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2.2  Toggle Coverage

Toggle coverage answers the question, "Did this bit of this wire/register change from a value of zero (0) to one (1) and back from one (1) to zero (0) during simulation?"

A bit is said to be fully covered when it toggles back and forth at least once. This metric does not indicate to the user that every value of a multi-bit vector was seen. For example, if we have a two bit vector called "foo", toggle coverage will not tell you that the value of foo was set to the values of 0, 1, 2 and 3. However, it will tell you that all bits in that vector were toggled back and forth.

2.2.1 Example

Example

Suppose that a DUT was comprised of the following module

module test;

reg [2:0] a;

initial
  begin
    a = 3'b0;
    #10;
    a = 3'b110;
    #10;
    a = 3'b010;
    #10;
  end

endmodule

The register called "a", after being simulated, would have achieved a total toggle percentage of 50% (or 3 out of 6). Can you see which toggles are missing?

Bit 0 has never toggled to 1 (and has never been at a value of 1 and toggled to 0); therefore, bit-0 has toggled 0% (0 out of 2). Bit 1 has toggled from a value of 0 to 1, but has not toggled from a value of 1 to 0; therefore, bit-1 has toggled 50% (0 out of 2). Bit 2 is the only bit that has fully toggled, achieving a toggle percentage of 100% (2 out of 2). If you add all of the possible toggles to the number of achieved toggles, you end up with 3 out of 6 (or 50% total toggle coverage).

2.2.2 Recommendations

Recommendations

For a design to pass full coverage, it is recommended that the toggle coverage for all modules in a design received 100% coverage. If a bit is never changes value, it is usually an indication that a mode is not being exercised in the design or a datapath has a stuck-at issue.

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