-译码器输出是低电平有效。所以每一次只有一个低电平。

--KEY1键和KEY2键和KEY3键作为 A b C信号的输入。LED灯作为输出显示状态

--按键的默认状态是1 高电平 --当按键按下时 对应的I/O为低电平（0），

--为了得到不通的值，三个按键不按下时，都是111.表示7；三个按键都按下时,都是000.表示0-- The output of the decoder is active low. So every time only a low level.- KEY1 and KEY2 key key and KEY3 key as A B C signal input. LED light output shows the default state of the state- the key is a high level - When the button is pressed, the corresponding I/O is low (0),- in order to get the value does not make sense, three keys are not when pressed, are 111. said 7　three buttons are pressed, are 000. 0