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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