实现位选功能的VHDL 为了使数码管显示数值，首先要产生位选信号，即选中哪一个数码管来显示数值；其次，要给定段选信号，即数码管显示出什么数值或者符号-Choice function VHDL to digital display value, we must first generate the bit-select signal, that is, select a digital tube to display the value　Secondly, to give a given segment select signal, the digital tube display what numerical or symbolic