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44个vhdl实例 注1： 含有不可综合语句

----------------------------------------------------------------
--
-- Copyright (c) 1992,1993,1994, Exemplar Logic Inc. All rights reserved.
-- 
----------------------------------------------------------------
--
-- State machine description using enumerated types
--
-- This is a typical example of a state machine description.
--   Two processes, one to update the state on a clock plus
--   handles reset, and one to calculate the next state and
--   the outputs in a case statement
--
-- State encoding on state_t will be done by CORE. Binary encoding is the
-- default, option -encoding provides alternatives (onehot, gray, random)
-- 
-- This description implements a traffic light controller
--
--     Version 1.1 : Original Creation 
--     Version 1.2 : Modified to std_logic types
--     Version 2.1 : Improved design with CASE instead of if-then-else
--		
--	download from: www.pld.com.cn & www.fpga.com.cn 
----------------------------------------------------------------

LIBRARY ieee;
USE ieee.std_logic_1164.all;

ENTITY traffic IS
    PORT (clock, sensor1, sensor2, reset : IN std_logic;
	  red1, yellow1, green1, red2, yellow2, green2 : OUT std_logic);
END ;

ARCHITECTURE eXemplar OF traffic IS
    TYPE state_t IS ( ST0, ST1, ST2, ST3, ST4, ST5, ST6, ST7 );
    SIGNAL state, nxstate : state_t;
BEGIN

update_state :	-- Update the state on the clock edge
    PROCESS (reset, clock)
    BEGIN
        IF (reset='1') THEN
	    state <= ST0 ;
        ELSIF clock'event and clock='1' THEN
	    state <= nxstate ;
	END IF ;
    END PROCESS;

transitions :	-- set the outputs and next state
    PROCESS (state, sensor1, sensor2)
    BEGIN
        -- Default values for the outputs
        red1 <= '0'; yellow1 <= '0'; green1 <= '0';
        red2 <= '0'; yellow2 <= '0'; green2 <= '0';

        -- Make sure to always set a value for nxstate, 
        -- or unwanted latches will occur.
	CASE state IS
	    WHEN ST0 =>
	        green1 <= '1';
	        red2 <= '1';
	        IF sensor2 = sensor1 THEN
		    nxstate <= ST1;
	        ELSIF (sensor1 = '0' AND sensor2 = '1') THEN
		    nxstate <= ST2;
	        ELSE
		    nxstate <= ST0;
	        END IF;
	    WHEN ST1 =>       
	        green1 <= '1';
	        red2 <= '1';
	        nxstate <= ST2;
	    WHEN ST2 =>       
	        green1 <= '1';
	        red2 <= '1';
	        nxstate <= ST3;
	    WHEN ST3 =>       
	        yellow1 <= '1';
	        red2 <= '1';
	        nxstate <= ST4;
	    WHEN ST4 =>       
	        red1 <= '1';
	        green2 <= '1';
	        IF (sensor1 = '0' AND sensor2 = '0') THEN
		    nxstate <= ST5;
	        ELSIF (sensor1 = '1' AND sensor2 = '0') THEN
		    nxstate <= ST6;
	        ELSE
		    nxstate <= ST4;
	        END IF;
	    WHEN ST5 =>       
	        red1 <= '1';
	        green2 <= '1';
	        nxstate <= ST6;
	    WHEN ST6 =>       
	        red1 <= '1';
	        green2 <= '1';
	        nxstate <= ST7;
	    WHEN ST7 =>       
	        red1 <= '1';
	        yellow2 <= '1';
	        nxstate <= ST0;
	END CASE;
    END PROCESS;
END eXemplar;