STM32 定时器更新中断最小周期是多少？不是1us

TIM_HandleTypeDef htim14;

/* TIM14 init function */
void MX_TIM14_Init(void)
{

  /* USER CODE BEGIN TIM14_Init 0 */

  /* USER CODE END TIM14_Init 0 */

  /* USER CODE BEGIN TIM14_Init 1 */

  /* USER CODE END TIM14_Init 1 */
  htim14.Instance = TIM14;
  htim14.Init.Prescaler = 42-1;
  htim14.Init.CounterMode = TIM_COUNTERMODE_UP;
  htim14.Init.Period = 4-1;
  htim14.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;
  htim14.Init.AutoReloadPreload = TIM_AUTORELOAD_PRELOAD_DISABLE;
  if (HAL_TIM_Base_Init(&htim14) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN TIM14_Init 2 */

  /* USER CODE END TIM14_Init 2 */
void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim)
{
	if(htim->Instance == TIM14)
	{
		HAL_GPIO_TogglePin(GPIOC,GPIO_PIN_13);

	}
}

}

以STM32F405RGT6为例，使用外部有源晶振。
中断里只做翻转IO操作。
TIM14的时钟是84MHz，故
更新周期为：time = 4*(42/84000000)*1000000 = 2us

逻辑分析仪采集检测正确。但当
htim14.Init.Prescaler = 21-1;
htim14.Init.CounterMode = TIM_COUNTERMODE_UP;
htim14.Init.Period = 4-1;
更新周期理论为：time = 4*(21/84000000)*1000000 = 1us
实际采集并不是1us。
所以想通过更新中断输出高精度PWM波形的家人们还是选择硬件Timer输出PWM吧