Question

Now that I have managed to shift my text while writing, I want to implement another feature, scrolling the text 1 digit per second. So for example I will will write “STACK” from keyboard, and then when I toggle a pin it will start floating on the seven segment display. I am getting multiple clocks error as I expected. Now, I got over that error with a counter but the text is not scrolling properly, random characters appear on random locations.

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
use IEEE.numeric_std.all;

entity my_shifter is
        port(clk      : in  std_logic;
            doShift : in std_logic;--shift mode
            Scan_Dav : in  std_logic;--from keyboard module, new data
            Data_in  : in  std_logic_vector (7 downto 0);--scancode of the key pressed
            O1 : out std_logic_vector(7 downto 0);
            O2 : out std_logic_vector(7 downto 0);
            O3 : out std_logic_vector(7 downto 0);
            O4 : out std_logic_vector(7 downto 0)
            );
end my_shifter;

architecture bhv of my_shifter is

signal bytes : std_logic_vector(63 downto 0):=(others => '0');
signal Scan_Dav_Sync: std_logic_vector(1 downto 0):="00";
signal Previous_Scan_Dav: std_logic:='0';
signal shift : std_logic:='0';
signal flag : std_logic:='0';
signal first_letter: std_logic_vector(7 downto 0):="00000000";
begin
    process(clk)
        variable var:integer range 0 to 50000000 :=0;
        begin
            if rising_edge(clk) then
                if var = 50000000 then
                    var:=0;
                    flag<='0';
                    shift <= '1';
                else
                    flag <= '1';
                    var:=var+1;
                    shift <= '0';
                end if;
            end if;
    end process;

    process (clk, doShift) 
        begin
            case doShift is

                when '0' =>
                    if rising_edge(clk) then
                        Scan_Dav_Sync(0) <= Scan_Dav;
                        Scan_Dav_Sync(1) <= Scan_Dav_Sync(0);
                        Previous_Scan_Dav <= Scan_Dav_Sync(1);
                        if (Previous_Scan_Dav = '0') and (Scan_Dav_Sync(1) = '1') then
                            bytes <= bytes (bytes'high-8 downto 0) & Data_in;
                        end if;
                    end if;--till here it works fine.

                when '1' => -- this is where it messes up
                    if (shift = '1' and flag = '0' ) then
                        first_letter <= bytes(bytes'high downto bytes'high-7);
                        bytes <= bytes (bytes'high-8 downto 0) & first_letter;
                    end if; 

                when others =>--ignore here
                    bytes <= bytes (bytes'high-8 downto 0) & Data_in;

            end case;
    end process;
    O1 <= bytes(31 downto 24);
    O2 <= bytes(23 downto 16);
    O3 <= bytes(15 downto 8);
    O4 <= bytes(7 downto 0);
end bhv;

I wonder how I can overcome this issue? What or where is the error?

Answer 1

Most likely you can get away with this working if you make your second process a sane clocked process.

Something like:

process (clk) 
begin
    if rising_edge(clk) then
        case doShift is
            when '0' =>
                Scan_Dav_Sync(0) <= Scan_Dav;
                Scan_Dav_Sync(1) <= Scan_Dav_Sync(0);
                Previous_Scan_Dav <= Scan_Dav_Sync(1);
                if (Previous_Scan_Dav = '0') and (Scan_Dav_Sync(1) = '1') then
                    bytes <= bytes (bytes'high-8 downto 0) & Data_in;
                end if;

            when '1' =>
                if (shift = '1' and flag = '0' ) then
                    first_letter <= bytes(bytes'high downto bytes'high-7);
                    bytes <= bytes (bytes'high-8 downto 0) & first_letter;
                end if; 

            when others =>
                bytes <= bytes (bytes'high-8 downto 0) & Data_in;

        end case;
    end if;
end process;