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+//////////////////////////////////////////////////////////////////////////////
+// Copyright (c) 2011, Andrew "bunnie" Huang
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, 
+// are permitted provided that the following conditions are met:
+//
+//  * Redistributions of source code must retain the above copyright notice, 
+//    this list of conditions and the following disclaimer.
+//  * Redistributions in binary form must reproduce the above copyright notice, 
+//    this list of conditions and the following disclaimer in the documentation and/or 
+//    other materials provided with the distribution.
+//
+//    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY 
+//    EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 
+//    OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT 
+//    SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, 
+//    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 
+//    LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR 
+//    PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, 
+//    WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
+//    ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 
+//    POSSIBILITY OF SUCH DAMAGE.
+//
+//////////////////////////////////////////////////////////////////////////////
+// this module does simple PWM modulation to create the "breathing" LED effect
+//
+
+`timescale 1 ns / 1 ps
+
+module pwm(
+	   input wire clk812k, // use clock from device DNA block, 812.5kHz
+	   input wire reset,
+	   output reg pwmout,
+	   input wire [11:0] bright,
+	   input wire [11:0] dim
+	   );
+
+   reg [9:0] 		    pwm_count;
+   reg 			    pwmreg;
+   reg [11:0] 		    interpolate;
+   reg 			    countdn;
+   wire [9:0] 		    interp;
+      
+   always @(posedge clk812k or posedge reset) begin
+      if( reset ) begin
+	 pwm_count <= 0;
+	 interpolate[11:0] <= dim[11:0];
+	 countdn <= 0;
+      end else begin
+	 if( interpolate[11:0] >= bright[11:0] ) begin
+	    countdn <= 1;
+	 end else if( interpolate[11:0] <= dim[11:0] ) begin
+	    countdn <= 0;
+	 end else begin
+	    countdn <= countdn;
+	 end
+	 
+	 if( pwm_count[9:0] == 10'h0 ) begin
+	    if( countdn == 1'b1 ) begin
+	       interpolate[11:0] <= interpolate[11:0] - 12'b1;
+	    end else begin
+	       interpolate[11:0] <= interpolate[11:0] + 12'b1;
+	    end
+	 end else begin
+	    interpolate[11:0] <= interpolate[11:0];
+	 end
+	 
+	 pwm_count[9:0] <= pwm_count[9:0] + 10'b1;
+      end
+
+      pwmreg <= (pwm_count[9:0] < interp[9:0]);
+
+   end // always @ (posedge clk812k or posedge reset)
+
+   assign interp[9:0] = interpolate[11:2];
+
+   always @(posedge clk812k) begin
+      // make it registered to ease up routing congestion to the edge
+      pwmout <= !pwmreg;
+   end
+
+endmodule // pwm