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--##############################################################################

--

--  butterworth3

--      3rd order Butterworth lowpass filter

--

--      This circuit simulates an analog ladder filter by replacing the integral

--      relations of the LC elements by digital accumulators.

--

--------------------------------------------------------------------------------

--

--  Versions / Authors

--      1.1 Francois Corthay    added additional w(0) AND w(filterOrder+1)

--      1.0 Romain Cheviron     first implementation

--

--  Provided under GNU LGPL licence: <http://www.gnu.org/copyleft/lesser.html>

--

--  by the electronics group of "HES-SO//Valais Wallis", in Switzerland:

--  <http://isi.hevs.ch/switzerland/robust-electronics.html>.

--

--------------------------------------------------------------------------------

--

--  Usage

--      Set the input signal bit number with the generic "inputBitNb".

--

--      Set the output signal bit number with the generic "outputBitNb". This

--      value must be greater or equal than "inputBitNb". The additional bits

--      are added as LSBs. They allow to increas the resolution as the bandwidth

--      is reduced.

--

--      Define the cutoff frequency with the generic "shiftBitNb". Every

--      increment in this value shifts the cutoff frequency down by an octave

--      (a factor of 2).

--

--      The input samples are read from the signal "filterIn" at the rising edge

--      of "clock" when "en" is '1'.

--

--      With this, a new output sample is calculated and provided on

--      "filterOut". The output changes at the rising edge of "clock" when "en"

--      is '1'. It remains stable until the next time a sample is calculated.

--

--      The "reset" signal is active high.

--

--------------------------------------------------------------------------------

--

--  Synthesis results

--      A circuit with 16 bit input, 16 bit output and 4 bit shift gives the

--      following synthesis result on a Xilinx Spartan3-1000:

--          Number of Slice Flip Flops:            67 out of  15,360    1%

--          Number of 4 input LUTs:               141 out of  15,360    1%

--          Average Fanout of Non-Clock Nets:    2.20

--

--##############################################################################

LIBRARY ieee;

  USE ieee.std_logic_1164.all;

  USE ieee.numeric_std.all;

ENTITY butterworth3 IS

  GENERIC(

    inputBitNb  : positive := 16;

    outputBitNb : positive := 16;

    shiftBitNb  : positive := 4

  );

  PORT(

    clock     : IN     std_ulogic;

    reset     : IN     std_ulogic;

    en        : IN     std_ulogic;

    filterIn  : IN     signed (inputBitNb-1 DOWNTO 0);

    filterOut : OUT    signed (outputBitNb-1 DOWNTO 0)

  );

END butterworth3;

--==============================================================================

ARCHITECTURE RTL OF butterworth3 IS

  constant filterOrder : natural := 3;

  type natural_vector_t is array(1 to filterOrder) of natural;

  constant t : natural_vector_t := (0, 1, 0);

  constant additionalinternalWBitNb: positive := 2;

  constant internalWBitNb: positive := filterOut'length + additionalinternalWBitNb;

  signal inputSignalScaled : signed(internalWBitNb-1 downto 0);

  constant internalaccumulatorBitNb : positive := internalWBitNb + shiftBitNb;

  type signed_vector_accumulator is array(1 to filterOrder)

    of signed(internalaccumulatorBitNb-1 downto 0);

  type signed_vector_w is array(0 to filterOrder+1)

    of signed(internalWBitNb-1 downto 0);

  signal accumulator : signed_vector_accumulator;

  signal w : signed_vector_w;

BEGIN

  ------------------------------------------------------------------------------

                          -- Scale input signal to internal state variables size

  inputSignalScaled <= SHIFT_LEFT(

    RESIZE(filterIn, inputSignalScaled'length),

    filterOut'length - filterIn'length

  );

  ------------------------------------------------------------------------------

                                                            -- Accumulator chain

  process(reset, clock)

  begin

    if reset = '1' then

      accumulator <= (others => (others => '0'));

    elsif rising_edge(clock) then

      if en = '1' then

        for index in 1 to filterOrder loop

          accumulator(index) <= accumulator(index) + (

            RESIZE(w(index-1), w(index)'length+1) -

            RESIZE(w(index+1), w(index)'length+1)

          );

        end loop;

      end if;

    end if;

  end process;

  ------------------------------------------------------------------------------

                                                -- Analog filter state variables

  process(accumulator, w, inputSignalScaled)

  begin

    for index in 1 to filterOrder loop

      w(index) <= RESIZE(

        SHIFT_RIGHT(

          accumulator(index),

          t(index) + shiftBitNb

        ),

        w(index)'length

      );

    end loop;

                           -- w(0) combines input and w(1) for first accumulator

    w(0) <= inputSignalScaled - w(1);

            -- w(filterOrder+1) is a copy of w(filterOrder) for last accumulator

    w(filterOrder+1) <= w(filterOrder);

  end process;

  ------------------------------------------------------------------------------

                                    -- Scale last state variables to output size

  filterOut <= RESIZE(w(w'high), filterOut'length);

END ARCHITECTURE RTL;