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

////                                                              ////

////  File name "pciw_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 $

//

/* 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 pci_pciw_fifo_control

(

    rclock_in,

    wclock_in,

    renable_in,

    wenable_in,

    reset_in,

    almost_full_out,

    full_out,

    almost_empty_out,

    empty_out,

    waddr_out,

    raddr_out,

    rallow_out,

    wallow_out,

    two_left_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;

// almost full and empy status outputs

output almost_full_out, almost_empty_out;

// full and empty status outputs

output full_out, 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 ;

// two locations left output indicator

output two_left_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_minus1 ; // previous

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_minus2 ; // two before current

reg [(ADDR_LENGTH - 1):0] rgrey_minus1 ; // one before current

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] ;

// write allow - writes are allowed when fifo is not full

assign wallow_out = wenable_in & ~full_out ;

// clear generation for FFs and registers

wire clear = reset_in ;

//rallow generation

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

// 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 ;

// read address mux - when read is performed, next address is driven, so next data is available immediately after read

// this is convenient for zero wait stait bursts

assign raddr_out = rallow_out ? raddr_plus_one : raddr ;

always@(posedge rclock_in or posedge clear)

begin

    if (clear)

    begin

        // initial values seem a bit odd - they are this way to allow easier grey pipeline implementation and to allow min fifo size of 8

        raddr_plus_one <= #`FF_DELAY 5 ;

        raddr          <= #`FF_DELAY 4 ;

    end

    else if (rallow_out)

    begin

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

        raddr          <= #`FF_DELAY raddr_plus_one ;

    end

end

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

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

There are 4 Grey addresses:

    - rgrey_minus2 is Grey Code of address two before current address

    - rgrey_minus1 is Grey Code of address one before current address

    - rgrey_addr is Grey Code of current read address

    - rgrey_next is Grey Code of next read address

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

// grey coded address pipeline for status generation in read clock domain

always@(posedge rclock_in or posedge clear)

begin

    if (clear)

    begin

        rgrey_minus2 <= #1 0 ;

        rgrey_minus1 <= #`FF_DELAY 1 ;

        rgrey_addr   <= #1 3 ;

        rgrey_next   <= #`FF_DELAY 2 ;

    end

    else

    if (rallow_out)

    begin

        rgrey_minus2 <= #1 rgrey_minus1 ;

        rgrey_minus1 <= #`FF_DELAY rgrey_addr ;

        rgrey_addr   <= #1 rgrey_next ;

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

    end

end

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

Write address control consists of write address counter and 3 Grey Code Registers:

    - wgrey_minus1 represents previous Grey coded write address

    - wgrey_addr   represents current Grey Coded write address

    - wgrey_next   represents Grey Coded next write address

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

// grey coded address pipeline for status generation in write clock domain

always@(posedge wclock_in or posedge clear)

begin

    if (clear)

    begin

        wgrey_minus1 <= #`FF_DELAY 1 ;

        wgrey_addr   <= #1 3 ;

        wgrey_next   <= #`FF_DELAY 2 ;

    end

    else

    if (wallow_out)

    begin

        wgrey_minus1 <= #`FF_DELAY wgrey_addr ;

        wgrey_addr   <= #1 wgrey_next ;

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

    end

end

// write address counter - nothing special except initial value

always@(posedge wclock_in or posedge clear)

begin

    if (clear)

        // initial value 5

        waddr <= #`FF_DELAY 4 ;

    else

    if (wallow_out)

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

end

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

Gray coded address of read address decremented by two is synchronized to write clock domain and compared to:

- previous grey coded write address - if they are equal, the fifo is full

- gray coded write address. If they are equal, fifo is almost full.

- grey coded next write address. If they are equal, the fifo has two free locations left.

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

wire [(ADDR_LENGTH - 1):0] wclk_sync_rgrey_minus2 ;

reg  [(ADDR_LENGTH - 1):0] wclk_rgrey_minus2 ;

synchronizer_flop #(ADDR_LENGTH, 0) i_synchronizer_reg_rgrey_minus2

(

    .data_in        (rgrey_minus2),

    .clk_out        (wclock_in),

    .sync_data_out  (wclk_sync_rgrey_minus2),

    .async_reset    (clear)

) ;

always@(posedge wclock_in or posedge clear)

begin

    if (clear)

    begin

        wclk_rgrey_minus2 <= #`FF_DELAY 0 ;

    end

    else

    begin

        wclk_rgrey_minus2 <= #`FF_DELAY wclk_sync_rgrey_minus2 ;

    end

end

assign full_out        = (wgrey_minus1 == wclk_rgrey_minus2) ;

assign almost_full_out = (wgrey_addr   == wclk_rgrey_minus2) ;

assign two_left_out = (wgrey_next   == wclk_rgrey_minus2) ;

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

Empty control:

Gray coded write address pointer is synchronized to read clock domain and compared to Gray coded read address pointer.

If they are equal, fifo is empty.

Almost empty control:

Synchronized write pointer is also compared to Gray coded next read address. If these two are

equal, fifo is almost empty.

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

wire [(ADDR_LENGTH - 1):0] rclk_sync_wgrey_addr ;

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

synchronizer_flop #(ADDR_LENGTH, 3) i_synchronizer_reg_wgrey_addr

(

    .data_in        (wgrey_addr),

    .clk_out        (rclock_in),

    .sync_data_out  (rclk_sync_wgrey_addr),

    .async_reset    (clear)

) ;

always@(posedge rclock_in or posedge clear)

begin

    if (clear)

        rclk_wgrey_addr <= #`FF_DELAY 3 ;

    else

        rclk_wgrey_addr <= #`FF_DELAY rclk_sync_wgrey_addr ;

end

assign almost_empty_out = (rgrey_next == rclk_wgrey_addr) ;

assign empty_out        = (rgrey_addr == rclk_wgrey_addr) ;

endmodule