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

//

// Filename:    fwb_slave.v

//

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

//

// Purpose:     This file describes the rules of a wishbone interaction from the

//              perspective of a wishbone slave.  These formal rules may be used

//      with yosys-smtbmc to *prove* that the slave properly handles outgoing

//      responses to (assumed correct) incoming requests.

//

//      This module contains no functional logic.  It is intended for formal

//      verification only.  The outputs returned, the number of requests that

//      have been made, the number of acknowledgements received, and the number

//      of outstanding requests, are designed for further formal verification

//      purposes *only*.

//

//      This file is different from a companion formal_master.v file in that

//      assumptions are made about the inputs to the slave: i_wb_cyc,

//      i_wb_stb, i_wb_we, i_wb_addr, i_wb_data, and i_wb_sel, while full

//      assertions are made about the outputs: o_wb_stall, o_wb_ack, o_wb_data,

//      o_wb_err.  In the formal_master.v, assertions are made about the

//      master outputs (slave inputs)), and assumptions are made about the

//      master inputs (the slave outputs).

//

//

//

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

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

//

// Copyright (C) 2017, 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

//

module  fwb_slave(i_clk, i_reset,

                // The Wishbone bus

                i_wb_cyc, i_wb_stb, i_wb_we, i_wb_addr, i_wb_data, i_wb_sel,

                        i_wb_ack, i_wb_stall, i_wb_idata, i_wb_err,

                // Some convenience output parameters

                f_nreqs, f_nacks, f_outstanding);

        parameter               AW=32, DW=32;

        parameter               F_MAX_STALL = 0,

                                F_MAX_ACK_DELAY = 0;

        parameter               F_LGDEPTH = 4;

        parameter [(F_LGDEPTH-1):0] F_MAX_REQUESTS = 0;

        //

        // If true, allow the bus to be kept open when there are no outstanding

        // requests.  This is useful for any master that might execute a

        // read modify write cycle, such as an atomic add.

        parameter [0:0]           F_OPT_RMW_BUS_OPTION = 1;

        //

        // 

        // If true, allow the bus to issue multiple discontinuous requests.

        // Unlike F_OPT_RMW_BUS_OPTION, these requests may be issued while other

        // requests are outstanding

        parameter       [0:0]     F_OPT_DISCONTINUOUS = 0;

        //

        //

        localparam [(F_LGDEPTH-1):0] MAX_OUTSTANDING = {(F_LGDEPTH){1'b1}};

        localparam      MAX_DELAY = (F_MAX_STALL > F_MAX_ACK_DELAY)

                                ? F_MAX_STALL : F_MAX_ACK_DELAY;

        localparam      DLYBITS= (MAX_DELAY < 4) ? 2

                                : ((MAX_DELAY <    16) ? 4

                                : ((MAX_DELAY <    64) ? 6

                                : ((MAX_DELAY <   256) ? 8

                                : ((MAX_DELAY <  1024) ? 10

                                : ((MAX_DELAY <  4096) ? 12

                                : ((MAX_DELAY < 16384) ? 14

                                : ((MAX_DELAY < 65536) ? 16

                                : 32)))))));

        //

        input   wire                    i_clk, i_reset;

        // Input/master bus

        input   wire                    i_wb_cyc, i_wb_stb, i_wb_we;

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

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

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

        //

        input   wire                    i_wb_ack;

        input   wire                    i_wb_stall;

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

        input   wire                    i_wb_err;

        //

        output  reg     [(F_LGDEPTH-1):0]        f_nreqs, f_nacks;

        output  wire    [(F_LGDEPTH-1):0]        f_outstanding;

        //

        // Let's just make sure our parameters are set up right

        //

        assert property(F_MAX_REQUESTS < {(F_LGDEPTH){1'b1}});

        //

        // Wrap the request line in a bundle.  The top bit, named STB_BIT,

        // is the bit indicating whether the request described by this vector

        // is a valid request or not.

