simple.vhd

来自「<数字逻辑与VHDL设计>代码 作者：STEPHEN BROWN」· VHDL 代码 · 共 45 行

LIBRARY ieee ;
USE ieee.std_logic_1164.all ;

ENTITY simple IS
	PORT ( 	Clock, Resetn, w 	: IN	STD_LOGIC ;
			z					: OUT 	STD_LOGIC ) ;
END simple ;

ARCHITECTURE Behavior OF simple IS
	SIGNAL y_present, y_next : STD_LOGIC_VECTOR(1 DOWNTO 0);
	CONSTANT A 	: STD_LOGIC_VECTOR(1 DOWNTO 0) := "00" ;
	CONSTANT B 	: STD_LOGIC_VECTOR(1 DOWNTO 0) := "01" ;
	CONSTANT C 	: STD_LOGIC_VECTOR(1 DOWNTO 0) := "11" ;
BEGIN
	PROCESS ( w, y_present )
	BEGIN
		CASE y_present IS
			WHEN A =>
				IF w = '0' THEN y_next <= A ;
				ELSE y_next <= B ;
				END IF ;
			WHEN B =>
				IF w = '0' THEN y_next <= A ;
				ELSE y_next <= C ;
				END IF ;
			WHEN C =>
				IF w = '0' THEN y_next <= A ;
				ELSE y_next <= C ;
				END IF ;
			WHEN OTHERS =>
				y_next <= A ;
		END CASE ;
	END PROCESS ;
	PROCESS ( Clock, Resetn )
	BEGIN
		IF Resetn = '0' THEN
			y_present <= A ;
		ELSIF (Clock'EVENT AND Clock = '1') THEN
			y_present <= y_next ;
		END IF ;
	END PROCESS ;
	z <= '1' WHEN y_present = C ELSE '0' ;
END Behavior ;