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-- File : arbiter.vhd
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-- Description : Sub-block for allocator
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-- Designer : Hannu Penttinen 29.08.2006
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--
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-- Note: If there's problems with synthesis concerning the carry
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-- chain. Try dublicating the first arbiter and
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-- connecting '0' to the carry in of the first arbiter and
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-- ORing the grants of the first and second arbiter.
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-- Done with carry1 and carry2
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--
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-- Req and hold must be asserted simultanesouly. When granted, req can be
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-- de-asserted
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--
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-- The structure shown in Dally,Towles, fig 18.5, 18.6 and 18.7
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--
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-- Note: arb_type_g 0 - round-robin
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-- 1 - fixed priority
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-- 2 - variable priority
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-----------------------------------------------------------------
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-------------------------------------------------------------------------------
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-- Copyright (c) 2011 Tampere University of Technology
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-------------------------------------------------------------------------------
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-- This file is part of Transaction Generator.
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--
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-- Transaction Generator is free software: you can redistribute it and/or
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-- modify it under the terms of the Lesser GNU General Public License as
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-- published by the Free Software Foundation, either version 3 of the License,
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-- or (at your option) any later version.
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--
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-- Transaction Generator is distributed in the hope that it will be useful,
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-- but WITHOUT ANY WARRANTY; without even the implied warranty of
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-- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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-- Lesser GNU General Public License for more details.
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--
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-- You should have received a copy of the Lesser GNU General Public License
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-- along with Transaction Generator. If not, see
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-- <http://www.gnu.org/licenses/>.
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-------------------------------------------------------------------------------
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library ieee;
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use ieee.std_logic_1164.all;
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use ieee.numeric_std.all;
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entity arbiter is
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generic (
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arb_width_g : integer;
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arb_type_g : integer := 0
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);
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port(
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clk : in std_logic;
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rst_n : in std_logic;
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req_in : in std_logic_vector(arb_width_g - 1 downto 0);
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hold_in : in std_logic_vector(arb_width_g - 1 downto 0);
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grant_out : out std_logic_vector(arb_width_g - 1 downto 0)
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);
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end arbiter;
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architecture rtl of arbiter is
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signal priority_r : std_logic_vector (arb_width_g - 1 downto 0);
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signal carry_1 : std_logic_vector (arb_width_g - 1 downto 0);
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signal carry_2 : std_logic_vector (arb_width_g - 1 downto 0);
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signal gc : std_logic_vector (arb_width_g - 1 downto 0);
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signal grant_i : std_logic_vector (arb_width_g - 1 downto 0);
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signal last_grant_r : std_logic_vector (arb_width_g - 1 downto 0);
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begin -- rtl
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grant_out <= grant_i;
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-- Generate intermediate grant with aid of carry signals
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-- Carry means that no higher prior has asserted any requests
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-- Carry logic is dupliacted to avoid combinatorial loop,
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-- only difference is in the bit carry_x(0)
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arbiter : process (rst_n, priority_r, req_in, carry_1, carry_2)
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begin -- process arbiter
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if rst_n = '0' then
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carry_1 <= (others => '0');
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carry_2 <= (others => '0');
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gc <= (others => '0');
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else
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-- generate carry signal
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carry_1(0) <= '0';
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for i in 1 to arb_width_g - 1 loop
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carry_1(i) <= ( (priority_r(i-1) or carry_1(i-1)) and not(req_in(i-1)));
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end loop; -- i
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carry_2(0) <= ((priority_r(arb_width_g - 1) or carry_1(arb_width_g - 1))
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and not(req_in(arb_width_g - 1)));
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for i in 1 to arb_width_g - 1 loop
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carry_2(i) <= ( (priority_r(i-1) or carry_2(i-1)) and not(req_in(i-1)));
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end loop; -- i
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-- generate intermediate grant signal
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for i in 0 to arb_width_g - 1 loop
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gc(i) <= ((priority_r(i) or carry_1(i)) and req_in(i)) or
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((priority_r(i) or carry_2(i)) and req_in(i));
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end loop; -- i
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end if;
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end process arbiter;
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-- grant-hold circuit
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-- a) Previous grant remains if hold is active
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-- b) new grant is given if no holds are asserted
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grant_hold_async : process(gc, last_grant_r, hold_in)
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variable anyhold_v : std_logic;
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begin -- process grant_hold_async
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anyhold_v := '0';
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for i in 0 to arb_width_g - 1 loop
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anyhold_v := anyhold_v or (hold_in(i) and last_grant_r(i));
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end loop; -- i
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for i in 0 to arb_width_g - 1 loop
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grant_i(i) <= (last_grant_r(i) and hold_in(i))
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or (gc(i) and not(anyhold_v));
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end loop; -- i
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end process grant_hold_async;
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-- grant-hold: register previous grant signal (one-hot)
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grant_hold_sync : process (clk, rst_n)
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begin -- process grant_hold_sync
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if rst_n = '0' then -- asynchronous reset (active low)
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last_grant_r <= (others => '0');
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elsif clk'event and clk = '1' then -- rising clock edge
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last_grant_r <= grant_i;
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end if;
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end process grant_hold_sync;
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-- purpose: update priority register(round-robin)
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-- Input that got grant is put to the lowest priority
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-- One-hot encoded: one bit shows the highest priority,
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-- if prior(i)=1, i has highest prior, i+1 has 2nd highest and so on
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pri_round_robin : process (clk, rst_n)
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variable anyg_v : std_logic;
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begin -- process pri_round_robin
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if rst_n = '0' then -- asynchronous reset (active low)
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priority_r <= std_logic_vector (to_unsigned(1, arb_width_g));
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elsif clk'event and clk = '1' then -- rising clock edge
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case arb_type_g is
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when 0 =>
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-- round-robin
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anyg_v := '0';
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for i in 0 to arb_width_g - 1 loop
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anyg_v := anyg_v or grant_i(i);
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end loop; -- i
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priority_r(0) <= ( priority_r(0) and not(anyg_v) ) or grant_i(arb_width_g - 1);
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for i in 1 to arb_width_g - 1 loop
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priority_r(i) <= (priority_r(i) and not(anyg_v)) or grant_i(i-1);
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end loop; -- i
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when 1 =>
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-- fixed_priority
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priority_r <= priority_r;
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when 2 =>
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-- variable priority
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if grant_i /= std_logic_vector(to_unsigned(0, grant_i'length)) then
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priority_r <= priority_r(priority_r'length - 2 downto 0) & priority_r(priority_r'length - 1);
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else
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priority_r <= priority_r;
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end if;
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when others => null;
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end case;
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end if;
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end process pri_round_robin;
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end rtl;
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