I'm missing something in how to setup a dist constraint. I'm trying to get an equal weight between two ranges [0:31] and [32:65535]. I've set up a simple test:
class data;
rand bit [15:0] field1;
constraint c_f1 { field1 dist {[0:31] := 1, [32:65535] := 1};}
endclass
module testbench;
initial begin
data test_h;
test_h = new();
repeat(10) begin
assert(test_h.randomize());
$display("field1 = %h", test_h.field1);
end
end
endmodule
However, I'm seeing most values > 32. What have I misunderstood here?
1 Answer
Use the :/ operator instead of the := operator. Change:
constraint c_f1 { field1 dist {[0:31] := 1, [32:65535] := 1};}
to:
constraint c_f1 { field1 dist {[0:31] :/ 1, [32:65535] :/ 1};}
Refer to IEEE Std 1800-2017, section 18.5.4 Distribution.
The :/ operator assigns the specified weight to the item or, if the item is a range, to the range as a whole. If there are n values in the range, the weight of each value is range_weight / n .
This gives more values between 0-31:
field1 = 0012
field1 = 001f
field1 = 000f
field1 = a138
field1 = 0018
field1 = 001c
field1 = 9d48
field1 = 0009
field1 = 85bf
field1 = f930
