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。