Subversion Repositories astron_r2sdf_fft

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--   Author: Raj Thilak Rajan : rajan at astron.nl: Nov 2009
--   Copyright (C) 2009-2010
--   ASTRON (Netherlands Institute for Radio Astronomy)
--   P.O.Box 2, 7990 AA Dwingeloo, The Netherlands
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--   This file is part of the UniBoard software suite.
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--
--   This program is distributed in the hope that it will be useful,
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--   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
--   GNU General Public License for more details.
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--   You should have received a copy of the GNU General Public License
--   along with this program.  If not, see <http://www.gnu.org/licenses/>.
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-- Purpose: Get twiddles from ROM
-- Description:
--   The twiddles ROM is generated twiddlesPkg.vhd.
-- Remark:
-- . Default use g_lat=1 as for synthesis to improve fmax.
-- . Use g_lat=0 for no digital latency to show the pure functional behavior
--   of a pipelined FFT.  
-- . When the pipelined FFT is used in a Wideband FFT configuration the 
--   rTwoWeights unit compensates for this by estimating the virtual stage and
--   applying the twiddle-offset and the stage-offset. 
 
library ieee, common_pkg_lib;
use IEEE.std_logic_1164.all;
use common_pkg_lib.common_pkg.all;
use work.twiddlesPkg.all;
 
entity rTwoWeights is
  generic (
    g_stage          : natural := 4; -- The stage number of the pft
    g_lat            : natural := 1; -- latency 0 or 1
    g_twiddle_offset : natural := 0; -- The twiddle offset: 0 for normal FFT. Other than 0 in wideband FFT
    g_stage_offset   : natural := 0  -- The Stage offset: 0 for normal FFT. Other than 0 in wideband FFT
  );
  port (
    clk       : in  std_logic;
    in_wAdr   : in  std_logic_vector;
    weight_re : out wTyp;
    weight_im : out wTyp
  );
end;
 
architecture rtl of rTwoWeights is
 
  constant c_virtual_stage : integer := g_stage + g_stage_offset; -- Virtual stage based on the real stage and the stage_offset.
  constant c_nof_shifts    : integer := -1*g_stage_offset;        -- Shift factor when fft is used in wfft configuration  
 
  signal nxt_weight_re : wTyp;
  signal nxt_weight_im : wTyp; 
  signal wAdr_shift    : std_logic_vector(c_virtual_stage-1 downto 1);
  signal wAdr_unshift  : std_logic_vector(c_virtual_stage-1 downto 1); 
  signal wAdr_tw_offset: integer := 0; 
 
begin
 
  -- Estimate the correct twiddle address. 
  -- In case of a wfft configuration the address will be shifted and the twiddle offset will be added. 
  wAdr_unshift   <= RESIZE_UVEC(in_wAdr, wAdr_unshift'length);
  wAdr_shift     <= SHIFT_UVEC(wAdr_unshift, c_nof_shifts) when in_wAdr'length > 0 else (others => '0');
  wAdr_tw_offset <= TO_UINT(wAdr_shift) + g_twiddle_offset when in_wAdr'length > 0 else g_twiddle_offset;
 
  -- functionality
  p_get_weights : process(wAdr_tw_offset)
  begin
    if c_virtual_stage=1 then
      nxt_weight_re <= wRe(wMap(0, 1));
      nxt_weight_im <= wIm(wMap(0, 1));
    else
      nxt_weight_re <= wRe(wMap(wAdr_tw_offset, c_virtual_stage));
      nxt_weight_im <= wIm(wMap(wAdr_tw_offset, c_virtual_stage));
    end if;
  end process;
 
  -- latency
  no_reg : if g_lat=0 generate
    weight_re <= nxt_weight_re;
    weight_im <= nxt_weight_im;
  end generate;
 
  gen_reg : if g_lat=1 generate
    p_clk : process(clk)
    begin
      if rising_edge(clk) then
        weight_re <= nxt_weight_re;
        weight_im <= nxt_weight_im;
      end if;
    end process;
  end generate;
 
  assert g_lat<=1 report "rTwoWeights : g_lat must be 0 or 1" severity failure;
 
end rtl;