VLSI DV Interview Puzzles · All levels

Three-way cross with only custom bins scored

Find the scored denominator and percentage for this 3-D cross model.

Puzzle

Difficulty: Hard · Puzzle 6 of 6 · Topic: Cross Coverage Puzzles

Find the scored denominator and percentage for this 3-D cross model.

Code

systemverilog

covergroup cg with function sample(bit [1:0] port, bit prio, bit drop);
  cp_port: coverpoint port {
    bins p0 = {0};
    bins p1 = {1};
    bins p2 = {2};
  }
  cp_prio: coverpoint prio {
    bins lo = {0};
    bins hi = {1};
  }
  cp_drop: coverpoint drop {
    bins no  = {0};
    bins yes = {1};
  }
  x3: cross cp_port, cp_prio, cp_drop {
    bins p0_hi_drop = binsof(cp_port.p0) && binsof(cp_prio.hi) && binsof(cp_drop.yes);
    bins p1_lo_no   = binsof(cp_port.p1) && binsof(cp_prio.lo) && binsof(cp_drop.no);
    bins p2_hi_no   = binsof(cp_port.p2) && binsof(cp_prio.hi) && binsof(cp_drop.no);
    ignore_bins others = default;
  }
endgroup

initial begin
  cg c = new();
  c.sample(0,1,1);
  c.sample(1,0,0);
  c.sample(2,1,0);
  c.sample(2,0,1);
end

Hint

default ignore means only three named bins are scored.

Step-by-step solution

diagram

1) Denominator is exactly 3 scored bins.
2) First three samples hit p0_hi_drop, p1_lo_no, and p2_hi_no.
3) Final sample (2,0,1) is ignored by default rest space.
4) Coverage = 3/3 = 100%.

Answer

Answer: Scored bins = 3 and coverage = 100%.

Why candidates get it wrong

In high-dimensional crosses, default ignore can make closure look easy; ensure selected bins truly represent spec risk.

Interviewer follow-up

Which additional named bin would you add first if silicon escapes were seen on p2 low-priority drops?

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