----------------------------------------------------------------------
-- HAMMING DECODER (5,2) & (6,3) & (7,4)
----------------------------------------------------------------------

LIBRARY IEEE;
USE  IEEE.STD_LOGIC_1164.all;

entity hamm67dec34 is
generic( Nbits : Integer range 2 to 4 := 4);
port
        (code   : in  std_logic_vector(Nbits+3 downto 0);
         data   : out std_logic_vector(Nbits-1 downto 0);
         status : out std_logic_vector(1 downto 0)
        );
end hamm67dec34 ;

architecture a of hamm67dec34 is
signal syndrom : std_logic_vector(2 downto 0);
signal parity, parity_ok, syndrom_nez  : std_logic;
-- this code was generated by a program of Rolf Schneider
begin
h2: if Nbits=2 generate
  -- PARITY -------------------------------------------------------------------
  parity <= code( 0) xor code( 1) xor code( 2) xor code( 4) ;
  parity_ok <= parity xor code(5);
  -- SYNDROM ------------------------------------------------------------------
  syndrom(0) <= code( 0) xor code( 2) xor code( 4) ;
  syndrom(1) <= code( 1) xor code( 2) ;
  syndrom(2) <= '0' ;
 -- CORRECTED DATA -----------------------------------------------------------
  data( 0) <= code( 2) xor (syndrom(0) and syndrom(1) and (not syndrom(2)) );
  data( 1) <= code( 4) xor (syndrom(0) and (not syndrom(1)) and syndrom(2) );
        end generate;

h3: if Nbits=3 generate
  -- PARITY -------------------------------------------------------------------
  parity <=     code( 0) xor code( 1) xor code( 2) xor code( 3)
            xor code( 4) xor code( 5) ;
  parity_ok <= parity xor code(6);
  -- SYNDROM ------------------------------------------------------------------
  syndrom(0) <= code( 0) xor code( 2) xor code( 4) ;
  syndrom(1) <= code( 1) xor code( 2) xor code( 5) ;
  syndrom(2) <= code( 3) xor code( 4) xor code( 5) ;
  -- CORRECTED DATA -----------------------------------------------------------
  data( 0) <= code( 2) xor (syndrom(0) and syndrom(1) and (not syndrom(2)) );
  data( 1) <= code( 4) xor (syndrom(0) and (not syndrom(1)) and syndrom(2) );
  data( 2) <= code( 5) xor ((not syndrom(0)) and syndrom(1) and syndrom(2) );
        end generate;

h4: if Nbits=4 generate
  -- PARITY -------------------------------------------------------------------
  parity <=     code( 0) xor code( 1) xor code( 2) xor code( 3)
            xor code( 4) xor code( 5) xor code( 6) ;
  parity_ok <= parity xor code(7);
  -- SYNDROM ------------------------------------------------------------------
  syndrom(0) <= code( 0) xor code( 2) xor code( 4) xor code( 6);
  syndrom(1) <= code( 1) xor code( 2) xor code( 5) xor code( 6);
  syndrom(2) <= code( 3) xor code( 4) xor code( 5) xor code( 6);
  -- CORRECTED DATA -----------------------------------------------------------
  data( 0) <= code( 2) xor (syndrom(0) and syndrom(1) and (not syndrom(2)) );
  data( 1) <= code( 4) xor (syndrom(0) and (not syndrom(1)) and syndrom(2) );
  data( 2) <= code( 5) xor ((not syndrom(0)) and syndrom(1) and syndrom(2) );
  data( 3) <= code( 6) xor (syndrom(0) and syndrom(1) and syndrom(2) );
        end generate;

  -- STATUS -------------------------------------------------------------------
  syndrom_nez <= syndrom(0) or syndrom(1) or syndrom(2) ;
  status <= syndrom_nez & parity_ok;    -- "00" : everything ok
                                        -- "01" : data ok, parity wrong
                                        -- "11" : data corrected
                                        -- "10" : data wrong, two bit error
end;