cnt3.vhd

HDB3编解码

--********************************************************************************--
--                                                                                --
--  3-bit Bi-direction counter                                                    --
--                                                                                --
--********************************************************************************--



library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_unsigned.all;


entity cnt3 is
	port
		(
		clk :in std_logic;
		dir :in std_logic;
		reset :in std_logic;
		q :out std_logic_vector (2 downto 0)
		);
begin
end cnt3;

architecture behavior of cnt3 is
	signal result: std_logic_vector(2 downto 0);
begin

    process(reset, clk)
    begin
	if (reset='0') then
		result<="000";
	elsif (clk'event) and (clk='1') then 
		case dir is 
                    when '1' => 
        		    if (result/=7) then result<=result+1;
         		    end if;
	            when '0' =>
                            if (result/=0) then result<=result-1;
                            end if;
                    when others =>
		end case;
	end if;
    end process;
end behavior;


architecture structure of cnt3 is
	signal add,sub,d,result:std_logic_vector (2 downto 0);
	signal top,bottom:std_logic;
begin
	q<=result;
	top<=result(0) and result(1) and result(2);
	bottom<=result(0) or result(1) or result(2);
	
	add(0)<=(not result(0)) or top;
	sub(0)<=(not result(0)) and bottom;
	add(1)<=(result(0) xor result(1)) or top;
	sub(1)<=not(result(0) xor result(1)) and bottom;
	add(2)<=((result(0) and result(1)) xor result(2)) or top;
	sub(2)<=not((result(0) or result(1)) xor result(2)) and bottom;
	
	d(0)<=(sub(0) and (not dir)) or (add(0) and dir);
	d(1)<=(sub(1) and (not dir)) or (add(1) and dir);
	d(2)<=(sub(2) and (not dir)) or (add(2) and dir);

	process(reset, clk)
	begin
		if (reset='0') then 
			result<="000";
		elsif (clk'event)and(clk='1') then
			result<=d;
		end if;
	end process;
end structure;

configuration conf of cnt3 is
	for behavior
	end for;
end conf;