四人抢答器

				library ieee;
				use ieee.std_logic_1164.all;
				use ieee.std_logic_unsigned.all;
				entity js is 
				  port (clr, en,clk:in std_logic;
						sp:out std_logic;
				        qa:out std_logic_vector (3 downto 0);
				        qb:out std_logic_vector (3 downto 0));
				end entity js;
				architecture art of js is
				begin 
				process(clk,clr)is
				  variable tmpa:std_logic_vector (3 downto 0);
				  variable tmpb:std_logic_vector (3 downto 0);
				  begin 
				if tmpa="0000"and tmpb="0000"then
				sp				else sp				end if;
				  if clr='1'then tmpa:= "0010";tmpb:= "0000";
				     elsif clk'event and clk='1'then
				    	if  en='1'then
				          if tmpb="0000" then tmpb:="1001";tmpa:=tmpa-1;
				          else tmpb:=tmpb-1;
				          end if ;
				       end if;
				      end if;
				qa				end process;
				end;