LIBRARY ieee;
USE ieee.std_logic_1164.ALL;

-- simple arbiter. Implements round robin priority scheme

-- Author: B. Reese, Summer'97 VHDL class

entity  arbiter is


  port (
	clk     : in     std_logic;                   -- input clock
	reset     : in     std_logic;                   -- input reset
	breq    : in     std_logic_vector(7 downto 0);  -- bus request
	bgrant  : out    std_logic_vector(7 downto 0);  -- bus grant
	bbusy    : in    std_logic                    -- bus busy
	);

end arbiter;



architecture behv of arbiter is



  type fsmstate is (NOGRANTS,ONEGRANT_WAIT,ONEGRANT,OVERLAP,OVERLAP2);
  signal pstate,nstate : fsmstate;
  
  signal breq_int: std_logic_vector(7 downto 0);
  signal bgrant_int: std_logic_vector(7 downto 0);
  signal pending_next, pending: std_logic_vector(2 downto 0);
  signal current_next, current: std_logic_vector(2 downto 0);
  signal current_mux :std_logic;
  signal startp_next, startp: std_logic_vector(2 downto 0) := "000";
  signal request,request_int: std_logic_vector(2 downto 0);
  signal valid_req: std_logic;
  signal load_pending, load_current, load_startp: std_logic;

  signal bgrant_set, bgrant_clr: std_logic_vector(7 downto 0);
  signal bgrant_set_index, bgrant_clr_index:std_logic_vector (2 downto 0);
  signal bgrant_set_all: std_logic;
  signal bgrant_load : std_logic;
  signal bgrant_clr_en : std_logic;
  signal bgrant_set_en : std_logic;

  signal c:std_logic_vector(3 downto 0);
begin



  -- dumb 3 bit incrementer
  inc3bit:process (startp) 
    begin
      startp_next <= "000";
      if (startp = "000") then
	startp_next <= "001";
      end if;
      if (startp = "001") then
	startp_next <= "010";
      end if;
      if (startp = "010") then
	startp_next <= "011";
      end if;
      if (startp = "011") then
	startp_next <= "100";
      end if;
      if (startp = "100") then
	startp_next <= "101";
      end if;
      if (startp = "101") then
	startp_next <= "110";
      end if;
      if (startp = "110") then
	startp_next <= "111";
      end if;
      if (startp = "111") then
	startp_next <= "000";
      end if;
    end process inc3bit;

  -- basic priority process, determines priority pending
  -- will give NEXT pending request is current grant is asserted
  rcheck: process (breq_int,bgrant_int)
    begin
      valid_req <= '0';
      request_int <= "000";
      if ((breq_int(7) = '0')) then
	valid_req <= '1';
	request_int <= "111";
      end if;
      if ((breq_int(6) = '0')) then
	valid_req <= '1';
	request_int <= "110";
      end if;
      if ((breq_int(5) = '0')) then
	valid_req <= '1';
	request_int <= "101";
      end if;
      if ((breq_int(4) = '0')) then
	valid_req <= '1';
	request_int <= "100";
      end if;
      if ((breq_int(3) = '0')) then
	valid_req <= '1';
	request_int <= "011";
      end if;
      if ((breq_int(2) = '0')) then
	valid_req <= '1';
	request_int <= "010";
      end if;
      if ((breq_int(1) = '0')) then
	valid_req <= '1';
	request_int <= "001";
      end if;
      if ((breq_int(0) = '0')) then
	valid_req <= '1';
	request_int <= "000";
      end if;
    end process rcheck;

  bgrant_reg: process(clk,reset)
    begin
      if (reset = '0') then
	bgrant_int <= "00000000";
      elsif (clk'event and clk = '1') then
	if (bgrant_load = '1') then
	  for i in bgrant'range loop
	    if (bgrant_clr(i) = '1') then
	      bgrant_int(i) <= '0' ;
	    end if;
	    if (bgrant_set(i) = '1') then
	      bgrant_int(i) <= '1' ;
	    end if;
	  end loop;
	end if;
      end if;
    end process bgrant_reg;
      

