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///////////////////////////////////////////////////////////////////////////

//

// Filename:    wbarbiter.v

//

// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core

//

// Purpose:     At some point in time, I might wish to have two masters connect

//              to the same wishbone bus.  As an example, I might wish to have

//              both the instruction fetch and the load/store operators

//              of my Zip CPU access the the same bus.  How shall they both

//              get access to the same resource?  This module allows the

//              wishbone interfaces from two sources to drive the bus, while

//              guaranteeing that only one drives the bus at a time.

//

//              The core logic works like this:

//

//              1. If 'A' or 'B' asserts the o_cyc line, a bus cycle will begin,

//                      with acccess granted to whomever requested it.

//              2. If both 'A' and 'B' assert o_cyc at the same time, only 'A'

//                      will be granted the bus.  (If the alternating parameter 

//                      is set, A and B will alternate who gets the bus in

//                      this case.)

//              3. The bus will remain owned by whomever the bus was granted to

//                      until they deassert the o_cyc line.

//              4. At the end of a bus cycle, o_cyc is guaranteed to be

//                      deasserted (low) for one clock.

//              5. On the next clock, bus arbitration takes place again.  If

//                      'A' requests the bus, no matter how long 'B' was

//                      waiting, 'A' will then be granted the bus.  (Unless

//                      again the alternating parameter is set, then the

//                      access is guaranteed to switch to B.)

//

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

///////////////////////////////////////////////////////////////////////////

//

// Copyright (C) 2015, Gisselquist Technology, LLC

//

// This program is free software (firmware): you can redistribute it and/or

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

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

// your option) any later version.

//

// This program is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

// for more details.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

///////////////////////////////////////////////////////////////////////////

//

`define WBA_ALTERNATING

module  wbarbiter(i_clk, i_rst,

        // Bus A

        i_a_adr, i_a_dat, i_a_we, i_a_stb, i_a_cyc, o_a_ack, o_a_stall, o_a_err,

        // Bus B

        i_b_adr, i_b_dat, i_b_we, i_b_stb, i_b_cyc, o_b_ack, o_b_stall, o_b_err,

        // Both buses

        o_adr, o_dat, o_we, o_stb, o_cyc, i_ack, i_stall, i_err);

        // 18 bits will address one GB, 4 bytes at a time.

        // 19 bits will allow the ability to address things other than just

        // the 1GB of memory we are expecting.

        parameter                       DW=32, AW=19;

        // Wishbone doesn't use an i_ce signal.  While it could, they dislike

        // what it would (might) do to the synchronous reset signal, i_rst.

        input                           i_clk, i_rst;

        input           [(AW-1):0]       i_a_adr, i_b_adr;

        input           [(DW-1):0]       i_a_dat, i_b_dat;

        input                           i_a_we, i_a_stb, i_a_cyc;

        input                           i_b_we, i_b_stb, i_b_cyc;

        output  wire                    o_a_ack, o_b_ack, o_a_stall, o_b_stall,

                                        o_a_err, o_b_err;

        output  wire    [(AW-1):0]       o_adr;

        output  wire    [(DW-1):0]       o_dat;

        output  wire                    o_we, o_stb, o_cyc;

        input                           i_ack, i_stall, i_err;

        // All the fancy stuff here is done with the three primary signals:

        //      o_cyc

        //      w_a_owner

        //      w_b_owner

        // These signals are helped by r_cyc, r_a_owner, and r_b_owner.

        // If you understand these signals, all else will fall into place.

        // r_cyc just keeps track of the last o_cyc value.  That way, on

        // the next clock we can tell if we've had one non-cycle before

        // starting another cycle.  Specifically, no new cycles will be

        // allowed to begin unless r_cyc=0.

        reg     r_cyc;

        always @(posedge i_clk)

                if (i_rst)

                        r_cyc <= 1'b0;

                else

                        r_cyc <= o_cyc;

        // Go high immediately (new cycle) if ...

        //      Previous cycle was low and *someone* is requesting a bus cycle

        // Go low immadiately if ...

