VLSI DV Interview Puzzles · All levels

within and Alternative Windows

Property intent: ack ##1 done must complete within either a 4-cycle or 5-cycle request window. Timeline: C60 start=1 req=1, C62 ack=1, C63 d1, C64 end_win=1. Determine the verdict.

Puzzle

Difficulty: Hard · Puzzle 6 of 6 · Topic: Sequence Puzzles

Property intent: ack ##1 done must complete within either a 4-cycle or 5-cycle request window. Timeline: C60 start=1 req=1, C62 ack=1, C63 done=1, C64 end_win=1. Determine the verdict.

Code

systemverilog

sequence resp_seq;
  ack ##1 done;
endsequence

sequence win4;
  req ##[1:4] end_win;
endsequence

sequence win5;
  req ##[1:5] end_win;
endsequence

property p_within_or;
  @(posedge clk) start |-> (resp_seq within (win4 or win5));
endproperty

Hint

Check containment of the full resp sequence, not just ack.

Step-by-step solution

diagram

1) start launches matching at C60.
2) resp_seq spans C62 to C63.
3) win4 can match as req at C60 and end_win at C64, spanning C60 to C64.
4) resp_seq is fully contained inside that window; or is satisfied by win4 alone.

Answer

Answer: PASS - resp_seq is fully enclosed by a valid request window, so within succeeds via the win4 branch of the or composition.

Why candidates get it wrong

Engineers often evaluate only start alignment and forget that within requires full-sequence containment.

Interviewer follow-up

If done moved to C65 while end_win stayed at C64, what verdict would you expect?

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