I'm trying to make a 8-bit shift register using D flipflop. The problem is that when simulating it takes two clock rising edges for the register to shift, one for the D input to change, the other for the Q to change. I don't know why.
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;
In the process, there is clock edge sensitive condition with the expression:
clk'event and clk = '1'
The process thereby implements an additional level of sequential logic (flip flops), but you probably wanted to create a process for purely combinatorial design, like:
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;
Note that VHDL-2008 all is used as sensitivity list above, to automatically
include all signals used in a process for combinatorial design.