Subversion Repositories ccsds_rxtxsoc

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---- Project: EurySPACE CCSDS RX/TX with wishbone interface

---- Design Name: ccsds_rxtx_lfsr

---- Version: 1.0.0

---- Description:

---- Linear Feedback Shift Register

---- Input: none

---- Timing requirements: CCSDS_RXTX_LFSR_DATA_BUS_SIZE+1 clock cycles for valid output data

---- Output: dat_val_o <= "1" / dat_o <= "LFSRSEQUENCE"

---- Ressources requirements: TODO

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---- Author(s):

---- Guillaume REMBERT

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---- Licence:

---- MIT

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---- Changes list:

---- 2016/11/05: initial release

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-- Test ressources:

-- GNURADIO GLFSR block

-- CCSDS parameters

-- Width = 8

-- Mode = Fibonacci ('0')

-- Polynomial = x"A9"

-- Seed = x"FF"

-- Result = "1111111101001000000011101100000010011010"

-- Width = 8

-- Mode = Galois ('1')

-- Polynomial = x"A9"

-- Seed = x"FF"

-- Result = "101001011011000001011000110110"

-- libraries used

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

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

-- Entity declaration for ccsds_tx / unitary tx randomizer inputs and outputs

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

entity ccsds_rxtx_lfsr is

  generic(

    constant CCSDS_RXTX_LFSR_DATA_BUS_SIZE: integer; -- in bits

                constant CCSDS_RXTX_LFSR_MEMORY_SIZE: integer range 2 to 256 := 8; -- in bits

                constant CCSDS_RXTX_LFSR_MODE: std_logic := '0'; -- 0: Fibonacci / 1: Galois

                constant CCSDS_RXTX_LFSR_POLYNOMIAL: std_logic_vector   := x"A9"; -- Polynomial / MSB <=> lower polynome (needs to be '1')

                constant CCSDS_RXTX_LFSR_SEED: std_logic_vector := x"FF"                -- Initial Value

  );

  port(

    -- inputs

    clk_i: in std_logic;

    rst_i: in std_logic;

    -- outputs

    dat_o: out std_logic_vector(CCSDS_RXTX_LFSR_DATA_BUS_SIZE-1 downto 0);

    dat_val_o: out std_logic

  );

end ccsds_rxtx_lfsr;

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

-- architecture declaration / internal components and connections

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

architecture structure of ccsds_rxtx_lfsr is

-- internal constants

-- internal variable signals

        signal lfsr_memory: std_logic_vector(CCSDS_RXTX_LFSR_MEMORY_SIZE-1 downto 0) := CCSDS_RXTX_LFSR_SEED;

-- components instanciation and mapping

  begin

-- presynthesis checks

          CHKLFSRP0 : if CCSDS_RXTX_LFSR_POLYNOMIAL'length /= CCSDS_RXTX_LFSR_MEMORY_SIZE generate

                  process

                  begin

                          report "ERROR: LFSR_POLYNOMIAL LENGTH MUST BE EQUAL TO MEMORY SIZE (SHORTENED VERSION / DON'T PUT MANDATORY HIGHER POLYNOME '1')" severity failure;

                          wait;

                  end process;

          end generate CHKLFSRP0;

          CHKLFSRP1 : if CCSDS_RXTX_LFSR_MEMORY_SIZE <= 1 generate

                  process

                  begin

                          report "ERROR: LFSR_MEMORY_SIZE MUST BE BIGGER THAN 1" severity failure;

                          wait;

                  end process;

          end generate CHKLFSRP1;

          CHKLFSRP2 : if CCSDS_RXTX_LFSR_SEED'length /= CCSDS_RXTX_LFSR_MEMORY_SIZE generate

                  process

                  begin

                          report "ERROR: LFSR_SEED LENGTH MUST BE EQUAL TO LFSR_MEMORY_SIZE" severity failure;

                          wait;

                  end process;

          end generate CHKLFSRP2;

    CHKLFSRP3 : if CCSDS_RXTX_LFSR_POLYNOMIAL(CCSDS_RXTX_LFSR_MEMORY_SIZE-1) = '0' generate

      process

      begin

        report "ERROR: LFSR POLYNOMIAL MSB MUST BE EQUAL TO 1" severity failure;

        wait;

      end process;

    end generate CHKLFSRP3;

-- internal processing

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

    -- Begin of crcp

    -- Compute CRC based on input data

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

    -- read: rst_i

    -- write: dat_o, dat_val_o

    -- r/w: lfsr_memory

    LFSRP: process (clk_i)

    variable output_pointer: integer range -1 to (CCSDS_RXTX_LFSR_DATA_BUS_SIZE-1) := CCSDS_RXTX_LFSR_DATA_BUS_SIZE-1;

    variable feedback_register: std_logic := '0';

    begin

      -- on each clock rising edge

      if rising_edge(clk_i) then

        -- reset signal received

        if (rst_i = '1') then

          lfsr_memory <= CCSDS_RXTX_LFSR_SEED;

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

          dat_val_o <= '0';

          output_pointer := CCSDS_RXTX_LFSR_DATA_BUS_SIZE-1;

          feedback_register := '0';

        else

          -- generation is finished

          if (output_pointer = -1) then

            dat_val_o <= '1';

          -- generating sequence

          else

            dat_val_o <= '0';

            -- Fibonacci

            if (CCSDS_RXTX_LFSR_MODE = '0') then

              dat_o(output_pointer) <= lfsr_memory(CCSDS_RXTX_LFSR_MEMORY_SIZE-1);

              output_pointer := output_pointer - 1;

              feedback_register := lfsr_memory(CCSDS_RXTX_LFSR_MEMORY_SIZE-1);

              for i in 0 to CCSDS_RXTX_LFSR_MEMORY_SIZE-2 loop

                if (CCSDS_RXTX_LFSR_POLYNOMIAL(i) = '1') then

                  feedback_register := feedback_register xor lfsr_memory(i);

                end if;

              end loop;

              lfsr_memory <= std_logic_vector(resize(unsigned(lfsr_memory),CCSDS_RXTX_LFSR_MEMORY_SIZE-1)) & feedback_register;

            -- Galois

            else

              dat_o(output_pointer) <= lfsr_memory(CCSDS_RXTX_LFSR_MEMORY_SIZE-1);

              output_pointer := output_pointer - 1;

              lfsr_memory(0) <= lfsr_memory(CCSDS_RXTX_LFSR_MEMORY_SIZE-1);

              for i in 1 to CCSDS_RXTX_LFSR_MEMORY_SIZE-1 loop

                if (CCSDS_RXTX_LFSR_POLYNOMIAL(i) = '1') then

                  lfsr_memory(i) <= lfsr_memory(i-1) xor lfsr_memory(CCSDS_RXTX_LFSR_MEMORY_SIZE-1);

                else

                  lfsr_memory(i) <= lfsr_memory(i-1);

                end if;

              end loop;

            end if;

          end if;

        end if;

      end if;

    end process;

end structure;