downcnt.vhd

来自「一本很好的关于学习VHDL的书,Fundamentals of Digital 」· VHDL 代码 · 共 26 行

LIBRARY ieee ;
USE ieee.std_logic_1164.all ;

ENTITY downcnt IS
	GENERIC ( modulus : INTEGER := 8 ) ;
	PORT (	Clock, L, E : IN 	STD_LOGIC ;
			Q 			: OUT 	INTEGER RANGE 0 TO modulus-1 ) ;
END downcnt ;

ARCHITECTURE Behavior OF downcnt IS
	SIGNAL Count : INTEGER RANGE 0 TO modulus-1 ;
BEGIN
	PROCESS
	BEGIN
		WAIT UNTIL (Clock'EVENT AND Clock = '1') ;
		IF E = '1' THEN
			IF L = '1' THEN
				Count <= modulus-1 ;
			ELSE
				Count <= Count-1 ;
			END IF ;
		END IF ;
	END PROCESS;
	Q <= Count ;
END Behavior ;