I have a custom designed shift register that has as input DL(leftmost input), DR(rightmost), CLR that clears and loads DR, S that shifts right and W that loads leftmost. After testing it, the rightmost is being loaded but not the left. I have reread the code multiple times, but I can't figure out what is wrong. Here's the code:
library IEEE;
use IEEE.std_logic_1164.all;
entity shiftregister is
port (
CLK, CLR: in STD_LOGIC;
S: in STD_LOGIC; --Shift right
W: in STD_LOGIC; --Write
Cin: in STD_LOGIC; --possible carry in from the addition
DL: in STD_LOGIC_VECTOR (7 downto 0); --left load for addition result
DR: in STD_LOGIC_VECTOR (7 downto 0); --right load for initial multiplier
Q: out STD_LOGIC_VECTOR (15 downto 0)
);
end shiftregister ;
architecture shiftregister of shiftregister is
signal IQ: std_logic_vector(15 downto 0):= (others => '0');
begin
process (CLK)
begin
if(CLK'event and CLK='1') then
if CLR = '1' then
IQ(7 downto 0) <= DR; --CLR clears and initializes the multiplier
IQ(15 downto 8) <= (others => '0');
else
if (S='1') then
IQ <= Cin & IQ(15 downto 1);
elsif (W='1') then
IQ(15 downto 8) <= DL;
end if;
end if;
end if;
end process;
Q<=IQ;
end shiftregister;
Waveform
TestBench
library IEEE;
use IEEE.std_logic_1164.all;
entity register_tb is
end register_tb;
architecture register_tb of register_tb is
component shiftregister is port (
CLK, CLR: in STD_LOGIC;
S: in STD_LOGIC; --Shift right
W: in STD_LOGIC; --Write
Cin: in STD_LOGIC; --possible carry in from the addition
DL: in STD_LOGIC_VECTOR (7 downto 0); --left load for addition result
DR: in STD_LOGIC_VECTOR (7 downto 0); --right load for initial multiplier
Q: out STD_LOGIC_VECTOR (15 downto 0)
);
end component;
signal CLK: std_logic:='0';
signal CLR: std_logic:='1';
signal Cin: std_logic:='0';
signal S: std_logic:='1';
signal W: std_logic:='0';
signal DL, DR: std_logic_vector(7 downto 0):="00000000";
signal Q: std_logic_vector(15 downto 0):="0000000000000000";
begin
U0: shiftregister port map (CLK, CLR, S, W, Cin, DL,DR,Q);
CLR <= not CLR after 20 ns;
CLK <= not CLK after 5 ns;
W <= not W after 10 ns;
DL <= "10101010" after 10 ns;
DR <= "00110011" after 10 ns;
end register_tb;
Your simulation shows that your
S
input is always high. The way you have your conditions setup, this means that the last elsif statement will not execute becauseS
has priority overW
. If you want your write to have priority over your shift operation, you should switch your conditionsBased on your comment for the desired behaviour, you could do something like this: