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

//

// Filename:    zipjiffies.v

//

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

//

// Purpose:     This peripheral is motivated by the Linux use of 'jiffies'.

//      A process, in Linux, can request to be put to sleep until a certain

//      number of 'jiffies' have elapsed.  Using this interface, the CPU can

//      read the number of 'jiffies' from this peripheral (it only has the

//      one location in address space), add the sleep length to it, and

//      write the result back to the peripheral.  The zipjiffies peripheral

//      will record the value written to it only if it is nearer the current

//      counter value than the last current waiting interrupt time.  If no

//      other interrupts are waiting, and this time is in the future, it will

//      be enabled.  (There is currrently no way to disable a jiffie interrupt

//      once set.)  The processor may then place this sleep request into a

//      list among other sleep requests.  Once the timer expires, it would

//      write the next jiffy request to the peripheral and wake up the process

//      whose timer had expired.

//

//      Quite elementary, really.

//

// Interface:

//      This peripheral contains one register: a counter.  Reads from the

//      register return the current value of the counter.  Writes within

//      the (N-1) bit space following the current time set an interrupt.

//      Writes of values that occurred in the last 2^(N-1) ticks will be

//      ignored.  The timer then interrupts when it's value equals that time. 

//      Multiple writes cause the jiffies timer to select the nearest possible

//      interrupt.  Upon an interrupt, the next interrupt time/value is cleared

//      and will need to be reset if the CPU wants to get notified again.  With

//      only the single interface, there is no way of knowing when the next

//      interrupt is scheduled for, neither is there any way to slow down the

//      interrupt timer in case you don't want it overflowing as often and you

//      wish to wait more jiffies than it supports.  Thus, currently, if you

//      have a timer you wish to wait upon that is more than 2^31 into the

//      future, you would need to set timers along the way, wake up on those

//      timers, and set further timer's until you finally get to your

//      destination.

//

//

// Creator:     Dan Gisselquist, Ph.D.

//              Gisselquist Technology, LLC

//

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

//

// Copyright (C) 2015-2016, 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  zipjiffies(i_clk, 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;

        input                           i_clk, 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;

        //

        // Our counter logic: The counter is always counting up--it cannot

        // be stopped or altered.  It's really quite simple.  Okay, not quite.

        // We still support the clock enable line.  We do this in order to

        // support debugging, so that if we get everything running inside a

        // debugger, the timer's all slow down so that everything can be stepped

        // together, one clock at a time.

        //

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

        always @(posedge i_clk)

                if (i_ce)

                        r_counter <= r_counter+1;

        //

        // Writes to the counter set an interrupt--but only if they are in the

        // future as determined by the signed result of an unsigned subtract.

        //

        reg                             int_set,  new_set;

        reg             [(BW-1):0]       int_when, new_when;

        wire    signed  [(BW-1):0]       till_when, till_wb;

        assign  till_when = int_when-r_counter;

        assign  till_wb   = new_when-r_counter;

        initial new_set = 1'b0;

        always @(posedge i_clk)

        begin

                // Delay things by a clock to simplify our logic

                new_set <= ((i_wb_cyc)&&(i_wb_stb)&&(i_wb_we));

                // new_when is a don't care when new_set = 0, so don't worry

                // about setting it at all times.

                new_when<= i_wb_data;

        end

        initial o_int   = 1'b0;

        initial int_set = 1'b0;

        always @(posedge i_clk)

        begin

                o_int <= 1'b0;

                if ((i_ce)&&(int_set)&&(r_counter == int_when))

                        // Interrupts are self-clearing

                        o_int <= 1'b1;  // Set the interrupt flag for one clock

                else if ((new_set)&&(till_wb <= 0))

                        o_int <= 1'b1;

                if ((new_set)&&(till_wb > 0))

                        int_set <= 1'b1;

                else if ((i_ce)&&(r_counter == int_when))

                        int_set <= 1'b0;

                if ((new_set)&&(till_wb > 0)&&((till_wb<till_when)||(~int_set)))

                        int_when <= new_when;

        end

        //

        // Acknowledge any wishbone accesses -- everything we did took only

        // one clock anyway.

        //

        always @(posedge i_clk)

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

        assign  o_wb_data = r_counter;

        assign  o_wb_stall = 1'b0;

endmodule