Recently I was going through coverage topic where an expression like a+b was being covered. And earlier I had a doubt whether i can partially cover a variable. So went out to checkout all them doubs with a simple code.
module top;
logic clk=0;
logic a;
logic [1:0] b,c;
logic [7:0] d;
covergroup cg@(posedge clk);
option.auto_bin_max=32;
a_cov : coverpoint a;
b_cov : coverpoint b;
c_cov : coverpoint c;
d_cov : coverpoint d;
sum : coverpoint c+a;
expr : coverpoint (b&&c) + |d;
narrow: coverpoint d[3:0];
con : coverpoint {c+a,a};
crs : cross a_cov,sum;
endgroup
cg cg_h=new();
initial forever #5 clk=~clk;
always_ff @(posedge clk) {a,b,c,d} = $random();
initial #100 $finish;
endmodule
Note: option.auto_bin_max set to 32
sum coverpoint will have same bins as b_cov or c_cov, but will differ from c_cov because of the summation with a variable.
expr coverpoint will have same bins as sum because b&&c will result in a bit and |d into a bit and their sum as another bit. This way we can have any complex expression. This type of coverage is thus useful to check whether a case statement or loop and if blocks with complex expressions are being covered or not.
narrow coverpoint will only have 16 autobins unlike d_cov because it only covers d variable partially
Can we do cross coverage of two variable without using cross keyword?
Yes, ofcourse. But it will be cumbersome without cross coverage. We can just concatenate variables.