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[/] [zipcpu/] [trunk/] [rtl/] [peripherals/] [ziptimer.v] - Blame information for rev 2

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1 2 dgisselq
///////////////////////////////////////////////////////////////////////////
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//
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// Filename:    ziptimer.v
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//
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// Project:     Zip CPU -- a small, lightweight, RISC CPU soft core
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//
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// Purpose:
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//
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// Interface:
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//      Two options:
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//      1. One combined register for both control and value, and ...
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//              The reload value is set any time the timer data value is "set".
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//              Reading the register returns the timer value.  Controls are
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//              set so that writing a value to the timer automatically starts
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//              it counting down.
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//      2. Two registers, one for control one for value.
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//              The control register would have the reload value in it.
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//      On the clock when the interface is set to zero the interrupt is set.
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//              Hence setting the timer to zero will disable the timer without
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//              setting any interrupts.  Thus setting it to five will count
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//              5 clocks: 5, 4, 3, 2, 1, Interrupt.
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//
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//
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//      Control bits:
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//              Start_n/Stop.  Writing a '0' starts the timer, '1' stops it.
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//                      Thus, ignoring this bit sets it to start.
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//              AutoReload.  If set, then on reset the timer automatically
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//                      loads the last set value and starts over.  This is
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//                      useful for distinguishing between a one-time interrupt
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//                      timer, and a repetitive interval timer.
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//              (COUNT: If set, the timer only ticks whenever an external
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//                      line goes high.  What this external line is ... is
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//                      not specified here.  This, however, breaks my 
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//                      interface ideal of having our peripheral set not depend
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//                      upon anything.  Hence, this is an advanced option
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//                      enabled at compile time only.)
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//              (INTEN.  Interrupt enable--reaching zero always creates an
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//                      interrupt, so this control bit isn't needed.  The
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//                      interrupt controller can be used to mask the interrupt.)
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//              (COUNT-DOWN/UP: This timer is *only* a count-down timer.
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//                      There is no means of setting it to count up.)
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//      WatchDog
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//              This timer can be implemented as a watchdog timer simply by
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//              connecting the interrupt line to the reset line of the CPU.
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//              When the timer then expires, it will trigger a CPU reset.
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//
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//
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// Creator:     Dan Gisselquist, Ph.D.
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//              Gisselquist Tecnology, LLC
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//
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///////////////////////////////////////////////////////////////////////////
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//
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// Copyright (C) 2015, Gisselquist Technology, LLC
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//
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// This program is free software (firmware): you can redistribute it and/or
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// modify it under the terms of  the GNU General Public License as published
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// by the Free Software Foundation, either version 3 of the License, or (at
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// your option) any later version.
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//
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// This program is distributed in the hope that it will be useful, but WITHOUT
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// ANY WARRANTY; without even the implied warranty of MERCHANTIBILITY or
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// FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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// for more details.
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//
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// License:     GPL, v3, as defined and found on www.gnu.org,
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//              http://www.gnu.org/licenses/gpl.html
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//
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//
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///////////////////////////////////////////////////////////////////////////
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//
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module  ziptimer(i_clk, i_rst, i_ce,
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                i_wb_cyc, i_wb_stb, i_wb_we, i_wb_data,
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                        o_wb_ack, o_wb_stall, o_wb_data,
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                o_int);
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        parameter       BW = 32, VW = (BW-2);
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        input                   i_clk, i_rst, i_ce;
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        // Wishbone inputs
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        input                   i_wb_cyc, i_wb_stb, i_wb_we;
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        input   [(BW-1):0]       i_wb_data;
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        // Wishbone outputs
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        output  reg                     o_wb_ack;
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        output  wire                    o_wb_stall;
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        output  wire    [(BW-1):0]       o_wb_data;
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        // Interrupt line
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        output  reg             o_int;
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        reg                     r_auto_reload, r_running;
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        reg     [(VW-1):0]       r_reload_value;
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        initial r_running = 1'b0;
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        initial r_auto_reload = 1'b0;
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        always @(posedge i_clk)
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                if (i_rst)
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                begin
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                        r_running <= 1'b0;
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                        r_auto_reload <= 1'b0;
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                end else if ((i_wb_cyc)&&(i_wb_stb)&&(i_wb_we))
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                begin
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                        r_running <= (~i_wb_data[(BW-1)])&&(|i_wb_data[(BW-2):0]);
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                        r_auto_reload <= (i_wb_data[(BW-2)]);
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                        // If setting auto-reload mode, and the value to other
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                        // than zero, set the auto-reload value
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                        if ((i_wb_data[(BW-2)])&&(|i_wb_data[(BW-3):0]))
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                                r_reload_value <= i_wb_data[(BW-3):0];
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                end
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        reg     [(VW-1):0]       r_value;
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        initial r_value = 0;
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        always @(posedge i_clk)
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                if ((r_running)&&(|r_value)&&(i_ce))
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                begin
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                        r_value <= r_value - 1;
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                end else if ((r_running)&&(r_auto_reload))
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                        r_value <= r_reload_value;
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                else if ((~r_running)&&(i_wb_cyc)&&(i_wb_stb)&&(i_wb_we))
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                        r_value <= i_wb_data[(VW-1):0];
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        // Set the interrupt on our last tick.
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        initial o_int   = 1'b0;
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        always @(posedge i_clk)
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                if (i_ce)
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                o_int <= (r_running)&&(r_value == { {(VW-1){1'b0}}, 1'b1 });
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                else
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                        o_int <= 1'b0;
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        initial o_wb_ack = 1'b0;
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        always @(posedge i_clk)
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                o_wb_ack <= (i_wb_cyc)&&(i_wb_stb);
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        assign  o_wb_stall = 1'b0;
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        assign  o_wb_data = { ~r_running, r_auto_reload, r_value };
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endmodule

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