Understanding Code Coverage vs Functional Coverage in SystemVerilog – Beyond the Numbers
In design verification, achieving coverage is not just about hitting 100%—it's about what that 100% actually represents.
Code Coverage
- Measures how much of the RTL code is exercised during simulation
- Includes statements, branches, FSM transitions, toggles, expressions
- Tool-generated
- Ensures your testbench is covering all implementation paths.
Functional Coverage
- Assesses how well the design’s functionality aligns with the specification
- Covers features, configurations, inputs, and protocol scenarios.
- User-defined using covergroups and coverpoints.
- Confirms whether the test plan goals are fully met.
Why Both Matter
100% Code but <100% Functional?
– Missing stimulus
– Uncovered bins
– RTL feature not fully implemented
100% Functional but <100% Code?
– Dead/defensive code
– Legacy logic
– Some RTL paths not exercised
Debug Checklist:
Identify missing bins or RTL lines
Analyze if the stimulus can be improved
Check feature intent vs design
Confirm if code is dead/defensive, or missed in functional coverage
Takeaway:
To ensure robust verification, balance both metrics.
- Code Coverage shows what was executed.
- Functional Coverage shows what was truly verified.
Don’t just simulate.
Verify with intent. Validate with strategy.
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