Describe in VHDL a generator parallel 4 bits to serial 1 bit

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isamel85

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Hello,
I want to describe in VHDL a generator parallel 4 bits to serial 1 bit.
Indeed, at each clock edge (250 kHz), we take only one bit starting with the least significant bit (LSB).
Example:
Input = "0101" (over 4 bits)
So at first clock edge, output = '1' (LSB)
Second clock edge, output = '0'
Third clock edge, output = '1'
Fourth clock edge, output = '0' (MSB)
There's someone there who can help me?
And thank you
 

Ignoring all the declarations and the other verbose stuff...
Code VHDL - [expand]
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process (clk)
begin
  if rising_edge(clk) then
    if (load_sr) then
      sr <= parallel_data;
    else
      sr <= '0' & sr(3 downto 1);
    end
  end if;
end process;
lsb_out <= sr(0);
 

I tried it but it still gives me a single output state (either 0 or 1) even if I have a new rising edge, the value remains the same

Code VHDL - [expand]
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library IEEE;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
 
entity generator_bits is
port ( Output_data      : out  std_logic;
        Clock_250kHz        : in std_logic;
        Load_sr : in std_logic;
        Input_datas : in std_logic_vector(3 downto 0)
        );
 
end generator_bits;
 
architecture Behavioral of generator_bits is
signal  register_temp : std_logic_vector (3 downto 0);
begin
process (Clock_250kHz)
 
begin
  if rising_edge(Clock_250kHz) then
    if (Load_sr = '1') then
      register_temp <= Input_datas;
    else
      register_temp <= '0' & register_temp(3 downto 1);
    end if;
     
  end if;
end process;
Output_data <= register_temp(0);
 
end Behavioral;
 
Last edited by a moderator:

This is my test bench:
Code: 
LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
 
 
ENTITY test_gen IS
END test_gen;
 
ARCHITECTURE behavior OF test_gen IS 
 
    -- Component Declaration for the Unit Under Test (UUT)
 
    COMPONENT generateur_bits
    PORT(
         Output_data : OUT  std_logic;
         Clock_250kHz : IN  std_logic;
         Load_sr : IN  std_logic;
         Input_datas : IN  std_logic_vector(3 downto 0)
        );
    END COMPONENT;
    

   --Inputs
   signal Clock_250kHz : std_logic := '0';
   signal Load_sr : std_logic := '0';
   signal Input_datas : std_logic_vector(3 downto 0) := (others => '0');

 	--Outputs
   signal Output_data : std_logic;

   -- Clock period definitions
   constant Clock_250kHz_period : time := 4 us;
 
BEGIN
 
	-- Instantiate the Unit Under Test (UUT)
   uut: generateur_bits PORT MAP (
          Output_data => Output_data,
          Clock_250kHz => Clock_250kHz,
          Load_sr => Load_sr,
          Input_datas => Input_datas
        );

   -- Clock process definitions
   Clock_250kHz_process :process
   begin
		Clock_250kHz <= '0';
		wait for Clock_250kHz_period/2;
		Clock_250kHz <= '1';
		wait for Clock_250kHz_period/2;
   end process;
 

   -- Stimulus process
   stim_proc: process
   begin		
      -- hold reset state for 100 ns.
      --wait for 16 us;	
      
      wait for Clock_250kHz_period*4;
      Load_sr <= '1';
		Input_datas <= "1100";
      -- insert stimulus here 

      wait;
   end process;

END;
Show please the result of simulation
 

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Data is shifted when "Load_sr" is '0' and loaded when it is '1'.
You must put it back to '0' after loading the parallel data.
 

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