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

//

// Filename:    ufifo.v

//

// Project:     wbuart32, a full featured UART with simulator

//

// Purpose:     

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

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

//

// Copyright (C) 2015-2017, Gisselquist Technology, LLC

//

// This program is free software (firmware): you can redistribute it and/or

// modify it under the terms of  the GNU General Public License as published

// by the Free Software Foundation, either version 3 of the License, or (at

// your option) any later version.

//

// This program is distributed in the hope that it will be useful, but WITHOUT

// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or

// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

// for more details.

//

// You should have received a copy of the GNU General Public License along

// with this program.  (It's in the $(ROOT)/doc directory.  Run make with no

// target there if the PDF file isn't present.)  If not, see

// <http://www.gnu.org/licenses/> for a copy.

//

// License:     GPL, v3, as defined and found on www.gnu.org,

//              http://www.gnu.org/licenses/gpl.html

//

//

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

//

//

module ufifo(i_clk, i_rst, i_wr, i_data, o_empty_n, i_rd, o_data, o_status, o_err);

        parameter       BW=8;   // Byte/data width

        parameter [3:0]  LGFLEN=4;

        parameter       RXFIFO=1'b0;

        input                   i_clk, i_rst;

        input                   i_wr;

        input   [(BW-1):0]       i_data;

        output  wire            o_empty_n;      // True if something is in FIFO

        input                   i_rd;

        output  wire [(BW-1):0]  o_data;

        output  wire    [15:0]   o_status;

        output  wire            o_err;

        localparam      FLEN=(1<<LGFLEN);

        reg     [(BW-1):0]       fifo[0:(FLEN-1)];

        reg     [(LGFLEN-1):0]   r_first, r_last, r_next;

        wire    [(LGFLEN-1):0]   w_first_plus_one, w_first_plus_two,

                                w_last_plus_one;

        assign  w_first_plus_two = r_first + {{(LGFLEN-2){1'b0}},2'b10};

        assign  w_first_plus_one = r_first + {{(LGFLEN-1){1'b0}},1'b1};

        assign  w_last_plus_one  = r_next; // r_last  + 1'b1;

        reg     will_overflow;

        initial will_overflow = 1'b0;

        always @(posedge i_clk)

                if (i_rst)

                        will_overflow <= 1'b0;

                else if (i_rd)

                        will_overflow <= (will_overflow)&&(i_wr);

                else if (i_wr)

                        will_overflow <= (w_first_plus_two == r_last);

                else if (w_first_plus_one == r_last)

                        will_overflow <= 1'b1;

        // Write

        reg     r_ovfl;

        initial r_first = 0;

        initial r_ovfl  = 0;

        always @(posedge i_clk)

                if (i_rst)

                begin

                        r_ovfl <= 1'b0;

                        r_first <= { (LGFLEN){1'b0} };

                end else if (i_wr)

                begin // Cowardly refuse to overflow

                        if ((i_rd)||(!will_overflow)) // (r_first+1 != r_last)

                                r_first <= w_first_plus_one;

                        else

                                r_ovfl <= 1'b1;

                end

        always @(posedge i_clk)

                if (i_wr) // Write our new value regardless--on overflow or not

                        fifo[r_first] <= i_data;

        // Reads

        //      Following a read, the next sample will be available on the

        //      next clock

        //      Clock   ReadCMD ReadAddr        Output

        //      0        0        0                fifo[0]

        //      1       1       0                fifo[0]

        //      2       0        1               fifo[1]

        //      3       0        1               fifo[1]

        //      4       1       1               fifo[1]

        //      5       1       2               fifo[2]

        //      6       0        3               fifo[3]

        //      7       0        3               fifo[3]

        reg     will_underflow;

        initial will_underflow = 1'b1;

        always @(posedge i_clk)

                if (i_rst)

                        will_underflow <= 1'b1;

                else if (i_wr)

                        will_underflow <= (will_underflow)&&(i_rd);

                else if (i_rd)

                        will_underflow <= (w_last_plus_one == r_first);

                else

                        will_underflow <= (r_last == r_first);

        //

        // Don't report FIFO underflow errors.  These'll be caught elsewhere

        // in the system, and the logic below makes it hard to reset them.

        // We'll still report FIFO overflow, however.

