Functional Coverage Part-XX

Coverage Options

Like in Vera, Systemverilog provides ways to control the behavior of the covergroup, coverpoint and cross. One of the most common usage of these coverage options is setting weightage of a covergroup. In a advanced testbench there could be many covergroups, and from the verification point of view some covergroups have high priority, and they migh be is less number, on other there hand there could be covergroups which are of low priority, and they are in large number. There is no way simulator is going to know this priority information, so Systemverilog provides options to communicate this to simulator. This way, even if don't have good coverage on low priority covegroup, it won't effect overall coverage in big way.

There are two types of options

covergroup instance specific options
All instance specific options

Below table shows all the options available in systemverilog to control behaviour of coverage group

Options Name	Default	Description
weight=number	1	If set at the covergroup syntactic level, it specifies the weight of this covergroup instance for computing the overall instance coverage of the simulation. If set at the coverpoint (or cross) syntactic level, it specifies the weight of a coverpoint (or cross) for computing the instance coverage of the enclosing covergroup.
goal=number	90	Specifies the target goal for a covergroup instance or for a coverpoint or a cross of an instance.
name=string	unique name	Specifies a name for the covergroup instance.
comment=string		A comment that appears with a covergroup instance or with a coverpoint or cross of the covergroup instance
at_least=number	1	Minimum number of times a bin needs to hit before it is declared as hit
detect_overlap=boolean	0	When true, a warning is issued if there is an overlap between the range list (or transition list) of two bins of a coverpoint.
auto_bin_max=number	64	Maximum number of automatically created bins when no bins are explicitly defined for a coverpoint.
cross_num_print_missing = number	0	Number of missing (not covered) cross product bins that must be saved to the coverage database and printed in the coverage report.
per_instance=boolean	0	Each instance contributes to the overall coverage information for the covergroup type. When true, coverage information for this covergroup instance is tracked as well.

Out of above, below list of options could improve quality of coverag convergence.

weight=number
goal=number
at_least=number
per_instance=boolean

Note : Refer to LRM if you need more information on this

Example : Coverage options


  1 module test();
  2 
  3 logic [2:0] addr;
  4 wire [2:0] addr2;
  5 reg ce;
  6 
  7 assign addr2 = addr + 1;
  8 
  9 covergroup address_cov () @ (posedge ce);
 10   option.name         = "address_cov";
 11   option.comment      = "This is cool";
 12   option.per_instance = 1;
 13   option.goal         = 100;
 14   option.weight       = 50;
 15   ADDRESS : coverpoint addr {
 16     option.auto_bin_max = 100;
 17   }
 18   ADDRESS2 : coverpoint addr2 {
 19     option.auto_bin_max = 10;
 20   }
 21 endgroup
 22 
 23 address_cov my_cov = new();
 24 
 25 initial begin
 26   my_cov.ADDRESS.option.at_least = 1;
 27   my_cov.ADDRESS2.option.at_least = 2;
 28   ce   <= 0;
 29   $monitor("ce %b addr 8'h%x addr2 8'h%x",ce,addr,addr2);
 30   repeat (10) begin
 31     ce <= 1;
 32     addr <= $urandom_range(0,7);
 33      #10 ;
 34     ce <= 0;
 35      #10 ;
 36   end
 37 end
 38 
 39 endmodule

You could download file coverage_options.sv here

Simulation : Coverage options

 ce 1 addr 8'h6 addr2 8'h7
 ce 0 addr 8'h6 addr2 8'h7
 ce 1 addr 8'h4 addr2 8'h5
 ce 0 addr 8'h4 addr2 8'h5
 ce 1 addr 8'h5 addr2 8'h6
 ce 0 addr 8'h5 addr2 8'h6
 ce 1 addr 8'h2 addr2 8'h3
 ce 0 addr 8'h2 addr2 8'h3
 ce 1 addr 8'h3 addr2 8'h4
 ce 0 addr 8'h3 addr2 8'h4
 ce 1 addr 8'h7 addr2 8'h0
 ce 0 addr 8'h7 addr2 8'h0
 ce 1 addr 8'h0 addr2 8'h1
 ce 0 addr 8'h0 addr2 8'h1
 ce 1 addr 8'h7 addr2 8'h0
 ce 0 addr 8'h7 addr2 8'h0
 ce 1 addr 8'h6 addr2 8'h7
 ce 0 addr 8'h6 addr2 8'h7
 ce 1 addr 8'h3 addr2 8'h4
 ce 0 addr 8'h3 addr2 8'h4

