d-filp-flop_hct175.txt

有C编写的红绿灯的控制

-- Quad D-Type Flip-flop
-- This example shows how a conditional signal assignment statement could be used to describe sequential logic (it is more common to use a process). The keyword 'unaffected' is equivalent to the 'null' statement in the sequential part of the language. The model would work exactly the same without the clause 'else unaffected' attached to the end of the statement. 
-- uses 1993 std VHDL
-- download from: www.pld.com.cn & www.fpga.com.cn


library IEEE;
use IEEE.Std_logic_1164.all;
entity HCT175 is  
   port(D : in std_logic_vector(3 downto 0);
         Q : out std_logic_vector(3 downto 0);
         CLRBAR, CLK : in std_logic);
end HCT175;

architecture VER1 of HCT175 is
begin
   Q <= (others => '0') when (CLRBAR = '0') 
            else D when rising_edge(CLK)
            else unaffected;
end VER1;