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

//

// Filename:    wbwatchdog.v

//

// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core

//

// Purpose:     A Zip timer, redesigned to be a bus watchdog

//

//      This is a **really** stripped down Zip Timer.  All options for external

//      control have been removed.  This timer may be reset, and ... that's 

//      about it.  The goal is that this stripped down timer be used as a bus

//      watchdog element.  Even at that, it's not really fully featured.  The

//      rest of the important features can be found in the zipsystem module.

//

//      As a historical note, the wishbone watchdog timer began as a normal

//      timer, with some fixed inputs.  This makes sense, if you think about it:

//      if the goal is to interrupt a stalled wishbone transaction by inserting

//      a bus error, then you can't use the bus to set it up or configure it

//      simply because the bus in question is ... well, unreliable.  You're

//      trying to make it reliable.

//

//      The problem with using the ziptimer in a stripped down implementation

//      was that the fixed inputs caused the synthesis tool to complain about

//      the use of registers values would never change.  This solves that

//      problem by explicitly removing the cruft that would otherwise

//      just create synthesis warnings and errors.

//

//

// 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  wbwatchdog(i_clk, i_rst, i_ce, i_timeout, o_int);

        parameter       BW = 32;

        input                   i_clk, i_rst, i_ce;

        // Inputs (these were at one time wishbone controlled ...)

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

        // Interrupt line

        output  reg             o_int;

        reg     [(BW-1):0]       r_value;

        initial r_value = 0;

        always @(posedge i_clk)

                if (i_rst)

                        r_value <= i_timeout[(BW-1):0];

                else if ((i_ce)&&(~o_int))

                        r_value <= r_value + {(BW){1'b1}}; // r_value - 1;

        // Set the interrupt on our last tick.

        initial o_int   = 1'b0;

        always @(posedge i_clk)

                if ((i_rst)||(~i_ce))

                        o_int <= 1'b0;

                else

                        o_int <= (r_value == { {(BW-1){1'b0}}, 1'b1 });

endmodule