Subversion Repositories astron_r2sdf_fft

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--

-- Copyright 2020

-- ASTRON (Netherlands Institute for Radio Astronomy) <http://www.astron.nl/>

-- P.O.Box 2, 7990 AA Dwingeloo, The Netherlands

-- 

-- Licensed under the Apache License, Version 2.0 (the "License");

-- you may not use this file except in compliance with the License.

-- You may obtain a copy of the License at

-- 

--     http://www.apache.org/licenses/LICENSE-2.0

-- 

-- Unless required by applicable law or agreed to in writing, software

-- distributed under the License is distributed on an "AS IS" BASIS,

-- WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

-- See the License for the specific language governing permissions and

-- limitations under the License.

--

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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;