Behavioral into FlipFlop Structural

Question

In this code, when reset equals 1 the s becomes 1000 and when reset equals 0 the s becomes 0100 then 0010 then 0001 and it starts all over again with 1000 as the start value, only if the clock is up.

library IEEE;
use IEEE.std_logic_1164.all;
use ieee.std_logic_unsigned.all; 


entity clock_behav is
  port (clock  : in  std_logic;
          reset   : in  std_logic;
          s  : out std_logic_vector (3 downto 0));
end clock_behav;

architecture behav of clock_behav is
begin
process(clock,reset)
variable shift_counter: integer := 0;
begin
if (reset='1') then
    s<="1000";
    shift_counter := 1;
else
    if(clock'event and clock='1') then
        if(shift_counter =1) then
            s<="0100";
            shift_counter := 2;
        elsif(shift_counter =2) then
            s<="0010";
            shift_counter := 3;
        elsif(shift_counter =3) then
            s<="0001";
            shift_counter := 0;
        else
            s<="1000";
            shift_counter := 1;
        end if;
    end if;

end if;
end process;
end behav;

I want to create this enter image description here With FlipFlops as you can see, one Set and 3 Reset. But, I struggle to move from behavioral into structural, because in VHDL we can't have in process port maps. Of course I tried many things, as you can see below, but it's impossible to recreate it with flipflops if the port maps are not inside the process. As you can clearly understand , my knowledge about VHDL it's not that great. Also, I want you to tell me if I did right when I changed the flipflop D and Q types, it was std_logic and I changed it to std_logic_vector . I did this for the purpose of this exercise.

library IEEE;
use IEEE.std_logic_1164.all;
use ieee.std_logic_unsigned.all; 


entity clock_structural is
  port (clock  : in  std_logic;
          reset   : in  std_logic;
          s  : out std_logic_vector (3 downto 0));
end clock_structural;


architecture behavior of clock_structural is
signal t,t1,t2,t3 : std_logic_vector (3 downto 0);
component flipflop_new
port
 (D : in std_logic_vector (3 downto 0);
 CLK : in std_logic;
 CLR : in std_logic;
 Q : out std_logic_vector (3 downto 0));
end component;

component flipflop_set
port
 (D_s : in std_logic_vector (3 downto 0);
 CLK_s : in std_logic;
 CLR_s : in std_logic;
 Q_s : out std_logic_vector (3 downto 0));
end component;
begin
process(clock,reset)
variable shift_counter: integer := 0;
begin
if (reset='1') then
    t<="1000";  

    shift_counter := 1;
else
    if(clock'event and clock='1') then
        if(shift_counter =1) then
            shift_counter := 2;
        elsif(shift_counter =2) then
            shift_counter := 3;
        elsif(shift_counter =3) then
            shift_counter := 0;
        else
            shift_counter := 1;
        end if;
end if;

end if;

end process;

FFS1: flipflop_set port map(t,clock,reset,t1);
s<=t1;
FFR1: flipflop_new port map(t1,clock, reset,t2);
s<=t2;
FFR2: flipflop_new port map(t2,clock, reset,t3);
s<=t3;
FFR3: flipflop_new port map(t3,clock, reset,s);

end behavior ;



library IEEE;
use IEEE.std_logic_1164.all;
use ieee.std_logic_unsigned.all; 

entity flipflop_new is
 port ( D : in std_logic_vector (3 downto 0);
 CLK : in std_logic;
 CLR : in std_logic;
 Q : out std_logic_vector (3 downto 0)
 );
end flipflop_new; 


architecture behavior of flipflop_new is
begin
process(CLK)
begin
if CLR='0' then null;
elsif RISING_EDGE(CLK) then
Q <= D;
end if;
end process ;
end behavior ;

library IEEE;
use IEEE.std_logic_1164.all;
use ieee.std_logic_unsigned.all; 

entity flipflop_set is
 port ( D_s : in std_logic_vector (3 downto 0);
 CLK_s : in std_logic;
 CLR_s : in std_logic;
 Q_s : out std_logic_vector (3 downto 0)
 );
end flipflop_set; 


architecture behavior of flipflop_set is
begin
process(CLK_s)
begin
if CLR_s='1' then null;
elsif RISING_EDGE(CLK_s) then
Q_s <= D_s;
end if;
end process ;
end behavior ;

score 1 · Accepted Answer · answered May 18 '15 at 15:14

There are several things to change or improve. A structural VHDL model should describe your schematic, which you don't really do.

First, why do you have shift_counter in your structural? You don't need that process.

Second, you instantiate 4 of your flip-flop entity which are each 4 bits wide, while your schematic's has 4 flip-flops. Basically, you instantiate a total of 16 registers when you need 4. Why is your flip-flop model 4 bits wide? It should be a single bit.

Third, look at your flip-flop description:

process(CLK)
begin
    if CLR='0' then
        null;
    elsif RISING_EDGE(CLK) then
        Q <= D;
    end if;
end process ;

Does it seems what a flip-flop do? The Q <= D when the clock rises is fine, but does nothing happens when the clr of the flip-flop is active? Your output should reset/set in that case, which is not what your VHDL describe.

Another error is that you assign your output s 3 times. s must be assigned once, but you can assign bit individually like s(0) <= t1.

Finally, you don't describe the feedback. The output of your last flip-flop is s, while the input of the first flip-flop is t. From your schematic, they should be connected together.

One question before I proceed , do I need at all the process, or should I use only port map? — Manuel Pap, May 18 '15 at 15:25
You don't need a process in the structural. You of course need one in the flip-flop description. — Jonathan Drolet, May 18 '15 at 15:26
I have a problem with flipflop, because my teacher gave the entity with these variables and we had to write the architecture. And you said that I need set/reset, which I don't have. — Manuel Pap, May 18 '15 at 15:39
You have it, `CLR` for the reset flipflop and `CLR_s` for the set flipflop — Jonathan Drolet, May 18 '15 at 15:40

Behavioral into FlipFlop Structural

1 Answers1