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////////////////////////////////////////////////////////////////////////////////
//
// Filename: 	rxemin.v
//
// Project:	OpenArty, an entirely open SoC based upon the Arty platform
//
// Purpose:	To force the minimum received packet size of an ethernet frame
//		to be a minimum of 64 bytes.  Packets less than 64-bytes
//		(including CRC) need to be dropped.  This module handles that
//	logic.
//
// Creator:	Dan Gisselquist, Ph.D.
//		Gisselquist Technology, LLC
//
////////////////////////////////////////////////////////////////////////////////
//
// Copyright (C) 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.
//
// 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 rxemin(i_clk, i_ce, i_en, i_cancel, i_v, i_d, o_err);
	parameter	MINNIBBLES=120;
	localparam	LGNCOUNT=(MINNIBBLES<63)? 6
				:((MINNIBBLES<127)? 7:((MINNIBBLES<255)? 8:9));
	input			i_clk, i_ce, i_en, i_cancel;
	input			i_v;	// Valid
	input		[3:0]	i_d;	// Data nibble
	output	reg		o_err;
 
	reg	last_v;
	reg	[(LGNCOUNT-1):0]	r_ncnt;
	initial	last_v = 1'b0;
	always @(posedge i_clk)
	if (i_ce)
	begin
		last_v <= i_v;
 
		if ((!i_v)||(i_cancel))
		begin
			// Here's our reset.  If th input isn't valid (i.e., no
			// packet present), or if we are cancelling the packet,
			// then we come in here and reset our interface.
			r_ncnt <= 0;
			o_err  <= 0;
		end else if (i_v)
		begin
			r_ncnt <= (r_ncnt<MINNIBBLES) ? r_ncnt+1'b1 : r_ncnt;
		end else if (last_v)
			o_err <= (i_en)&&(r_ncnt < MINNIBBLES);
	end
 
endmodule