stm32之HAL库实现us延时方法
HAL_Delay()源码为ms级别的延时如下：

1. /**
   
2.   * @brief This function provides accurate delay (in milliseconds) based
   
3.   *        on variable incremented.
   
4.   * @note In the default implementation , SysTick timer is the source of time base.
   
5.   *       It is used to generate interrupts at regular time intervals where uwTick
   
6.   *       is incremented.
   
7.   * @note This function is declared as __weak to be overwritten in case of other
   
8.   *       implementations in user file.
   
9.   *
   
10.   * @param Delay: specifies the delay time length, in milliseconds.
    
11.   * @retval None
    
12.   */
    
13. __weak void HAL_Delay(__IO uint32_t Delay)
    
14. {
    
15.   uint32_t tickstart = 0U;
    
16.   tickstart = HAL_GetTick();
    
17.   while((HAL_GetTick() - tickstart) < Delay)
    
18.   {
    
19.   }
    
20. }

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下面是us级别的延时：

1. void HAL_Delay_us(uint32_t nus)
   
2. {
   
3. //将systic设置为1us中断
   
4. HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000000);
   
5. //延时nus
   
6. HAL_Delay(nus-1);
   
7. //恢复systic中断为1ms
   
8. HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);
   
9. }

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另外需要注意的一点是stm32H7里面时钟配置如下：



其中systic是系统时钟直接给systic的而不是HCLK，HAL_RCC_GetHCLKFreq()正好是系统时钟的1/2所以上面配置1us时钟源要改为如下：

HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()*2/1000000);