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////////////////////////////////////////////////////////////////////////////////
//
// Filename: 	ufifo.v
//
// Project:	wbuart32, a full featured UART with simulator
//
// Purpose:	
//
// Creator:	Dan Gisselquist, Ph.D.
//		Gisselquist Technology, LLC
//
////////////////////////////////////////////////////////////////////////////////
//
// Copyright (C) 2015-2017, 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.
//
// You should have received a copy of the GNU General Public License along
// with this program.  (It's in the $(ROOT)/doc directory.  Run make with no
// target there if the PDF file isn't present.)  If not, see
// <http://www.gnu.org/licenses/> for a copy.
//
// License:	GPL, v3, as defined and found on www.gnu.org,
//		http://www.gnu.org/licenses/gpl.html
//
//
////////////////////////////////////////////////////////////////////////////////
//
//
module ufifo(i_clk, i_rst, i_wr, i_data, o_empty_n, i_rd, o_data, o_status, o_err);
	parameter	BW=8;	// Byte/data width
	parameter [3:0]	LGFLEN=4;
	parameter 	RXFIFO=1'b0;
	input			i_clk, i_rst;
	input			i_wr;
	input	[(BW-1):0]	i_data;
	output	wire		o_empty_n;	// True if something is in FIFO
	input			i_rd;
	output	wire [(BW-1):0]	o_data;
	output	wire	[15:0]	o_status;
	output	wire		o_err;
 
	localparam	FLEN=(1<<LGFLEN);
 
	reg	[(BW-1):0]	fifo[0:(FLEN-1)];
	reg	[(LGFLEN-1):0]	r_first, r_last, r_next;
 
	wire	[(LGFLEN-1):0]	w_first_plus_one, w_first_plus_two,
				w_last_plus_one;
	assign	w_first_plus_two = r_first + {{(LGFLEN-2){1'b0}},2'b10};
	assign	w_first_plus_one = r_first + {{(LGFLEN-1){1'b0}},1'b1};
	assign	w_last_plus_one  = r_next; // r_last  + 1'b1;
 
	reg	will_overflow;
	initial	will_overflow = 1'b0;
	always @(posedge i_clk)
		if (i_rst)
			will_overflow <= 1'b0;
		else if (i_rd)
			will_overflow <= (will_overflow)&&(i_wr);
		else if (i_wr)
			will_overflow <= (w_first_plus_two == r_last);
		else if (w_first_plus_one == r_last)
			will_overflow <= 1'b1;
 
	// Write
	reg	r_ovfl;
	initial	r_first = 0;
	initial	r_ovfl  = 0;
	always @(posedge i_clk)
		if (i_rst)
		begin
			r_ovfl <= 1'b0;
			r_first <= { (LGFLEN){1'b0} };
		end else if (i_wr)
		begin // Cowardly refuse to overflow
			if ((i_rd)||(!will_overflow)) // (r_first+1 != r_last)
				r_first <= w_first_plus_one;
			else
				r_ovfl <= 1'b1;
		end
	always @(posedge i_clk)
		if (i_wr) // Write our new value regardless--on overflow or not
			fifo[r_first] <= i_data;
 
	// Reads
	//	Following a read, the next sample will be available on the
	//	next clock
	//	Clock	ReadCMD	ReadAddr	Output
	//	0	0	0		fifo[0]
	//	1	1	0		fifo[0]
	//	2	0	1		fifo[1]
	//	3	0	1		fifo[1]
	//	4	1	1		fifo[1]
	//	5	1	2		fifo[2]
	//	6	0	3		fifo[3]
	//	7	0	3		fifo[3]
	reg	will_underflow;
	initial	will_underflow = 1'b1;
	always @(posedge i_clk)
		if (i_rst)
			will_underflow <= 1'b1;
		else if (i_wr)
			will_underflow <= (will_underflow)&&(i_rd);
		else if (i_rd)
			will_underflow <= (w_last_plus_one == r_first);
		else
			will_underflow <= (r_last == r_first);
 
