上计数，每来一个时间脉冲就计数加一，而Systick是每来一个就向下计数加一。理论上可以计2^24个值，减为0的时候产生中断，恢复成某个设定的的数，然后重新向下递减。

Systick的作用：

• 精确延时：HAL_Delay函数，用于延时x毫秒
• 给系统提供一个单独的时钟节拍（时间基准）（相当于是芯片的一个心跳，上电之后就会一直工作）

下面结合程序看一下Systick初始化的流程：
看一下HAL_Init();函数的实现：

HAL_StatusTypeDef HAL_Init(void)
{
  HAL_StatusTypeDef  status = HAL_OK;
  /* Configure Flash prefetch, Instruction cache, Data cache */
  /* Default configuration at reset is:                      */
  /* - Prefetch disabled                                     */
  /* - Instruction cache enabled                             */
  /* - Data cache enabled                                    */
#if (INSTRUCTION_CACHE_ENABLE == 0U)
  __HAL_FLASH_INSTRUCTION_CACHE_DISABLE();
#endif /* INSTRUCTION_CACHE_ENABLE */

#if (DATA_CACHE_ENABLE == 0U)
  __HAL_FLASH_DATA_CACHE_DISABLE();
#endif /* DATA_CACHE_ENABLE */

#if (PREFETCH_ENABLE != 0U)
  __HAL_FLASH_PREFETCH_BUFFER_ENABLE();
#endif /* PREFETCH_ENABLE */
//上面这些宏定义和if，endif都是一些初始化的东西，不用管
  /* Set Interrupt Group Priority */
  HAL_NVIC_SetPriorityGrouping(NVIC_PRIORITYGROUP_4);
//上面这句是配置中断优先级的组
  /* Use SysTick as time base source and configure 1ms tick (default clock after Reset is HSI) */
  if (HAL_InitTick(TICK_INT_PRIORITY) != HAL_OK)
  {
    status = HAL_ERROR;
  }
  else
  {
    /* Init the low level hardware */
    HAL_MspInit();
  }

  /* Return function status */
  return status;
//上面这段就是将Systick配置成一个1ms的时间基准，并且将上电默认的时钟设置为HSI。
}

点开HAL_InitTick（）这个函数可以看到：

__weak HAL_StatusTypeDef HAL_InitTick(uint32_t TickPriority)
{
  HAL_StatusTypeDef  status = HAL_OK;

  if (uwTickFreq != 0U)
  {
    /* Configure the SysTick to have interrupt in 1ms time basis*/
    //这个函数最主要的部分就是下面这句，设置了一个溢出的标志位
    if (HAL_SYSTICK_Config(SystemCoreClock / (1000U / uwTickFreq)) == 0U)
    {
      /* Configure the SysTick IRQ priority */
      if (TickPriority < (1UL << __NVIC_PRIO_BITS))
      {
        HAL_NVIC_SetPriority(SysTick_IRQn, TickPriority, 0U);
        uwTickPrio = TickPriority;
      }
      else
      {
        status = HAL_ERROR;
      }
    }
    else
    {
      status = HAL_ERROR;
    }
  }
  else
  {
    status = HAL_ERROR;
  }

  /* Return function status */
  return status;
}

再打开HAL_SYSTICK_Config（）这个函数中的SysTick_Config（）函数：

__STATIC_INLINE uint32_t SysTick_Config(uint32_t ticks)
{//可以看到就是很重要的一步：配置了重装载值
  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 */
}

最后就是要明白一点：对于Systick来说，一定要知道的就是uwTick这个变量的意义：就是每过1ms他就会自加1。
uwTick这个变量在程序中是定义为__IO uint32_t uwTick;,是一个32位的无符号数，所以他一共可以计$2^{32}$这么多毫秒，差不多是49天左右，所以不用担心会有溢出的问题，即使溢出了，他也会自动归零。

Systick程序设计

HAL_Delay延时会阻塞程序，使其他程序实时性下降。
除了HAl_Delay延时函数之外，可以将Systick用于程序调度。
我们以LED灯的程序为例写出如下调度代码：

//由于uwTick的类型是32位无符号整型，所以我们自定义的ledTick也定义为这样的类型：
__IO uint32_t ledTick =0;
//下面这个变量的定义是上一篇文章中提到的对led灯哪个亮哪个不亮的控制，初始化为0xff就是使所有灯都亮
u8 led_ctrl=0xff;
void LED_Process(void)
{
//由于uwTick在上电之后就会每1ms加一，而ledTick一开始初始化为0，所以uwTick-ledTick的值就是uwTick本身，当uwTick大于500，即程序上电500毫秒之后，执行后面的代码，否则就return,也就是退出这个函数
	if(uwTick-ledTick<500)return;
//	把uwTick的值赋给ledTick，为了下次进入这个函数还是500毫秒之后执行该程序（因为uwTick的值是一直递增的，所以想要控制uwTick与ledTick的差值始终为500就需要每进一次这个函数更新一次ledTick）
	ledTick=uwTick;
	//控制led灯的亮灭
	LED_Control(led_ctrl);
	//每进一次这个函数就把led_ctrl取反，相当于每500毫秒，灯整体亮或者灭
	led_ctrl=~led_ctrl;
}

