说明：  msp430g2553用定时器去捕捉波形的频率， 采用上升沿捕捉方法 同时可以测量方波 正弦波 精确到0.3 。
(msp430g2553 use a timer to capture the frequency of the waveform, the rising edge of the capture method can measure the square wave sine wave is accurate to 0.3 .)