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-- $Id: nco.vhdl,v 1.4 2008-06-26 06:16:04 arif_endro Exp $
-------------------------------------------------------------------------------
-- Title       : NCO (Numerical Controlled Oscillator)
-- Project     : FM Receiver 
-------------------------------------------------------------------------------
-- File        : nco.vhdl
-- Author      : "Arif E. Nugroho" <arif_endro@yahoo.com>
-- Created     : 2004/10/27
-- Last update : 
-- Simulators  : 
-- Synthesizers: 
-- Target      : 
-------------------------------------------------------------------------------
-- Description : Works like VCO in analog PLL
-------------------------------------------------------------------------------
-- Copyright (C) 2004 Arif Endro Nugroho
-------------------------------------------------------------------------------
-- 
-- 	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 BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
-- IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
-- MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO
-- EVENT SHALL THE AUTHOR 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.
-- 
-------------------------------------------------------------------------------
 
library IEEE;
use IEEE.STD_LOGIC_1164.all;
 
entity nco is
      port (
           clock       : in  bit;
	   clear       : in  bit;
           input_nco   : in  bit_vector (17 downto 0);
           offset      : in  bit_vector (17 downto 0);
           output_nco  : out bit_vector (07 downto 0)
           );
end nco;
 
architecture structural of nco is
   component addacc 
     port (
           clock       : in  bit;
           acc         : in  bit_vector (17 downto 0);
           result      : out bit_vector (17 downto 0);
           offset      : in  bit_vector (17 downto 0)
           );
   end component;
 
   component rom
     port (
           address     : in  bit_vector (09 downto 0);
           data        : out bit_vector (07 downto 0)
           );
   end component;
 
   signal adder_output : bit_vector (17 downto 0);
   signal address_in   : bit_vector (09 downto 0);
   signal output_rom   : bit_vector (07 downto 0);
 
begin
 
 myaddacc  : addacc
     port map (
              clock                 => clock,
              acc                   => input_nco,
              result (17 downto 0)  => adder_output,
              offset                => offset
              );
 myrom     : rom
     port map (
              address (09 downto 0) => address_in,
              data    (07 downto 0) => output_rom
              );
 
   address_in (09) <= (adder_output(17));
   address_in (08) <= (adder_output(16));
   address_in (07) <= (adder_output(15));
   address_in (06) <= (adder_output(14));
   address_in (05) <= (adder_output(13));
   address_in (04) <= (adder_output(12));
   address_in (03) <= (adder_output(11));
   address_in (02) <= (adder_output(10));
   address_in (01) <= (adder_output(09));
   address_in (00) <= (adder_output(08));
 
--   process (clock, clear)
   process (clock)
 
   begin
 
-- 20080625
-- fixme
-- how to enable clear signal in here... :(
 
--   if    (clear = '1') then
   if ((clock = '1') and clock'event) then
 
--	output_nco      <= (others => '0');
 
--   elsif (((clock = '1') and (not(clear) = '1')) and clock'event) then
 
	output_nco (07) <= (output_rom(07));
	output_nco (06) <= (output_rom(06));
	output_nco (05) <= (output_rom(05));
	output_nco (04) <= (output_rom(04));
	output_nco (03) <= (output_rom(03));
	output_nco (02) <= (output_rom(02));
	output_nco (01) <= (output_rom(01));
	output_nco (00) <= (output_rom(00));
 
   end if;
 
   end process;
 
end structural;