Thanks Dave.
The complete functional coverage code in my test bench is this -:
covergroup covport;
option.auto_bin_max = 4; //Functional Coverage for Input top_left
coverpoint top_left;
coverpoint top_center;
coverpoint top_right;
coverpoint middle_left;
coverpoint middle_center;
endgroup
covport ck = new;
initial begin
repeat(100)@(ganglion_if.cb)
begin
initialize();
driver();
ck.sample();
end
end
endprogram