Assertions

Assertions are statements embedded in (or alongside) RTL that specify expected design behavior, automatically checked during simulation and formal verification. SystemVerilog Assertions (SVA), written using assert property, assume property, and cover property constructs, let designers and verification engineers express temporal relationships — "if signal A is asserted, signal B must be asserted within N cycles" — using a concise, declarative syntax built on sequences and properties.

For an RTL designer, assertions serve as executable documentation of design intent and as an early bug-detection mechanism. Simple combinational assertions (immediate assertions, assert (condition)) can catch invalid states the moment they occur — for example, asserting that a one-hot signal never has more than one bit set, or that a FIFO's read pointer never exceeds its write pointer. Concurrent assertions, evaluated relative to clock edges using SVA's temporal operators (## for cycle delay, |-> and |=> for implication, throughout, until), capture protocol-level requirements like "a request must be followed by an acknowledge within 4 cycles, and the request signal must remain high until the acknowledge."

Assertions integrated directly into RTL (sometimes called "white-box" assertions) are particularly valuable for internal FSM invariants, interface protocol checks between sub-modules, and CDC-related properties (e.g., asserting a signal is only ever a single bit wide when crossing a synchronizer). Many of these properties can also feed formal verification tools directly — the same SVA written for simulation checking can often be used as a formal property to be proven exhaustively, rather than merely checked against simulated stimulus.

A practical RTL coding consideration is keeping assertions synthesis-transparent: assertions must be ignored by synthesis (most tools automatically strip assert/cover statements, but designers should verify this) while remaining active in simulation, and assertion density should be balanced against simulation performance overhead, especially in large block-level or SoC-level regressions.

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