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Distributions Constraints
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When weight based randomization is required in Systemverilog, we can use Distribution. |
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Syntax |
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constraint_block ::=
...
| expression dist { dist_list } ;
dist_list ::= dist_item { , dist_item }
dist_item ::= value_range [ dist_weight ]
dist_weight ::=
:= expression
| :/ expression
dist_item ::=
value_range := expression
| value_range :/ expression
expression_or_dist ::= expression [ dist { dist_list } ]
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Where |
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- expression : Can be any Systemverilog expression that refers to at least one variable declared as rand.
- dist: The dist operator returns true if the expression is contained in the set; otherwise it returns false.
- := weight : The := operator assigns weight to the item, min, or if there is a range (that is, min:max), to every value in the range (that is, all) elements in the range get the weight. weight may be any Systemverilog expression).
- :/ weight : The :/ operator divides the value of weight and assigns equally among all elements in the range. If there are n values in the range, the weight of each value is range_weight/n. (weight may be any Systemverilog expression).
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Example : Distributions
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1 program distribution;
2 class frame_t;
3 rand bit [7:0] src_port;
4 rand bit [7:0] des_port;
5 rand bit [15:0] length;
6 constraint len {
7 length dist {
8 [64 : 127 ] := 10,
9 [128 : 511 ] := 10,
10 [512 : 2048] := 10
11 };
12 }
13 constraint src {
14 src_port dist {
15 0 := 1,
16 1 := 1,
17 2 := 5,
18 4 := 1
19 };
20 }
21 constraint des {
22 des_port dist {
23 [0 : 5 ] :/ 5,
24 [6 : 100 ] := 1,
25 [101 : 200 ] := 1,
26 [201 : 255 ] := 1
27 };
28 }
29
30 function void post_randomize();
31 begin
32 $display("src port : %0x",src_port);
33 $display("des port : %0x",des_port);
34 $display("length : %0x",length);
35 end
36 endfunction
37 endclass
38
39 initial begin
40 frame_t frame = new();
41 integer i,j = 0;
42 for (j=0;j < 4; j++) begin
43 $display("-------------------------------");
44 $display("Randomize Value");
45 $display("-------------------------------");
46 i = frame.randomize();
47 end
48 $display("-------------------------------");
49 end
50
51 endprogram
You could download file distribution.sv here
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Simulation Output : Distributions
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-------------------------------
Randomize Value
-------------------------------
src port : 0
des port : 36
length : bd
-------------------------------
Randomize Value
-------------------------------
src port : 2
des port : 5e
length : 43b
-------------------------------
Randomize Value
-------------------------------
src port : 2
des port : 83
length : 4fe
-------------------------------
Randomize Value
-------------------------------
src port : 2
des port : e4
length : 6d9
-------------------------------
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Copyright © 1998-2014 |
Deepak Kumar Tala - All rights reserved |
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Do you have any Comment? mail me at:deepak@asic-world.com
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