基于msp430g2553的频率计， 测频波形：尽可能使50 占空比的矩形波 测频幅度范围：尽可能取 0~3.6V-Based frequency meter of msp430g2553, measured frequency waveform: as much as possible so that the 50 duty cycle of the square wave frequency measurement amplitude range: as much as possi