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

////                                                             ////

////  Universal FIFO Single Clock                                ////

////                                                             ////

////                                                             ////

////  Author: Rudolf Usselmann                                   ////

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

////                                                             ////

////                                                             ////

////  D/L from: http://www.opencores.org/cores/generic_fifos/    ////

////                                                             ////

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

////                                                             ////

//// Copyright (C) 2000-2002 Rudolf Usselmann                    ////

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

////                         rudi@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: generic_fifo_sc_b.v,v 1.1.1.1 2002/09/25 05:42:04 rudi Exp $

//

//  $Date: 2002/09/25 05:42:04 $

//  $Revision: 1.1.1.1 $

//  $Author: rudi $

//  $Locker:  $

//  $State: Exp $

//

// Change History:

//               $Log: generic_fifo_sc_b.v,v $

//               Revision 1.1.1.1  2002/09/25 05:42:04  rudi

//               Initial Checkin

//

//

//

//

//

//

//

//

//

//

//

`include "timescale.v"

/*

Description

===========

I/Os

----

rst     low active, either sync. or async. master reset (see below how to select)

clr     synchronous clear (just like reset but always synchronous), high active

re      read enable, synchronous, high active

we      read enable, synchronous, high active

din     Data Input

dout    Data Output

full    Indicates the FIFO is full (combinatorial output)

full_r  same as above, but registered output (see note below)

empty   Indicates the FIFO is empty

empty_r same as above, but registered output (see note below)

full_n          Indicates if the FIFO has space for N entries (combinatorial output)

full_n_r        same as above, but registered output (see note below)

empty_n         Indicates the FIFO has at least N entries (combinatorial output)

empty_n_r       same as above, but registered output (see note below)

level           indicates the FIFO level:

                2'b00   0-25%    full

                2'b01   25-50%   full

                2'b10   50-75%   full

                2'b11   %75-100% full

combinatorial vs. registered status outputs

-------------------------------------------

Both the combinatorial and registered status outputs have the same

basic functionality. The registered outputs are de-asserted with a

1 cycle delay for full_r and empty_r, and a 2 cycle delay for full_n_r

and empty_n_r.

The combinatorial outputs however, pass through several levels of

logic before they are output. The registered status outputs are

direct outputs of a flip-flop. The reason both are provided, is

that the registered outputs require additional logic inside the

FIFO. If you can meet timing of your device with the combinatorial

outputs, use them ! The FIFO will be smaller. If the status signals

are in the critical pass, use the registered outputs, they have a

much smaller output delay (actually only Tcq).

Parameters

----------

The FIFO takes 3 parameters:

dw      Data bus width

aw      Address bus width (Determines the FIFO size by evaluating 2^aw)

n       N is a second status threshold constant for full_n and empty_n

        If you have no need for the second status threshold, do not

        connect the outputs and the logic should be removed by your

        synthesis tool.

Synthesis Results

-----------------

In a Spartan 2e a 8 bit wide, 8 entries deep FIFO, takes 85 LUTs and runs

at about 116 MHz (IO insertion disabled). The registered status outputs

are valid after 2.1NS, the combinatorial once take out to 6.5 NS to be

available.

Misc

----

This design assumes you will do appropriate status checking externally.

IMPORTANT ! writing while the FIFO is full or reading while the FIFO is

empty will place the FIFO in an undefined state.

