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

-- File         : arbiter.vhd

-- Description  : Sub-block for allocator

-- Designer     : Hannu Penttinen 29.08.2006

--

-- Note: If there's problems with synthesis concerning the carry

--       chain. Try dublicating the first arbiter and

--       connecting '0' to the carry in of the first arbiter and

--       ORing the grants of the first and second arbiter.

--       Done with carry1 and carry2

--

--      Req and hold must be asserted simultanesouly. When granted, req can be

--      de-asserted

--

--      The structure shown in Dally,Towles, fig 18.5, 18.6 and 18.7

--      

-- Note:          arb_type_g  0 - round-robin

--                            1 - fixed priority

--                            2 - variable priority

-- 2012-01-22     alhonen fixed names

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

library ieee;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

entity sdram_arbiter is

  generic (

    arb_width_g : integer;

    arb_type_g  : integer := 0

    );

  port(

    clk       : in  std_logic;

    rst_n     : in  std_logic;

    req_in    : in  std_logic_vector(arb_width_g - 1 downto 0);

    hold_in   : in  std_logic_vector(arb_width_g - 1 downto 0);

    grant_out : out std_logic_vector(arb_width_g - 1 downto 0)

    );

end sdram_arbiter;

architecture rtl of sdram_arbiter is

  signal priority_r   : std_logic_vector (arb_width_g - 1 downto 0);

  signal carry_1      : std_logic_vector (arb_width_g - 1 downto 0);

  signal carry_2      : std_logic_vector (arb_width_g - 1 downto 0);

  signal gc           : std_logic_vector (arb_width_g - 1 downto 0);

  signal grant_i      : std_logic_vector (arb_width_g - 1 downto 0);

  signal last_grant_r : std_logic_vector (arb_width_g - 1 downto 0);

begin  -- rtl

  grant_out <= grant_i;

  -- Generate intermediate grant with aid of carry signals

  -- Carry means that no higher prior has asserted any requests

  -- Carry logic is dupliacted to avoid combinatorial loop,

  -- only difference is in the bit carry_x(0)

  arbiter : process (rst_n, priority_r, req_in, carry_1, carry_2)

  begin  -- process arbiter

    if rst_n = '0' then

      carry_1 <= (others => '0');

      carry_2 <= (others => '0');

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

    else

      -- generate carry signal

      carry_1(0) <= '0';

      for i in 1 to arb_width_g - 1 loop

        carry_1(i) <= ( (priority_r(i-1) or carry_1(i-1)) and not(req_in(i-1)));

      end loop;  -- i

      carry_2(0) <= ((priority_r(arb_width_g - 1) or carry_1(arb_width_g - 1))

                   and not(req_in(arb_width_g - 1)));

      for i in 1 to arb_width_g - 1 loop

        carry_2(i) <= ( (priority_r(i-1) or carry_2(i-1)) and not(req_in(i-1)));

      end loop;  -- i

      -- generate intermediate grant signal

      for i in 0 to arb_width_g - 1 loop

        gc(i) <= ((priority_r(i) or carry_1(i)) and req_in(i)) or

                 ((priority_r(i) or carry_2(i)) and req_in(i));

      end loop;  -- i

    end if;

  end process arbiter;

  -- grant-hold circuit

  -- a) Previous grant remains if hold is active

  -- b) new grant is given if no holds are asserted

  grant_hold_async : process(gc, last_grant_r, hold_in)

    variable anyhold_v : std_logic;

  begin  -- process grant_hold_async

    anyhold_v := '0';

    for i in 0 to arb_width_g - 1 loop

      anyhold_v := anyhold_v or (hold_in(i) and last_grant_r(i));

    end loop;  -- i

    for i in 0 to arb_width_g - 1 loop

      grant_i(i) <= (last_grant_r(i) and hold_in(i))

                    or (gc(i) and not(anyhold_v));

    end loop;  -- i

  end process grant_hold_async;

  -- grant-hold: register previous grant signal (one-hot)

  grant_hold_sync : process (clk, rst_n)

  begin  -- process grant_hold_sync

    if rst_n = '0' then                 -- asynchronous reset (active low)

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

    elsif clk'event and clk = '1' then  -- rising clock edge

      last_grant_r <= grant_i;

    end if;

  end process grant_hold_sync;

  -- purpose: update priority register(round-robin)

  -- Input that got grant is put to the lowest priority

  -- One-hot encoded: one bit shows the highest priority,

  -- if prior(i)=1, i has highest prior, i+1 has 2nd highest and so on

  pri_round_robin : process (clk, rst_n)

    variable anyg_v : std_logic;

  begin  -- process pri_round_robin

    if rst_n = '0' then                 -- asynchronous reset (active low)

      priority_r <= std_logic_vector (to_unsigned(1, arb_width_g));

    elsif clk'event and clk = '1' then  -- rising clock edge

      case arb_type_g is

        when 0 =>

          -- round-robin

          anyg_v := '0';

          for i in 0 to arb_width_g - 1 loop

            anyg_v := anyg_v or grant_i(i);

          end loop;  -- i

          priority_r(0) <= ( priority_r(0) and not(anyg_v) ) or grant_i(arb_width_g - 1);

          for i in 1 to arb_width_g - 1 loop

            priority_r(i) <= (priority_r(i) and not(anyg_v)) or grant_i(i-1);

          end loop;  -- i

        when 1 =>

          -- fixed_priority

          priority_r <= priority_r;

        when 2 =>

          -- variable priority

          if grant_i /= std_logic_vector(to_unsigned(0, grant_i'length)) then

            priority_r <= priority_r(priority_r'length - 2 downto 0) & priority_r(priority_r'length - 1);

          else

            priority_r <= priority_r;

          end if;

        when others => null;

      end case;

    end if;

  end process pri_round_robin;

end rtl;