dsp: new rounding. more complex, but better properties

2 files changed, 80 insertions, 3 deletions

diff --git a/usrp2/sdr_lib/round.v b/usrp2/sdr_lib/round.v
index 7a137d702..5b83981c6 100644
--- a/usrp2/sdr_lib/round.v
+++ b/usrp2/sdr_lib/round.v
@@ -2,7 +2,7 @@
 //
 //  USRP - Universal Software Radio Peripheral
 //
-//  Copyright (C) 2007 Matt Ettus
+//  Copyright (C) 2011 Matt Ettus
 //
 //  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
@@ -24,12 +24,28 @@
 
 module round
   #(parameter bits_in=0,
-    parameter bits_out=0)
+    parameter bits_out=0,
+    parameter round_to_zero=0,       // original behavior
+    parameter round_to_nearest=1,    // lowest noise
+    parameter trunc=0)               // round to negative infinity
     (input [bits_in-1:0] in,
      output [bits_out-1:0] out,
      output [bits_in-bits_out:0] err);
 
-   assign out = in[bits_in-1:bits_in-bits_out] + (in[bits_in-1] & |in[bits_in-bits_out-1:0]);
+   wire 			 round_corr;
+   wire 			 round_corr_trunc = 0;
+   wire 			 round_corr_rtz = (in[bits_in-1] & |in[bits_in-bits_out-1:0]);
+   wire 			 round_corr_nearest = in[bits_in-bits_out-1];
+   wire 			 round_corr_nearest_safe = (~in[bits_in-1] & (&in[bits_in-2:bits_out])) ? 0 :
+				 round_corr_nearest;
+      
+   assign round_corr = round_to_nearest ? round_corr_nearest_safe :
+		       trunc ? round_corr_trunc : 
+		       round_to_zero ? round_corr_rtz :
+		       0;  // default to trunc
+      
+   assign out = in[bits_in-1:bits_in-bits_out] + round_corr;
+   
    assign err = in - {out,{(bits_in-bits_out){1'b0}}};
    
 endmodule // round
diff --git a/usrp2/sdr_lib/round_tb.v b/usrp2/sdr_lib/round_tb.v
new file mode 100644
index 000000000..ddc464f4a
--- /dev/null
+++ b/usrp2/sdr_lib/round_tb.v
@@ -0,0 +1,61 @@
+// -*- verilog -*-
+//
+//  USRP - Universal Software Radio Peripheral
+//
+//  Copyright (C) 2011 Matt Ettus
+//
+//  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., 51 Franklin Street, Boston, MA  02110-1301  USA
+//
+
+// Rounding "macro"
+// Keeps the topmost bits, does proper 2s comp round to zero (unbiased truncation)
+
+module round_tb();
+
+   localparam IW=8;
+   localparam OW=4;
+   localparam EW=IW-OW+1;
+   
+   reg signed [IW-1:0] in;
+   wire signed [OW-1:0] out;
+   wire signed [EW-1:0] err;
+
+   round #(.bits_in(IW),
+	   .bits_out(OW),
+	   .round_to_zero(0),       // original behavior
+	   .round_to_nearest(1),    // lowest noise
+	   .trunc(0))               // round to negative infinity
+   round (.in(in),.out(out),.err(err));
+
+   initial $dumpfile("round_tb.vcd");
+   initial $dumpvars(0,round_tb);
+
+   wire signed [IW-1:0] out_round = {out,{IW-OW{1'b0}}};
+   
+   initial
+     begin
+	in <= -129;
+	#1;
+	repeat (260)
+	  begin
+	     in <= in + 1;
+	     #1;
+	     $display("In %d, out %d, out_rnd %d, err %d, real err %d",in,out,out_round,-err,out_round-in);
+	     #1;
+	  end
+	$finish;
+     end
+     
+endmodule // round