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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;