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

//

// Filename:    ziptimer.v

//

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

//

// Purpose:

//

// Interface:

//      Two options:

//      1. One combined register for both control and value, and ...

//              The reload value is set any time the timer data value is "set".

//              Reading the register returns the timer value.  Controls are

//              set so that writing a value to the timer automatically starts

//              it counting down.

//      2. Two registers, one for control one for value.

//              The control register would have the reload value in it.

//      On the clock when the interface is set to zero the interrupt is set.

//              Hence setting the timer to zero will disable the timer without

//              setting any interrupts.  Thus setting it to five will count

//              5 clocks: 5, 4, 3, 2, 1, Interrupt.

//

//

//      Control bits:

//              Start_n/Stop.  Writing a '0' starts the timer, '1' stops it.

//                      Thus, ignoring this bit sets it to start.

//              AutoReload.  If set, then on reset the timer automatically

//                      loads the last set value and starts over.  This is

//                      useful for distinguishing between a one-time interrupt

//                      timer, and a repetitive interval timer.

//              (COUNT: If set, the timer only ticks whenever an external

//                      line goes high.  What this external line is ... is

//                      not specified here.  This, however, breaks my 

//                      interface ideal of having our peripheral set not depend

//                      upon anything.  Hence, this is an advanced option

//                      enabled at compile time only.)

//              (INTEN.  Interrupt enable--reaching zero always creates an

//                      interrupt, so this control bit isn't needed.  The

//                      interrupt controller can be used to mask the interrupt.)

//              (COUNT-DOWN/UP: This timer is *only* a count-down timer.

//                      There is no means of setting it to count up.)

//      WatchDog

//              This timer can be implemented as a watchdog timer simply by

//              connecting the interrupt line to the reset line of the CPU.

//              When the timer then expires, it will trigger a CPU reset.

//

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Tecnology, LLC

//

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

//

// Copyright (C) 2015, 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.

//

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

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

//

//

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

//

module  ziptimer(i_clk, i_rst, i_ce,

                i_wb_cyc, i_wb_stb, i_wb_we, i_wb_data,

                        o_wb_ack, o_wb_stall, o_wb_data,

                o_int);

        parameter       BW = 32, VW = (BW-2);

        input                   i_clk, i_rst, i_ce;

        // Wishbone inputs

        input                   i_wb_cyc, i_wb_stb, i_wb_we;

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

        // Wishbone outputs

        output  reg                     o_wb_ack;

        output  wire                    o_wb_stall;

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

        // Interrupt line

        output  reg             o_int;

        reg                     r_auto_reload, r_running;

        reg     [(VW-1):0]       r_reload_value;

        initial r_running = 1'b0;

        initial r_auto_reload = 1'b0;

        always @(posedge i_clk)

                if (i_rst)

                begin

                        r_running <= 1'b0;

                        r_auto_reload <= 1'b0;

                end else if ((i_wb_cyc)&&(i_wb_stb)&&(i_wb_we))

                begin

                        r_running <= (~i_wb_data[(BW-1)])&&(|i_wb_data[(BW-2):0]);

                        r_auto_reload <= (i_wb_data[(BW-2)]);

                        // If setting auto-reload mode, and the value to other

                        // than zero, set the auto-reload value

                        if ((i_wb_data[(BW-2)])&&(|i_wb_data[(BW-3):0]))

                                r_reload_value <= i_wb_data[(BW-3):0];

                end

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

        initial r_value = 0;

        always @(posedge i_clk)

                if ((r_running)&&(|r_value)&&(i_ce))

                begin

                        r_value <= r_value - 1;

                end else if ((r_running)&&(r_auto_reload))

                        r_value <= r_reload_value;

                else if ((~r_running)&&(i_wb_cyc)&&(i_wb_stb)&&(i_wb_we))

                        r_value <= i_wb_data[(VW-1):0];

        // Set the interrupt on our last tick.

        initial o_int   = 1'b0;

        always @(posedge i_clk)

                if (i_ce)

                o_int <= (r_running)&&(r_value == { {(VW-1){1'b0}}, 1'b1 });

                else

                        o_int <= 1'b0;

        initial o_wb_ack = 1'b0;

        always @(posedge i_clk)

                o_wb_ack <= (i_wb_cyc)&&(i_wb_stb);

        assign  o_wb_stall = 1'b0;

        assign  o_wb_data = { ~r_running, r_auto_reload, r_value };

endmodule