1KHZ方波从P1.0输出0.2秒，接着0.2秒从P1.0输出电平信号，如此循环下去，就形成我们所需的报警声了。-1KHZ square wave output from P1.0 0.2 seconds, followed by 0.2 seconds from the P1.0 output level signal, so the cycle continues, we need to form the sound of the alarm.