Sequence End Points: Triggered and Matched

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WhyWhat IfCompareScale It

1. Sequence End Points: Triggered and Matched
2. WhyWhy End Points ExistL2 Mid-Level

   Why add end-point methods at all? Can a property not just start from the sequence?

   Reveal the Model Answer

   A property that starts from a sequence looks forward from its end. Some rules look backward instead: "when X happens, Y must have just finished". End points let a property check that backward rule at the moment X happens. They also let you build a sequence out of other sequences' end points.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 A property that starts from a sequence looks forward from its end. Key point 2 Some rules look backward instead: "when X happens, Y must have just finished". Key point 3 End points let a property check that backward rule at the moment X happens. Key point 4 They also let you build a sequence out of other sequences' end points.

   Half-Truth

   "End points are only syntax sugar."

   They express backward rules that a forward implication cannot, as the early-done case shows.

   Source Mehta, SystemVerilog Assertions and Functional Coverage (3rd ed.), §13.1

   • Lesson A5.1Assertions in Procedures, Sequences as Events
3. What IfWaiting on a Sequence from Testbench CodeL2 Mid-Level

   Asked if they mention testbench code

   What if a test task needs to wait until the bus has finished a full handshake sequence?

   Reveal the Model Answer

   Declare the handshake as a named sequence and write wait (hs.triggered); in the task. The task wakes in the time step where the handshake ends, with no hand-written state machine. .matched cannot be used in a wait; only .triggered can.

   attempt startspassfail

   1. Edge 2 · t=15

      a at 15 starts a match of a ##2 b.

   2. Edge 3 · t=25

      a again at 25: a second match overlaps the first.

   3. Edge 4 · t=35

      b at 35 completes the first match. always @(s_ab) wakes at 35.

   4. Edge 5 · t=45

      b at 45 completes the second match: another wake-up.

   5. Edge 7 · t=65

      a at 65 starts a third match.

   6. Edge 9 · t=85

      b is low at 85, so this match dies. The lane fails, and the block does not wake.

   Read the lanes as text

   • a @15, from edge 2 (t=15): passes at edge 4 (t=35).
   • a @25, from edge 3 (t=25): passes at edge 5 (t=45).
   • a @65, from edge 7 (t=65): fails at edge 9 (t=85).

   Sampled values, edge 1 to 9: a 0 1 1 0 0 0 1 0 0; b 0 0 0 1 1 0 0 0 0.

   The program behind the lanes

   The lanes come from running this program on Verilator 5.052. Each lane is a separate copy of b_two_later that can start only at that lane's edge, so every verdict and its edge are read from that copy's own output.

   module tb;
     logic clk = 0;
     always #5 clk = ~clk;                 // rising edges at 5, 15, 25, ...
     logic a = 0, b = 0;
     sequence s_ab; @(posedge clk) a ##2 b; endsequence
   
     b_two_later: assert property (@(posedge clk) a |-> ##2 b);
     always @(s_ab) $display("s_ab ended at %0t", $time);
   
     initial begin
       @(negedge clk); a = 1;              // a at 15 and 25
       @(negedge clk);
       @(negedge clk); a = 0; b = 1;       // b at 35 and 45
       @(negedge clk);
       @(negedge clk); b = 0;
       @(negedge clk); a = 1;              // a at 75
       @(negedge clk); a = 0;              // no b at 85
       @(negedge clk);
       @(negedge clk);
       $finish;
     end
   endmodule
   

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Declare the handshake as a named sequence and write wait (hs.triggered); in the task. Key point 2 The task wakes in the time step where the handshake ends, with no hand-written state machine. Key point 3 .matched cannot be used in a wait; only .triggered can.

   Half-Truth

   "Use @(hs) and .triggered interchangeably."

   @(hs) waits for the next end point after the wait starts. wait (hs.triggered) also returns if the end point is in the current time step.

   Source IEEE 1800-2017 §9.4.2.4

   • Lesson A5.1Assertions in Procedures, Sequences as Events
4. CompareTriggered vs the Old Ended MethodL2 Mid-Level

   Old code uses s.ended. Compare it with s.triggered.

   Reveal the Model Answer

   They mean the same thing. The 2005 standard used .ended inside sequences and .triggered elsewhere. Later standards allow .triggered everywhere and removed .ended. When you port old code, replace .ended with .triggered.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 They mean the same thing. Key point 2 The 2005 standard used .ended inside sequences and .triggered elsewhere. Key point 3 Later standards allow .triggered everywhere and removed .ended. Key point 4 When you port old code, replace .ended with .triggered.

   Misconception

   ".ended waits for the sequence to end; .triggered fires when it starts."

   Both mark the end point of a match. Neither says anything about the start.

   Source Mehta, SystemVerilog Assertions and Functional Coverage (3rd ed.), §13.1

   • Lesson A5.1Assertions in Procedures, Sequences as Events
5. Scale ItBuilding a Protocol Checker from Named SequencesL3 Senior

   How would you structure the assertions for a complex protocol, with many phases, so they stay readable and reusable?

   Reveal the Model Answer

   Write each protocol phase once as a named sequence with arguments, such as address phase, data beat and response. Build properties from those pieces, using end points for "this phase must have just finished" rules. Keep them in a package or a checker that every agent binds. A spec change then touches one sequence, not twenty properties.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Write each protocol phase once as a named sequence with arguments, such as address phase, data beat and response. Key point 2 Build properties from those pieces, using end points for "this phase must have just finished" rules. Key point 3 Keep them in a package or a checker that every agent binds. Key point 4 A spec change then touches one sequence, not twenty properties.

   What Interviewers Listen For (Senior)

   A good senior answer treats the sequence library as shared code with an owner and reviews. It notes which pieces the team's tools cannot run, and keeps a fallback for them.

   Wrong Altitude

   "One long property per rule is clearer than shared sequences."

   Long properties repeat the same phases many times. One spec change then needs many edits, and the copies drift.

   • Lesson A7.1Packaging and Binding
   • Lesson A8.1Capstone: An Assertion Suite
   • Problem 6.6.13A Tagged Channel Assertion Suite