VHDL design for 8 bit shift register

Altera_Forum · ‎10-21-2014

I need to program an 8 bit shift register, which will be used in a 4-bit multiplier.

Basically, this is what it should do:

1.- Receive inputs x (multiplier), sum (this comes from the adder segment of the multiplier), done, ad, sh, load

2.- WHen it receives the load signal, it must take the input values x

3.- when it gets the done signal, it must stop working

4.- when it gets the ad signal, it must replace the last 4 bits with the sum, also, when it gets sh it must shift

However, I don't seem to get the expected results. This is the code:

LIBRARY IEEE;USE ieee.std_logic_1164.all;
Entity registro8 IS
    PORT (load, sh, ad, Clock, done, z : IN  STD_logic;
            x,sum  : IN  STD_logic_vector (3 downto 0);
            m : OUT STD_logic;
            w : OUT STD_logic_vector(7 downto 0));
END registro8;
architecture reg of registro8 IS
signal s : std_logic_vector (7 downto 0);
begin
        process (load, x, sum, clock, sh, done, ad)
            begin
            if(done = '0') THEN
                if (load = '1') THEN
                    for i in 4 to 7 loop
                    s(i) <= x(i-4); 
                    end loop;
                    for i in 0 to 3 loop
                    s(i) <= '0';
                    end loop;
                elsif (clock'event and clock = '1') THEN
                    if (ad = '1') THEN
                        for i in 0 to 3 loop
                        s(i) <= sum(3-i);
                        end loop;
                    elsif (sh = '1') THEN
                    s <= s(6 downto 0) & z;
                    END IF;
                end if;
                m <= s(7);
                w <= s;
            end if;
        end process;
end reg;

The main problem is that, after loading, the register seems to be deleted. Also, it continues to work, even if the done signal is 1.

Thanks for the help

Altera_Forum · ‎10-21-2014

Read a good book about VHDL, it isn't generic programming language like you are trying to use it.

Your code of the process should look like this:

process (clock, reset)

begin

if(reset = '1') THEN

-- here are the initial conditions

elsif (clock'event and clock = '1') THEN

-- here you write what your shift register does

-- if(done = '0') THEN

-- if (load = '1') THEN

-- if (ad = '1') THEN

-- else etc bla bla bla

end if;

end process;