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// $Id: //acds/rel/11.0/ip/merlin/altera_avalon_mm_clock_crossing_bridge/altera_avalon_mm_clock_crossing_bridge.v#2 $
// $Revision: #2 $
// $Date: 2011/03/09 $
// $Author: jyeap $
// --------------------------------------
// Avalon-MM clock crossing bridge
//
// Clock crosses MM commands and responses with the
// help of asynchronous FIFOs.
//
// This bridge will stop emitting read commands when
// too many read commands are in flight to avoid 
// response FIFO overflow.
// --------------------------------------

module altera_avalon_mm_clock_crossing_bridge
#(
    parameter DATA_WIDTH            = 32,
    parameter SYMBOL_WIDTH          = 8,
    parameter ADDRESS_WIDTH         = 10,
    parameter BURSTCOUNT_WIDTH      = 1,

    parameter COMMAND_FIFO_DEPTH    = 4,
    parameter RESPONSE_FIFO_DEPTH   = 4,

    parameter MASTER_SYNC_DEPTH     = 2,
    parameter SLAVE_SYNC_DEPTH      = 2,

    // --------------------------------------
    // Derived parameters
    // --------------------------------------
    parameter BYTEEN_WIDTH = DATA_WIDTH / SYMBOL_WIDTH
)
(
    input                           s0_clk,
    input                           s0_reset,

    input                           m0_clk,
    input                           m0_reset,

    output                          s0_waitrequest,
    output [DATA_WIDTH-1:0]         s0_readdata,
    output                          s0_readdatavalid,
    input  [BURSTCOUNT_WIDTH-1:0]   s0_burstcount,
    input  [DATA_WIDTH-1:0]         s0_writedata,
    input  [ADDRESS_WIDTH-1:0]      s0_address, 
    input                           s0_write,  
    input                           s0_read,  
    input  [BYTEEN_WIDTH-1:0]       s0_byteenable,  
    input                           s0_debugaccess,

    input                           m0_waitrequest,
    input  [DATA_WIDTH-1:0]         m0_readdata,
    input                           m0_readdatavalid,
    output [BURSTCOUNT_WIDTH-1:0]   m0_burstcount,
    output [DATA_WIDTH-1:0]         m0_writedata,
    output [ADDRESS_WIDTH-1:0]      m0_address, 
    output                          m0_write,  
    output                          m0_read,  
    output [BYTEEN_WIDTH-1:0]       m0_byteenable,
    output                          m0_debugaccess
);

    localparam CMD_WIDTH = BURSTCOUNT_WIDTH + DATA_WIDTH + ADDRESS_WIDTH 
                    + BYTEEN_WIDTH 
                    + 3;        // read, write, debugaccess

    localparam NUMSYMBOLS    = DATA_WIDTH / SYMBOL_WIDTH;
    localparam RSP_WIDTH     = DATA_WIDTH;
    localparam MAX_BURST     = (1 << (BURSTCOUNT_WIDTH-1));
    localparam COUNTER_WIDTH = $clog2(RESPONSE_FIFO_DEPTH) + 1;
    localparam NON_BURSTING  = (MAX_BURST == 1);
    localparam BURST_WORDS_W = BURSTCOUNT_WIDTH;

    // --------------------------------------
    // Signals
    // --------------------------------------
    wire [CMD_WIDTH-1:0]     s0_cmd_payload;
    wire [CMD_WIDTH-1:0]     m0_cmd_payload;
    wire                     s0_cmd_valid;
    wire                     m0_cmd_valid;
    wire                     m0_internal_write;
    wire                     m0_internal_read;
    wire                     s0_cmd_ready;
    wire                     m0_cmd_ready;
    reg  [COUNTER_WIDTH-1:0] pending_read_count;
    wire [COUNTER_WIDTH-1:0] space_avail;
    wire                     stop_cmd;
    reg                      stop_cmd_r;
    wire                     m0_read_accepted;
    wire                     m0_rsp_ready;
    reg                      old_read;
    wire [BURST_WORDS_W-1:0] m0_burstcount_words;

    // --------------------------------------
    // Command FIFO
    // --------------------------------------
    (* altera_attribute = "-name ALLOW_ANY_RAM_SIZE_FOR_RECOGNITION ON" *) altera_avalon_dc_fifo
    #(
        .SYMBOLS_PER_BEAT (1),
        .BITS_PER_SYMBOL  (CMD_WIDTH),
        .FIFO_DEPTH       (COMMAND_FIFO_DEPTH),
        .WR_SYNC_DEPTH    (MASTER_SYNC_DEPTH),
        .RD_SYNC_DEPTH    (SLAVE_SYNC_DEPTH)
    ) 
    cmd_fifo
    (
        .in_clk          (s0_clk),
        .in_reset_n      (~s0_reset),
        .out_clk         (m0_clk),
        .out_reset_n     (~m0_reset),

