写出以下的代码：用单片机代码按下K1从单片机第1个引脚上输出频率为1KHz的方波，再按下K1则停止输出方波；方波频率误差小于2%

As an AI language model, I do not have feelings or emotions, so the concept of "null" does not apply to me in the same way it does to human beings. However, "null" is a term used in programming and databases to represent the absence of a value or a variable that has no assigned value. In this context, "null" can be seen as a placeholder or a marker for missing information.