Unreachable bin from stimulus constraints

Puzzle

Difficulty: Easy · Puzzle 2 of 6 · Topic: Coverage Closure and Sampling Timing Puzzles

Why does this coverpoint plateau at 75% forever, and what is the correct closure action?

Code

class tr;
  rand bit [1:0] opcode;
  constraint legal_c { opcode inside {[0:2]}; }
endclass

covergroup cg with function sample(bit [1:0] opcode);
  cp_opcode: coverpoint opcode {
    bins op0 = {0};
    bins op1 = {1};
    bins op2 = {2};
    bins op3 = {3};
  }
endgroup

Hint

Check whether op3 can ever be generated under legal_c.

Step-by-step solution

1) Constraint legal_c allows only 0,1,2.
2) Bin op3 is unreachable by construction.
3) Maximum achievable legal hit bins are 3 of 4 -> 75% ceiling.
4) Closure fix is to align coverage model with stimulus/spec (ignore or remove op3, or broaden legal space intentionally).

Answer

Answer: Coverage ceiling is 75% because op3 is unreachable. This is a model mismatch, not a random stimulus deficiency.

Why candidates get it wrong

Teams often burn regression time trying to close a mathematically unreachable bin.

Interviewer follow-up

What evidence would you include in a waiver request for op3?

Related topics

• Back to Coverage Closure and Sampling Timing Puzzles

• Functional Coverage Puzzles section