使用msp430的定时器生成PWM波通过硬件滤波器实现DAC功能，一阶滤波为直流电平，二阶可以生成任意波形，如正弦波，锯齿波等-MSP430 timer used to generate PWM wave through the hardware filter DAC functions, first-order filter for the DC-ping, the second order can generate arbitrary waveform, such as sine wave,