Payload Stable While Stalled
Assert that a source cannot change its payload while valid is high and ready is low.
Task
On a stalled cycle, require valid to remain high and data to stay stable on the next cycle.
Examples
- Implement the requested verification structure or behavior.
Constraints
- Use SystemVerilog.
- Keep the code focused on the requested verification concept.
What this problem is testing
SVA questions test temporal reasoning: exactly when signals are sampled, overlapped versus non-overlapped implication, reset disabling, and whether an assertion can pass vacuously.
Interview habit: before coding, state reset behavior, the state that must be stored, and the conditions that update each output.
Verification plan
- Write one passing trace and one deliberately failing trace.
- Check the exact cycle where the antecedent becomes true and where the consequent starts.
- Exercise reset while the property is active.
- Use cover or directed stimulus when useful to prove the antecedent can actually occur.
Common mistakes
- Choosing |-> versus |=> by habit.
- Forgetting disable iff or mishandling reset.
- Allowing vacuous success because the antecedent never occurs.
- Using unbounded eventuality when the protocol has a finite response requirement.
Interview follow-up questions
- Would overlapped or non-overlapped implication be correct here, and why?
- How would you detect or avoid vacuous success?
Concepts to review
sva, ready-valid, stable
Open Payload Stable While Stalled in the HDLForge interactive editor.
Related problems
- Valid Must Eventually Handshake — Medium
- Reset Clears State — Easy
- Ready/Valid Skid Buffer — Hard
- Backpressure-Safe Ready/Valid Source — Medium
- Valid-Ready Streaming Accumulator — Hard
Explore the SystemVerilog Assertions (SVA) Practice hub.