Using the cover directive in SystemVerilog and functional coverage

📌 What is the Cover Directive?

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In SystemVerilog, the cover directive is used in the design or testbench environment to check whether a certain behavior has occurred. It is a critical mechanism for functional coverage tracking.

Example:

cover property (@(posedge clk) req |-> ack);

In this example:

• When the req signal is active, it checks whether the ack signal is active in the next cycle.
• It does not produce an assertion failure; it only triggers a coverage event when the condition is met.

🛠️ Why Use Cover?

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✅ Testbench Activity: Used to see whether every path in the design (e.g., state transitions or handshake scenarios) has been exercised.

✅ Coverage Analysis: Measures test coverage, reporting which behaviors have been tested and which are missing.

✅ Combination with Assertions: The same property can be used with both assert and cover directives:

assert property (p_handshake);
cover property (p_handshake);

🔍 Use Cases for Cover

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• FSM (Finite State Machine) Coverage: Ensures all state transitions are tested.

• Handshake Protocols: Checks whether every combination of request-acknowledge, valid-ready, etc. has occurred.

• Protocol Coverage: Monitors different transaction combinations in protocols like PCIe and AMBA.

📝 Considerations When Using Cover

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• Simulation Dependent: The cover directive only checks for behavior occurrences during simulation. For formal verification, assertion coverage depends on the assertion coverage tool.

• No Error Messages: If the cover condition is not met, the test does not fail; only coverage is reported as missing.

🔧 Simple Example

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property p_valid_ready;
  @(posedge clk) valid |-> ready;
endproperty

cover property(p_valid_ready);

In this example:

• When valid is asserted, ready is checked in the next cycle.
• If this scenario occurs, the coverage is ticked.