Coverage Methods in SystemVerilog

Key Coverage Methods in SystemVerilog

1. void sample()

The sample() method is one of the most fundamental tools in a covergroup. It triggers the evaluation of all coverpoints and cross coverage within the associated covergroup. By default, sampling occurs when covergroup variables are assigned new values during a simulation, but you can manually call sample() to control when coverage is collected.

Example:

class Testbench;
    rand bit [7:0] addr;
    rand bit [3:0] opcode;

    covergroup cg @(posedge clk);
        coverpoint addr;
        coverpoint opcode;
        cross addr, opcode;
    endgroup

    function void run_test();
        cg c_inst = new();
        foreach (addr, opcode) begin
            addr = $random;
            opcode = $random;
            c_inst.sample(); // Explicit sampling
        end
    endfunction
endclass

In this example:

  • The sample() method ensures that coverage is recorded for the addr and opcode values at every iteration.

2. void set_inst_name(string name)

The set_inst_name() method assigns a unique name to a specific instance of a covergroup. This is particularly useful in simulations where multiple instances of the same covergroup are created, as it helps in differentiating and analyzing coverage data.

Example:
cg c_inst1 = new();
cg c_inst2 = new();
c_inst1.set_inst_name("Instance1");
c_inst2.set_inst_name("Instance2");

Here:

  • c_inst1 and c_inst2 are instances of the same covergroup.
  • Assigning unique names allows the simulator to distinguish their individual contributions to coverage.

3. real get_coverage()

The get_coverage() method returns the overall coverage percentage of a covergroup. This includes all coverpoints and cross coverage within the group, aggregated into a single percentage value. It is a quick way to determine how much of the intended functionality has been tested.

Example:
cg c_inst = new();
// Run the simulation and sample coverage
$display("Overall coverage: %f%%", c_inst.get_coverage());
  • The output provides a percentage value, such as 75.0, indicating that 75% of the coverage goals have been achieved.

4. real get_inst_coverage()

The get_inst_coverage() method retrieves the coverage percentage for a specific instance of a covergroup. This is useful when multiple instances are used, and you want to analyze their contributions separately.

Example:
cg c_inst1 = new();
cg c_inst2 = new();
c_inst1.sample();
$display("Coverage for Instance1: %f%%", c_inst1.get_inst_coverage());
$display("Coverage for Instance2: %f%%", c_inst2.get_inst_coverage());

In this case:

  • c_inst1.get_inst_coverage() and c_inst2.get_inst_coverage() provide individual coverage statistics for their respective instances.

Additional Coverage Methods

  • void stop(): Prevents further sampling for a covergroup. Useful when you want to freeze coverage collection after a certain point in the simulation.
c_inst.stop();
  • void start(): Resumes sampling for a covergroup that was previously stopped.
c_inst.start();
  • void reset(): Clears all coverage data for the covergroup, effectively resetting it to an unsampled state.
c_inst.reset();

Practical Applications of Coverage Methods

  1. Selective Sampling: Using sample() only during specific conditions or events in the testbench, such as when the DUT enters a certain state.
  2. Instance Tracking: Naming covergroup instances with set_inst_name() for better coverage analysis in multi-instance designs.
  3. Coverage Metrics: Using get_coverage() and get_inst_coverage() to monitor progress during regression runs.
  4. Debugging: Stopping (stop()) and restarting (start()) coverage collection to isolate specific parts of the simulation.
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