//======================================================================
//
// clkmgr_mmcm_ctrl.v
// ------------------
// PLL reset generator and lock monitor.
//
//
// Author: Pavel Shatov
// Copyright (c) 2019, NORDUnet A/S All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// - Redistributions of source code must retain the above copyright
//   notice, this list of conditions and the following disclaimer.
//
// - Redistributions in binary form must reproduce the above copyright
//   notice, this list of conditions and the following disclaimer in the
//   documentation and/or other materials provided with the distribution.
//
// - Neither the name of the NORDUnet nor the names of its contributors may
//   be used to endorse or promote products derived from this software
//   without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
// IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
// TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
// PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// HOLDER 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.
//
//======================================================================

module clkmgr_mmcm_ctrl
(
    input  clk_in,
    input  reset_n_in,
    input  locked_in,
    output reset_out
);

    //
    // Timer Settings
    //
    localparam [13:0] tmr_zero = {14{1'b0}};
    localparam [13:0] tmr_last = {14{1'b1}};
    reg        [13:0] tmr      = tmr_zero;
    wire       [13:0] tmr_next = tmr + 1'b1;
    wire       [ 9:0] tmr_msb  = tmr[13:4];
    
    wire              tmr_about_to_wrap = tmr == tmr_last;
    wire              tmr_msb_is_zero   = tmr_msb == 10'd0;
    
    //
    // Lock Counting and Monitoring
    //
    reg  [1:0] pll_seen_locks      = 2'd0;
    wire [1:0] pll_seen_locks_next = (pll_seen_locks < 2'd3) ? pll_seen_locks + 1'b1 : pll_seen_locks;

    (* SHREG_EXTRACT="NO" *)
    (* EQUIVALENT_REGISTER_REMOVAL="NO" *)
    reg  [3:0] pll_stable_lock_shreg = 4'h0;
    wire       pll_stable_lock = pll_stable_lock_shreg[3];
        
    wire pll_needs_reset = (pll_seen_locks != 2'd1) || !pll_stable_lock;

    //
    // Output Register
    //
    reg reset_out_r = 1'b1;
    assign reset_out = reset_out_r;
    
    //
    // Timer Increment/Wrap Logic
    //
    always @(posedge clk_in or negedge reset_n_in)
        //
        if (!reset_n_in)              tmr <= tmr_zero;
        else begin
            if (!tmr_about_to_wrap)   tmr <= tmr_next;
            else if (pll_needs_reset) tmr <= tmr_zero;
        end

    //
    // Lock Counter
    //
    always @(posedge locked_in or posedge reset_out)
        //
        if (reset_out) pll_seen_locks <= 2'd0;
        else           pll_seen_locks <= pll_seen_locks_next;

    //
    // Lock Monitor
    //
    always @(posedge clk_in or negedge locked_in)
        //
        if (!locked_in) pll_stable_lock_shreg <= 4'h0;
        else            pll_stable_lock_shreg <= {pll_stable_lock_shreg[2:0], 1'b1};

    //
    // Reset Generator
    //
    always @(posedge clk_in or negedge reset_n_in)
        //
        if (!reset_n_in) reset_out_r <= 1'b1;
        else             reset_out_r <= tmr_msb_is_zero;

endmodule

//======================================================================
// EOF clkmgr_mmcm_ctrl.v
//======================================================================