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-- Design Name: 
-- Module Name:    LFSR4_wZERO - Behavioral 
-- Project Name: 
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-- Description: 
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-- Revision: 
-- Revision 0.01 - File Created
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-- ===================================================================================================
-- LFSR -- modeled after the version available in OpenCores except that I have added the all 0 case
-- and changed the shift right to a shift left.  If set_seed is 1, then load seed. If set_seed is 0 
-- and gen_bit is 1, do a 1-bit shift. 
-- ===================================================================================================

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

entity LFSR4_wZERO is
   generic (LFSR_LEN_NB: integer := 4);
   port (
      Clk: in std_logic;
      RESET: in std_logic;
      set_seed: in std_logic; 
      gen_bit: in std_logic; 
      seed: in std_logic_vector(LFSR_LEN_NB-1 downto 0);
      LFSR_out: out std_logic_vector(LFSR_LEN_NB-1 downto 0)
      );
end LFSR4_wZERO;

architecture beh of LFSR4_wZERO is
   signal LFSR_reg, LFSR_next: std_logic_vector(LFSR_LEN_NB-1 downto 0);
   signal next_bit: std_logic;
   signal nor_bit: std_logic;
   signal request: std_logic_vector(1 downto 0);
   begin

-- state and register logic
   process(Clk, RESET)
      begin
      if ( RESET = '1' ) then
         LFSR_reg <= (LFSR_LEN_NB-1 downto 1 =>'0') & (0=>'1');
      elsif ( Clk'event and Clk = '1' ) then
         LFSR_reg <= LFSR_next;
      end if; 
   end process;

-- Set seed takes priority if both are set
   request <= set_seed & gen_bit;

-- Add the 'all zero' state. Simply the bitwise nor of the lower 3 bits. See
-- LFSR_C_Validation/VHDL_NORM_wZERO.c for a C implementation of this LFSR.
   nor_bit <= not(LFSR_reg(2) or LFSR_reg(1) or LFSR_reg(0));

-- Confirmed this is a primitive in a C code implementation. NOTE: Implementation in
-- /home/research/research/FPGAs/HELP/VHDL_HELP/contrib/lfsr_randgen/trunk/lfsr_pkg.vhd
-- shifts the new bit on the right, as we do here. This is important -- if reversed,
-- you need to reverse the tap points.
--    xor_out := rand(3) xor rand(2);
   next_bit <= LFSR_reg(3) xor LFSR_reg(2) xor nor_bit;

-- Preserve contents or put the seed in or shift the LSFR to generate the new bit. Reversed
-- bit shift on 9/27/2013 to match that in above C file.
   LFSR_next <= LFSR_reg when (request = "00") else
                seed when (request(1) = '1') else
                LFSR_reg(LFSR_LEN_NB-2 downto 0) & next_bit; 

   LFSR_out <= LFSR_reg;

end beh;