upcount.vhd

<数字逻辑与VHDL设计>代码 作者：STEPHEN BROWN

LIBRARY ieee ;
USE ieee.std_logic_1164.all ;

ENTITY upcount IS
	PORT (	R 					: IN 		INTEGER RANGE 0 TO 15 ;
			Clock, Resetn, L 	: IN 		STD_LOGIC ;
			Q 					: BUFFER 	INTEGER RANGE 0 TO 15 ) ; 
END upcount ;

ARCHITECTURE Behavior OF upcount IS
BEGIN
	PROCESS ( Clock, Resetn )
	BEGIN
		IF Resetn = '0' THEN
			Q <= 0 ;
		ELSIF (Clock'EVENT AND Clock = '1') THEN
			IF L = '1' THEN
				Q <= R ;
			ELSE
				Q <= Q + 1 ;
			END IF;
		END IF;
	END PROCESS;
END Behavior;