VLSI DV Interview Puzzles · All levels

binsof-selected tuples with default ignore

Determine scored bin count and final percentage when only selected tuples are named.

Puzzle

Difficulty: Medium · Puzzle 3 of 6 · Topic: Cross Coverage Puzzles

Determine scored bin count and final percentage when only selected tuples are named.

Code

systemverilog

covergroup cg with function sample(bit [2:0] size, bit [1:0] qos);
  cp_size: coverpoint size {
    bins small = {[0:3]};
    bins big   = {[4:7]};
  }

  cp_qos: coverpoint qos {
    bins low    = {0};
    bins high   = {1};
    bins urgent = {2};
  }

  x_sz_qos: cross cp_size, cp_qos {
    bins big_high   = binsof(cp_size.big)   && binsof(cp_qos.high);
    bins big_urgent = binsof(cp_size.big)   && binsof(cp_qos.urgent);
    bins small_low  = binsof(cp_size.small) && binsof(cp_qos.low);
    ignore_bins rest = default;
  }
endgroup

initial begin
  cg c = new();
  c.sample(6,1);
  c.sample(7,2);
  c.sample(2,2);
end

Hint

Only the three named bins are scored; one sample lands in ignored space.

Step-by-step solution

diagram

1) Denominator is 3 scored bins: big_high, big_urgent, small_low.
2) sample(6,1) hits big_high.
3) sample(7,2) hits big_urgent.
4) sample(2,2) is rest and ignored.
5) Coverage = 2/3 = 66.67%.

Answer

Answer: 3 bins are scored and coverage is 66.67% (2 hit of 3).

Why candidates get it wrong

Many candidates incorrectly include all 2x3=6 possible tuples in denominator even with ignore default.

Interviewer follow-up

What single stimulus would close this model to 100%?

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