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

////                                                              ////

////  File name "wbr_fifo_control.v"                              ////

////                                                              ////

////  This file is part of the "PCI bridge" project               ////

////  http://www.opencores.org/cores/pci/                         ////

////                                                              ////

////  Author(s):                                                  ////

////      - Miha Dolenc (mihad@opencores.org)                     ////

////                                                              ////

////  All additional information is avaliable in the README       ////

////  file.                                                       ////

////                                                              ////

////                                                              ////

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

////                                                              ////

//// Copyright (C) 2001 Miha Dolenc, mihad@opencores.org          ////

////                                                              ////

//// This source file may be used and distributed without         ////

//// restriction provided that this copyright statement is not    ////

//// removed from the file and that any derivative work contains  ////

//// the original copyright notice and the associated disclaimer. ////

////                                                              ////

//// 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 2.1 of the License, or (at your option) any   ////

//// later version.                                               ////

////                                                              ////

//// This source 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 Lesser General Public License for more ////

//// details.                                                     ////

////                                                              ////

//// You should have received a copy of the GNU Lesser General    ////

//// Public License along with this source; if not, download it   ////

//// from http://www.opencores.org/lgpl.shtml                     ////

////                                                              ////

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

//

// CVS Revision History

//

// $Log: not supported by cvs2svn $

// Revision 1.2  2001/10/05 08:14:30  mihad

// Updated all files with inclusion of timescale file for simulation purposes.

//

// Revision 1.1.1.1  2001/10/02 15:33:47  mihad

// New project directory structure

//

//

/* FIFO_CONTROL module provides read/write address and status generation for

   FIFOs implemented with standard dual port SRAM cells in ASIC or FPGA designs */

`include "pci_constants.v"

// synopsys translate_off

`include "timescale.v"

// synopsys translate_on

module WBR_FIFO_CONTROL

(

    rclock_in,

    wclock_in,

    renable_in,

    wenable_in,

    reset_in,

    flush_in,

    empty_out,

    waddr_out,

    raddr_out,

    rallow_out,

    wallow_out

) ;

parameter ADDR_LENGTH = 7 ;

// independent clock inputs - rclock_in = read clock, wclock_in = write clock

input  rclock_in, wclock_in;

// enable inputs - read address changes on rising edge of rclock_in when reads are allowed

//                 write address changes on rising edge of wclock_in when writes are allowed

input  renable_in, wenable_in;

// reset input

input  reset_in;

// flush input

input flush_in ;

// empty status output

output empty_out;

// read and write addresses outputs

output [(ADDR_LENGTH - 1):0] waddr_out, raddr_out;

// read and write allow outputs

output rallow_out, wallow_out ;

// read address register

reg [(ADDR_LENGTH - 1):0] raddr ;

// write address register

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

assign waddr_out = waddr ;

// grey code registers

// grey code pipeline for write address

reg [(ADDR_LENGTH - 1):0] wgrey_addr ; // current

reg [(ADDR_LENGTH - 1):0] wgrey_next ; // next

// next write gray address calculation - bitwise xor between address and shifted address

wire [(ADDR_LENGTH - 2):0] calc_wgrey_next  = waddr[(ADDR_LENGTH - 1):1] ^ waddr[(ADDR_LENGTH - 2):0] ;

// grey code pipeline for read address

reg [(ADDR_LENGTH - 1):0] rgrey_addr ; // current

reg [(ADDR_LENGTH - 1):0] rgrey_next ; // next

// next read gray address calculation - bitwise xor between address and shifted address

wire [(ADDR_LENGTH - 2):0] calc_rgrey_next  = raddr[(ADDR_LENGTH - 1):1] ^ raddr[(ADDR_LENGTH - 2):0] ;

// FF for registered empty flag

reg empty ;

// write allow wire

wire wallow = wenable_in ;

// write allow output assignment

assign wallow_out = wallow ;

// read allow wire

wire rallow ;

// clear generation for FFs and registers

wire clear = reset_in || flush_in ;

reg wclock_nempty_detect ;

always@(posedge reset_in or posedge wclock_in)

begin

    if (reset_in)

        wclock_nempty_detect <= #`FF_DELAY 1'b0 ;

    else

        wclock_nempty_detect <= #`FF_DELAY (rgrey_addr != wgrey_addr) ;

end

// special synchronizing mechanism for different implementations - in synchronous imp., empty is prolonged for 1 clock edge if no write clock comes after initial write

reg stretched_empty ;

always@(posedge rclock_in or posedge clear)

begin

    if(clear)

        stretched_empty <= #`FF_DELAY 1'b1 ;

    else

        stretched_empty <= #`FF_DELAY empty && ~wclock_nempty_detect ;

end

// empty output is actual empty + 1 read clock cycle ( stretched empty )

assign empty_out = empty  || stretched_empty ;

