实现8051上的方波输出，使用定时方式，通过改变P1.1口的高低电平进行输出，文件中包括proteus仿真和keil/汇编语言。-8051 on the realization of the square wave output, use the regular way, by changing the P1.1 port of the high-low to output file, including proteus simulation and keil/assembly language