	//
	// Don't report FIFO underflow errors.  These'll be caught elsewhere
	// in the system, and the logic below makes it hard to reset them.
	// We'll still report FIFO overflow, however.
	//
	// reg		r_unfl;
	// initial	r_unfl = 1'b0;
	initial	r_last = 0;
	always @(posedge i_clk)
		if (i_rst)
		begin
			r_last <= 0;
			r_next <= { {(LGFLEN-1){1'b0}}, 1'b1 };
			// r_unfl <= 1'b0;
		end else if (i_rd)
		begin
			if ((i_wr)||(!will_underflow)) // (r_first != r_last)
			begin
				r_last <= r_next;
				r_next <= r_last +{{(LGFLEN-2){1'b0}},2'b10};
				// Last chases first
				// Need to be prepared for a possible two
				// reads in quick succession
				// o_data <= fifo[r_last+1];
			end
			// else r_unfl <= 1'b1;
		end
 
	reg	[7:0]	fifo_here, fifo_next, r_data;
	always @(posedge i_clk)
		fifo_here <= fifo[r_last];
	always @(posedge i_clk)
		fifo_next <= fifo[r_next];
	always @(posedge i_clk)
		r_data <= i_data;
 
	reg	[1:0]	osrc;
	always @(posedge i_clk)
		if (will_underflow)
			// o_data <= i_data;
			osrc <= 2'b00;
		else if ((i_rd)&&(r_first == w_last_plus_one))
			osrc <= 2'b01;
		else if (i_rd)
			osrc <= 2'b11;
		else
			osrc <= 2'b10;
	assign o_data = (osrc[1]) ? ((osrc[0])?fifo_next:fifo_here) : r_data;
 
	// wire	[(LGFLEN-1):0]	current_fill;
	// assign	current_fill = (r_first-r_last);
 
	reg	r_empty_n;
	initial	r_empty_n = 1'b0;
	always @(posedge i_clk)
		if (i_rst)
			r_empty_n <= 1'b0;
		else case({i_wr, i_rd})
			2'b00: r_empty_n <= (r_first != r_last);
			2'b11: r_empty_n <= (r_first != r_last);
			2'b10: r_empty_n <= 1'b1;
			2'b01: r_empty_n <= (r_first != w_last_plus_one);
		endcase
 
	wire	w_full_n;
	assign	w_full_n = will_overflow;
 
	//
	// If this is a receive FIFO, the FIFO count that matters is the number
	// of values yet to be read.  If instead this is a transmit FIFO, then 
	// the FIFO count that matters is the number of empty positions that
	// can still be filled before the FIFO is full.
	//
	// Adjust for these differences here.
	reg	[(LGFLEN-1):0]	r_fill;
	always @(posedge i_clk)
		if (RXFIFO!=0) begin
			// Calculate the number of elements in our FIFO
			//
			// Although used for receive, this is actually the more
			// generic answer--should you wish to use the FIFO in
			// another context.
			if (i_rst)
				r_fill <= 0;
			else case({i_wr, i_rd})
			2'b01:   r_fill <= r_first - r_next;
			2'b10:   r_fill <= r_first - r_last + 1'b1;
			default: r_fill <= r_first - r_last;
			endcase
		end else begin
			// Calculate the number of elements that are empty and
			// can be filled within our FIFO
			if (i_rst)
				r_fill <= { (LGFLEN){1'b1} };
			else case({i_wr, i_rd})
			2'b01:   r_fill <= r_last - r_first;
			2'b10:   r_fill <= r_last - w_first_plus_two;
			default: r_fill <= r_last - w_first_plus_one;
			endcase
		end
 
	// We don't report underflow errors.  These
	assign o_err = (r_ovfl); //  || (r_unfl);
 
	wire	[3:0]	lglen;
	assign lglen = LGFLEN;
 
	wire	[9:0]	w_fill;
	assign	w_fill[(LGFLEN-1):0] = r_fill;
	generate if (LGFLEN < 10)
		assign w_fill[9:(LGFLEN)] = 0;
	endgenerate
 
	wire	w_half_full;
	assign	w_half_full = r_fill[(LGFLEN-1)];
 
	assign	o_status = {
		// Our status includes a 4'bit nibble telling anyone reading
		// this the size of our FIFO.  The size is then given by
		// 2^(this value).  Hence a 4'h4 in this position means that the
		// FIFO has 2^4 or 16 values within it.
		lglen,
		// The FIFO fill--for a receive FIFO the number of elements
		// left to be read, and for a transmit FIFO the number of
		// empty elements within the FIFO that can yet be filled.
		w_fill,
		// A '1' here means a half FIFO length can be read (receive
		// FIFO) or written to (not a receive FIFO).
		// receive FIFO), or be written to (if it isn't).
		(RXFIFO!=0)?w_half_full:w_half_full,
		// A '1' here means the FIFO can be read from (if it is a
		// receive FIFO), or be written to (if it isn't).
		(RXFIFO!=0)?r_empty_n:w_full_n
	};
 
	assign	o_empty_n = r_empty_n;
 
endmodule