option.weight zero removes metric influence

Puzzle

Difficulty: Medium · Puzzle 6 of 6 · Topic: Coverage Options Puzzles

Compute each coverpoint percentage and final group percentage when one coverpoint has weight 0.

Code

covergroup cg with function sample(bit [1:0] main_mode, bit debug_mode);
  cp_main: coverpoint main_mode {
    option.weight = 1;
    bins m[] = {[0:2]};
  }
  cp_debug: coverpoint debug_mode {
    option.weight = 0;
    bins d0 = {0};
    bins d1 = {1};
  }
endgroup

initial begin
  cg c = new();
  c.sample(0,0);
  c.sample(1,0);
end

Hint

Calculate cp_main and cp_debug first, then apply weights in group aggregation.

Step-by-step solution

1) cp_main bins are 0,1,2 with hits on 0 and 1 -> 2/3 = 66.67%.
2) cp_debug bins are d0 and d1 with only d0 hit -> 1/2 = 50%.
3) Group aggregation uses weights: (1*66.67 + 0*50) / (1+0).
4) Final group coverage remains 66.67%.

Answer

Answer: cp_main=66.67%, cp_debug=50%, group coverage=66.67% because cp_debug has zero weight.

Why candidates get it wrong

A low-quality auxiliary metric can look harmless in top-line numbers if its weight is 0; reviewers must inspect component coverage too.

Interviewer follow-up

When is setting weight 0 justified, and what governance rule would you add to prevent abuse?

Related topics

• Back to Coverage Options Puzzles

• Functional Coverage Puzzles section