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

-- Package

--

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

use IEEE.MATH_REAL.ALL;

package bus_arbiter_pack is

  component bus_arbiter_N_way

    generic(

      LOCKING : integer; -- Nonzero to hold until ack_i is received.

      N_VALUE : integer; -- Number of bus requestors.

      LOG2_N  : integer  -- Bit width of msel_o

    );

    port (

      -- System Clock and Clock Enable

      sys_rst_n   : in  std_logic;

      sys_clk     : in  std_logic;

      sys_clk_en  : in  std_logic;

      -- Bus Access Request Inputs

      req_i       : in  unsigned(N_VALUE-1 downto 0);

      -- Status

      cyc_o       : out std_logic;

      hold_o      : out std_logic; -- Bus lock

      -- Ram Access Acknowledge

      ack_i       : in  std_logic; -- Releases lock

      -- Ram Selection (Use to control external muxes)

      msel_o      : out unsigned(LOG2_N-1 downto 0);

      msel_new_o  : out std_logic

    );

  end component;

  component bus_arbiter_dataflow_N_way

    generic(

      LOCKING : integer; -- Nonzero to hold until ack_i is received.

      N_VALUE : integer; -- Number of bus requestors.

      LOG2_N  : integer  -- Bit width of msel_o

    );

    port (

      -- System Clock and Clock Enable

      sys_rst_n   : in  std_logic;

      sys_clk     : in  std_logic;

      sys_clk_en  : in  std_logic;

      -- Bus Access Request Inputs

      req_i       : in  unsigned(N_VALUE-1 downto 0);

      -- Status

      cyc_o       : out std_logic;

      hold_o      : out std_logic; -- Bus lock

      -- Ram Access Acknowledge

      ack_i       : in  std_logic; -- Releases lock

      -- Ram Selection (Use to control external muxes)

      msel_o      : out unsigned(LOG2_N-1 downto 0);

      msel_new_o  : out std_logic

    );

  end component;

  component bus_requester_N_way

    generic(

      N_VALUE : integer -- Number of bus requestors.

    );

    port (

      -- System Clock and Clock Enable

      sys_rst_n   : in  std_logic;

      sys_clk     : in  std_logic;

      sys_clk_en  : in  std_logic;

      -- Single "master" Bus Access Request and acknowledge

      req_i       : in  std_logic;

      ack_o       : out std_logic;

      -- A multiplicity of subordinate bus requests

      n_req_o     : out unsigned(N_VALUE-1 downto 0);

      -- A multiplicity of subordinate Bus Access Acknowledge signals

      n_ack_i     : in  unsigned(N_VALUE-1 downto 0)

    );

  end component;

end bus_arbiter_pack;

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

-- Bus Access Arbitration Module

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

--

-- Author: John Clayton

-- Date  : July 13, 2011 Copied code from "bus_arbiter_4_way" to begin.

--                       I am hoping to derive a more parameterized

--                       module.

--         Sept 11, 2012 After much successful use in hardware, I have

--                       revisited this module and simplified it so that

--                       requests are immediately driven out to msel.

--                       The intent is to eliminate wasted clock cycles.

--         Nov. 21, 2012 I have discovered that I was using an external

--                       mux for the bus_cyc signal, which is fairly

--                       understandable, since I'm using external muxes

--                       for nearly everything...  However, in the case

--                       of the bys_cyc signal, there is an unnecessary

--                       "opportunity for error" since bus_cyc is always

--                       set to the currently selected request line. It

--                       occurred to me that this can be moved inside

--                       the module, thereby eliminating the possibility

--                       of connecting it up incorrectly outside the

--                       module.  Thus, I'm adding the cyc_o signal.

--         May  15, 2013 While simulating a design, I discovered that

--                       while one request is active, another request

--                       may arrive which alters the state of msel_next,

--                       thus altering the output msel_o.  This is a flaw

--                       since requesters may be depending on the value

--                       of msel_o in order to receive acknowledgments,

--                       and could therefore erroneously see a ack when

--                       msel_o changed.  A new statement was added to

--                       "lock out" changes to msel_next while the current

--                       request is still active.

