利用stm32f407作为测试板，作用为利用TIM3定时器和中断服务来进行精确的延时，进过测试可以精确到3us，如果延时时间小于3us会造成比较大的误差。-Stm32f407 use as a test board, the role for the use of TIM3 timer and interrupt service to perform accurate delay, been to test can be accurate to 3us, if the delay time is less than 3us will cause relatively large errors.