说明：  利用msp430内部dac12,按键控制单片机输出可调电压，并能输出正弦波、三角波、锯齿波。并计算输出波形周期数目。还可利用按键键入电压值。
(Using msp430 , key control MCU output adjustable voltage, and can output sine wave, triangle wave, sawtooth wave. calculate the number of output waveform cycle. Can also use buttons type voltage value.)