--         May  23, 2013 Because accesses through multiple arbiters using

--                       a single cycle seemed "aggressive" I put in a

--                       one cycle delay here.  This arbiter has the

--                       advantage now of not synthesizing any latches...

--                       The "dataflow" version could still be used, perhaps,

--                       reliably with a delayed acknowledge signal!

--         Feb.  6, 2014 Added cyc_l for fast clearing of cyc_o.

--         Feb. 11, 2014 Added LOCKING generic and hold_o output, to

--                       permit locking the arbiter until ack_i is

--                       received.

--                 

--

-- Description

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

-- This module is an N way request arbitration unit.  There are N high

-- asserted request inputs.  When any subset of these is asserted, the arbitrator

-- causes its "msel" (multiplexer select address) output to control muxes that

-- route the appropriate request to the bus.

--

-- This unit is meant to coordinate access to a single resource bus, by N requesters.

-- The access is not exactly fair, since there is no timeout implemented here.

-- Thus, a requester must implement its own timeout, and if it never receives

-- an acknoledge signal, and never deasserts its request line, then the bus

-- will be "hogged" by that requester for ever.  The purpose of this module

-- is not to rip away the bus from a misbehaving or irresponsible requester.

-- Rather, this module gives out access to the bus in turns.  Each turn must

-- be fairly conducted by the requester.

--

-- There is a single acknowledge signal from the single bus.  The assertion of

-- the ack_i signal for one sys_clk edge is sufficient to terminate the current

-- access cycle, and cause this unit to change the msel_o output to route signals

-- from the next requester to the memory bus.  This same termination condition

-- should be respected by all requesters using this module.

--

-- The reason that external muxes are used to send the bus signals to the RAM is

-- to enable other muxes to be controlled for additional processing, e.g. an

-- offset engine which modifies the address presented to the bus, with the offset

-- being different depending on which requester is using the bus.

--

-- The policy of "fairness" followed by this module is of the "round robin" type.

-- In other words, each request when completed, is followed by a grant to the next

-- request, in order 0,1,2,...N-1,0,1...  There is no requirement for the request

-- lines to be returned to the inactive state before they can be asserted and

-- recognized again as valid requests.

--

-- There is a "parking" philosophy implemented in this module, which works in the

-- following way:  Whenever a request is completed, if there are no other pending

-- requests, the msel_o output remains in its current state.  This is probably

-- not an important fact, except to note that the most recent requester, already

-- having control of the memory bus, gets its next request granted one cycle faster

-- than usual, since its new request doesn't need to cause any change in the muxes

-- to grant it access to the memory bus.

--

-- Following reset, the msel_o output is set to zero, meaning that the bus is parked

-- for quickest access by requester zero.  Note that it is the user's responsibility

-- to ensure that the connections to the external muxes are made in such a way as to

-- correspond to the request inputs, according to this mapping:

--

--      Request Input         Corresponding msel_o output

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

--        req_i(0)                       00b

--        req_i(1)                       01b

--         .....                         ...

--        req_i(N-1)                     unsigned(N-1)

--

-- All storage elements within this module are clocked according to the positive edge

-- of the sys_clk input, qualified by the sys_clk_en input being asserted high.  In

-- this way, sys_clk_en can be used to run this module at slower rates than sys_clk.

--

-- The sys_rst_n input is an asynchronous reset.

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

use IEEE.MATH_REAL.ALL;

entity bus_arbiter_N_way is

    generic(

      LOCKING : integer := 0; -- Nonzero to hold until ack_i is received.

      N_VALUE : integer := 4; -- Number of bus requestors.

