Weight, Goal and Minimum Hits

Where the interviewer goes next. Open a node to practise that follow-up.

WhyBreak ItCompareScale It

1. Weight, Goal and Minimum Hits
2. WhyWhy Ask for More Than One HitL1 JuniorL2 Mid-Level

   Why would you set at_least above 1? What does this coverpoint report?

   module tb;
     bit [1:0] v;
     covergroup cg;
       cp: coverpoint v { bins b[] = {[0:1]}; option.at_least = 2; }
     endgroup
     cg c = new();
     initial begin
       v = 0; c.sample();
       v = 0; c.sample();     // bin b[0]: 2 hits
       v = 1; c.sample();     // bin b[1]: 1 hit, not enough
       $display("coverage=%0.2f with at_least=2", c.get_coverage());
     end
   endmodule
   

   Reveal the Model Answer

   It reports 50%, because b[1] has one hit and needs two. One hit shows a case happened once. It does not show the design handles it again, for example after it has already recovered once. Raise at_least for cases like error recovery or a full FIFO, where a second hit tests a different state. Don't raise it everywhere, or closure slows down for no gain.

   RUNVerilator (coverage build) · simulator service

   Edit the code, then run it

   OutputWaves

   RunReset codeExpected: coverage=50.00 with at_least=2

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 It reports 50%, because b[1] has one hit and needs two. Key point 2 One hit shows a case happened once. Key point 3 It does not show the design handles it again, for example after it has already recovered once. Key point 4 Raise at_least for cases like error recovery or a full FIFO, where a second hit tests a different state. Key point 5 Don't raise it everywhere, or closure slows down for no gain.

   Wrong Altitude

   "Set at_least to 10 everywhere to be safe."

   Every bin then needs ten hits, and rare bins take ten times longer. Use it where repeats test something new.

   Source Spear & Tumbush, SystemVerilog for Verification (3rd ed.), §9.10.3 (PDF p394)

   • Lesson C3Choosing Bins
3. Scale ItDifferent Targets for Different FeaturesL2 Mid-LevelL3 Senior

   The plan says error paths need at least 10 hits each, and normal traffic needs one. How do you express that in a large model?

   Reveal the Model Answer

   Put error paths in their own coverpoints or covergroup, with option.at_least = 10 set on those coverpoints. Keep normal traffic at the default. Then each covergroup maps to a line of the plan, and the report shows each feature against its own target. Don't encode the difference in weights, which only blurs one total.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 Put error paths in their own coverpoints or covergroup, with option.at_least = 10 set on those coverpoints. Key point 2 Keep normal traffic at the default. Key point 3 Then each covergroup maps to a line of the plan, and the report shows each feature against its own target. Key point 4 Don't encode the difference in weights, which only blurs one total.

   Missed Corner

   "Set at_least = 10 once on the covergroup."

   That raises the bar for normal traffic too. Some tools also ignore the covergroup-level setting, including this site's coverage build.

   Source Spear & Tumbush, SystemVerilog for Verification (3rd ed.), §9.10.3 (PDF p394)

   • Lesson C7Closing Holes
   • Lesson U15.1Coverage Architecture and Planning
4. CompareWeight Zero vs Deleting the CoverpointL1 Junior

   Compare setting option.weight = 0 on a coverpoint with deleting it.

   Reveal the Model Answer

   With weight 0, the coverpoint still collects data and appears in the report, but it adds nothing to the covergroup number. That suits diagnostic coverpoints, and coverpoints that exist only to build a cross. Deleting it loses the data, and any cross that uses it no longer compiles.

   	option.weight = 0	Coverpoint deleted
   Collects data	Yes	No
   Shows in the report	Yes	No
   Counts in the covergroup number	No	No
   A cross can still use it	Yes	No: the cross no longer compiles

   Weight 0 keeps the data and the cross, and only drops the item from the total.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 With weight 0, the coverpoint still collects data and appears in the report, but it adds nothing to the covergroup number. Key point 2 That suits diagnostic coverpoints, and coverpoints that exist only to build a cross. Key point 3 Deleting it loses the data, and any cross that uses it no longer compiles.

   Misconception

   "Weight 0 also removes its crosses from the total."

   Weight is per coverpoint or cross. A cross built on a zero-weight coverpoint keeps its own weight of 1.

   Source Mehta, Introduction to SystemVerilog, Table 15.3 (PDF p614)

   • Lesson C5Cross Coverage
5. Break ItWeights That Make the Number LieL2 Mid-LevelL3 Senior

   How can weights make a coverage number misleading?

   Reveal the Model Answer

   A large weight on an easy coverpoint pulls the total up early. The group then looks nearly done while a hard cross is empty. A zero weight on a hard cross hides it completely. Weights also behave differently between tools. Keep the default weights, and report hard items, such as key crosses, as their own numbers.

   Self-Check

   Tick each key point you covered in your answer.

   Key point 1 A large weight on an easy coverpoint pulls the total up early. Key point 2 The group then looks nearly done while a hard cross is empty. Key point 3 A zero weight on a hard cross hides it completely. Key point 4 Weights also behave differently between tools. Key point 5 Keep the default weights, and report hard items, such as key crosses, as their own numbers.

   What Interviewers Listen For (Senior)

   A good senior answer treats any weight that is not the default as a review item. The plan must give a reason for it.

   Delivery

   "Weights let you show management the important progress."

   They change the arithmetic, not the testing. A tuned total is easy to misread and hard to compare between projects.

   Source Mehta, SystemVerilog Assertions and Functional Coverage (3rd ed.), §26.19 guidelines (PDF p477)

   • Lesson C7Closing Holes