Notes on SystemVerilog Functional Coverage and Bins

Introduction

• Overview of defining bins in SystemVerilog functional coverage.
• Recap of definitions from the last video: Bin, Cover Point, Cover Group.

What are Bins?

• Bins are used to track/record occurrences of specific value in a cover point expression.
• Example: For a 2-bit logic variable, possible values are 0 to 3 (total 4).
• Auto Bins: If not defined explicitly, the simulator defines bins ranging from 0 to 3 automatically.

Auto Bin Max

• Default Auto Bin Max is 64.
• When variable size is less than or equal to 64:
  • Independent bins created.
• When variable size exceeds 64 (e.g., 8-bit variable, which has 256 possible values):
  • Grouping occurs.
  • Example of grouping: 256 values / 64 = 4 groups (bins 0-3, 4-7, etc.).

Cover Group Definition

• Structure for defining a cover group in SystemVerilog.
• Example Cover Group:

  covergroup CG @(posedge clock);
      coverpoint X;
      coverpoint Y;
  endgroup
  

• Can redefine Auto Bin Max using option.auto_bin_max.
  • Can be set globally for the cover group or individually for cover points.

Explicit Bins Definition

• For example, defining 2-bit variables X and Y:
  • Total bins would equal 4.
• Explicitly define cover points and bins:

  covergroup CG;
      coverpoint X {
          bins b0 = 0;
          bins b1 = 1;
          bins b2 = 2;
          bins b3 = 3;
      }
      coverpoint Y;
  endgroup
  

Scalar and Vector Bins

• Scalar Bins Syntax:
  • bins b0 = {0};
  • Individual values covered by separate bins.
• Vector Bins Syntax:
  • bins b = {[0:3]}; // covers numbers from 0 to 3.

Ranges in Vector Bins

• Ranges can be defined using the $ symbol.
• Example: For 4-bit variable, range from 0 to 15 would be defined as:

  bins b = {[0:$]} 
  

Example Code

• Example module defining cover groups:

  module example;
      logic [3:0] X;
      logic [1:0] Y;
      covergroup CG @(posedge clock);
          coverpoint X;
          coverpoint Y;
      endgroup
  endmodule
  

• This will automatically create bins as per the variable size.

• Running the code produces:

  • 16 auto bins for variable X.
  • 4 auto bins for variable Y.

Coverage Results

• Coverage can be reviewed after executing the code:
  • If values are covered multiple times, the coverage percentage increases.
  • Adjusting the sample counts (e.g., from 50 to 10) affects results (drops coverage).

Conclusion

• Understanding invocation and creation of auto bins.
• The mechanisms of groups vs. independent bins based on the variable sizes.
• Detailed insights into weighted coverage will be discussed in later videos.

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