      LOG2_N  : integer := 2  -- Bit width of msel_o

    );

    port (

      -- System Clock and Clock Enable

      sys_rst_n   : in  std_logic;

      sys_clk     : in  std_logic;

      sys_clk_en  : in  std_logic;

      -- Access Request Inputs

      req_i       : in  unsigned(N_VALUE-1 downto 0);

      -- Status

      cyc_o       : out std_logic;

      hold_o      : out std_logic; -- Bus lock

      -- Ram Access Acknowledge

      ack_i       : in  std_logic; -- Releases lock

      -- Selection (Use to control external muxes)

      msel_o      : out unsigned(LOG2_N-1 downto 0);

      msel_new_o  : out std_logic

    );

end bus_arbiter_N_way;

architecture beh of bus_arbiter_N_way is

  -- Constants

  -- Functions & associated types

  -- Signal Declarations

  signal msel_l        : unsigned(LOG2_N-1 downto 0);

  signal msel_prior    : unsigned(LOG2_N-1 downto 0);

  signal req_asserted  : std_logic;

  signal cyc_l         : std_logic;

  signal hold_l        : std_logic;

begin

-- Combine all incoming request signals into a single signal indicating

-- there is at least one request active.

req_asserted <= '1' when (req_i/=0) else '0';

process (sys_clk, sys_clk_en, sys_rst_n)

variable l : integer := 0;

variable k : integer := 0;

begin

  if (sys_rst_n='0') then

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

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

    cyc_l  <= '0';

    hold_l <= '0';

  elsif (sys_clk'event and sys_clk='1') then

    if (sys_clk_en='1') then

      for j in 0 to N_VALUE-1 loop

        l := j+to_integer(msel_l);

        -- Use an if statement to apply "modulo N_VALUE"

        if (l>=N_VALUE) then

          k := l-N_VALUE;

        else

          k := l;

        end if;

        if (req_i(k)='1') then

          msel_l <= to_unsigned(k,LOG2_N);

        end if;

      end loop;

      -- Lock out changes to msel_l when current request is still active

      if LOCKING=0 then

        if req_i(to_integer(msel_l))='1' then

          msel_l <= msel_l;

        end if;

      else

        if hold_l='1' and ack_i='0' then

          msel_l <= msel_l;

        end if;

      end if;

      -- Handle hold_l signal

      if LOCKING=1 then

        -- clear hold

        if (hold_l='1' and ack_i='1') then

          hold_l <= '0';

        end if;

        -- Setting hold has higher priority than clearing hold

        if req_i(to_integer(msel_l))='1' and (hold_l='0' or (hold_l='1' and ack_i='1')) then

          hold_l <= '1';

        end if;

      end if;

      -- Save previous msel_l value, to support msel_new_o

      msel_prior <= msel_l;

      -- Provide the currently selected request line as an

      -- arbiter-wide bus cycle signal

      cyc_l <= req_i(to_integer(msel_l));

    end if; -- sys_clk_en

  end if; -- sys_clk

end process;

msel_o <= msel_l;

msel_new_o <= '1' when (msel_prior/=msel_l) else '0';

-- Clear the bus cycle output immediately upon removal of request.

cyc_o <= cyc_l and req_asserted;

-- Provide hold output

hold_o <= hold_l;

end beh;

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

-- Bus Access Arbitration Module - Dataflow version

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

--

-- Author: John Clayton

-- Date  : July 13, 2011 Copied code from "bus_arbiter_4_way" to begin.

--                       I am hoping to derive a more parameterized

--                       module.

--         Sept 11, 2012 After much successful use in hardware, I have

--                       revisited this module and simplified it so that

--                       requests are immediately driven out to msel.

--                       The intent is to eliminate wasted clock cycles.

--         Nov. 21, 2012 I have discovered that I was using an external

--                       mux for the bus_cyc signal, which is fairly

--                       understandable, since I'm using external muxes

--                       for nearly everything...  However, in the case

--                       of the bys_cyc signal, there is an unnecessary

--                       "opportunity for error" since bus_cyc is always

--                       set to the currently selected request line. It

--                       occurred to me that this can be moved inside

--                       the module, thereby eliminating the possibility

--                       of connecting it up incorrectly outside the

--                       module.  Thus, I'm adding the cyc_o signal.

