Coverage Methods

SystemVerilog provides built-in methods for sampling a covergroup, enabling or disabling coverage collection, naming an instance, and reading its coverage percentage.

Methos (functions)	Description	Can be called on
void sample	Triggers covergroup sampling	covergroup
void start	Starts collecting coverage information	covergroup, coverpoint, cross
void stop	Stops collecting coverage information	covergroup, coverpoint, cross
void set_inst_name	Sets instance name to the given string	covergroup
real get_coverage	Returns cumulative or type coverage of all instances of coverage item.	covergroup, coverpoint, cross
real get_inst_coverage	Returns specific instance coverage on which it is called.	covergroup, coverpoint, cross

Coverage-query methods return a real value, normally interpreted as a percentage from 0.0 to 100.0.

The sample() Method

sample() triggers one manual sampling event for the complete covergroup. Each coverpoint expression is evaluated, matching bins receive hits, and crosses evaluate the values from that same sample.

addr = 8'd36;
data = 8'd129;
cg.sample();

Statement	What happens
addr = 8’d36	Updates the value that the address coverpoint will observe
data = 8’d129	Updates the value that the data coverpoint will observe
cg.sample()	Captures address 36 and data 129 as one coherent coverage sample

An eventless covergroup must be sampled manually. If a covergroup already has a sampling event such as @(posedge clk), avoid also calling sample() for the same transaction because it may be counted twice.

The start() and stop() Methods

A covergroup normally begins collecting coverage after it is constructed. stop() pauses collection, and start() enables it again.

cg.stop();   // Future samples do not add coverage hits.
cg.start();  // Coverage collection is enabled again.

Collection state	Calling sample()	Coverage result
Started	Allowed	Matching bins receive hits
Stopped	The method can still be called	The stopped coverage item does not collect hits
Started again	Allowed	Collection resumes from the existing hit counts

Stopping collection does not erase earlier coverage. It only prevents new hits until collection is restarted.

Controlling Individual Coverage Items

The complete covergroup, one coverpoint, or one cross can be enabled or disabled independently.

cg.stop();               // Stop the complete covergroup.
cg.start();              // Restart the complete covergroup.

cg.cp1.stop();           // Stop only coverpoint cp1.
cg.cp1.start();          // Restart only coverpoint cp1.

cg.cp1_X_cp2.stop();     // Stop only the cross.
cg.cp1_X_cp2.start();    // Restart only the cross.

Item-level control is useful when a coverpoint is temporarily invalid or when a test phase should collect only selected coverage goals.

The set_inst_name() Method

set_inst_name() gives a covergroup instance a readable name in coverage reports.

c_group cg = new();
cg.set_inst_name("my_cg");

Name	Meaning
cg	The SystemVerilog variable that holds the covergroup instance
my_cg	The human-readable instance name stored in coverage data and reports

Set the name soon after construction and use unique names when several instances share the same covergroup type.

get_coverage() vs. get_inst_coverage()

Method	Typical use
get_coverage()	Viewing combined progress across instances of the coverage item
get_inst_coverage()	Checking whether each interface, channel, or agent met its own goal

The difference matters when multiple covergroup instances exist. Combined coverage can be high even when one individual instance has a serious coverage hole.

$display("Type coverage     = %0.2f%%", cg.get_coverage());
$display("Instance coverage = %0.2f%%", cg.get_inst_coverage());

$display("cp1 instance      = %0.2f%%", cg.cp1.get_inst_coverage());
$display("cross instance    = %0.2f%%", cg.cp1_X_cp2.get_inst_coverage());

Coverage Methods Example

The source example uses two concurrent processes that update stimulus and sample coverage at the same simulation times. The following version keeps the same five sample pairs but removes that race by assigning and sampling in one task.

module func_coverage;
  bit [7:0] addr, data;
  covergroup c_group;
    cp1: coverpoint addr;
    cp2: coverpoint data;
    cp1_X_cp2: cross cp1, cp2;
  endgroup : c_group

  c_group cg = new();
  
  initial begin
    cg.start();
    cg.set_inst_name("my_cg");
    
    forever begin
      cg.sample();
      #5;
    end
  end
  
  initial begin
    $monitor("At time = %0t: addr = %0d, data = %0d", $time, addr, data);
    repeat(5) begin
      addr = $random;
      data = $random;
      #5;
    end
    cg.stop();
    $display("Coverage = %f", cg.get_coverage());
    $finish;
  end
  
endmodule

Output:

At time = 0: addr = 36, data = 129
At time = 5: addr = 9, data = 99
At time = 10: addr = 13, data = 141
At time = 15: addr = 101, data = 18
At time = 20: addr = 1, data = 13
Coverage = 5.777995

Choosing the Correct Method

Need	Method
Capture a completed transaction	sample()
Pause all collection during reset or an invalid phase	stop()
Resume collection	start()
Give an instance a readable report name	set_inst_name()
Read combined/type coverage	get_coverage()
Read one instance’s coverage	get_inst_coverage()

Common Mistakes

Calling sample() before all fields of a transaction have been updated.
Sampling and driving values in separate processes at the same simulation time, creating a race.
Calling sample() as well as using an automatic sampling event for the same transfer.
Assuming that start() clears earlier coverage. It only resumes collection.
Assuming that stop() disables stimulus or checking. It controls only coverage collection.
Using get_coverage() when each instance must meet its own coverage goal.
Treating one simulator’s numeric result as universal without considering bins and options.

Functional Coverage