Report : Coverage options

 ===========================================================
 Group : test::address_cov
 ===========================================================
 SCORE  INSTANCES WEIGHT GOAL   
  87.50  62.50    1      100    
 
 1 Instances:
 
 SCORE  WEIGHT GOAL   NAME        
  62.50 50     100    address_cov 
 -----------------------------------------------------------
 Summary for Group   test::address_cov
 
 CATEGORY  EXPECTED UNCOVERED COVERED PERCENT 
 Variables 16       2         14      87.50   
 
 Variables for Group  test::address_cov
 
 VARIABLE EXPECTED UNCOVERED COVERED PERCENT GOAL WEIGHT 
 ADDRESS  8        1         7       87.50   100  1      
 ADDRESS2 8        1         7       87.50   100  1      
 -----------------------------------------------------------
 Summary for Variable ADDRESS
 
 CATEGORY                     EXPECTED UNCOVERED COVERED PERCENT 
 Automatically Generated Bins 8        1         7       87.50   
 
 Automatically Generated Bins for ADDRESS
 
 Uncovered bins
 
 NAME      COUNT AT LEAST NUMBER 
 [auto[1]] 0     1        1      
 
 Covered bins
 
 NAME    COUNT AT LEAST  
 auto[0] 1     1        
 auto[2] 1     1        
 auto[3] 2     1        
 auto[4] 1     1        
 auto[5] 1     1        
 auto[6] 2     1        
 auto[7] 2     1        
 -----------------------------------------------------------
 Summary for Variable ADDRESS2
 
 CATEGORY                     EXPECTED UNCOVERED COVERED PERCENT 
 Automatically Generated Bins 8        1         7       87.50   
 
 Automatically Generated Bins for ADDRESS2
 
 Uncovered bins
 
 NAME      COUNT AT LEAST NUMBER 
 [auto[2]] 0     1        1      
 
 Covered bins
 
 NAME    COUNT AT LEAST  
 auto[0] 2     1        
 auto[1] 1     1        
 auto[3] 1     1        
 auto[4] 2     1        
 auto[5] 1     1        
 auto[6] 1     1        
 auto[7] 2     1        
 ===========================================================
 Group Instance : address_cov
 ===========================================================
 Comment: This is cool
 SCORE  WEIGHT GOAL   
  62.50 50     100    
 
 Group:
 
 SCORE  INSTANCES WEIGHT GOAL   NAME              
  87.50  62.50    1      100    test::address_cov 
 -----------------------------------------------------------
 Summary for Group Instance   address_cov
 
 CATEGORY  EXPECTED UNCOVERED COVERED PERCENT 
 Variables 16       6         10      62.50   
 
 Variables for Group Instance  address_cov
 
 VARIABLE EXPECTED UNCOVERED COVERED PERCENT GOAL WEIGHT 
 ADDRESS  8        1         7       87.50   100  1      
 ADDRESS2 8        5         3       37.50   100  1      
 -----------------------------------------------------------
 Summary for Variable ADDRESS
 
 CATEGORY                     EXPECTED UNCOVERED COVERED PERCENT 
 Automatically Generated Bins 8        1         7       87.50   
 
 Automatically Generated Bins for ADDRESS
 
 Uncovered bins
 
 NAME      COUNT AT LEAST NUMBER 
 [auto[1]] 0     1        1      
 
 Covered bins
 
 NAME    COUNT AT LEAST  
 auto[0] 1     1        
 auto[2] 1     1        
 auto[3] 2     1        
 auto[4] 1     1        
 auto[5] 1     1        
 auto[6] 2     1        
 auto[7] 2     1        
 -----------------------------------------------------------
 Summary for Variable ADDRESS2
 
 CATEGORY                     EXPECTED UNCOVERED COVERED PERCENT 
 Automatically Generated Bins 8        5         3       37.50   
 
 Automatically Generated Bins for ADDRESS2
 
 Uncovered bins
 
 NAME      COUNT AT LEAST NUMBER 
 [auto[2]] 0     2        1      
 auto[1]   1     2        1      
 auto[3]   1     2        1      
 auto[5]   1     2        1      
 auto[6]   1     2        1      
 
 Covered bins
 
 NAME    COUNT AT LEAST  
 auto[0] 2     2        
 auto[4] 2     2        
 auto[7] 2     2

Do you have any Comment? mail me at:deepak@asic-world.com