--         May  15, 2013 While simulating a design, I discovered that

--                       while one request is active, another request

--                       may arrive which alters the state of msel_next,

--                       thus altering the output msel_o.  This is a flaw

--                       since requesters may be depending on the value

--                       of msel_o in order to receive acknowledgments,

--                       and could therefore erroneously see a ack when

--                       msel_o changed.  A new statement was added to

--                       "lock out" changes to msel_next while the current

--                       request is still active.

--         Feb. 11, 2014 Added LOCKING generic and hold_o output, to

--                       permit locking the arbiter until ack_i is

--                       received.

--                 

--

-- Description

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

-- This module is an N way request arbitration unit.  There are N high

-- asserted request inputs.  When any subset of these is asserted, the arbitrator

-- causes its "msel" (multiplexer select address) output to control muxes that

-- route the appropriate request to the bus.

--

-- This unit is meant to coordinate access to a single resource bus, by N requesters.

-- The access is not exactly fair, since there is no timeout implemented here.

-- Thus, a requester must implement its own timeout, and if it never receives

-- an acknoledge signal, and never deasserts its request line, then the bus

-- will be "hogged" by that requester for ever.  The purpose of this module

-- is not to rip away the bus from a misbehaving or irresponsible requester.

-- Rather, this module gives out access to the bus in turns.  Each turn must

-- be fairly conducted by the requester.

--

-- There is a single acknowledge signal from the single bus.  The assertion of

-- the ack_i signal for one sys_clk edge is sufficient to terminate the current

-- access cycle, and cause this unit to change the msel_o output to route signals

-- from the next requester to the memory bus.  This same termination condition

-- should be respected by all requesters using this module.

--

-- The reason that external muxes are used to send the bus signals to the RAM is

-- to enable other muxes to be controlled for additional processing, e.g. an

-- offset engine which modifies the address presented to the bus, with the offset

-- being different depending on which requester is using the bus.

--

-- The policy of "fairness" followed by this module is of the "round robin" type.

-- In other words, each request when completed, is followed by a grant to the next

-- request, in order 0,1,2,...N-1,0,1...  There is no requirement for the request

-- lines to be returned to the inactive state before they can be asserted and

-- recognized again as valid requests.

--

-- There is a "parking" philosophy implemented in this module, which works in the

-- following way:  Whenever a request is completed, if there are no other pending

-- requests, the msel_o output remains in its current state.  This is probably

-- not an important fact, except to note that the most recent requester, already

-- having control of the memory bus, gets its next request granted one cycle faster

-- than usual, since its new request doesn't need to cause any change in the muxes

-- to grant it access to the memory bus.

--

-- Following reset, the msel_o output is set to zero, meaning that the bus is parked

-- for quickest access by requester zero.  Note that it is the user's responsibility

-- to ensure that the connections to the external muxes are made in such a way as to

-- correspond to the request inputs, according to this mapping:

--

--      Request Input         Corresponding msel_o output

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

--        req_i(0)                       00b

--        req_i(1)                       01b

--         .....                         ...

--        req_i(N-1)                     unsigned(N-1)

--

-- All storage elements within this module are clocked according to the positive edge

-- of the sys_clk input, qualified by the sys_clk_en input being asserted high.  In

-- this way, sys_clk_en can be used to run this module at slower rates than sys_clk.

--

-- The sys_rst_n input is an asynchronous reset.

--

-- This version of the bus arbiter is a "dataflow" version in the sense that the

-- msel_o outputs change immediately upon a request input going high, thus allowing

-- for a bus cycle to be arbitrated and acknowledged in fewer sys_clk periods.

--

-- However, it synthesizes a latch in the msel_next process.  If this is nettlesome,

-- then do not use it...

