Coverage is big part of “CRV” (constrained random verification). If randomization answers the question “does it work?”, Something needs to say when to stop “randomizing” and answer the question “are done?”.

SV Standard define combine several Semantics languages (mixed bag) for OOP, SVA, Randomization and most importantly Functional Coverage. This is small(or abridged if you like fancy words) intro to Coverage semantics. So, LRM reading is a must to understand all features and details about functional coverage semantics.

The mandatory LRM tour guide Link to heading

There are 4 language features to be know here:

• covergroup
• coverpoint
• bin
• cross bin

Covergroup Link to heading

LRM define the covergroup

logic clk;
logic [3:0] addr;

covergroup cg @(posedge clk);
    cp1: coverpoint addr;
endcover

covergroup can be defined inside design units (module, interface, bla bla bla). but it shines when combined with randomization inside a class.

class cls;

covergroup cg @(posedge clk);
endcover

cg cg_u = new;

endclass

Also, covergroup can be constructed in class constructor.

function new();
cg_u = new()
endfunction

covergroup can take event for triggering or can triggered by calling .sample(). Event can be clock or bus enable ro anything that indicate new values to sample. anther use-case, calling .sample() from monitor or scoreboard or coverage-driven component.

function get_transction();
    cg.sample();
endfunction

Coverpoint Link to heading

coverpoint is the way define variables we want to sample to make sure all(needed values) are covered. In this example, add is is sampled on each clock cycle.

logic clk;
logic [3:0] addr;

covergroup cg @(posedge clk);
    cp1: coverpoint addr;
endcover

as addr is 4 bits, there are 16 possible values. So, The simulator will auto-create 16 bins. and will keep track which values were hit.

bins Link to heading

as mentioned, if there are no bins defined, simulator will create auto bins. but we and group values to define that specific condition was hit instead indivisual values.

logic clk;
logic [3:0] addr;

covergroup cg @(posedge clk);
    cp1: coverpoint addr{
        bins border = {0,15};
        bins others = {[1:14]};
    }
endcover

cross bins Link to heading

LRM also defines cross which create M*N bins where M and N are bins coverpoint defined in cross.

In this example, 2 bins are created to cover all combinations in these two coverpoints

• <cp1.border,cp2.large>
• <cp1.others,cp2.large>

logic clk;
logic [3:0] addr;

covergroup cg @(posedge clk);
    cp1: coverpoint addr{
        bins border = {0,15};
        bins others = {[2:4]};
    }
    cp2: coverpoint addr{
        bins large = {[4:14]};
    }
    cross cps
endcover