（VHDL小程序005）用VHDL设计多路器

该设计为四输入多路器，当控制信号端OEbar低电平有效时，高电平有效的选择控制端R_sel,D_sel,uPC_sel,stack_sel所对应的输入数据R,D,uPC,reg_file(sp)之一送到输出Y。其中，整数sp为指向reg_file堆栈单元的指针。

 

源文件如下：

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_unsigned.all;

use ieee.std_logic_arith.all;          --一般不清楚用什么的情况下，这三个包都use即可。

 

entity multiplexer is

     port(

          OEbar:in std_logic;

          R_sel,D_sel,uPC_sel,stack_sel:in std_logic;

          sp:in integer range 0 to 5;

          R,D,uPC:in std_logic_vector(11 downto 0);

          Y:out std_logic_vector(11 downto 0)

          );

end multiplexer;

 

architecture func of multiplexer is

type std_logic_array is array(5 downto 0) of std_logic_vector(11 downto 0);

begin

     muxr:block-------------------------------------------------------------块block必须加标识符，不然无法通过编译；

     signal temp :std_logic_vector(11 downto 0);

     signal reg_file:std_logic_array:=(                                  --需要另作定义std_logic_array

                                                                       ("0000 0000 0000"),

                                                                       ("1111 1111 1111"),

                                                                       ("0000 0000 0000"),

                                                                       ("1111 1111 1111"),

                                                                       ("0000 0000 0000"),

                                                                       ("1111 1111 1111")

                                                                      );

     begin

          temp<=R when R_sel='1' else

                    D when D_sel='1' else

                    uPC when uPC_sel='1' else

                    reg_file(sp) when stack_sel='1' else

                    "0000 0000 0000";

 

          Y<=temp when OEbar='0' else

               "ZZZZ ZZZZ ZZZZ";

     end block muxr;

end func;

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虽然temp和Y赋值是同时进行，OEbar和X_sel数据选择控制信号是同时有效，但是由于vhdl的delta延时语法现象，

可出现如下现象：

                              temp                               Y

当前               0000 0000 0000            0000 0000 0000

一个delta延迟后1111 1111 1111            0000 0000 0000

两个delta延迟后1111 1111 1111            1111 1111 1111

 

 二维数组需要用“（）”括号分开，如下：

type oooo is array(2 downto 0) of std_logic_vector(3 downto 0);
type oo is array(2 downto 0) of oooo;
        
signal ooooo:oo:=(
     ("0000","0000","0000"),
     ("1111","1111","1111"),
     ("0000","0000","0000")
     );

 

常用数据类型定义例子如下：

--------------------------------------------
subtype abcd is integer range 0 to 555;  ---integer 范围-2**31-1 to 2**31
type ddd is range 600 to 700;
--------------------------------------------
type color is (aa,b,c,d,e,f);
--------------------------------------------
type current is range 0 to 1000000000
units
 na;
 ua=1000na;
 ma=1000ua;
 a=1000ma;
end units;
---------------------------------------------
type oooo is array(2 downto 0) of std_logic_vector(3 downto 0);
type oo is array(2 downto 0) of oooo;
        
signal ooooo:oo:=(
     ("0000","0000","0000"),
     ("1111","1111","1111"),
     ("0000","0000","0000")
     );
---------------------------------------------
type myrecord is record
 ADDR: STD_LOCIG_VECTOR(31 DOWNTO 0);
    DATA: STD_LOGIC_VECTOR(31 DOWNTO 0);
end record;
---------------------------------------------