ud_cnt.vhd

硬盘接口的硬件实现

---------------------------------------------------------------------
----                                                             ----
----  Generic Up/Down counter (ripple carry architecture)        ----
----                                                             ----
----  Author: Richard Herveille                                  ----
----          [e-mail]                                   ----
----          www.asics.ws                                       ----
----                                                             ----
---------------------------------------------------------------------
----                                                             ----
---- Copyright (C) 2001, 2002 Richard Herveille                  ----
----                          [e-mail]                   ----
----                                                             ----
---- This source file may be used and distributed without        ----
---- restriction provided that this copyright statement is not   ----
---- removed from the file and that any derivative work contains ----
---- the original copyright notice and the associated disclaimer.----
----                                                             ----
----     THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY     ----
---- EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED   ----
---- TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS   ----
---- FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL THE AUTHOR      ----
---- OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,         ----
---- INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES    ----
---- (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE   ----
---- GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR        ----
---- BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF  ----
---- LIABILITY, WHETHER IN  CONTRACT, STRICT LIABILITY, OR TORT  ----
---- (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT  ----
---- OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE         ----
---- POSSIBILITY OF SUCH DAMAGE.                                 ----
----                                                             ----
---------------------------------------------------------------------

--
--  CVS Log
--
--  $Id: ud_cnt.vhd,v 1.1 2002/03/01 03:48:22 rherveille Exp $
--
--  $Date: 2002/03/01 03:48:22 $
--  $Revision: 1.1 $
--  $Author: rherveille $
--  $Locker:  $
--  $State: Exp $
--
-- Change History:
--               $Log: ud_cnt.vhd,v $
--               Revision 1.1  2002/03/01 03:48:22  rherveille
--               Changed internal counter libraries.
--               Split counter.vhd into separate files.
--               Core is in same state as Verilog version now.
--


library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;

entity ud_cnt is
	generic(
		SIZE : natural := 8;
		RESD : natural := 0
	);
	port(
		clk    : in  std_logic;                  -- master clock
		nReset : in  std_logic := '1';           -- asynchronous active low reset
		rst    : in  std_logic := '0';           -- synchronous active high reset

		cnt_en : in  std_logic := '1';           -- count enable
		ud     : in  std_logic := '0';           -- up / not down
		nld    : in  std_logic := '1';           -- synchronous active low load
		d      : in  unsigned(SIZE -1 downto 0); -- load counter value
		q      : out unsigned(SIZE -1 downto 0); -- current counter value
		
		rci    : in  std_logic := '1';           -- carry input
		rco    : out std_logic                   -- carry output
	);
end entity ud_cnt;

architecture structural of ud_cnt is
	signal Qi : unsigned(SIZE -1 downto 0);
	signal val : unsigned(SIZE downto 0);
begin
	val <= ( ('0' & Qi) + rci) when (ud = '1') else ( ('0' & Qi) - rci);

	regs: process(clk, nReset)
	begin
		if (nReset = '0') then
			Qi <= conv_unsigned(RESD, SIZE);
		elsif (clk'event and clk = '1') then
			if (rst = '1') then
				Qi <= conv_unsigned(RESD, SIZE);
			else
				if (nld = '0') then
					Qi <= D;
				elsif (cnt_en = '1') then
					Qi <= val(SIZE -1 downto 0);
				end if;
			end if;
		end if;
	end process regs;

	-- assign outputs
	Q <= Qi;
	rco <= val(SIZE);
end architecture structural;