 -- set one bit of the bgrant register based on 'bgrant_val', other bits
 -- get old values
 set_bgrant: process (bgrant_clr_index, bgrant_set_index, bgrant_set_all,bgrant_clr_en,bgrant_set_en)
   begin

     bgrant_clr <= "00000000";
     bgrant_set <= "00000000";
     if (bgrant_set_all = '1') then
       bgrant_set <= "11111111";
     end if;
     if (bgrant_set_en = '1') then
	 if (bgrant_set_index = "000") then
	   bgrant_set(0) <= '1';
	 end if;
	 if (bgrant_set_index = "001") then
	   bgrant_set(1) <= '1';
	 end if;
	 if (bgrant_set_index = "010") then
	   bgrant_set(2) <= '1';
	 end if;
	 if (bgrant_set_index = "011") then
	   bgrant_set(3) <= '1';
	 end if;
	 if (bgrant_set_index = "100") then
	   bgrant_set(4) <= '1';
	 end if;
	 if (bgrant_set_index = "101") then
	   bgrant_set(5) <= '1';
	 end if;
	 if (bgrant_set_index = "110") then
	   bgrant_set(6) <= '1';
	 end if;
	 if (bgrant_set_index = "111") then
	   bgrant_set(7) <= '1';
	 end if;
     end if;
     if (bgrant_clr_en = '1') then

	 if (bgrant_clr_index = "000") then
	   bgrant_clr(0) <= '1';
	 end if;
	 if (bgrant_clr_index = "001") then
	   bgrant_clr(1) <= '1';
	 end if;
	 if (bgrant_clr_index = "010") then
	   bgrant_clr(2) <= '1';
	 end if;
	 if (bgrant_clr_index = "011") then
	   bgrant_clr(3) <= '1';
	 end if;
	 if (bgrant_clr_index = "100") then
	   bgrant_clr(4) <= '1';
	 end if;
	 if (bgrant_clr_index = "101") then
	   bgrant_clr(5) <= '1';
	 end if;
	 if (bgrant_clr_index = "110") then
	   bgrant_clr(6) <= '1';
	 end if;
	 if (bgrant_clr_index = "111") then
	   bgrant_clr(7) <= '1';
	 end if;
     end if;
   end process set_bgrant;

  -- map input request signals to internal request
  breq_map: process (breq,startp, bgrant_int)
    begin
      -- for right now, just do straight mapping
      -- internal breq is masked by bgrant
      -- if bgrant is low, then internal breq are high
      for i in 0 to 7 loop
	breq_int(i) <= breq(i) or (not bgrant_int(i));
      end loop;
      if (startp = "001") then
	for i in 0 to 7 loop
	  breq_int(i) <= breq((i+1) mod 8) or (not bgrant_int((i+1) mod 8));
	end loop;
      end if;
      if (startp = "010") then
	for i in 0 to 7 loop
	  breq_int(i) <= breq((i+2) mod 8) or (not bgrant_int((i+2) mod 8));
	end loop;
      end if;
      if (startp = "011") then
	for i in 0 to 7 loop
	  breq_int(i) <= breq((i+3) mod 8) or (not bgrant_int((i+3) mod 8));
	end loop;
      end if;
      if (startp = "100") then
	for i in 0 to 7 loop
	  breq_int(i) <= breq((i+4) mod 8) or (not bgrant_int((i+4) mod 8));
	end loop;
      end if;
      if (startp = "101") then
	for i in 0 to 7 loop
	  breq_int(i) <= breq((i+5) mod 8) or (not bgrant_int((i+5) mod 8));
	end loop;
      end if;
      if (startp = "110") then
	for i in 0 to 7 loop
	  breq_int(i) <= breq((i+6) mod 8) or (not bgrant_int((i+6) mod 8));
	end loop;
      end if;
      if (startp = "111") then
	for i in 0 to 7 loop
	  breq_int(i) <= breq((i+7) mod 8) or (not bgrant_int((i+7) mod 8));
	end loop;
      end if;

    end process breq_map;

