-- Copyright(C) 2004 by Xilinx, Inc. All rights reserved. -- The files included in this design directory contain proprietary, confidential information of -- Xilinx, Inc., are distributed under license from Xilinx, Inc., and may be used, copied -- and/or disclosed only pursuant to the terms of a valid license agreement with Xilinx, Inc. -- This copyright notice must be retained as part of this text at all times. JC2_VHD is a bi-directional 4-bit Johnson counter with stop control.DESIGN FLOW: Stand-alone VHDL design (JC2_TOP.VHD)TARGET DEVICE: XC9572XL-CS48 (any speed)I/O PINS:	CLK       : input free-running clock 	LEFT      : input signal to shift left (active-low switch)	RIGHT     : input signal to shift right (active-low switch)	STOP      : input signal to stop counting (active-low switch)	Q<3>-Q<0> : counter outputs (active-low LEDs; Q<0> is right-most)OPERATION:	The counter is triggered on the rising edge of the clock (CLK). 	A low pulse on the LEFT input will cause the counter to start 	shifting left from its current state. A low pulse on the RIGHT	input will cause the counter to start shifting right from its 	current state. A low pulse on the STOP input will cause the 	counter to hold its current state until LEFT or RIGHT is pulsed.        The following sequence appears on outputs Q<3>-Q<0> when shifting left:		0000		0001		0011		0111		1111		1110		1100		1000		0000 (repeats)        The following sequence appears on outputs Q<3>-Q<0> when shifting right:		0000		1000		1100		1110		1111		0111		0011		0001		0000 (repeats)	After power-up, the counter is stopped with all outputs low (LEDs lit). SIMULATION:	VHDL test bench is provided in JC2_TEST.VHD.Using the jc2_top.ucf file, the Johnson Counter design used in this example can be downloaded to the xc9572-cs48 FAE Demo board.  (note: If using Synplify, Synplify Pro, or LeonardoSpectrum for synthesisthe jc.ucf file will need to be edited as indicated in the file.)For support information and contacts please see:	http://www.xilinx.com/supportor	http://www.xilinx.com/support/services/contact_info.htm