        //

        localparam      STB_BIT = 2+AW+DW+DW/8-1;

        wire    [STB_BIT:0]      f_request;

        assign  f_request = { i_wb_stb, i_wb_we, i_wb_addr, i_wb_data, i_wb_sel };

        //

        // A quick register to be used later to know if the $past() operator

        // will yield valid result

        reg     f_past_valid;

        initial f_past_valid = 1'b0;

        always @(posedge i_clk)

                f_past_valid <= 1'b1;

        always @(*)

                if (!f_past_valid)

                        assume(i_reset);

        //

        //

        // Assertions regarding the initial (and reset) state

        //

        //

        //

        // Assume we start from a reset condition

        initial assume(i_reset);

        initial assume(!i_wb_cyc);

        initial assume(!i_wb_stb);

        //

        initial assert(!i_wb_ack);

        initial assert(!i_wb_err);

        always @(posedge i_clk)

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

        begin

                assume(!i_wb_cyc);

                assume(!i_wb_stb);

                //

                assert(!i_wb_ack);

                assert(!i_wb_err);

        end

        // Things can only change on the positive edge of the clock

        always @($global_clock)

        if ((f_past_valid)&&(!$rose(i_clk)))

        begin

                assume($stable(i_reset));

                assume($stable(i_wb_cyc));

                assume($stable(f_request));

                if (i_wb_we)

                        assume($stable(f_request)); // The entire request should b stabl

                else

                        assume($stable(f_request[(2+AW-1):(DW+DW/8)]));

                //

                assert($stable(i_wb_ack));

                assert($stable(i_wb_stall));

                assert($stable(i_wb_idata));

                assert($stable(i_wb_err));

        end

        //

        //

        // Bus requests

        //

        //

        // Following any bus error, the CYC line should be dropped to abort

        // the transaction

        always @(posedge i_clk)

        if ((f_past_valid)&&($past(i_wb_err))&&($past(i_wb_cyc)))

                assume(!i_wb_cyc);

        // STB can only be true if CYC is also true

        always @(posedge i_clk)

                if (i_wb_stb)

                        assume(i_wb_cyc);

        // If a request was both outstanding and stalled on the last clock,

        // then nothing should change on this clock regarding it.

        always @(posedge i_clk)

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

                        &&($past(i_wb_stall))&&(i_wb_cyc))

        begin

                assume(i_wb_stb);

                assume($stable(f_request));

        end

        // Within any series of STB/requests, the direction of the request

        // may not change.

        always @(posedge i_clk)

                if ((f_past_valid)&&($past(i_wb_stb))&&(i_wb_stb))

                        assume(i_wb_we == $past(i_wb_we));

        // Within any given bus cycle, the direction may *only* change when

        // there are no further outstanding requests.

        always @(posedge i_clk)

                if ((f_past_valid)&&(f_outstanding > 0))

                        assume(i_wb_we == $past(i_wb_we));

        // Write requests must also set one (or more) of i_wb_sel

        always @(posedge i_clk)

                if ((i_wb_stb)&&(i_wb_we))

                        assume(|i_wb_sel);

        //

        //

        // Bus responses

        //

        //

        // If CYC was low on the last clock, then both ACK and ERR should be

        // low on this clock.

        always @(posedge i_clk)

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

        begin

                assert(!i_wb_ack);

                assert(!i_wb_err);

                // Stall may still be true--such as when we are not

                // selected at some arbiter between us and the slave

        end

        // ACK and ERR may never both be true at the same time

        always @(*)

                assume((!i_wb_ack)||(!i_wb_err));

        generate if (F_MAX_STALL > 0)

        begin : MXSTALL

                //

                // Assume the slave cannnot stall for more than F_MAX_STALL

                // counts.  We'll count this forward any time STB and STALL

                // are both true.

