udasyncounter.vhd

Up-down Asynchronous counter in Behavioral Model

library ieee;
use ieee.std_logic_1164.all;

Entity UDasyncounter is
 port (ud,reset,clk : in bit; Z : out bit_vector(3 downto 0));
end UDasyncounter;

Architecture beh of UDasyncounter is
signal Q : bit_vector(3 downto 0);
begin
-- Q <= "0000";
 process(ud,reset,clk,Q)
 begin
  if reset = '1' then Q <= "0000";
  else
   if ud = '0' then
    if clk'event and clk = '0' then
     Q(0) <= not Q(0);
    end if;
    if Q(0)'event and Q(0) = '0' then
     Q(1) <= not Q(1);
    end if;
    if Q(1)'event and Q(1) = '0' then
     Q(2) <= not Q(2);
    end if;
    if Q(2)'event and Q(2) = '0' then
     Q(3) <= not Q(3);
    end if;    
   else
    if clk'event and clk = '1' then
     Q(0) <= not Q(0);
    end if;
    if Q(0)'event and Q(0) = '1' then
     Q(1) <= not Q(1);
    end if;
    if Q(1)'event and Q(1) = '1' then
     Q(2) <= not Q(2);
    end if;
    if Q(2)'event and Q(2) = '1' then
     Q(3) <= not Q(3);
    end if;    
   end if;
  end if;
 end process;
 Z <= Q;
end beh;