proper shifting for the cic decim now keeps signals roughly leveled

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@4784 221aa14e-8319-0410-a670-987f0aec2ac5

2 files changed, 103 insertions, 20 deletions

diff --git a/sdr_lib/cic_dec_shifter.v b/sdr_lib/cic_dec_shifter.v
new file mode 100644
index 000000000..a213303c8
--- /dev/null
+++ b/sdr_lib/cic_dec_shifter.v
@@ -0,0 +1,100 @@
+// -*- verilog -*-
+//
+//  USRP - Universal Software Radio Peripheral
+//
+//  Copyright (C) 2003 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
+//
+
+
+// NOTE   This only works for N=4, max decim rate of 128
+// NOTE   signal "rate" is ONE LESS THAN the actual rate
+
+module cic_dec_shifter(rate,signal_in,signal_out);
+   parameter bw = 16;
+   parameter maxbitgain = 28;
+   
+   input [7:0] rate;
+   input       wire [bw+maxbitgain-1:0] signal_in;
+   output      reg [bw-1:0] signal_out;
+
+   function [4:0] bitgain;
+      input [7:0] rate;
+      case(rate)
+	// Exact Cases -- N*log2(rate)
+	8'd4 : bitgain = 8;
+	8'd8 : bitgain = 12;
+	8'd16 : bitgain = 16;
+	8'd32 : bitgain = 20;
+	8'd64 : bitgain = 24;
+	8'd128 : bitgain = 28;
+	
+	// Nearest without overflow -- ceil(N*log2(rate))
+	8'd5 : bitgain = 10;
+	8'd6 : bitgain = 11;
+	8'd7 : bitgain = 12;
+	8'd9 : bitgain = 13;
+	8'd10,8'd11 : bitgain = 14;
+	8'd12,8'd13 : bitgain = 15;
+	8'd14,8'd15 : bitgain = 16;
+	8'd17,8'd18,8'd19 : bitgain = 17;
+	8'd20,8'd21,8'd22 : bitgain = 18;
+	8'd23,8'd24,8'd25,8'd26 : bitgain = 19;
+	8'd27,8'd28,8'd29,8'd30,8'd31 : bitgain = 20;
+	8'd33,8'd34,8'd35,8'd36,8'd37,8'd38 : bitgain = 21;
+	8'd39,8'd40,8'd41,8'd42,8'd43,8'd44,8'd45 : bitgain = 22;
+	8'd46,8'd47,8'd48,8'd49,8'd50,8'd51,8'd52,8'd53 : bitgain = 23;
+	8'd54,8'd55,8'd56,8'd57,8'd58,8'd59,8'd60,8'd61,8'd62,8'd63 : bitgain = 24;
+	8'd65,8'd66,8'd67,8'd68,8'd69,8'd70,8'd71,8'd72,8'd73,8'd74,8'd75,8'd76 : bitgain = 25;
+	8'd77,8'd78,8'd79,8'd80,8'd81,8'd82,8'd83,8'd84,8'd85,8'd86,8'd87,8'd88,8'd89,8'd90 : bitgain = 26;
+	8'd91,8'd92,8'd93,8'd94,8'd95,8'd96,8'd97,8'd98,8'd99,8'd100,8'd101,8'd102,8'd103,8'd104,8'd105,8'd106,8'd107 : bitgain = 27;
+	default : bitgain = 28;
+      endcase // case(rate)
+   endfunction // bitgain
+   
+   wire [4:0] 	  shift = bitgain(rate+1);
+   
+   // We should be able to do this, but can't ....
+   // assign 	  signal_out = signal_in[shift+bw-1:shift];
+   
+   always @*
+     case(shift)
+       5'd8  : signal_out = signal_in[8+bw-1:8];
+       5'd10 : signal_out = signal_in[10+bw-1:10];
+       5'd11 : signal_out = signal_in[11+bw-1:11];
+       5'd12 : signal_out = signal_in[12+bw-1:12];
+       5'd13 : signal_out = signal_in[13+bw-1:13];
+       5'd14 : signal_out = signal_in[14+bw-1:14];
+       5'd15 : signal_out = signal_in[15+bw-1:15];
+       5'd16 : signal_out = signal_in[16+bw-1:16];
+       5'd17 : signal_out = signal_in[17+bw-1:17];
+       5'd18 : signal_out = signal_in[18+bw-1:18];
+       5'd19 : signal_out = signal_in[19+bw-1:19];
+       5'd20 : signal_out = signal_in[20+bw-1:20];
+       5'd21 : signal_out = signal_in[21+bw-1:21];
+       5'd22 : signal_out = signal_in[22+bw-1:22];
+       5'd23 : signal_out = signal_in[23+bw-1:23];
+       5'd24 : signal_out = signal_in[24+bw-1:24];
+       5'd25 : signal_out = signal_in[25+bw-1:25];
+       5'd26 : signal_out = signal_in[26+bw-1:26];
+       5'd27 : signal_out = signal_in[27+bw-1:27];
+       5'd28 : signal_out = signal_in[28+bw-1:28];
+       
+       default : signal_out = signal_in[28+bw-1:28];
+     endcase // case(shift)
+
+endmodule // cic_dec_shifter
+
diff --git a/sdr_lib/cic_decim.v b/sdr_lib/cic_decim.v
index d7e5310b8..6c20416f1 100755
--- a/sdr_lib/cic_decim.v
+++ b/sdr_lib/cic_decim.v
@@ -24,7 +24,7 @@ module cic_decim
   ( clock,reset,enable,rate,strobe_in,strobe_out,signal_in,signal_out);
    parameter bw = 16;
    parameter N = 4;
-   parameter log2_of_max_rate = 8;
+   parameter log2_of_max_rate = 7;
    parameter maxbitgain = N * log2_of_max_rate;
    
    input clock;
@@ -82,25 +82,8 @@ module cic_decim
       
    wire [bw+maxbitgain-1:0] signal_out_unnorm = pipeline[N-1];
 
-   // Output Scaling to same width as input
-   function [2:0] log_ceil;
-      input [7:0] val;
-      log_ceil = val[6] ? 3'd7 : val[5] ? 3'd6 : val[4] ? 3'd5 :
-		 val[3] ? 3'd4 : val[2] ? 3'd3 : val[1] ? 3'd2 : 3'd1; 
-   endfunction // log_ceil
-   
-   wire [2:0] shift = log_ceil(rate);
-   
-   always @*
-     case(shift)
-       3'd2 : signal_out = signal_out_unnorm[2*N+bw-1:2*N]; //  Decim by 4
-       3'd3 : signal_out = signal_out_unnorm[3*N+bw-1:3*N];
-       3'd4 : signal_out = signal_out_unnorm[4*N+bw-1:4*N];
-       3'd5 : signal_out = signal_out_unnorm[5*N+bw-1:5*N];
-       3'd6 : signal_out = signal_out_unnorm[6*N+bw-1:6*N];
-       3'd7 : signal_out = signal_out_unnorm[7*N+bw-1:7*N];
-       default : signal_out = signal_out_unnorm[7*N+bw-1:7*N];
-     endcase // case(shift)
+   cic_dec_shifter #(bw)
+	cic_dec_shifter(rate,signal_out_unnorm,signal_out);
    
 endmodule // cic_decim