VHDL_LIB之DFF

1　　D-Flip-Flop with async reset or set

 1 library IEEE;
 2 use ieee.std_logic_1164.all;
 3 
 4 entity FFD is
 5     generic
 6     (
 7         ResetVal    : std_logic := '0';        --! select async set / async reset
 8         ClockFall   : boolean := False         --! select clock edge
 9     );
10     port
11     (
12         RST  :    in    std_logic;             　--! Async Reset
13         C    :    in    std_logic;               --! Clock
14         D    :    in    std_logic;               --! Input
15         Q    :    out    std_logic               --! Output
16     );
17 end FFD;
18 
19 architecture beh of FFD is
20     signal clock : std_logic;                    --! active clock
21 
22 begin
23 
24 gckr: if ClockFall = false generate
25     clock <= C;
26 end generate;
27 gckf: if ClockFall = true generate
28     clock <= not(C);
29 end generate;
30 
31 pFF:    process(RST,clock)
32         begin
33             if(RST = '1') then
34                 Q <= ResetVal;
35             else
36                 if(clock'Event and clock = '1') then
37                     Q    <=    D;
38                 end if;
39             end if;
40         end process pFF;
41 end beh;

2　　D-Flip-Flop with clock enable

1 -- Port declaration
2 CE    :    in    std_logic;           --! Clock enable
3 
4 -- Clock event
5     if(clock'Event and clock = '1') then
6         if (CE = '1') then　　Q <= D; 
7         end if;        
8     end if;