Disabling single signals in expression coverage?

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%.

Steve,

thanks for your response.

Unfortunately it does not seem to help me, since atpg_mode is a signal that is driven from a flop inside the circuit.

The set_com command appears to apply only to signals that are explicitly constant (assign sig = 1) and not those that simply do not toggle in the simulation.

But, if set_com would apply to all signals that do not toggle, coverage analysis would report an overly optimistic (aka useless) toggle coverage.

I really need a way to explicltly list those signals that I want to be ignored.

thanks!

christian.

You can ignore named (or pattern-matched) signals from toggle coverage, but I don't believe this will have any effect on expression coverage, because the ignore is simply saying you don't care about the toggle, not that you expect it to be stuck at a certain value.

You might wish to consider using the formal coverage reachability app; this takes a simulation (merged) coverage database, and produces a cover property for each uncovered block / expression / toggle. The formal engine then looks to see if it's even possible to reach that hole given any kind of stimulus. If formal says the line can't be reach by any means, it automatically creates ignore marks for you. It works really well, even for quite large designs that you might not normally consider for formal.

Depending on how the D input to your scan control flop is driven you might need a simple constraint to prevent the scan mode being enabled.

My apologies for never responding to this.

So this is what I ended up doing:

`ifdef COVERAGE_SIMULATION
  assign atpg_mode = 0;
`else
  assign aptg_mode = atpg_mode_reg;
`endif

This was pretty much the only way that ended up working for me.

Thanks for your help, Stephen!