定时器和外部中断0实现脉宽测量，很好的中断和定时器例子，T0 对/INT0输入信号脉宽测量，能实现很精确的测量功能(The timer and external interruption 0 realize pulse width measurement, a good example of interruption and timer. T0 can measure the pulse width of the /INT0 input signal and can achieve a very accurate measurement function.)