counter.vhd

基于vhdl语言描述的dpll

--------------------------------------------------------------------------------
-- Company: GLXT
-- Engineer: Fuqi LIU
--
-- Create Date:    2003.11.08
-- Design Name:    counter
-- Module Name:    counter 
-- Project Name:   dpll
-- Target Device:  
-- Tool versions:  
-- Description:
--
-- Dependencies:
-- 
-- Revision:
-- Revision 0.01 - File Created
-- Additional Comments:
-- 
--------------------------------------------------------------------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;


entity counter is
	GENERIC
	(
		mode : std_logic_vector(3 downto 0) := "1111"		
	);
port(clk : in std_logic;
	 updown : in std_logic; 	 
	 clkout : out std_logic);
end counter;

architecture Behavioral of counter is
	signal cnt : std_logic_vector(3 downto 0);
	signal flag : std_logic; --每一个clkout时针周期只做一次调整.
begin

process(clk, cnt)
begin	
 	if clk'event and clk = '1' then
		if cnt = '0' & mode(2 downto 0) then
			clkout <= '1';
		elsif cnt = mode then
			clkout <= '0';
			flag <= '0';			
		end if;
		
		if updown = '1' and cnt = "0100" and flag = '0' then
			cnt <= cnt;
			flag <= '1';
		elsif updown = '0' and cnt = "1010" and flag = '0' then 
			cnt <= cnt + 2;	
			flag <= '1';
		else
			cnt <= cnt +1;	
		end if;		
	end if;
end process;
			
end Behavioral;