*/

// Selecting Sync. or Async Reset

// ------------------------------

// Uncomment one of the two lines below. The first line for

// synchronous reset, the second for asynchronous reset

`define SC_FIFO_ASYNC_RESET                             // Uncomment for Syncr. reset

//`define SC_FIFO_ASYNC_RESET   or negedge rst          // Uncomment for Async. reset

module generic_fifo_sc_b(clk, rst, clr, din, we, dout, re,

                        full, empty, full_r, empty_r,

                        full_n, empty_n, full_n_r, empty_n_r,

                        level);

parameter dw=8;

parameter aw=8;

parameter n=32;

parameter max_size = 1<<aw;

input                   clk, rst, clr;

input   [dw-1:0] din;

input                   we;

output  [dw-1:0] dout;

input                   re;

output                  full, full_r;

output                  empty, empty_r;

output                  full_n, full_n_r;

output                  empty_n, empty_n_r;

output  [1:0]            level;

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

//

// Local Wires

//

reg     [aw:0]   wp;

wire    [aw:0]   wp_pl1;

reg     [aw:0]   rp;

wire    [aw:0]   rp_pl1;

reg             full_r;

reg             empty_r;

wire    [aw:0]   diff;

reg     [aw:0]   diff_r;

reg             re_r, we_r;

wire            full_n, empty_n;

reg             full_n_r, empty_n_r;

reg     [1:0]    level;

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

//

// Memory Block

//

generic_dpram  #(aw,dw) u0(

        .rclk(          clk             ),

        .rrst(          !rst            ),

        .rce(           1'b1            ),

        .oe(            1'b1            ),

        .raddr(         rp[aw-1:0]       ),

        .do(            dout            ),

        .wclk(          clk             ),

        .wrst(          !rst            ),

        .wce(           1'b1            ),

        .we(            we              ),

        .waddr(         wp[aw-1:0]       ),

        .di(            din             )

        );

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

//

// Misc Logic

//

always @(posedge clk `SC_FIFO_ASYNC_RESET)

        if(!rst)        wp <= #1 {aw+1{1'b0}};

        else

        if(clr)         wp <= #1 {aw+1{1'b0}};

        else

        if(we)          wp <= #1 wp_pl1;

assign wp_pl1 = wp + { {aw{1'b0}}, 1'b1};

always @(posedge clk `SC_FIFO_ASYNC_RESET)

        if(!rst)        rp <= #1 {aw+1{1'b0}};

        else

        if(clr)         rp <= #1 {aw+1{1'b0}};

        else

        if(re)          rp <= #1 rp_pl1;

assign rp_pl1 = rp + { {aw{1'b0}}, 1'b1};

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

//

// Combinatorial Full & Empty Flags

//

assign empty = (wp == rp);

assign full  = (wp[aw-1:0] == rp[aw-1:0]) & (wp[aw] != rp[aw]);

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

//

// Registered Full & Empty Flags

//

always @(posedge clk)

        empty_r <= #1 (wp == rp) | (re & (wp == rp_pl1));

always @(posedge clk)

        full_r <= #1 ((wp[aw-1:0] == rp[aw-1:0]) & (wp[aw] != rp[aw])) |

        (we & (wp_pl1[aw-1:0] == rp[aw-1:0]) & (wp_pl1[aw] != rp[aw]));

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

//

// Combinatorial Full_n & Empty_n Flags

//

assign diff = wp-rp;

assign empty_n = diff < n;

assign full_n  = !(diff < (max_size-n+1));

always @(posedge clk)

        level <= #1 {2{diff[aw]}} | diff[aw-1:aw-2];

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

//

// Registered Full_n & Empty_n Flags

//

always @(posedge clk)

        re_r <= #1 re;

always @(posedge clk)

        diff_r <= #1 diff;

always @(posedge clk)

        empty_n_r <= #1 (diff_r < n) | ((diff_r==n) & (re | re_r));

always @(posedge clk)

        we_r <= #1 we;

always @(posedge clk)

        full_n_r <= #1 (diff_r > max_size-n) | ((diff_r==max_size-n) & (we | we_r));

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

//

// Sanity Check

//

// synopsys translate_off

always @(posedge clk)

        if(we & full)

                $display("%m WARNING: Writing while fifo is FULL (%t)",$time);

always @(posedge clk)

        if(re & empty)

                $display("%m WARNING: Reading while fifo is EMPTY (%t)",$time);

// synopsys translate_on

endmodule