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// ============================================================================

//        __

//   \\__/ o\    (C) 2005-2018  Robert Finch, Waterloo

//    \  __ /    All rights reserved.

//     \/_//     robfinch<remove>@finitron.ca

//       ||

//

// This source file is free software: you can redistribute it and/or modify 

// it under the terms of the GNU Lesser General Public License as published 

// by the Free Software Foundation, either version 3 of the License, or     

// (at your option) any later version.                                      

//                                                                          

// This source file is distributed in the hope that it will be useful,      

// but WITHOUT ANY WARRANTY; without even the implied warranty of           

// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the            

// GNU General Public License for more details.                             

//                                                                          

// You should have received a copy of the GNU General Public License        

// along with this program.  If not, see <http://www.gnu.org/licenses/>.    

//

// ============================================================================

//

module round_robin(rst, clk, ce, req, lock, sel);

parameter N=12;

localparam B=$clog2(N);

input rst;                              // reset

input clk;                              // clock

input ce;                               // clock enable

input [N-1:0] req;               // request

input [N-1:0] lock;              // lock selection

output [N-1:0] sel;              // select

integer n;

reg [N-1:0] sel;

reg [B-1:0] rot;                 // forward rotate applied to request lines

reg [B-1:0] amt;                 // how much to rotate forward after a grant

reg [N-1:0] rgrnt;               // rotated value of grant

wire [N-1:0] nextGrant;  // unrotated value of grant

wire [N-1:0] rr1;                        // rotated request imtermediate

wire [N-1:0] ng1;                        // intermediate grant rotation

wire [N-1:0] rreq;               // rotated request

// rotate the request lines to set priority

wire [2*N-1:0] rreq1 = {req,{N{1'b0}}} >> rot;

assign rreq = rreq1[2*N-1:N]|rreq1[N-1:0];

// rotate the rotated grant value back into place

wire [2*N-1:0] rgnt1 = {{N{1'b0}},rgrnt} << rot;

assign nextGrant = rgnt1[2*N-1:N]|rgnt1[N-1:0];

// If there is a request, determine how far the request

// lines should be rotated when there is a grant

always @*

begin

        amt <= 0;

        for (n = N-1; n >= 0; n = n - 1)

                if (rreq[n])

                        amt <= n;

end

// set grant (if request present) based on which request

// was honored.

always @*

        rgrnt <= {{N{1'b0}},|rreq} << ((amt-1) % N);

// rotate the priorities on a grant

always @(posedge clk)

if (rst)

        rot = 0;

else if (ce)

        if (!(lock & sel))

                rot = rot + amt;

// Assign the next owner, if bus isn't locked

always @(posedge clk)

if (rst)

        sel = 0;

else if (ce)

        if (!(lock & sel))

                sel = nextGrant;

endmodule