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

//

// Filename:    flashcache.v

//

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

//

// Purpose:     Since my Zip CPU has primary access to a flash, which requires

//              nearly 24 clock cycles per read, this 'cache' module

//              is offered to minimize the effect.  The CPU may now request

//              some amount of flash to be copied into this on-chip RAM,

//              and then access it with nearly zero latency.

//

// Status:      This file is no longer being used as an active file within

//              the ZipCPU project.  It's an older file from an idea that 

//      never really caught traction.

//

// Interface:

//      FlashCache sits on the Wishbone bus as both a slave and a master.

//      Slave requests for memory will get mapped to a local RAM, from which

//      reads and writes may take place.

//

//      This cache supports a single control register: the base wishbone address

//      of the device to copy memory from.  The bottom bit if this address must

//      be zero (or it will be silently rendered as zero).  When read, this

//      bottom bit will indicate 1) that the controller is still loading memory

//      into the cache, or 0) that the cache is ready to be used.

//

//      Writing to this register will initiate a memory copy from the (new)

//      address.  Once done, the loading bit will be cleared and an interrupt

//      generated.

//

//      Where this memory is placed on the wishbone bus is entirely up to the

//              wishbone bus control logic.  Setting the memory base to an

//              address controlled by this flashcache will produce unusable

//              results, and may well hang the bus.

//      Reads from the memory before complete will return immediately with

//              the value if read address is less than the current copy

//              address, or else they will stall until the read address is

//              less than the copy address.

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

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

//

// Copyright (C) 2015,2017, 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

//

//

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

//

//

module  flashcache(i_clk,

                // Wishbone contrl interface

                i_wb_cyc, i_wb_stb,i_wb_ctrl_stb, i_wb_we, i_wb_addr, i_wb_data,

                        o_wb_ack, o_wb_stall, o_wb_data,

                // Wishbone copy interface

                o_cp_cyc, o_cp_stb, o_cp_we, o_cp_addr, o_cp_data,

                        i_cp_ack, i_cp_stall, i_cp_data,

                o_int);

        parameter       LGCACHELEN=10; // 4 kB

        input                   i_clk;

        // Control interface, CPU interface to cache

        input                   i_wb_cyc, i_wb_stb,i_wb_ctrl_stb, i_wb_we;

        input           [(LGCACHELEN-1):0]       i_wb_addr;

        input           [31:0]   i_wb_data;

        output  reg             o_wb_ack;

        output  wire            o_wb_stall;

        output  wire    [31:0]   o_wb_data;

        // Interface to peripheral bus, including flash

        output  reg             o_cp_cyc, o_cp_stb;

        output  wire            o_cp_we;

        output  reg     [31:0]   o_cp_addr;

        output  wire    [31:0]   o_cp_data;

        input                   i_cp_ack, i_cp_stall;

        input           [31:0]   i_cp_data;

        // And an interrupt to send once we complete

        output  reg             o_int;

        reg             loading;

        reg     [31:0]   cache_base;

        reg     [31:0]   cache   [0:((1<<LGCACHELEN)-1)];

        // Decouple writing the cache base from the highly delayed bus lines

        reg             wr_cache_base_flag;

        reg     [31:0]   wr_cache_base_value;

        always @(posedge i_clk)

                wr_cache_base_flag <= ((i_wb_cyc)&&(i_wb_ctrl_stb)&&(i_wb_we));

        always @(posedge i_clk)

                wr_cache_base_value<= { i_wb_data[31:1], 1'b0 };

        initial cache_base = 32'hffffffff;

        always @(posedge i_clk)

                if (wr_cache_base_flag)

                        cache_base <= wr_cache_base_value;

        reg     new_cache_base;

        initial new_cache_base = 1'b0;

        always @(posedge i_clk)

                if ((wr_cache_base_flag)&&(cache_base != wr_cache_base_value))

                        new_cache_base <= 1'b1;

                else

                        new_cache_base <= 1'b0;

        reg     [(LGCACHELEN-1):0]       rdaddr;

        initial loading = 1'b0;

        always @(posedge i_clk)

                if (new_cache_base)

                begin

                        loading <= 1'b1;

                        o_cp_cyc <= 1'b0;

                end else if ((~o_cp_cyc)&&(loading))

                begin

                        o_cp_cyc <= 1'b1;

                end else if (o_cp_cyc)

                begin

                        // Handle the ack/read line

                        if (i_cp_ack)

                        begin

                                if (&rdaddr)

                                begin

                                        o_cp_cyc <= 1'b0;

                                        loading <= 1'b0;

                                end

                        end

                end

        always @(posedge i_clk)

                if (~o_cp_cyc)

                        o_cp_addr <= cache_base;

                else if ((o_cp_cyc)&&(o_cp_stb)&&(~i_cp_stall))

                        o_cp_addr <= o_cp_addr + 1;;

        always @(posedge i_clk)

                if ((~o_cp_cyc)&&(loading))

                        o_cp_stb  <= 1'b1;

                else if ((o_cp_cyc)&&(o_cp_stb)&&(~i_cp_stall))

                begin

                        // We've made our last request

                        if (o_cp_addr >= cache_base + { {(32-LGCACHELEN-1){1'b0}}, 1'b1, {(LGCACHELEN){1'b0}}})

                                o_cp_stb <= 1'b0;

                end

        always @(posedge i_clk)

                if (~loading)

                        rdaddr    <= 0;

                else if ((o_cp_cyc)&&(i_cp_ack))

                        rdaddr <= rdaddr + 1;

        initial o_int = 1'b0;

        always @(posedge i_clk)

                if ((o_cp_cyc)&&(i_cp_ack)&&(&rdaddr))

                        o_int <= 1'b1;

                else

                        o_int <= 1'b0;

        assign  o_cp_we = 1'b0;

        assign  o_cp_data = 32'h00;

        //

        //      Writes to our cache ... always delayed by a clock.

        //              Clock 0 :       Write request

        //              Clock 1 :       Write takes place

        //              Clock 2 :       Available for reading

        //

        reg                             we;

        reg     [(LGCACHELEN-1):0]       waddr;

        reg     [31:0]                   wval;

        always @(posedge i_clk)

                we <= (loading)?((o_cp_cyc)&&(i_cp_ack)):(i_wb_cyc)&&(i_wb_stb)&&(i_wb_we);

        always @(posedge i_clk)

                waddr <= (loading)?rdaddr:i_wb_addr;

        always @(posedge i_clk)

                wval <= (loading)?i_cp_data:i_wb_data;

        always @(posedge i_clk)

                if (we)

                        cache[waddr] <= wval;

        reg     [31:0]   cache_data;

        always @(posedge i_clk)

                if ((i_wb_cyc)&&(i_wb_stb))

                        cache_data <= cache[i_wb_addr];

        always @(posedge i_clk)

                o_wb_ack <= (i_wb_cyc)&&(

                                ((i_wb_stb)&&(~loading))

                                ||(i_wb_ctrl_stb));

        reg     ctrl;

        always @(posedge i_clk)

                ctrl <= i_wb_ctrl_stb;

        assign  o_wb_data = (ctrl)?({cache_base[31:1],loading}):cache_data;

        assign  o_wb_stall = (loading)&&(~o_wb_ack);

endmodule