        .in_data         (s0_cmd_payload),
        .in_valid        (s0_cmd_valid),
        .in_ready        (s0_cmd_ready),

        .out_data        (m0_cmd_payload),
        .out_valid       (m0_cmd_valid),
        .out_ready       (m0_cmd_ready)
    );

    // --------------------------------------
    // Command payload
    // --------------------------------------
    assign s0_waitrequest = ~s0_cmd_ready;
    assign s0_cmd_valid   = s0_write | s0_read;

    assign s0_cmd_payload = {s0_address, 
                             s0_burstcount, 
                             s0_read, 
                             s0_write, 
                             s0_writedata, 
                             s0_byteenable,
                             s0_debugaccess};
    assign {m0_address, 
            m0_burstcount, 
            m0_internal_read, 
            m0_internal_write,
            m0_writedata, 
            m0_byteenable,
            m0_debugaccess} = m0_cmd_payload;

    assign m0_cmd_ready = ~m0_waitrequest & 
                            ~(m0_internal_read & stop_cmd_r & ~old_read);
    assign m0_write =  m0_internal_write & m0_cmd_valid;
    assign m0_read  =  m0_internal_read & m0_cmd_valid & (~stop_cmd_r | old_read);
    assign m0_read_accepted = m0_read & ~m0_waitrequest;


    // ---------------------------------------------
    // the non-bursting case
    // ---------------------------------------------
    generate if (NON_BURSTING)
    begin
        always @(posedge m0_clk, posedge m0_reset) begin
            if (m0_reset) begin
                pending_read_count <= 0;
            end
            else begin
                if (m0_read_accepted)
                    pending_read_count <= pending_read_count + 1;
                if (m0_readdatavalid)
                    pending_read_count <= pending_read_count - 1;
                if (m0_read_accepted & m0_readdatavalid)
                    pending_read_count <= pending_read_count;
            end
        end
    end
    // ---------------------------------------------
    // the bursting case
    // ---------------------------------------------
    else begin
        assign m0_burstcount_words = m0_burstcount;

        always @(posedge m0_clk, posedge m0_reset) begin
            if (m0_reset) begin
                pending_read_count <= 0;
            end
            else begin
                if (m0_read_accepted)
                    pending_read_count <= pending_read_count + 
                                            m0_burstcount_words;
                if (m0_readdatavalid)
                    pending_read_count <= pending_read_count - 1;
                if (m0_read_accepted & m0_readdatavalid)
                    pending_read_count <= pending_read_count +
                                            m0_burstcount_words - 1;
            end
        end
    end
    endgenerate

    assign stop_cmd = (pending_read_count + 2*MAX_BURST) > space_avail;

    always @(posedge m0_clk, posedge m0_reset) begin
        if (m0_reset) begin
            stop_cmd_r <= 1'b0;
            old_read   <= 1'b0;
        end
        else begin
            stop_cmd_r <= stop_cmd;
            old_read   <= m0_read & m0_waitrequest;
        end
    end

    // --------------------------------------
    // Response FIFO
    // --------------------------------------
    (* altera_attribute = "-name ALLOW_ANY_RAM_SIZE_FOR_RECOGNITION ON" *) altera_avalon_dc_fifo
    #(
        .SYMBOLS_PER_BEAT   (1),
        .BITS_PER_SYMBOL    (RSP_WIDTH),
        .FIFO_DEPTH         (RESPONSE_FIFO_DEPTH),
        .WR_SYNC_DEPTH      (SLAVE_SYNC_DEPTH),
        .RD_SYNC_DEPTH      (MASTER_SYNC_DEPTH),
        .USE_SPACE_AVAIL_IF (1)
    ) 
    rsp_fifo
    (
        .in_clk           (m0_clk),
        .in_reset_n       (~m0_reset),
        .out_clk          (s0_clk),
        .out_reset_n      (~s0_reset),

        .in_data          (m0_readdata),
        .in_valid         (m0_readdatavalid),

        // ------------------------------------
        // must never overflow, or we're in trouble
        // (we cannot backpressure the response)
        // ------------------------------------
        .in_ready         (m0_rsp_ready),

        .out_data         (s0_readdata),
        .out_valid        (s0_readdatavalid),
        .out_ready        (1'b1),

        .space_avail_data (space_avail)
    );

// synthesis translate_off
    always @(posedge m0_clk) begin
        if (~m0_rsp_ready & m0_readdatavalid) begin
            $display("%t %m: internal error, response fifo overflow", $time);
        end

        if (pending_read_count > space_avail) begin
            $display("%t %m: internal error, too many pending reads", $time);
        end
    end
// synthesis translate_on

endmodule  