//rallow generation

assign rallow = renable_in && ~empty && ~stretched_empty ; // reads allowed if read enable is high and FIFO is not empty

// rallow output assignment

assign rallow_out = renable_in ;

// at any clock edge that rallow is high, this register provides next read address, so wait cycles are not necessary

// when FIFO is empty, this register provides actual read address, so first location can be read

reg [(ADDR_LENGTH - 1):0] raddr_plus_one ;

// address output mux - when FIFO is empty, current actual address is driven out, when it is non - empty next address is driven out

// done for zero wait state burst

assign raddr_out = rallow ? raddr_plus_one : raddr ;

always@(posedge rclock_in or posedge clear)

begin

    if (clear)

    begin

        // initial value is 3

        raddr_plus_one <= #`FF_DELAY 3 ;

    end

    else if (rallow)

        raddr_plus_one <= #`FF_DELAY raddr_plus_one + 1'b1 ;

end

// raddr is filled with raddr_plus_one on rising read clock edge when rallow is high

always@(posedge rclock_in or posedge clear)

begin

    if (clear)

        // initial value is 2

        raddr <= #`FF_DELAY 2 ;

    else if (rallow)

        raddr <= #`FF_DELAY raddr_plus_one ;

end

/*-----------------------------------------------------------------------------------------------

Read address control consists of Read address counter and Grey Address pipeline

There are 3 Grey addresses:

    - rgrey_addr is Grey Code of current read address

    - rgrey_next is Grey Code of next read address

--------------------------------------------------------------------------------------------------*/

// grey code register for read address - represents current Read Address

always@(posedge rclock_in or posedge clear)

begin

    if (clear)

    begin

        // initial value is 0

        rgrey_addr <= #`FF_DELAY 0 ;

    end

    else

    if (rallow)

        rgrey_addr <= #`FF_DELAY rgrey_next ;

end

// grey code register for next read address - represents Grey Code of next read address

always@(posedge rclock_in or posedge clear)

begin

    if (clear)

    begin

        // initial value is 1

        rgrey_next <= #`FF_DELAY 1 ;

    end

    else

    if (rallow)

        rgrey_next <= #`FF_DELAY {raddr[ADDR_LENGTH - 1], calc_rgrey_next} ;

end

/*--------------------------------------------------------------------------------------------

Write address control consists of write address counter and two Grey Code Registers:

    - wgrey_addr represents current Grey Coded write address

    - wgrey_next represents Grey Coded next write address

----------------------------------------------------------------------------------------------*/

// grey code register for write address

always@(posedge wclock_in or posedge clear)

begin

    if (clear)

    begin

        // initial value is 0

        wgrey_addr <= #`FF_DELAY 0 ;

    end

    else

    if (wallow)

        wgrey_addr <= #`FF_DELAY wgrey_next ;

end

// grey code register for next write address

always@(posedge wclock_in or posedge clear)

begin

    if (clear)

    begin

        // initial value is 1

        wgrey_next <= #`FF_DELAY 1 ;

    end

    else

    if (wallow)

        wgrey_next <= #`FF_DELAY {waddr[(ADDR_LENGTH - 1)], calc_wgrey_next} ;

end

// write address counter - nothing special except initial value

always@(posedge wclock_in or posedge clear)

begin

    if (clear)

        // initial value is 2

        waddr <= #`FF_DELAY 2 ;

    else

    if (wallow)

        waddr <= #`FF_DELAY waddr + 1'b1 ;

end

/*------------------------------------------------------------------------------------------------------------------------------

Registered empty control:

registered empty is set on rising edge of rclock_in,

when only one location is used and read in/from fifo. It's kept high until something is written to FIFO, which is registered on

the next read clock.

--------------------------------------------------------------------------------------------------------------------------------*/

// combinatorial input for registered emty FlipFlop

wire reg_empty = (rallow && (rgrey_next == wgrey_addr)) || (rgrey_addr == wgrey_addr) ;

always@(posedge rclock_in or posedge clear)

begin

    if (clear)

        empty <= #`FF_DELAY 1'b1 ;

        else

        empty <= #`FF_DELAY reg_empty ;

end

endmodule