The covergroup is the fundamental container for functional coverage. It encapsulates definitions of what you want to cover and specifies when sampling should occur.
A covergroup is similar to a class. It must be defined
and then instantiated using the new() constructor to be active.
It can be defined inside a module, program, interface, or class.
// Definition
covergroup cg_packet;
// Coverpoints go here
cp_length: coverpoint length;
endgroup
// Instantiation
cg_packet cg_inst = new();There are two main ways to tell a covergroup to sample data:
Specify an event (like a clock edge) in the definition. The covergroup automatically samples whenever the event occurs.
// Samples automatically on every posedge clk
covergroup cg_bus @(posedge clk);
coverpoint ready;
endgroupLeave the event blank and call the built-in sample() method when you are ready
explicitly.
class my_monitor;
covergroup cg_trans;
coverpoint addr;
endgroup
function new();
cg_trans = new(); // Construct
endfunction
task run_phase();
forever begin
@(posedge vif.clk);
if (vif.valid) begin
// Explicitly sample only when valid is high
cg_trans.sample();
end
end
endtask
endclass
You can pass arguments to the new() constructor of a covergroup. This allows
you
to write generic covergroups and reuse them for different signals.
// Generic covergroup accepting arguments
covergroup cg_signal (ref bit [3:0] signal, input int max_val);
cp_sig: coverpoint signal {
bins valid_range = {[0:max_val]};
}
endgroup
module tb;
bit [3:0] a, b;
initial begin
// Pass 'a' by reference, max_val = 10
cg_signal cg_a = new(a, 10);
// Pass 'b' by reference, max_val = 5
cg_signal cg_b = new(b, 5);
end
endmoduleUsually, you pass signals by ref so the covergroup sees the current value of
the signal when sample() is called. If you pass by value (default), it
would only sample the value present at construction time!
new(), the object doesn't
exist, and no coverage will be collected.