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+// noise_filter.v - Low pass noise filter.
+// hdlcounter - Incremental encoder counter on programmable logic. {{{
+//
+// Copyright (C) 2007 Nicolas Schodet
+//
+// Robot APB Team 2008.
+// Web: http://apbteam.org/
+// Email: team AT apbteam DOT org
+//
+// This program is free software; 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 2 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
+// MERCHANTABILITY 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; if not, write to the Free Software
+// Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+//
+// }}}
+`timescale 1ns / 1ps
+
+// To switch output, input should be stable for a number of cycle. This will
+// filter small spikes. As input is latched in only one flip-flop, this
+// module can be used directly on an input pin. See input_latch.v for more
+// details.
+
+module noise_filter(clk, rst, q, qf);
+ parameter size = 3;
+ input clk;
+ input rst;
+ input q;
+ output qf;
+
+ reg qf;
+ reg [size-1:0] hist;
+
+ always @(posedge clk or negedge rst) begin
+ if (!rst) begin
+ qf <= 0;
+ hist <= 0;
+ end
+ else begin
+ if (&hist)
+ qf <= 1;
+ else if (~|hist)
+ qf <= 0;
+ hist <= { hist[size-2:0], q };
+ end
+ end
+
+endmodule