-- **************************************************************************
-- Owner:		Xilinx Inc.
-- File:  		err_det.vhd
--
-- Purpose: 		16B/20B error detect module.  Determines if an error has
--			been detected in the transmission of data.  Determines the
--			difference between special characters and error flags
--			asserted in each 8B/10B decode module.
--
-- Author:		Jennifer Jenkins
-- Date:		3-31-2000
--
-- **************************************************************************


library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;

entity ERR_DET is
    port(
        
          clk			: in STD_LOGIC;
          rst			: in STD_LOGIC;
        
          di			: in STD_LOGIC_VECTOR(15 downto 0);	-- Incoming decoded data
 	  start_det		: in STD_LOGIC;   			-- Asserted starts detecting of errors
		
	  k1			: in STD_LOGIC;				-- KOUT of upper decoder
	  k2			: in STD_LOGIC;				-- KOUT of lower decoder
	  error1		: in STD_LOGIC;				-- ERROR_OUT of upper decoder
	  error2	  	: in STD_LOGIC;				-- ERROR_OUT of lower decoder

	  -- Output signals
	  ill_char_det		: out STD_LOGIC				-- Asserted when error has been detected

          );

end ERR_DET;



architecture BEHAVIOUR of ERR_DET is

-- ******************** CONSTANT DECLARATIONS ***********************
constant RESET_ACTIVE 	: STD_LOGIC := '0';


-- ********************* SIGNAL DECLARATIONS ************************

-- Define states for download state machine
type STATE is (IDLE, PREL, DETECT, DONE);
signal prs_state, nxt_state : STATE;

signal ill_cl1, ill_cl2, ill_cl3 : STD_LOGIC;

begin

	-- ******************** SIGNAL ASSIGNMENTS ********************
	ill_cl1 <= (((di(7) xor di(15)) or (di(6) xor di(14)) or 
		     (di(5) xor di(13))) and (k1 and k2))
		   when ((prs_state = PREL) or (prs_state = DETECT)) else '0';

	ill_cl2	<= (((di(4) xor di(12)) or (di(3) xor di(11)) or 
		     (di(2) xor di(10))) and (k1 and k2))
		   when ((prs_state = PREL) or (prs_state = DETECT)) else '0';

	ill_cl3 <= (((di(1) xor di(9)) or (di(0) xor di(8))) 
		    and (k1 and k2))	
		   when ((prs_state = PREL) or (prs_state = DETECT)) else '0';


	-- ***************** Process: SEQUENTIAL ************************
	-- Purpose:  	Synchronize ENC_FUNC target state machine
	-- Components: 	none
    
    	SEQUENTIAL: process (rst, clk)
    	begin
       	 	if rst = RESET_ACTIVE then	
         	   	prs_state <= IDLE;
            
       	 	elsif clk'event and (clk = '1') then
         	   	prs_state <= nxt_state;
            
        	end if;

    	end process SEQUENTIAL;
    
    
    
    	-- ******************** Process: ERROR_DET ************************
    	-- Purpose: 	Detect transmission errors between special characters
	--		and errors in each 8B/10B decode module.
	--		
    	-- Components:	none
        
    	ERROR_DET: process (prs_state, start_det)
    	begin
    
    	 	nxt_state <= prs_state;
		ill_char_det <= '0';
			    	
        	case prs_state is
        
        		----------------------- IDLE State -------------------------
        		when IDLE =>
 
        			-- Waits for go signal from decode logic
        			if start_det = '1' then
        				nxt_state <= PREL;
        			end if;

			----------------------- PREL State -------------------------
			when PREL =>
						
				-- Preliminary signal assignments
				nxt_state <= DETECT;
        		
			---------------------- DETECT State -------------------------
	      		when DETECT =>
			
				-- Determine if an illegal character has been detected
				ill_char_det <= ill_cl1 or ill_cl2 or ill_cl3 or
						(error1 or error2 or (k1 xor k2));
				
				nxt_state <= DONE;


			--------------------- DONE State -------------------------
			when DONE =>
		
				-- Wait for encoding signal to be deasserted
				if start_det = '0' then
					nxt_state <= IDLE;
				end if;


			--------------------- DEFAULT State ------------------------
			when OTHERS =>
				nxt_state <= IDLE;	      
        	
        	        	
        end case;  
        
    end process ERROR_DET;

end BEHAVIOUR;