  -- map grant signals just map straight through
  bgrant_map: process (bgrant_int,startp)
    begin
      -- for right now, just do straight mapping
      bgrant <= bgrant_int;

    end process bgrant_map;

  -- map request signals based on start priority
  request_map: process (request_int,startp,c)

    begin

      -- just add start priority to request number
      c(0) <= '0';
      for i in 0 to 2 loop
	request(i) <= request_int(i) xor startp(i) xor c(i);
	c(i+1) <= (request_int(i) and startp(i)) or 
		  (c(i) and (request_int(i) or startp(i)));
      end loop;
    end process request_map;

	  
  ff:process (clk,reset) 
  begin

    if (reset = '0') then
      pstate <= fsmstate'LEFT;
      pending <= "000";
      current <= "000";
      startp <= "000";
    elsif (clk'event and clk = '1') then
      pstate <= nstate ;
      if (load_pending = '1') then
	pending <= pending_next ;
      end if;
      if (load_current = '1') then
	if (current_mux = '0') then
	  current <= current_next ;
	else
	  current <= pending ;
	end if;
      end if;
      if (load_startp = '1') then
	startp <= startp_next ;
      end if;
    end if;

  end process ff;



  
  -- grant the request. 
--Converted from non-synthesizeable code which is commented out.
--
  fsm: process (breq_int,bbusy,pstate, valid_req,request, current,pending)

--    variable i,j: integer;
--    variable pendreq :boolean;

--    variable pending_grant : natural;
--    variable current_grant : natural;
--    variable start_priority: natural;

  begin

    load_pending <= '0';
    load_current <= '0';


    bgrant_clr_index <= "000";
    bgrant_set_index <= "000";
    bgrant_set_all <= '0';
    bgrant_load <= '0';
    current_mux <= '0';
    current_next <= "000";
    pending_next <= "000";
    load_startp <= '0';

    bgrant_set_en <= '0';
    bgrant_clr_en <= '0';
    nstate <= pstate;
    
    case pstate is
      when NOGRANTS =>
	-- no current grants, make sure all grants are high
	-- bgrant_int <= (others =>'1') after 4 ns;
	bgrant_set_all <= '1';
	bgrant_load <= '1';

-- 	i := 0;
-- 	j := start_priority;
-- 	while (i < N) loop
-- 	  if (breq_int(j) = '0') then
-- 	    current_grant := j;
-- 	    nstate <= ONEGRANT_WAIT;
-- 	    exit;              -- exit the loop
-- 	  end if;
-- 	  i := i + 1;
-- 	  j := (j+1) mod N;
-- 	end loop;

	if (valid_req = '1') then
	  load_current <= '1';
	  current_next <= request;
	  nstate <= ONEGRANT_WAIT;
	  -- because bus grant is registered, we need to cause bus grant
	  -- to be loaded from here, bus grant for 'request' will be granted
	  -- next state, and we will remember what grant was made.
	  -- this will cause the bgrant line indexed by 'request' to go low
	  bgrant_clr_index <= request;
	  bgrant_clr_en <= '1';
	  bgrant_set_all <= '0';
	  load_startp <= '1';
	  
	end if;

      when ONEGRANT_WAIT =>
--	if (pstate'event and pstate'last_value = NOGRANTS) then
--	  if (ROUND_ROBIN = TRUE ) then
--	    start_priority := (start_priority+1) mod N;
--	  end if;
--	end if;


	-- grant goes valid in clock cycle after seeing the breq_int
	-- done in previous state bgrant_int(current_grant) <= '0' after 4 ns;  -- grant bus
	-- one grant, wait for Bus Busy before allowing other grants
	if (bbusy = '0') then
	  nstate <= ONEGRANT;
	  if (valid_req = '1') then
	    load_pending <= '1';
	    pending_next <= request;
            nstate <= OVERLAP;
	    load_startp <= '1';
	    -- need to swap bus grants
	    bgrant_load <= '1';
	    bgrant_clr_en <= '1';
	    bgrant_set_en <= '1';
	    bgrant_set_index <= current;  -- negate current grant
	    bgrant_clr_index <= request;  -- assert pending grant
	  end if;

