Re: [sv-ec] Query on illegal/ignore_bins

Swapnajit,

An ignored coverbin does not modify the coverage space, it only indicates
that the specified bins not be counted.
Thus, your example would do the following:
    - Create 7 auto-bins (0..6).
    - Bins 0, 1, and 2 never take a hit.
And, if the sampled "a" takes on all values 0-7, the coverage is 4/7 => 57.1%

    Answer

----- Original Message ----- 
From: "Swapnajit Chakraborti" <swapnaj@cadence.com>
To: <sv-ec@eda.org>
Sent: Friday, April 29, 2005 7:17 AM
Subject: [sv-ec] Query on illegal/ignore_bins


  If the user specifies only ignore_bins with a coverpoint, what will
  be the behavior? Consider the following example:

  reg [2:0] a;
  coverpoint a {
     ignore_bins ig_bins = {0, 1, 2};
  }

  LRM clearly says that (section 20.4.4) values 0, 1 & 2 will not
  be considered(ignored) during sampling. What LRM does not clarify
  is what happens for the values which are not ignored, e.g. 3, 4,
  5, 6 & 7.
  Should we interpret the above declaration as an automatic bin
  declaration for coverpoint a and create bins for the values which
  are not ignored. This implies we create auto[3], auto[4] etc.
  depending on whether these values occurred during simulation
  and were sampled. This makes sense since if we do not create auto
  bins, specifying ignore_bins doesn't serve any purpose. But this
  is in contradiction with how automatic bins are specified. For your
  convenience, I have given a couple of examples of how automatic
  bins are specified.

  coverpoint a;
  coverpoint b {
    option.auto_bin_max = 7;
  }
  coverpoint c {
    type_option.weight = 7;
  }

  In all the above three cases, automatic bins are created.
  Note that the basic criteria is that no user-defined bins are
specified.

  Now as soon as user specifies a bin with  ignore_bins keyword, as per
  LRM its an user-defined bin. So if we create an automatic bin in this
  case, it may be confusing. This applies to illegal_bins as well. 

  I believe this needs to be clarified in LRM.

Thx,
Swapnajit.