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/*

 * File         : FIFO.v

 * Project      : University of Utah, XUM Project MIPS32 core

 * Creator(s)   : Grant Ayers (ayers@cs.utah.edu)

 *

 * Modification History:

 *   Rev   Date         Initials  Description of Change

 *   1.0   4-Apr-2010   GEA       Initial design.

 *

 * Standards/Formatting:

 *   Verilog 2001, 4 soft tab, wide column.

 *

 * Description:

 *   A synchronous FIFO of variable data width and depth. 'enQ' is ignored when

 *   the FIFO is full and 'deQ' is ignored when the FIFO is empty. If 'enQ' and

 *   'deQ' are asserted simultaneously, the FIFO is unchanged and the output data

 *   is the same as the input data.

 *

 *   This FIFO is "First word fall-through" meaning data can be read without

 *   asserting 'deQ' by merely supplying an address. However, when 'deQ' is

 *   asserted, the data is "removed" from the FIFO and one location is freed.

 *   If the FIFO is empty and 'enQ' and 'deQ' are not asserted simultaneously,

 *   the output data will be 0s.

 *

 * Variation:

 *   - None. This is the basic FIFO module.

 */

module FIFO(clock, reset, clear, enQ, deQ, data_in, data_out, empty, full);

   parameter DATA_WIDTH = 8;

   parameter ADDR_WIDTH = 8;

   parameter RAM_DEPTH = 1 << ADDR_WIDTH;

   input clock;

   input reset;

   input enQ;

   input deQ;

   input [(DATA_WIDTH-1):0] data_in;

   output [(DATA_WIDTH-1):0] data_out;

   output empty;

   output full;

   reg [(ADDR_WIDTH-1):0] enQ_ptr, deQ_ptr;     // Addresses for reading from and writing to internal memory

   reg [(ADDR_WIDTH):0] count;                  // How many elements are in the FIFO (0->256)

   assign empty = (count == 0);

   assign full = (count == (1 << ADDR_WIDTH));

   wire [(DATA_WIDTH-1):0] w_data_out;

   assign data_out = (empty) ? ((enQ & deQ) ? data_in : 0) : w_data_out;

   wire w_enQ = (full) ? 0 : enQ;   // Mask 'enQ' when the FIFO is full

   wire w_deQ = (empty) ? 0 : deQ;  // Mask 'deQ' when the FIFO is empty

   always @(posedge clock) begin

      if (reset) begin

         enQ_ptr <= 0;

         deQ_ptr <= 0;

         count <= 0;

      end

      else begin

         enQ_ptr <= (w_enQ) ? enQ_ptr +1 : enQ_ptr;

         deQ_ptr <= (w_deQ) ? deQ_ptr +1 : deQ_ptr;

         count <= (w_enQ ~^ w_deQ) ? count : ((w_enQ) ? count +1 : count -1);

      end

   end

   SRAM #(

      .DATA_WIDTH (DATA_WIDTH),

      .ADDR_WIDTH (ADDR_WIDTH),

      .RAM_DEPTH  (RAM_DEPTH))

      RAM(

      .clock   (clock),

      .wEn     (w_enQ),

      .rAddr   (deQ_ptr),

      .wAddr   (enQ_ptr),

      .dIn     (data_in),

      .dOut    (w_data_out)

   );

endmodule