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

////                                                             ////

//// FIFO 4 entries deep                                         ////

////                                                             ////

//// Authors: Rudolf Usselmann, Richard Herveille                ////

////          rudi@asics.ws     richard@asics.ws                 ////

////                                                             ////

////                                                             ////

//// Download from: http://www.opencores.org/projects/sasc       ////

////                http://www.opencores.org/projects/simple_spi ////

////                                                             ////

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

////                                                             ////

//// Copyright (C) 2000-2002 Rudolf Usselmann, Richard Herveille ////

////                         www.asics.ws                        ////

////                         rudi@asics.ws, richard@asics.ws     ////

////                                                             ////

//// 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 SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY     ////

//// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED   ////

//// TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS   ////

//// FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL THE AUTHOR      ////

//// OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,         ////

//// INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES    ////

//// (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE   ////

//// GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR        ////

//// BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF  ////

//// LIABILITY, WHETHER IN  CONTRACT, STRICT LIABILITY, OR TORT  ////

//// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT  ////

//// OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE         ////

//// POSSIBILITY OF SUCH DAMAGE.                                 ////

////                                                             ////

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

//  CVS Log

//

//  $Id: fifo4.v,v 1.1.1.1 2002-12-22 16:07:14 rherveille Exp $

//

//  $Date: 2002-12-22 16:07:14 $

//  $Revision: 1.1.1.1 $

//  $Author: rherveille $

//  $Locker:  $

//  $State: Exp $

//

// Change History:

//               $Log: not supported by cvs2svn $

//

// synopsys translate_off

`include "timescale.v"

// synopsys translate_on

// 4 entry deep fast fifo

module fifo4(clk, rst, clr,  din, we, dout, re, full, empty);

parameter dw = 8;

input           clk, rst;

input           clr;

input   [dw:1]  din;

input           we;

output  [dw:1]  dout;

input           re;

output          full, empty;

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

//

// Local Wires

//

reg     [dw:1]  mem[0:3];

reg     [1:0]   wp;

reg     [1:0]   rp;

wire    [1:0]   wp_p1;

wire    [1:0]   wp_p2;

wire    [1:0]   rp_p1;

wire            full, empty;

reg             gb;

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

//

// Misc Logic

//

always @(posedge clk or negedge rst)

        if(!rst)        wp <=  2'h0;

        else

        if(clr)         wp <=  2'h0;

        else

        if(we)          wp <=  wp_p1;

assign wp_p1 = wp + 2'h1;

assign wp_p2 = wp + 2'h2;

always @(posedge clk or negedge rst)

        if(!rst)        rp <=  2'h0;

        else

        if(clr)         rp <=  2'h0;

        else

        if(re)          rp <=  rp_p1;

assign rp_p1 = rp + 2'h1;

// Fifo Output

assign  dout = mem[ rp ];

// Fifo Input

always @(posedge clk)

        if(we)  mem[ wp ] <=  din;

// Status

assign empty = (wp == rp) & !gb;

assign full  = (wp == rp) &  gb;

// Guard Bit ...

always @(posedge clk)

        if(!rst)                        gb <=  1'b0;

        else

        if(clr)                         gb <=  1'b0;

        else

        if((wp_p1 == rp) & we)          gb <=  1'b1;

        else

        if(re)                          gb <=  1'b0;

endmodule