                //

                reg     [(DLYBITS-1):0]          f_stall_count;

                initial f_stall_count = 0;

                always @(posedge i_clk)

                        if ((!i_reset)&&(i_wb_stb)&&(i_wb_stall))

                                f_stall_count <= f_stall_count + 1'b1;

                        else

                                f_stall_count <= 0;

                always @(posedge i_clk)

                        if (i_wb_cyc)

                                assert(f_stall_count < F_MAX_STALL);

        end endgenerate

        generate if (F_MAX_ACK_DELAY > 0)

        begin : MXWAIT

                //

                // Assume the slave will respond within F_MAX_ACK_DELAY cycles,

                // counted either from the end of the last request, or from the

                // last ACK received

                //

                reg     [(DLYBITS-1):0]          f_ackwait_count;

                initial f_ackwait_count = 0;

                always @(posedge i_clk)

                        if ((!i_reset)&&(i_wb_cyc)&&(!i_wb_stb)

                                        &&(!i_wb_ack)&&(!i_wb_err))

                        begin

                                f_ackwait_count <= f_ackwait_count + 1'b1;

                                assert(f_ackwait_count < F_MAX_ACK_DELAY);

                        end else

                                f_ackwait_count <= 0;

        end endgenerate

        //

        // Count the number of requests that have been received

        //

        initial f_nreqs = 0;

        always @(posedge i_clk)

                if ((i_reset)||(!i_wb_cyc))

                        f_nreqs <= 0;

                else if ((i_wb_stb)&&(!i_wb_stall))

                        f_nreqs <= f_nreqs + 1'b1;

        //

        // Count the number of acknowledgements that have been returned

        //

        initial f_nacks = 0;

        always @(posedge i_clk)

                if (!i_wb_cyc)

                        f_nacks <= 0;

                else if ((i_wb_ack)||(i_wb_err))

                        f_nacks <= f_nacks + 1'b1;

        //

        // The number of outstanding requests is the difference between

        // the number of requests and the number of acknowledgements

        //

        assign  f_outstanding = (i_wb_cyc) ? (f_nreqs - f_nacks):0;

        always @(posedge i_clk)

                if ((i_wb_cyc)&&(F_MAX_REQUESTS > 0))

                begin

                        if (i_wb_stb)

                                assume(f_nreqs < F_MAX_REQUESTS);

                        else

                                assume(f_nreqs <= F_MAX_REQUESTS);

                        assert(f_nacks <= f_nreqs);

                        assert(f_outstanding < (1<<F_LGDEPTH)-1);

                end else

                        assert(f_outstanding < (1<<F_LGDEPTH)-1);

        always @(posedge i_clk)

                if ((i_wb_cyc)&&(f_outstanding == 0))

                begin

                        // If nothing is outstanding, then there should be

                        // no acknowledgements

                        assert(!i_wb_ack);

                        // The same is not true of errors.  It may be that an

                        // error is created before the request gets through

                        // assert(!i_wb_err);

                end

        // While the error signal may be asserted immediately before

        // anything is outstanding, it may only be asserted in

        // response to a transaction request--whether completed or

        // not.

        always @(posedge i_clk)

                if ((i_wb_cyc)&&(!i_wb_stb)&&(f_outstanding == 0))

                        assert(!i_wb_err);

        generate if (!F_OPT_RMW_BUS_OPTION)

        begin

                // If we aren't waiting for anything, and we aren't issuing

                // any requests, then then our transaction is over and we

                // should be dropping the CYC line.

                always @(posedge i_clk)

                        if (f_outstanding == 0)

                                assume((i_wb_stb)||(!i_wb_cyc));

                // Not all masters will abide by this restriction.  Some

                // masters may wish to implement read-modify-write bus

                // interactions.  These masters need to keep CYC high between

                // transactions, even though nothing is outstanding.  For

                // these busses, turn F_OPT_RMW_BUS_OPTION on.

        end endgenerate

        generate if ((!F_OPT_DISCONTINUOUS)&&(!F_OPT_RMW_BUS_OPTION))

        begin : INSIST_ON_NO_DISCONTINUOUS_STBS

                // Within my own code, once a request begins it goes to

                // completion and the CYC line is dropped.  The master

                // is not allowed to raise STB again after dropping it.

                // Doing so would be a *discontinuous* request.

                //

                // However, in any RMW scheme, discontinuous requests are

                // necessary, and the spec doesn't disallow them.  Hence we

                // make this check optional.

                always @(posedge i_clk)

                        if ((f_past_valid)&&($past(i_wb_cyc))&&(!$past(i_wb_stb)))

                                assume(!i_wb_stb);

        end endgenerate

endmodule