	  -- while we are waiting, see if we have overlapping grants
	  -- see if we should go to the OVERLAP state instead
-- 	  i := 0;
-- 	  j := start_priority;
-- 	  while (i < N) loop
-- 	    if ((breq_int(j) = '0') and (j /= current_grant)) then
-- 	      -- we have a pending grant, what do we do?
-- 	      -- ok, we need to negate the current grant, and assert 
-- 	      -- the pending grant
-- 	      pending_grant := j;
-- 	      nstate <= OVERLAP;
-- 	      exit;
-- 	    end if;
-- 	    i := i + 1;
-- 	    j := (j+1) mod N;
-- 	  end loop;

	end if;


      when ONEGRANT =>

	  -- ok, have done one grant and seen busy bus. Lets see if have any more pending grants

-- 	  i := 0;
-- 	  j := start_priority;
-- 	  while (i < N) loop
-- 	    if ((breq_int(j) = '0') and (j /= current_grant)) then
-- 	      -- we have a pending grant, what do we do?
-- 	      -- ok, we need to negate the current grant, and assert 
-- 	      -- the pending grant
-- 	      pending_grant := j;
-- 	      pendreq := TRUE;
-- 	      nstate <= OVERLAP;
-- 	      exit;
-- 	    end if;
-- 	    i := i + 1;
-- 	    j := (j+1) mod N;
-- 	  end loop;

	  if (valid_req = '1') then
	    load_pending <= '1';
	    pending_next <= request;

            nstate <= OVERLAP;
	    load_startp <= '1';

	    -- need to swap bus grants
	    bgrant_load <= '1';
	    bgrant_clr_en <= '1';
	    bgrant_set_en <= '1';

	    bgrant_set_index <= current;  -- negate current grant
	    bgrant_clr_index <= request;  -- assert pending grant
	  end if;


	-- regardless of whether or not we have a pending request
	-- if the master has released the bus we need to release the grant
	-- and go to the no grant state
	if (bbusy /= '0') then
	  -- release the grant
	  bgrant_load <= '1';
	  bgrant_set_all <= '1';
	  load_startp <= '0';
	  
	  nstate <= NOGRANTS;
	end if;

	-- we will bounce back/forth between OVERLAP, OVERLAP2 if we keep getting
	-- overlapped requests
      when OVERLAP =>

-- IN SYNTHESIZED model, we do this in states that TRANSISTION to this state
	-- on first entry to this state, we need to negate current grant, assert pending grant
--	if (pstate'event) then
--	  -- use pstate'event to make sure we only make this assignment once
--	  -- because the current_grant variable may change
--	  bgrant_int(current_grant) <= '1' after 4 ns;    -- negate pending grant
--	  bgrant_int(pending_grant) <= '0' after 4 ns;    -- assert pending grant
--	end if;

	-- ok, we have overlapped grants
	-- we need to wait until the current bus master releases
	if (bbusy /= '0') then
	  -- ok, what is actually happening here?  We are definititely going
	  -- to transision to either ONEGRANT_WAIT or OVERLAP.
	  -- either way we HAVE ALREADY granted the bus to the pending
	  -- bus master so we need to transfer the pending grant index
	  -- to the current grant index
	  
-- 	  if (bbusy /= '0' and bbusy'event) then
-- 	    -- ok, current bus master is swapping
-- 	    -- make sure swap only happens one time
-- 	    current_grant := pending_grant;
-- 	    if (ROUND_ROBIN = TRUE ) then
-- 	      start_priority := (start_priority+1) mod N;
-- 	    end if;

-- 	  end if;

	  current_mux <= '1';    --  current_grant := pending_grant;
	  load_current <= '1';

	  -- check to see if we have ANOTHER pending grant.

