Coverage

Functional coverage answers the question that code coverage cannot: has the design actually been exercised in the scenarios the specification cares about, not just which lines of RTL executed. SystemVerilog covergroup/coverpoint/bins constructs let you define exactly what to measure — for example, a coverpoint on a transaction's opcode field with explicit bins for each instruction type, or a cross between opcode and a "queue full" flag to confirm every instruction type has been tried while the queue was full. Coverage is sampled (often via sample() calls triggered by a clocking event) and aggregated across all tests in a regression into a merged coverage database.

Code coverage — statement, branch, expression, toggle, and FSM state/transition coverage — is collected automatically by the simulator from the RTL itself and answers "which parts of the design were activated at all." It's a necessary but insufficient metric: 100% toggle coverage on a register says nothing about whether it was toggled in the right context. Most projects track both: code coverage to catch dead code or unreachable logic, and functional coverage to confirm the verification plan's scenarios were actually hit.

Assertion coverage — cover property statements in SVA — records when specific temporal scenarios occur, useful for confirming corner-case timing relationships were exercised. Coverage-driven verification (CDV) closes the loop between constrained-random stimulus and the test plan: coverage holes (bins that remain unhit after running the regression) are analyzed to determine whether they're reachable, and if so, constraints are adjusted or new directed tests are written to hit them.

Coverage closure — typically targeting near-100% on both functional and code coverage with documented waivers for unreachable cases — is one of the primary exit criteria for declaring a block "verified" and ready for tape-out. Tools like Synopsys VCS/Verdi, Cadence IMC, and Siemens Questa provide coverage merging, ranking (identifying which tests contribute unique coverage), and exclusion management for this process.

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