Question

bit [7:0] v_a, v_b;

covergroup cg @(posedge clk);
    a: coverpoint v_a
    {
        bins a1 = { [0:63] };
        bins a2 = { [64:127] };
        bins a3 = { [128:191] };
        bins a4 = { [192:255] };
    }

    b: coverpoint v_b
    {
        bins b1 = {0};
        bins b2 = { [1:84] };
        bins b3 = { [85:169] };
        bins b4 = { [170:255] };
    }

    c : cross v_a, v_b
    {
        bins c1 = ! binsof(a) intersect {[100:200]};// 4 cross products
        bins c2 = binsof(a.a2) || binsof(b.b2);// 7 cross products
        bins c3 = binsof(a.a1) && binsof(b.b4);// 1 cross product
    }
endgroup

The example above defines a coverage-group named cg that samples its cover-points on the positive edge of signal clk (not shown). The coverage-group includes two cover-points, one for each of the two 8-bit variables, v_a and v_b. The coverage-point labeled ‘a’ associated with variable v_a, defines four equal-sized bins for each possible value of variable v_a. Likewise, the coverage-point labeled ‘b’ associated with variable v_b, defines four bins for each possible value of variable v_b. The cross definition labeled ‘c’, specifies the cross coverage of the two cover-points v_a and v_b. If the cross coverage of cover-points a and b were defined without any additional cross-bins (select expressions), then cross coverage of a and b would include 16 cross products corresponding to all combinations of bins a1 through a4 with bins b1 through b4, that is, cross products <a1, b1>, <a1,b2>, <a1,b3>, <a1,b4>...<a4, b1>, <a4,b2>, <a4,b3>, <a4,b4>.

The first user-defined cross bin, c1, specifies that c1should include only cross products of cover-point a that do not intersect the value range 100-200. This select expression excludes bins a2, a3, and a4. Thus, c1 will cover only four cross-products of <a1,b1>, <a1,b2>, <a1,b3>, and <a1,b4>.

The second user-defined cross bin, c2, specifies that bin c2 should include only cross products whose values are covered by bin a2 of cover-point a or cross products whose values are covered by bin b2 of cover-point b. This select expression includes the following 7 cross products: <a2, b1>, <a2,b2>, <a2,b3>, <a2,b4>, <a1, b2>, <a3,b2>, and <a4,b2>.

The final user-defined cross bin, c3, specifies that c3 should include only cross products whose values are covered by bin a1 of cover-point a and cross products whose values are covered by bin b4 of cover-point b. This select expression includes only one cross-product <a1, b4>.

When select expressions are specified on transition bins, the binsof operator uses the last value of the transition.