最终的主函数如下：

/* USER CODE BEGIN Header */
/**
  ******************************************************************************
  * @file           : main.c
  * @brief          : Main program body
  ******************************************************************************
  * @attention
  *
  * <h2><center>&copy; Copyright (c) 2021 STMicroelectronics.
  * All rights reserved.</center></h2>
  *
  * This software component is licensed by ST under BSD 3-Clause license,
  * the "License"; You may not use this file except in compliance with the
  * License. You may obtain a copy of the License at:
  *                        opensource.org/licenses/BSD-3-Clause
  *
  ******************************************************************************
  */
/* USER CODE END Header */

/* Includes ------------------------------------------------------------------*/
#include "main.h"
#include "gpio.h"
#include "led.h"
/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */

/* USER CODE END Includes */

/* Private typedef -----------------------------------------------------------*/
/* USER CODE BEGIN PTD */

/* USER CODE END PTD */

/* Private define ------------------------------------------------------------*/
/* USER CODE BEGIN PD */
/* USER CODE END PD */

/* Private macro -------------------------------------------------------------*/
/* USER CODE BEGIN PM */

/* USER CODE END PM */

/* Private variables ---------------------------------------------------------*/

/* USER CODE BEGIN PV */

/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */
//Led执行程序
__IO uint32_t ledTick =0;
u8 led_ctrl=0xff;
void LED_Process(void)
{
	if(uwTick-ledTick<500)return;
	ledTick=uwTick;
	LED_Control(led_ctrl);
	led_ctrl=~led_ctrl;
}
/* USER CODE END 0 */

/**
  * @brief  The application entry point.
  * @retval int
  */
int main(void)
{
  /* USER CODE BEGIN 1 */

  /* USER CODE END 1 */

  /* MCU Configuration--------------------------------------------------------*/

  /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
  HAL_Init();

  /* USER CODE BEGIN Init */

  /* USER CODE END Init */

  /* Configure the system clock */
  SystemClock_Config();

  /* USER CODE BEGIN SysInit */

  /* USER CODE END SysInit */

  /* Initialize all configured peripherals */
  MX_GPIO_Init();
  /* USER CODE BEGIN 2 */
	
	LCD_Init();
  /* USER CODE END 2 */

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
	
	LCD_Clear(Blue);
	LCD_SetBackColor(Blue);
	LCD_SetTextColor(White);
	
	LCD_DisplayStringLine(Line0, (uint8_t *)"                    ");
	LCD_DisplayStringLine(Line1, (uint8_t *)"                    ");
	LCD_DisplayStringLine(Line2, (uint8_t *)"      LCD Test      ");
	LCD_DisplayStringLine(Line3, (uint8_t *)"                    ");
	LCD_DisplayStringLine(Line4, (uint8_t *)"                    ");
	
	LCD_SetBackColor(White);
	LCD_SetTextColor(Blue);

	LCD_DisplayStringLine(Line5, (uint8_t *)"                    ");
	LCD_DisplayStringLine(Line6, (uint8_t *)"       HAL LIB      ");
	LCD_DisplayStringLine(Line7, (uint8_t *)"                    ");
	LCD_DisplayStringLine(Line8, (uint8_t *)"         @80        ");
	LCD_DisplayStringLine(Line9, (uint8_t *)"                    ");
	
  while (1)
  {
    /* USER CODE END WHILE */

    /* USER CODE BEGIN 3 */
    LED_Process();
  }
  /* USER CODE END 3 */
}

/**
  * @brief System Clock Configuration
  * @retval None
  */
void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};

  /** Configure the main internal regulator output voltage
  */
  HAL_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1);
  /** Initializes the CPU, AHB and APB busses clocks
  */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState = RCC_HSI_ON;
  RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
  RCC_OscInitStruct.PLL.PLLM = RCC_PLLM_DIV2;
  RCC_OscInitStruct.PLL.PLLN = 20;
  RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
  RCC_OscInitStruct.PLL.PLLQ = RCC_PLLQ_DIV2;
  RCC_OscInitStruct.PLL.PLLR = RCC_PLLR_DIV2;
  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_CLOCKTYPE_PCLK2;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_3) != HAL_OK)
  {
    Error_Handler();
  }
}

/* USER CODE BEGIN 4 */

/* USER CODE END 4 */

/**
  * @brief  This function is executed in case of error occurrence.
  * @retval None
  */
void Error_Handler(void)
{
  /* USER CODE BEGIN Error_Handler_Debug */
  /* User can add his own implementation to report the HAL error return state */

  /* USER CODE END Error_Handler_Debug */
}

#ifdef  USE_FULL_ASSERT
/**
  * @brief  Reports the name of the source file and the source line number
  *         where the assert_param error has occurred.
  * @param  file: pointer to the source file name
  * @param  line: assert_param error line source number
  * @retval None
  */
void assert_failed(uint8_t *file, uint32_t line)
{
  /* USER CODE BEGIN 6 */
  /* User can add his own implementation to report the file name and line number,
     tex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
  /* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */

/************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/

就可以实现500毫秒的间隔闪烁。这样做的好处就是不堵塞程序，而不是在while里面用delay。