        //

        // reg          r_unfl;

        // initial      r_unfl = 1'b0;

        initial r_last = 0;

        always @(posedge i_clk)

                if (i_rst)

                begin

                        r_last <= 0;

                        r_next <= { {(LGFLEN-1){1'b0}}, 1'b1 };

                        // r_unfl <= 1'b0;

                end else if (i_rd)

                begin

                        if ((i_wr)||(!will_underflow)) // (r_first != r_last)

                        begin

                                r_last <= r_next;

                                r_next <= r_last +{{(LGFLEN-2){1'b0}},2'b10};

                                // Last chases first

                                // Need to be prepared for a possible two

                                // reads in quick succession

                                // o_data <= fifo[r_last+1];

                        end

                        // else r_unfl <= 1'b1;

                end

        reg     [7:0]    fifo_here, fifo_next, r_data;

        always @(posedge i_clk)

                fifo_here <= fifo[r_last];

        always @(posedge i_clk)

                fifo_next <= fifo[r_next];

        always @(posedge i_clk)

                r_data <= i_data;

        reg     [1:0]    osrc;

        always @(posedge i_clk)

                if (will_underflow)

                        // o_data <= i_data;

                        osrc <= 2'b00;

                else if ((i_rd)&&(r_first == w_last_plus_one))

                        osrc <= 2'b01;

                else if (i_rd)

                        osrc <= 2'b11;

                else

                        osrc <= 2'b10;

        assign o_data = (osrc[1]) ? ((osrc[0])?fifo_next:fifo_here) : r_data;

        // wire [(LGFLEN-1):0]  current_fill;

        // assign       current_fill = (r_first-r_last);

        reg     r_empty_n;

        initial r_empty_n = 1'b0;

        always @(posedge i_clk)

                if (i_rst)

                        r_empty_n <= 1'b0;

                else case({i_wr, i_rd})

                        2'b00: r_empty_n <= (r_first != r_last);

                        2'b11: r_empty_n <= (r_first != r_last);

                        2'b10: r_empty_n <= 1'b1;

                        2'b01: r_empty_n <= (r_first != w_last_plus_one);

                endcase

        wire    w_full_n;

        assign  w_full_n = will_overflow;

        //

        // If this is a receive FIFO, the FIFO count that matters is the number

        // of values yet to be read.  If instead this is a transmit FIFO, then 

        // the FIFO count that matters is the number of empty positions that

        // can still be filled before the FIFO is full.

        //

        // Adjust for these differences here.

        reg     [(LGFLEN-1):0]   r_fill;

        always @(posedge i_clk)

                if (RXFIFO!=0) begin

                        // Calculate the number of elements in our FIFO

                        //

                        // Although used for receive, this is actually the more

                        // generic answer--should you wish to use the FIFO in

                        // another context.

                        if (i_rst)

                                r_fill <= 0;

                        else case({i_wr, i_rd})

                        2'b01:   r_fill <= r_first - r_next;

                        2'b10:   r_fill <= r_first - r_last + 1'b1;

                        default: r_fill <= r_first - r_last;

                        endcase

                end else begin

                        // Calculate the number of elements that are empty and

                        // can be filled within our FIFO

                        if (i_rst)

                                r_fill <= { (LGFLEN){1'b1} };

                        else case({i_wr, i_rd})

                        2'b01:   r_fill <= r_last - r_first;

                        2'b10:   r_fill <= r_last - w_first_plus_two;

                        default: r_fill <= r_last - w_first_plus_one;

                        endcase

                end

        // We don't report underflow errors.  These

        assign o_err = (r_ovfl); //  || (r_unfl);

        wire    [3:0]    lglen;

        assign lglen = LGFLEN;

        wire    [9:0]    w_fill;

        assign  w_fill[(LGFLEN-1):0] = r_fill;

        generate if (LGFLEN < 10)

                assign w_fill[9:(LGFLEN)] = 0;

        endgenerate

        wire    w_half_full;

        assign  w_half_full = r_fill[(LGFLEN-1)];

        assign  o_status = {

                // Our status includes a 4'bit nibble telling anyone reading

                // this the size of our FIFO.  The size is then given by

                // 2^(this value).  Hence a 4'h4 in this position means that the

                // FIFO has 2^4 or 16 values within it.

                lglen,

                // The FIFO fill--for a receive FIFO the number of elements

                // left to be read, and for a transmit FIFO the number of

                // empty elements within the FIFO that can yet be filled.

                w_fill,

                // A '1' here means a half FIFO length can be read (receive

                // FIFO) or written to (not a receive FIFO).

                // receive FIFO), or be written to (if it isn't).

                (RXFIFO!=0)?w_half_full:w_half_full,

                // A '1' here means the FIFO can be read from (if it is a

                // receive FIFO), or be written to (if it isn't).

                (RXFIFO!=0)?r_empty_n:w_full_n

        };

        assign  o_empty_n = r_empty_n;

endmodule