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Disabling single signals in expression coverage?

cmautner

I am running expression coverage on a design. There are many ATPG multiplexers implemented in RTL, such as

wire signal = aptg_mode ? something : something_else;

Expression coverage complains that, since atpg_mode is always 0 in RTL simulations, the ternary expression is only 50% tested. Is there something I can do about it? Like, a pragma that tells the coverage scoring that atpg_mode is considered to be constant '0'?

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  • The "set_com" command in your coverage control file should do this for you.

    Here's a small example:

    module flop (input clk, rst, scan, d, output q);
    bit q_;
    assign q = scan ? d : q_;
    always @(posedge clk)
      if (rst)
        q_ <= 1'b0;
      else
        q_ <= d;
    endmodule

    module tb;
    bit clk, rst, d, q;
    flop flop1(.clk(clk), .rst(rst), .scan(1'b0), .d(d), .q(q));
    initial
    begin
      rst=1'b1;
      #5 clk=1'b1; #5 clk=1'b0;
      rst=1'b0;
      #5 clk=1'b1; #5 clk=1'b0;
      d=1'b1;
      #5 clk=1'b1; #5 clk=1'b0;
      d=1'b0;
      #5 clk=1'b1; #5 clk=1'b0;
    end
    endmodule

    ## flop.ccf
    set_com -log
    select_coverage -e -module flop
    set_assign_scoring
    set_expr_scoring -all

    Run with "irun -covfile flop.ccf flop.sv tb.sv, then we do a quick & dirty check in IMC:

    Instance name: tb.flop1
    Type name: flop
    File name: flop.sv
    Number of covered expressions: 2 of 2
    Number of uncovered expressions: 0 of 2
    Number of excluded expressions: 2
    Number of unreachable expressions: 0

    index  | grade         | line   | expression                                         
    -------------------------------------------------------------------------------------
    1.1    | 100.00% (2/2/2) | 15     | scan ? d : q_                                      

    index: 1.1 grade: 100.00% (2/2/2) line: 15 source: assign q = scan ? d : q_;

    scan ? d : q_
    <1->   <2> <3>

    index     | hit   | rval | <1> <2> <3>
    ---------------------------------------
    1.1.1     | 1     | 1    | 0   -   1   
    1.1.2     | IGN   | 1    | 1   1   -   
    1.1.3     | 1     | 0    | 0   -   0   
    1.1.4     | IGN   | 0    | 1   0   -   

    You see the two lines where scan==1 show as "IGN" for ignored, and the grade is 100%.

Reply

  • The "set_com" command in your coverage control file should do this for you.

    Here's a small example:

    module flop (input clk, rst, scan, d, output q);
    bit q_;
    assign q = scan ? d : q_;
    always @(posedge clk)
      if (rst)
        q_ <= 1'b0;
      else
        q_ <= d;
    endmodule

    module tb;
    bit clk, rst, d, q;
    flop flop1(.clk(clk), .rst(rst), .scan(1'b0), .d(d), .q(q));
    initial
    begin
      rst=1'b1;
      #5 clk=1'b1; #5 clk=1'b0;
      rst=1'b0;
      #5 clk=1'b1; #5 clk=1'b0;
      d=1'b1;
      #5 clk=1'b1; #5 clk=1'b0;
      d=1'b0;
      #5 clk=1'b1; #5 clk=1'b0;
    end
    endmodule

    ## flop.ccf
    set_com -log
    select_coverage -e -module flop
    set_assign_scoring
    set_expr_scoring -all

    Run with "irun -covfile flop.ccf flop.sv tb.sv, then we do a quick & dirty check in IMC:

    Instance name: tb.flop1
    Type name: flop
    File name: flop.sv
    Number of covered expressions: 2 of 2
    Number of uncovered expressions: 0 of 2
    Number of excluded expressions: 2
    Number of unreachable expressions: 0

    index  | grade         | line   | expression                                         
    -------------------------------------------------------------------------------------
    1.1    | 100.00% (2/2/2) | 15     | scan ? d : q_                                      

    index: 1.1 grade: 100.00% (2/2/2) line: 15 source: assign q = scan ? d : q_;

    scan ? d : q_
    <1->   <2> <3>

    index     | hit   | rval | <1> <2> <3>
    ---------------------------------------
    1.1.1     | 1     | 1    | 0   -   1   
    1.1.2     | IGN   | 1    | 1   1   -   
    1.1.3     | 1     | 0    | 0   -   0   
    1.1.4     | IGN   | 0    | 1   0   -   

    You see the two lines where scan==1 show as "IGN" for ignored, and the grade is 100%.

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