-- 	  pendreq := FALSE;
-- 	  i := 0;
-- 	  j := start_priority;
-- 	  while (i < N) loop
-- 	    if ((breq_int(j) = '0') and (j /= current_grant)) then
-- 	      -- we have a pending grant, what do we do?
-- 	      -- ok, we need to negate the current grant, and assert 
-- 	      -- the pending grant
-- 	      pending_grant := j;
-- 	      pendreq := TRUE;
-- 	      nstate <= OVERLAP2;
-- 	      exit;
-- 	    end if;
-- 	    i := i + 1;
-- 	    j := (j+1) mod N;
-- 	  end loop;
-- 	  if ((pendreq = FALSE)) then
-- 	    -- we know we have at least one grant
-- 	    nstate <= ONEGRANT_WAIT;
-- 	  end if;

	  if (valid_req = '1') then
	    load_pending <= '1';
	    pending_next <= request;
	    load_startp <= '1';

            nstate <= OVERLAP2;
	    -- need to swap bus grants
	    bgrant_load <= '1';
	    bgrant_clr_en <= '1';
	    bgrant_set_en <= '1';

--	    bgrant_set_index <= current;  -- negate current grant
	    -- pending is actually the current grant because the current
	    -- grant has not been updated yet.

	    bgrant_set_index <= pending;  -- negate current grant
	    bgrant_clr_index <= request;  -- assert pending grant
	  else
	    nstate <= ONEGRANT_WAIT;
	  end if;



	end if;

      when OVERLAP2 =>
-- IN SYNTHESIZED model, we do this in states that TRANSISTION to this state
	-- on first entry to this state, we need to negate current grant, assert pending grant
-- 	if (pstate'event) then
-- 	  -- use pstate'event to make sure we only make this assignment once
-- 	  -- because the current_grant variable may change
-- 	  bgrant_int(current_grant) <= '1' after 4 ns;    -- negate pending grant
-- 	  bgrant_int(pending_grant) <= '0' after 4 ns;    -- assert pending grant
-- 	end if;

	-- ok, we have overlapped grants
	-- we need to wait until the current bus master releases
 	if (bbusy /= '0') then
-- 	  if (bbusy /= '0' and bbusy'event) then
-- 	    -- ok, current bus master is swapping
-- 	    -- make sure swap only happens one time
-- 	    current_grant := pending_grant;
-- 	    if (ROUND_ROBIN = TRUE ) then
-- 	      start_priority := (start_priority+1) mod N;
-- 	    end if;

-- 	  end if;

	  current_mux <= '1';    --  current_grant := pending_grant;
	  load_current <= '1';


	  -- check to see if we have ANOTHER pending grant.
-- 	  pendreq := FALSE;
-- 	  i := 0;
-- 	  j := start_priority;
-- 	  while (i < N) loop
-- 	    if ((breq_int(j) = '0') and (j /= current_grant)) then
-- 	      -- we have a pending grant, what do we do?
-- 	      -- ok, we need to negate the current grant, and assert 
-- 	      -- the pending grant
-- 	      pending_grant := j;
-- 	      pendreq := TRUE;
-- 	      nstate <= OVERLAP;
-- 	      exit;
-- 	    end if;
-- 	    i := i + 1;
-- 	    j := (j+1) mod N;
-- 	  end loop;

-- 	  if ((pendreq = FALSE)) then
-- 	    -- we know we have at least one grant
-- 	    nstate <= ONEGRANT_WAIT;
-- 	  end if;
-- 	end if;

	  if (valid_req = '1') then
	    load_pending <= '1';
	    pending_next <= request;

            nstate <= OVERLAP;
	    load_startp <= '1';

	    -- need to swap bus grants
	    bgrant_load <= '1';
	    bgrant_clr_en <= '1';
	    bgrant_set_en <= '1';

--	    bgrant_set_index <= current;  -- negate current grant
	    -- pending is actually the current grant because the current
	    -- grant has not been updated yet.

	    bgrant_set_index <= pending;  -- negate current grant
	    bgrant_clr_index <= request;  -- assert pending grant
	  else
	    nstate <= ONEGRANT_WAIT;
	  end if;

       end if;

	
    end case;

    
  end process;


  


end behv;