        //      We were just high and the owner no longer wants the bus

        // WISHBONE Spec recommends no logic between a FF and the o_cyc

        //      This violates that spec.  (Rec 3.15, p35)

        assign o_cyc = ((~r_cyc)&&((i_a_cyc)||(i_b_cyc))) || ((r_cyc)&&((w_a_owner)||(w_b_owner)));

        // Register keeping track of the last owner, wire keeping track of the

        // current owner allowing us to not lose a clock in arbitrating the

        // first clock of the bus cycle

        reg     r_a_owner, r_b_owner;

        wire    w_a_owner, w_b_owner;

`ifdef  WBA_ALTERNATING

        reg     r_a_last_owner;

`endif

        always @(posedge i_clk)

                if (i_rst)

                begin

                        r_a_owner <= 1'b0;

                        r_b_owner <= 1'b0;

                end else begin

                        r_a_owner <= w_a_owner;

                        r_b_owner <= w_b_owner;

`ifdef  WBA_ALTERNATING

                        if (w_a_owner)

                                r_a_last_owner <= 1'b1;

                        else if (w_b_owner)

                                r_a_last_owner <= 1'b0;

`endif

                end

        //

        // If you are the owner, retain ownership until i_x_cyc is no

        // longer asserted.  Likewise, you cannot become owner until o_cyc

        // is de-asserted for one cycle.

        //

        // 'A' is given arbitrary priority over 'B'

        // 'A' may own the bus only if he wants it.  When 'A' drops i_a_cyc,

        // o_cyc must drop and so must w_a_owner on the same cycle.

        // However, when 'A' asserts i_a_cyc, he can only capture the bus if

        // it's had an idle cycle.

        // The same is true for 'B' with one exception: if both contend for the

        // bus on the same cycle, 'A' arbitrarily wins.

`ifdef  WBA_ALTERNATING

        assign w_a_owner = (i_a_cyc)    // if A requests ownership, and either

                        && ((r_a_owner) // A has already been recognized or

                        || ((~r_cyc) // the bus is free and

                                &&((~i_b_cyc) // B has not requested, or if he 

                                ||(~r_a_last_owner)) )); // has, it's A's turn

        assign w_b_owner = (i_b_cyc)&& ((r_b_owner) || ((~r_cyc)&&((~i_a_cyc)||(r_a_last_owner)) ));

`else

        assign w_a_owner = (i_a_cyc)&& ((r_a_owner) ||  (~r_cyc) );

        assign w_b_owner = (i_b_cyc)&& ((r_b_owner) || ((~r_cyc)&&(~i_a_cyc)) );

`endif

        // Realistically, if neither master owns the bus, the output is a

        // don't care.  Thus we trigger off whether or not 'A' owns the bus.

        // If 'B' owns it all we care is that 'A' does not.  Likewise, if 

        // neither owns the bus than the values on the various lines are

        // irrelevant.

        assign o_adr = (w_a_owner) ? i_a_adr : i_b_adr;

        assign o_dat = (w_a_owner) ? i_a_dat : i_b_dat;

        assign o_we  = (w_a_owner) ? i_a_we  : i_b_we;

        assign o_stb = (o_cyc) && ((w_a_owner) ? i_a_stb : i_b_stb);

        // We cannot allow the return acknowledgement to ever go high if

        // the master in question does not own the bus.  Hence we force it

        // low if the particular master doesn't own the bus.

        assign  o_a_ack   = (w_a_owner) ? i_ack   : 1'b0;

        assign  o_b_ack   = (w_b_owner) ? i_ack   : 1'b0;

        // Stall must be asserted on the same cycle the input master asserts

        // the bus, if the bus isn't granted to him.

        assign  o_a_stall = (w_a_owner) ? i_stall : 1'b1;

        assign  o_b_stall = (w_b_owner) ? i_stall : 1'b1;

        //

        //

        assign  o_a_err = (w_a_owner) ? i_err : 1'b0;

        assign  o_b_err = (w_b_owner) ? i_err : 1'b0;

endmodule