The Monitor That Waited Zero Delays

Where the interviewer goes next. Open a node to practise that follow-up.

WhyWhat IfProve ItCompare

1. The Monitor That Waited Zero Delays
2. WhyWhy NBA Updates Wait for the Inactive RegionL2 Mid-Level

   Why is the NBA region placed after the Inactive region instead of before it?

   Reveal the Model Answer

   Every process that wakes at the edge must read pre-edge values, even one that waits with #0. Only then do nonblocking assignments model flops correctly. If NBA updates landed before Inactive, a #0 reader would see half-updated state. Then the result of <= would depend on process order again.

   1. Cycle 0

      Before the first edge, d is 11 and both pipelines hold 00.

   2. Cycle 1

      At t=5 the <= pipeline moves 11 into a1 only, and a2 takes the old 00. The blocking pipeline copies 11 into both stages.

   3. Cycle 2

      The value 11 reaches a2 one edge later. b2 never lags b1: the blocking version behaves as one register.

   4. Cycle 3

      After t=25, a2 holds 22 and b2 holds 33, as the last $strobe line shows.

   Data moves one stage per edge only when each stage reads the value from before the edge.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Every process that wakes at the edge must read pre-edge values, even one that waits with #0. Key point 2 Only then do nonblocking assignments model flops correctly. Key point 3 If NBA updates landed before Inactive, a #0 reader would see half-updated state. Key point 4 Then the result of <= would depend on process order again.

   Half-Truth

   "Inactive exists to let testbenches go last."

   It was a Verilog-era tool for ordering within the Active set. Using it as a "go last" lever fails as soon as a second process does the same.

   Source IEEE 1800-2017 §4.4.2.3

   • Lesson 6.2Scheduling Regions and Nonblocking Updates
3. CompareClocking Blocks vs Driving on the Falling EdgeL2 Mid-Level

   Compare a clocking block with driving stimulus on the falling edge as ways to avoid testbench races.

   Reveal the Model Answer

   Driving on the falling edge puts drives and samples in different time steps, so it is simple and race-free. But it only works for single-edge designs with a known duty cycle. A clocking block samples inputs just before the active edge (#1step, the Preponed region). It drives outputs after a set skew, so the testbench stays cycle-based on the real edge. Clocking blocks scale to agents and interfaces; falling-edge drives suit small directed benches.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Driving on the falling edge puts drives and samples in different time steps, so it is simple and race-free. Key point 2 But it only works for single-edge designs with a known duty cycle. Key point 3 A clocking block samples inputs just before the active edge (#1step, the Preponed region). Key point 4 It drives outputs after a set skew, so the testbench stays cycle-based on the real edge. Key point 5 Clocking blocks scale to agents and interfaces; falling-edge drives suit small directed benches.

   Spec Misquote

   "An input skew of #1step means one nanosecond before the edge."

   1step is one unit of the time precision, not one nanosecond. The sample is taken in the Preponed region, so it reads the values from just before the edge.

   Source IEEE 1800-2017 §14.4, §4.4.2.1

   • Lesson 6.3Clocking Blocks and Sampled Views
4. What IfReading a Clockvar After the Raw EdgeL2 Mid-Level

   Asked if they write a monitor with clocking blocks

   A monitor waits for @(posedge clk) and then reads cb.rdata. It sometimes reports the previous beat. What happened?

   clocking cb @(posedge clk);
     default input #1step;
     input rdata;
   endclocking
   initial forever begin
     @(posedge clk);
     $display("rdata=%h", cb.rdata);
   end

   Reveal the Model Answer

   Clockvars (the sampled signals of a clocking block) are updated when the clocking event is processed, in the Observed region. A process that resumes on the raw posedge clk can read cb.rdata before that update. It then gets the previous sample. Wait on @(cb) instead, so the read happens after the clockvars change.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Clockvars (the sampled signals of a clocking block) are updated when the clocking event is processed, in the Observed region. Key point 2 A process that resumes on the raw posedge clk can read cb.rdata before that update. Key point 3 It then gets the previous sample. Key point 4 Wait on @(cb) instead, so the read happens after the clockvars change.

   • Lesson 6.3Clocking Blocks and Sampled Views
   • Problem 6.3.1The Monitor That Read the Next Beat
5. Prove ItSolve 6.7.1: The Monitor That Saw the FutureSimulator-graded

   Show it, don't just say it: this Silicon Lab problem is graded by the simulator.

   Open Problem 6.7.1

6. What IfA Monitor That Waits for the Signal to ChangeL1 JuniorL2 Mid-Level

   Asked if they propose waiting on the signal itself

   What if the monitor waits with @(q) after the edge instead of using #0?

   Reveal the Model Answer

   It wakes on the NBA update and reads the new value, but only when the value actually changes. A beat that repeats the previous value never wakes it, so the monitor silently drops it. Sampling once per clocking event is the only scheme that sees every beat.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 It wakes on the NBA update and reads the new value, but only when the value actually changes. Key point 2 A beat that repeats the previous value never wakes it, so the monitor silently drops it. Key point 3 Sampling once per clocking event is the only scheme that sees every beat.

   Missed Corner

   "@(q) fires once per clock because q is a flop."

   An event control on a variable waits for a change of value, not for a write. Writing the same value again creates no event.

   Source IEEE 1800-2017 §9.4.2

   • Lesson 6.7Debug Case: A Racing Monitor