VLSI DV Interview Puzzles · All levels

Goto Repetition With Sparse Events

Property intent: after start, observe two occurrences of a (not necessarily consecutive), then one cycle later done. Timeline: C20 start=1, C22 a=1, C24 a=1, C25 d1. Determine the verdict.

Puzzle

Difficulty: Medium · Puzzle 2 of 6 · Topic: Sequence Puzzles

Property intent: after start, observe two occurrences of a (not necessarily consecutive), then one cycle later done. Timeline: C20 start=1, C22 a=1, C24 a=1, C25 done=1. Determine the verdict.

Code

systemverilog

property p_seq_goto;
  @(posedge clk) start |-> ##1 a[->2] ##1 done;
endproperty

Hint

Compare [->2] behavior to [*2].

Step-by-step solution

diagram

1) start triggers at C20; ##1 shifts matching window to C21 onward.
2) a[->2] consumes two a hits at C22 and C24, allowing gaps.
3) ##1 done then requires done at C25.
4) done is high at C25, satisfying consequent.

Answer

Answer: PASS - [->2] allows non-consecutive matches, so hits at C22/C24 are legal and done arrives exactly one cycle later.

Why candidates get it wrong

Interviewees frequently misread [->2] as consecutive repetition and incorrectly fail this waveform.

Interviewer follow-up

How would verdict change if the sequence used a[*2] instead of a[->2]?

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