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

use IEEE.MATH_REAL.ALL;

entity bus_arbiter_dataflow_N_way is

    generic(

      LOCKING : integer := 0; -- Nonzero to hold until ack_i is received.

      N_VALUE : integer := 4; -- Number of bus requestors.

      LOG2_N  : integer := 2  -- Bit width of msel_o

    );

    port (

      -- System Clock and Clock Enable

      sys_rst_n   : in  std_logic;

      sys_clk     : in  std_logic;

      sys_clk_en  : in  std_logic;

      -- Ram Access Request Inputs

      req_i       : in  unsigned(N_VALUE-1 downto 0);

      -- Status

      cyc_o       : out std_logic;

      hold_o      : out std_logic; -- Bus lock

      -- Ram Access Acknowledge

      ack_i       : in  std_logic; -- Releases lock

      -- Ram Selection (Use to control external muxes)

      msel_o      : out unsigned(LOG2_N-1 downto 0);

      msel_new_o  : out std_logic

    );

end bus_arbiter_dataflow_N_way;

architecture beh of bus_arbiter_dataflow_N_way is

  -- Constants

  -- Functions & associated types

  -- Signal Declarations

  -- type msel_type is integer range 0 to N_VALUE-1; -- changed to integer for the simulator...

  -- signal msel_l        : msel_type := 0;

  -- signal msel_prev     : msel_type := 0;

  signal msel_next     : unsigned(LOG2_N-1 downto 0);

  signal msel_l        : unsigned(LOG2_N-1 downto 0);

  signal msel_prior    : unsigned(LOG2_N-1 downto 0);

  signal req_asserted  : std_logic;

  signal hold_l        : std_logic;

begin

-- Combine all incoming request signals into a single signal indicating

-- there is at least one request active.

req_asserted <= '1' when (req_i/=0) else '0';

process (msel_l,req_i)

-- type msel_modulo_type is integer range 0 to 2*N_VALUE-1; -- changed to integer, to satisfy simulator...

-- variable l : msel_modulo_type := 0;

-- variable k : msel_type := 0;

variable l : integer := 0;

variable k : integer := 0;

begin

  -- Change mux selections based on request and ack inputs.

  msel_next <= msel_l; -- Default value, prevents latch formation

  for j in 0 to N_VALUE-1 loop

    l := j+to_integer(msel_l);

    -- Use an if statement to apply "modulo N_VALUE"

    if (l>=N_VALUE) then

      k := l-N_VALUE;

    else

      k := l;

    end if;

    if (req_i(k)='1') then

      msel_next <= to_unsigned(k,LOG2_N);

    end if;

  end loop;

  -- Lock out changes to msel_next when current request is still active

  if LOCKING=0 then

    if req_i(to_integer(msel_l))='1' then

      msel_next <= msel_next;

    end if;

  else

    if (hold_l='1' and ack_i='0') then

      msel_next <= msel_next;

    end if;

  end if;

end process;

process (sys_clk, sys_clk_en, sys_rst_n)

begin

  if (sys_rst_n='0') then

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

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

    hold_l <= '0';

  elsif (sys_clk'event and sys_clk='1') then

    if (sys_clk_en='1') then

      msel_prior <= msel_next;

      if LOCKING=0 then

        if req_asserted='1' then

          msel_l <= msel_next;

        end if;

      else

        if req_asserted='1' and (hold_l='0' or (hold_l='1' and ack_i='1')) then

          msel_l <= msel_next;

        end if;

      end if;

      -- Handle the hold_l signal

        -- clear hold

      if (hold_l='1' and ack_i='1') then

        hold_l <= '0';

      end if;

        -- Setting hold has higher priority than clearing hold

      if req_i(to_integer(msel_l))='1' and LOCKING=1 then

        if (hold_l='0' or (hold_l='1' and ack_i='1')) then

          hold_l <= '1';

        end if;

      end if;

    end if; -- sys_clk_en

  end if; -- sys_clk

end process;

msel_o <= msel_next;

msel_new_o <= '1' when (msel_prior/=msel_next) else '0';

-- Provide the currently selected request line as an

-- arbiter-wide bus cycle signal

cyc_o <= req_i(to_integer(msel_next));

hold_o <= '0';

end beh;

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

-- Multiple Bus Access Request Module

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

--

-- Author: John Clayton

-- Date  : Feb.  3, 2014 Copied code from "bus_arbiter_N_way" to begin.

