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

//

// Filename:    icontrol.v

//

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

//

// Purpose:     An interrupt controller, for managing many interrupt sources.

//

//      This interrupt controller started from the question of how best to

//      design a simple interrupt controller.  As such, it has a few nice

//      qualities to it:

//              1. This is wishbone compliant

//              2. It sits on a 32-bit wishbone data bus

//              3. It only consumes one address on that wishbone bus.

//              4. There is no extra delays associated with reading this

//                      device.

//              5. Common operations can all be done in one clock.

//

//      So, how shall this be used?  First, the 32-bit word is broken down as

//      follows:

//

//      Bit 31  - This is the global interrupt enable bit.  If set, interrupts

//              will be generated and passed on as they come in.

//      Bits 16-30      - These are specific interrupt enable lines.  If set,

//              interrupts from source (bit#-16) will be enabled.

//              To set this line and enable interrupts from this source, write

//              to the register with this bit set and the global enable set.

//              To disable this line, write to this register with global enable

//              bit not set, but this bit set.  (Writing a zero to any of these

//              bits has no effect, either setting or unsetting them.)

//      Bit 15 - This is the any interrupt pin.  If any interrupt is pending,

//              this bit will be set.

//      Bits 0-14       - These are interrupt bits.  When set, an interrupt is

//              pending from the corresponding source--regardless of whether

//              it was enabled.  (If not enabled, it won't generate an

//              interrupt, but it will still register here.)  To clear any

//              of these bits, write a '1' to the corresponding bit.  Writing

//              a zero to any of these bits has no effect.

//

//      The peripheral also sports a parameter, IUSED, which can be set

//      to any value between 1 and (buswidth/2-1, or) 15 inclusive.  This will

//      be the number of interrupts handled by this routine.  (Without the

//      parameter, Vivado was complaining about unused bits.  With it, we can

//      keep the complaints down and still use the routine).

//

//      To get access to more than 15 interrupts, chain these together, so

//      that one interrupt controller device feeds another.

//

//

// 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  icontrol(i_clk, i_reset, i_wr, i_proc_bus, o_proc_bus,

                i_brd_ints, o_interrupt_strobe);

        parameter       IUSED = 15;

        input                   i_clk, i_reset;

        input                   i_wr;

        input           [31:0]   i_proc_bus;

        output  wire    [31:0]   o_proc_bus;

        input           [(IUSED-1):0]    i_brd_ints;

        output  reg             o_interrupt_strobe;

        reg     [(IUSED-1):0]    r_int_state;

        reg     [(IUSED-1):0]    r_int_enable;

        wire    [(IUSED-1):0]    nxt_int_state;

        reg             r_any, r_interrupt, r_gie;

        assign  nxt_int_state = (r_int_state|i_brd_ints);

        initial r_int_state = 0;

        always @(posedge i_clk)

                if (i_reset)

                        r_int_state  <= 0;

                else if (i_wr)

                        r_int_state <= nxt_int_state & (~i_proc_bus[(IUSED-1):0]);

                else

                        r_int_state <= nxt_int_state;

        initial r_int_enable = 0;

        always @(posedge i_clk)

                if (i_reset)

                        r_int_enable <= 0;

                else if ((i_wr)&&(i_proc_bus[31]))

                        r_int_enable <= r_int_enable | i_proc_bus[(16+IUSED-1):16];

                else if ((i_wr)&&(~i_proc_bus[31]))

                        r_int_enable <= r_int_enable & (~ i_proc_bus[(16+IUSED-1):16]);

        initial r_gie = 1'b0;

        always @(posedge i_clk)

                if (i_reset)

                        r_gie <= 1'b0;

                else if (i_wr)

                        r_gie <= i_proc_bus[31];

        initial r_any = 1'b0;

        always @(posedge i_clk)

                r_any <= ((r_int_state & r_int_enable) != 0);

        initial r_interrupt = 1'b0;

        always @(posedge i_clk)

                r_interrupt <= r_gie & r_any;

        generate

        if (IUSED < 15)

        begin

                assign o_proc_bus = {

                                r_gie, { {(15-IUSED){1'b0}}, r_int_enable },

                                r_any, { {(15-IUSED){1'b0}}, r_int_state  } };

        end else begin

                assign o_proc_bus = { r_gie, r_int_enable, r_any, r_int_state };

        end endgenerate

        reg     int_condition;

        initial int_condition      = 1'b0;

        initial o_interrupt_strobe = 1'b0;

        always @(posedge i_clk)

                if (i_reset)

                begin

                        int_condition <= 1'b0;

                        o_interrupt_strobe <= 1'b0;

                end else if (~r_interrupt) // This might end up generating

                begin // many, many, (wild many) interrupts

                        int_condition <= 1'b0;

                        o_interrupt_strobe <= 1'b0;

                end else if ((~int_condition)&&(r_interrupt))

                begin

                        int_condition <= 1'b1;

                        o_interrupt_strobe <= 1'b1;

                end else

                        o_interrupt_strobe <= 1'b0;

endmodule