我正在尝试使用 D 触发器制作 8 位移位寄存器。问题在于,在仿真时,寄存器需要两个时钟上升沿来移动,一个用于D输入变化,另一个用于Q变化。我不知道为什么。
entity Registry_8 is
port (input : in std_logic;
output : out std_logic;
clk : in std_logic;
clear : in std_logic;
load : in std_logic;
LR : in std_logic;
pIn : in std_logic_vector (7 downto 0);
pOut : out std_logic_vector (7 downto 0);
shift : in std_logic);
end Registry_8;
architecture Behavioral of Registry_8 is
component D_flipflop
port(D, clk, clear, preset : in std_logic;
Q, Q_b : out std_logic);
end component;
signal D, Q : std_logic_vector (7 downto 0);
begin
GEN_FLIP :
for i in 0 to 7 generate
D_i : D_flipflop port map(clk => clk, preset => '0', clear => clear, D => D(i), Q => Q(i));
end generate GEN_FLIP;
process (clk, load, LR, shift)
begin
if (load = '1')
then D <= pIn;
end if;
if (clk'event and clk = '1' and shift = '1')
then
if (LR = '0')
then D(7 downto 0) <= Q(6 downto 0) & input;
output <= Q(7);
else
D(7 downto 0) <= input & Q(7 downto 1);
output <= Q(0);
end if;
end if;
end process;
pOut <= Q;
end Behavioral;
在此过程中,存在时钟边沿敏感条件,表达式为:
clk'event and clk = '1'
因此,该过程实现了附加级别的顺序逻辑(翻转flops),但您可能想为纯组合创建一个过程设计,如:
process (all) is
begin
if (load = '1') then
D <= pIn;
end if;
if shift = '1' then
if (LR = '0') then
D(7 downto 0) <= Q(6 downto 0) & input;
output <= Q(7);
else
D(7 downto 0) <= input & Q(7 downto 1);
output <= Q(0);
end if;
end if;
end process;
请注意,VHDL-2008 all
用作上面的灵敏度列表,以自动包括组合设计过程中使用的所有信号。