--

-- Description

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

-- This module is an N way bus request unit.  It is meant for those rare days

-- when a single bus cycle is issued to multiple buses, all of which must

-- acknowledge the cycle before the request can be considered fulfilled.

--

-- Asserting a single "master" req_i signal gives rise to the assertion of a

-- multiplicity of bus request req_o signals.  Each of these is then used to

-- generate a bus cycle, with its associated acknowledge ack_i handshake signal

-- in return.  When each ack_i handshake is received, the associated request

-- line is deasserted.  However, the main "master" acknowledge is not given

-- until all the ack_i inputs have been received.

--

-- This module does not implement a cycle timeout of any kind.  Therefore, if

-- any of the multiple requested cycles does not complete, the master cycle

-- will remain unacknowledged, and it can "hang" forever in this state.  You

-- have been warned!

--

-- This is a "dataflow" unit in the sense that asserting req_i immediately

-- asserts the req_o signals, without any clock delays.  The acknowledge

-- ack_i inputs do not, however, immediately deassert the req_o outputs.

-- Instead, the req_o outputs are deasserted after one clock cycle.

--

-- Isn't that just as "clear as mud?"

--

-- The sys_rst_n input is an asynchronous reset.

library IEEE;

use IEEE.STD_LOGIC_1164.ALL;

use IEEE.NUMERIC_STD.ALL;

use IEEE.MATH_REAL.ALL;

entity bus_requester_N_way is

    generic(

      N_VALUE : integer := 2 -- Number of bus requestors.

    );

    port (

      -- System Clock and Clock Enable

      sys_rst_n   : in  std_logic;

      sys_clk     : in  std_logic;

      sys_clk_en  : in  std_logic;

      -- Single "master" Bus Access Request and acknowledge

      req_i       : in  std_logic;

      ack_o       : out std_logic;

      -- A multiplicity of subordinate bus requests

      n_req_o     : out unsigned(N_VALUE-1 downto 0);

      -- A multiplicity of subordinate Bus Access Acknowledge signals

      n_ack_i     : in  unsigned(N_VALUE-1 downto 0)

    );

end bus_requester_N_way;

architecture beh of bus_requester_N_way is

  -- Constants

  -- Functions & associated types

  -- Signal Declarations

  signal req_l    : unsigned(N_VALUE-1 downto 0);

  signal req_msk  : unsigned(N_VALUE-1 downto 0);

begin

process (sys_clk, sys_clk_en, sys_rst_n)

variable k : integer := 0;

begin

  if (sys_rst_n='0') then

    req_l      <= (others=>'1');

  elsif (sys_clk'event and sys_clk='1') then

    if (sys_clk_en='1') then

      -- Clear internal request bits when acknowledged

      for k in 0 to N_VALUE-1 loop

        if (n_ack_i(k)='1') then

          req_l(k) <= '0';

        end if;

      end loop;

      -- Reset the internal request bits

      if (req_i='0') then

        req_l <= (others=>'1');

      end if;

    end if; -- sys_clk_en

  end if; -- sys_clk

end process;

n_req_gen : for i in 0 to N_VALUE-1 generate

  n_req_o(i) <= '1' when req_i='1' and req_l(i)='1' else '0';

end generate n_req_gen;

-- The masked version of req_l proleptically reflects the request

-- bits which are about to be cleared.

req_msk <= req_l and (not n_ack_i);

ack_o <= '1' when req_i='1' and req_msk=0 else '0';

end beh;