CODEWITHSUNDEEP
scrollconcatenationvhdlbit-shift
Yusif_Nurizade
Yusif_Nurizade

Reputation: 133

VHDL Shift with Concatenation

I am writing what I thought would be a fairly standard VHDL code but the execution keeps coming back on me.

The code is meant to be a simple game on a Nexsys 3 board. The first player selects a switch and the LEDs scroll down from it showing a blank when they are over the switch itself. The second player presses a button when the scroll gets to the switch selected. My part of the game is to get the LEDs scrolling. My code can be viewed below:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
use IEEE.NUMERIC_STD.ALL;

entity Two_Player_3_24_2013 is
port(   clk:        in   std_logic;
        rst:        in   std_logic;
        switches:   in   std_logic_vector(7 downto 0);
        led_indic:  out  std_logic_vector(7 downto 0)           
 );

end Two_Player_3_24_2013;

architecture Behavioral of Two_Player_3_24_2013 is

signal    count:    std_logic_vector(31 downto 0):= "00000000000000000000000000000000";     
signal    only_one: std_logic := '0';
signal    shifter:  std_logic_vector(7 downto 0)    := "00000000";

begin

only_one <= '1' when switches = "10000000" else
            '1' when switches = "01000000" else
            '1' when switches = "00100000" else
            '1' when switches = "00010000" else
            '1' when switches = "00001000" else
            '1' when switches = "00000100" else
            '1' when switches = "00000010" else
            '1' when switches = "00000001" else
            '0';

counting:   process(clk, rst, only_one)
    begin 
    if (rst = '1') then
    count <= "00000000000000000000000000000000";
    elsif (clk'event and  clk='1' and only_one = '1') then
    count <=  count + '1';
    end if;
end process counting;


shifting:   process(clk, rst, count, shifter)
    begin
    if (rst = '1') then
    shifter <= "00000000";
    elsif (clk'event and  clk='1' and count = "00000000000000010000000000000000") then
    shifter <= switches;
    elsif (clk'event and  clk='1' and count(25) = '1') then
    shifter <= shifter(0) & shifter(7 downto 1);
    end if;
end process shifting;

led_indic <= shifter;

end Behavioral;

Now I've checked the parts individually and they work. The only_one signal is true only when one switch is on. The counter begins at the right time and the first switch is triggered at the right time as well. For some reason I still can't figure out, however, it refuses to scroll. I've tried running this both on the board and in simulation, but I can't figure out why the scrolling doesn't work. The original setting only happens at a specific time, while the concatenation should occur at regular time intervals. I know the problem is with the 

shifter <= shifter(0) & shifter(7 downto 1);

but so far nothing I've tried has fixed it.

Any advice is greatly appreciated.

Thanks,
Yusif Nurizade

Upvotes: 0

Views: 885

Answers (1)

baldyHDL
baldyHDL

Reputation: 1387

I guess in simulation you just have to wait ;-)

when you use a 100MHz clock, it takes ~335ms until anything happens (count(25) is high). as soon and as long as count(25) is high, the LEDs will shift very fast for the next 335ms ending at the same position as they started. --> therefore adjust your counters.

for the game itself re-think the "shift while count(25)='1'" thing! it makes more sense to shift only at one counterposition (e.g. if count=X"01000000"...)

btw: for synthesis it is not good practice to implement gated clocks like

elsif (clk'event and  clk='1' and count(25) = '1')) then

you should rather use e.g.:

elsif (clk'event and  clk='1') then
   if count(25) = '1' then
      ...

Upvotes: 1

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