added comments, removed dead code

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@4779 221aa14e-8319-0410-a670-987f0aec2ac5

diff --git a/usrp/fpga/sdr_lib/cic_int_shifter.v b/usrp/fpga/sdr_lib/cic_int_shifter.v
index fd928373b4fee0ae0ca1a6390db0ceabb382c813..fd148c9790048752190c50e6cdefe7f19fd0420e 100644 (file)
--- a/usrp/fpga/sdr_lib/cic_int_shifter.v
+++ b/usrp/fpga/sdr_lib/cic_int_shifter.v
@@ -20,25 +20,21 @@
 //
 
 
+// NOTE   This only works for N=4, max interp rate of 128
+// NOTE   signal "rate" is ONE LESS THAN the actual rate
+
 module cic_int_shifter(rate,signal_in,signal_out);
    parameter bw = 16;
-   parameter N = 4;
-   parameter log2_of_max_rate = 7;
-   parameter maxbitgain = (N-1)*log2_of_max_rate;
+   parameter maxbitgain = 21;
    
    input [7:0] rate;
    input       wire [bw+maxbitgain-1:0] signal_in;
    output      reg [bw-1:0] signal_out;
 
-   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
-   
    function [4:0] bitgain;
       input [7:0] rate;
       case(rate)
+       // Exact Cases
        8'd4 : bitgain = 2*(N-1);
        8'd8 : bitgain = 3*(N-1);
        8'd16 : bitgain = 4*(N-1);
@@ -46,6 +42,7 @@ module cic_int_shifter(rate,signal_in,signal_out);
        8'd64 : bitgain = 6*(N-1);
        8'd128 : bitgain = 7*(N-1);
        
+       // Nearest without overflow
        8'd5 : bitgain = 7;
        8'd6 : bitgain = 8;
        8'd7 : bitgain = 9;
@@ -90,7 +87,6 @@ module cic_int_shifter(rate,signal_in,signal_out);
        5'd19 : signal_out = signal_in[19+bw-1:19];
        5'd20 : signal_out = signal_in[20+bw-1:20];
        
-       
        default : signal_out = signal_in[21+bw-1:21];
      endcase // case(shift)