3-8译码器与分频器

练习demo:
 

LIBRARY IEEE;  --38decoder
USE IEEE.std_logic_1164.ALL;
ENTITY decoder_38 IS
PORT(a,b,c,g1,g2a,g2b:IN std_logic;
                    y:OUT std_logic_vector(7 DOWNTO 0));
END decoder_38;
ARCHITECTURE behav OF decoder_38 IS
 SIGNAL indata:std_logic_vector(2 DOWNTO 0);
BEGIN
 indata<=c&b&a;
 PROCESS(indata,g1,g2a,g2b)
 BEGIN
     IF(g1='1' and g2a='0' AND g2b='0')THEN
       CASE indata IS
         WHEN "000"=> y<="11111110";
         WHEN "001"=> y<="11111101";
         WHEN "010"=> y<="11111011";
         WHEN "011"=> y<="11110111";
         WHEN "100"=> y<="11101111";
         WHEN "101"=> y<="11011111";
         WHEN "110"=> y<="10111111";
         WHEN "111"=> y<="01111111";
         WHEN OTHERS => y<="XXXXXXXX";
       END CASE;
     ELSE
        y<="11111111";
     END IF;
END PROCESS;
END behav;

         
         

分频器：
 

LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
ENTITY frequencies IS
  PORT(clk_50M:IN std_logic;
       clk_1:OUT std_logic);
END frequencies;
ARCHITECTURE behav OF frequencies IS
SIGNAL time:integer RANGE 0 TO 49999999;
SIGNAL m:integer RANGE 0 TO 1;
BEGIN
 PROCESS(clk_50M)
 BEGIN
  IF rising_edge(clk_50M) THEN
     time<=time+1;
     IF m=0 THEN
        clk_1<='1';
         IF time=24999999 THEN
           m<=1;
         END IF;
     ELSE
        clk_1<='0';
        IF time=49999999 THEN
           m<=0;
           time<=0;
        END IF;
     END IF;
  END IF;
 END PROCESS;
END behav;

library ieee;

use ieee.std_logic_1164.all; 
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;

entity led is

port(led_out : out std_logic_vector(7 downto 0); 
         clk : in std_logic;
         rst_n : in std_logic
);
end led;

architecture behavior of led is

signal light : std_logic_vector (7 downto 0); 
begin
 process(clk, rst_n) 
   begin
    if(rst_n='0')then
       light <= "00000010";
    elsif(clk' event and clk='1')then
     if(light = "00000000" ) then
        light <= "00000001";
     else
        if(light = "10000000" ) then
           light <= "00000001";
        else
           light <= light(6 downto 0)&'0'; 
        end if;
     end if;
    end if;
 end process; 
 led_out <= light;

end behavior;