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

//

// Filename:    wbarbiter.v

//

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

//

// Purpose:     This is a priority bus arbiter.  It allows two separate wishbone

//              masters to connect to the same bus, while also guaranteeing

//      that the last master can have the bus with no delay any time it is

//      idle.  The goal is to minimize the combinatorial logic required in this

//      process, while still minimizing access 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) 2015-2019, 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.

//

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

// with this program.  (It's in the $(ROOT)/doc directory.  Run make with no

// target there if the PDF file isn't present.)  If not, see

// <http://www.gnu.org/licenses/> for a copy.

//

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

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

//

//

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

//

//

`default_nettype        none

//

`define WBA_ALTERNATING

//

module  wbarbiter(i_clk, i_reset,

        // Bus A -- the priority bus

        i_a_cyc, i_a_stb, i_a_we, i_a_adr, i_a_dat, i_a_sel,

                o_a_ack, o_a_stall, o_a_err,

        // Bus B

        i_b_cyc, i_b_stb, i_b_we, i_b_adr, i_b_dat, i_b_sel,

                o_b_ack, o_b_stall, o_b_err,

        // Combined/arbitrated bus

        o_cyc, o_stb, o_we, o_adr, o_dat, o_sel, i_ack, i_stall, i_err

`ifdef  FORMAL

        ,

        f_a_nreqs, f_a_nacks, f_a_outstanding,

        f_b_nreqs, f_b_nacks, f_b_outstanding,

        f_nreqs,   f_nacks,   f_outstanding

`endif

        );

        parameter                       DW=32, AW=32;

        parameter                       SCHEME="ALTERNATING";

        parameter       [0:0]             OPT_ZERO_ON_IDLE = 1'b0;

        parameter                       F_MAX_STALL = 3;

        parameter                       F_MAX_ACK_DELAY = 3;

`ifdef  FORMAL

        parameter                       F_LGDEPTH=3;

`endif

        //

        input   wire                    i_clk, i_reset;

        // Bus A

        input   wire                    i_a_cyc, i_a_stb, i_a_we;

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

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

        input   wire    [(DW/8-1):0]     i_a_sel;

        output  wire                    o_a_ack, o_a_stall, o_a_err;

        // Bus B

        input   wire                    i_b_cyc, i_b_stb, i_b_we;

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

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

        input   wire    [(DW/8-1):0]     i_b_sel;

        output  wire                    o_b_ack, o_b_stall, o_b_err;

        //

        output  wire                    o_cyc, o_stb, o_we;

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

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

        output  wire    [(DW/8-1):0]     o_sel;

        input   wire                    i_ack, i_stall, i_err;

        //

`ifdef  FORMAL

        output  wire    [(F_LGDEPTH-1):0] f_nreqs, f_nacks, f_outstanding,

                        f_a_nreqs, f_a_nacks, f_a_outstanding,

                        f_b_nreqs, f_b_nacks, f_b_outstanding;

`endif

        // 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)

        reg     r_a_owner;

        assign o_cyc = (r_a_owner) ? i_a_cyc : i_b_cyc;

        initial r_a_owner = 1'b1;

        generate if (SCHEME == "PRIORITY")

        begin : PRI

                always @(posedge i_clk)

                        if (!i_b_cyc)

                                r_a_owner <= 1'b1;

                        // Allow B to set its CYC line w/o activating this

                        // interface

                        else if ((i_b_stb)&&(!i_a_cyc))

                                r_a_owner <= 1'b0;

        end else if (SCHEME == "ALTERNATING")

        begin : ALT

                reg     last_owner;

                initial last_owner = 1'b0;

                always @(posedge i_clk)

                        if ((i_a_cyc)&&(r_a_owner))

                                last_owner <= 1'b1;

                        else if ((i_b_cyc)&&(!r_a_owner))

                                last_owner <= 1'b0;

                always @(posedge i_clk)

                        if ((!i_a_cyc)&&(!i_b_cyc))

                                r_a_owner <= !last_owner;

                        else if ((r_a_owner)&&(!i_a_cyc))

                        begin

                                if (i_b_stb)

                                        r_a_owner <= 1'b0;

                        end else if ((!r_a_owner)&&(!i_b_cyc))

                        begin

                                if (i_a_stb)

                                        r_a_owner <= 1'b1;

                        end

        end else // if (SCHEME == "LAST")

        begin : LST

                always @(posedge i_clk)

                        if ((!i_a_cyc)&&(i_b_stb))

                                r_a_owner <= 1'b0;

                        else if ((!i_b_cyc)&&(i_a_stb))

                                r_a_owner <= 1'b1;

        end endgenerate

        // 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_we  = (r_a_owner) ? i_a_we  : i_b_we;

        generate if (OPT_ZERO_ON_IDLE)

        begin

                //

                // OPT_ZERO_ON_IDLE will use up more logic and may even slow

                // down the master clock if set.  However, it may also reduce

                // the power used by the FPGA by preventing things from toggling

                // when the bus isn't in use.  The option is here because it

                // also makes it a lot easier to look for when things happen

                // on the bus via VERILATOR when timing and logic counts

                // don't matter.

