外加时钟频率为2MHz时，通过8253产生方波，并使发光二极管按照亮1s，灭1s的间隔闪动。画出电路图、按要求编写程序，在仿真环境下调试通过。-plus the clock frequency is 2MHz, produces a square wave by 8253, and light-emitting diode in accordance with 1s, flash off intervals of 1s. Draw the circuit, according to the re