说明：  51系列单片机无PWM输出功能，可以采用定时器配合软件的方法输出。对精度要求不高的场合，非常实用。电路图见图一，采用了高速光隔（6N137）输出，并将PWM的信号倒相。
(51 series no PWM output, the timer can be used with the software, the method output. On occasions, less precision, very practical. Circuit shown in Figure 1, using high-speed optical compartment (6N137) output, and inverted phase PWM signal.)