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/trunk/Log2highacc.v
0,0 → 1,178
module Log2highacc
 
/*
A fast base-2 logarithm function, 24 bits (22 used) in, 12 bits out.
Designed and coded by: Michael Dunn, http://www.cantares.on.ca/
(more info at the web site - see "Extras")
Executes every cycle, with a latency of 3.
 
Compared to previous versions, this one has a larger, higher resolution
lookup table. This provides a smoother and more accurate output, though
a downside of having higher LUT resolution than the LUT depth fully
supports is that there are missing codes in the output. If this is a problem,
the lower 2 or 3 output bits can be ignored. In most cases, use all the
bits you can!
 
Valid input range = 000100 - FFFFFF. In effect, there is a binary point:
xxxx.yy. Logs of inputs below 1.00 are negative, and not handled by this design.
 
License: Free to use & modify, but please keep this header intact.
August 2, 2010, Kitchener, Ontario, Canada
*/
 
(
input [23:0] DIN,
input clk,
 
output [11:0] DOUT
);
 
 
// Comprises 4 main blocks: priority encoder, barrel shifter, LUT, and adder.
 
reg [3:0] priencout1;
reg [3:0] priencout2;
reg [3:0] priencout3;
reg [5:0] barrelout;
reg [20:0] barrelin;
reg [7:0] LUTout;
 
assign DOUT = {priencout3, LUTout}; // Basic top-level connectivity
 
always @(posedge clk)
begin
priencout2 <= priencout1;
priencout3 <= priencout2;
barrelin <= DIN[22:2];
end
 
 
wire [20:0] tmp1 = (barrelin << ~priencout1); // Barrel shifter - OMG, it's a primitive in Verilog!
always @(posedge clk)
begin
barrelout <= tmp1[20:15];
end
 
 
wire [15:0] priencin = DIN[23:8];
 
always @(posedge clk) // Priority encoder
 
casex (priencin)
 
16'b1xxxxxxxxxxxxxxx: priencout1 <= 15;
16'b01xxxxxxxxxxxxxx: priencout1 <= 14;
16'b001xxxxxxxxxxxxx: priencout1 <= 13;
16'b0001xxxxxxxxxxxx: priencout1 <= 12;
16'b00001xxxxxxxxxxx: priencout1 <= 11;
16'b000001xxxxxxxxxx: priencout1 <= 10;
16'b0000001xxxxxxxxx: priencout1 <= 9;
16'b00000001xxxxxxxx: priencout1 <= 8;
16'b000000001xxxxxxx: priencout1 <= 7;
16'b0000000001xxxxxx: priencout1 <= 6;
16'b00000000001xxxxx: priencout1 <= 5;
16'b000000000001xxxx: priencout1 <= 4;
16'b0000000000001xxx: priencout1 <= 3;
16'b00000000000001xx: priencout1 <= 2;
16'b000000000000001x: priencout1 <= 1;
16'b000000000000000x: priencout1 <= 0;
endcase
 
 
 
/*
LUT for log fraction lookup
- can be done with array or case:
 
case (addr)
0:out=0;
.
31:out=15;
endcase
 
OR
wire [3:0] lut [0:31];
assign lut[0] = 0;
.
assign lut[31] = 15;
 
Are there any better ways?
*/
 
// Let's try "case".
// The equation is: output = log2(1+input/64)*256
// For larger tables, better to generate a separate data file using a program!
 
always @(posedge clk)
case (barrelout)
 
0: LUTout <= 0;
1: LUTout <= 6;
2: LUTout <= 11;
3: LUTout <= 17;
4: LUTout <= 22;
5: LUTout <= 28;
6: LUTout <= 33;
7: LUTout <= 38;
8: LUTout <= 44;
9: LUTout <= 49;
10: LUTout <= 54;
11: LUTout <= 59;
12: LUTout <= 63;
13: LUTout <= 68;
14: LUTout <= 73;
15: LUTout <= 78;
16: LUTout <= 82;
17: LUTout <= 87;
18: LUTout <= 92;
19: LUTout <= 96;
20: LUTout <= 100;
21: LUTout <= 105;
22: LUTout <= 109;
23: LUTout <= 113;
24: LUTout <= 118;
25: LUTout <= 122;
26: LUTout <= 126;
27: LUTout <= 130;
28: LUTout <= 134;
29: LUTout <= 138;
30: LUTout <= 142;
31: LUTout <= 146;
32: LUTout <= 150;
33: LUTout <= 154;
34: LUTout <= 157;
35: LUTout <= 161;
36: LUTout <= 165;
37: LUTout <= 169;
38: LUTout <= 172;
39: LUTout <= 176;
40: LUTout <= 179;
41: LUTout <= 183;
42: LUTout <= 186;
43: LUTout <= 190;
44: LUTout <= 193;
45: LUTout <= 197;
46: LUTout <= 200;
47: LUTout <= 203;
48: LUTout <= 207;
49: LUTout <= 210;
50: LUTout <= 213;
51: LUTout <= 216;
52: LUTout <= 220;
53: LUTout <= 223;
54: LUTout <= 226;
55: LUTout <= 229;
56: LUTout <= 232;
57: LUTout <= 235;
58: LUTout <= 238;
59: LUTout <= 241;
60: LUTout <= 244;
61: LUTout <= 247;
62: LUTout <= 250;
63: LUTout <= 253;
 
endcase
 
endmodule

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