                //

                assign o_stb     = (o_cyc)? ((r_a_owner) ? i_a_stb : i_b_stb):0;

                assign o_adr     = (o_stb)? ((r_a_owner) ? i_a_adr : i_b_adr):0;

                assign o_dat     = (o_stb)? ((r_a_owner) ? i_a_dat : i_b_dat):0;

                assign o_sel     = (o_stb)? ((r_a_owner) ? i_a_sel : i_b_sel):0;

                assign o_a_ack   = (o_cyc)&&( r_a_owner) ? i_ack   : 1'b0;

                assign o_b_ack   = (o_cyc)&&(!r_a_owner) ? i_ack   : 1'b0;

                assign o_a_stall = (o_cyc)&&( r_a_owner) ? i_stall : 1'b1;

                assign o_b_stall = (o_cyc)&&(!r_a_owner) ? i_stall : 1'b1;

                assign o_a_err   = (o_cyc)&&( r_a_owner) ? i_err : 1'b0;

                assign o_b_err   = (o_cyc)&&(!r_a_owner) ? i_err : 1'b0;

        end else begin

                assign o_stb = (r_a_owner) ? i_a_stb : i_b_stb;

                assign o_adr = (r_a_owner) ? i_a_adr : i_b_adr;

                assign o_dat = (r_a_owner) ? i_a_dat : i_b_dat;

                assign o_sel = (r_a_owner) ? i_a_sel : i_b_sel;

                // 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   = ( r_a_owner) ? i_ack   : 1'b0;

                assign  o_b_ack   = (!r_a_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 = ( r_a_owner) ? i_stall : 1'b1;

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

                //

                //

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

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

        end endgenerate

        // Make Verilator happy

        // verilator lint_off UNUSED

        wire    unused;

        assign  unused = i_reset;

        // verilator lint_on  UNUSED

`ifdef  FORMAL

`ifdef  WBARBITER

`define ASSUME  assume

`else

`define ASSUME  assert

`endif

        reg     f_past_valid;

        initial f_past_valid = 1'b0;

        always @(posedge i_clk)

                f_past_valid <= 1'b1;

        initial `ASSUME(!i_a_cyc);

        initial `ASSUME(!i_a_stb);

        initial `ASSUME(!i_b_cyc);

        initial `ASSUME(!i_b_stb);

        initial `ASSUME(!i_ack);

        initial `ASSUME(!i_err);

        always @(*)

        if (!f_past_valid)

                `ASSUME(i_reset);

        always @(posedge i_clk)

        begin

                if (o_cyc)

                        assert((i_a_cyc)||(i_b_cyc));

                if ((f_past_valid)&&($past(o_cyc))&&(o_cyc))

                        assert($past(r_a_owner) == r_a_owner);

        end

        fwb_master #(.DW(DW), .AW(AW),

                        .F_MAX_STALL(F_MAX_STALL),

                        .F_LGDEPTH(F_LGDEPTH),

                        .F_MAX_ACK_DELAY(F_MAX_ACK_DELAY),

                        .F_OPT_RMW_BUS_OPTION(1),

                        .F_OPT_DISCONTINUOUS(1),

                        .F_OPT_CLK2FFLOGIC(1'b0))

                f_wbm(i_clk, i_reset,

                        o_cyc, o_stb, o_we, o_adr, o_dat, o_sel,

                        i_ack, i_stall, 32'h0, i_err,

                        f_nreqs, f_nacks, f_outstanding);

        fwb_slave  #(.DW(DW), .AW(AW),

                        .F_MAX_STALL(0),

                        .F_LGDEPTH(F_LGDEPTH),

                        .F_MAX_ACK_DELAY(0),

                        .F_OPT_RMW_BUS_OPTION(1),

                        .F_OPT_DISCONTINUOUS(1),

                        .F_OPT_CLK2FFLOGIC(1'b0))

                f_wba(i_clk, i_reset,

                        i_a_cyc, i_a_stb, i_a_we, i_a_adr, i_a_dat, i_a_sel,

                        o_a_ack, o_a_stall, 32'h0, o_a_err,

                        f_a_nreqs, f_a_nacks, f_a_outstanding);

        fwb_slave  #(.DW(DW), .AW(AW),

                        .F_MAX_STALL(0),

                        .F_LGDEPTH(F_LGDEPTH),

                        .F_MAX_ACK_DELAY(0),

                        .F_OPT_RMW_BUS_OPTION(1),

                        .F_OPT_DISCONTINUOUS(1),

                        .F_OPT_CLK2FFLOGIC(1'b0))

                f_wbb(i_clk, i_reset,

                        i_b_cyc, i_b_stb, i_b_we, i_b_adr, i_b_dat, i_b_sel,

                        o_b_ack, o_b_stall, 32'h0, o_b_err,

                        f_b_nreqs, f_b_nacks, f_b_outstanding);

        always @(posedge i_clk)

                if (r_a_owner)

                begin

                        assert(f_b_nreqs == 0);

                        assert(f_b_nacks == 0);

                        assert(f_a_outstanding == f_outstanding);

                end else begin

                        assert(f_a_nreqs == 0);

                        assert(f_a_nacks == 0);

                        assert(f_b_outstanding == f_outstanding);

                end

        always @(posedge i_clk)

        if ((f_past_valid)&&(!$past(i_reset))

                        &&($past(i_a_stb))&&(!$past(i_b_cyc)))

                assert(r_a_owner);

        always @(posedge i_clk)

        if ((f_past_valid)&&(!$past(i_reset))

                        &&(!$past(i_a_cyc))&&($past(i_b_stb)))

                assert(!r_a_owner);

        always @(posedge i_clk)

                if ((f_past_valid)&&(r_a_owner != $past(r_a_owner)))

                        assert(!$past(o_cyc));

`endif

endmodule