User guide

14-18
Negative Timing Checks
The timing violation, as represented by the X value, is lost to the
design. If a module path delay that is greater than ten time units was
used for the module instance, the X value would not appear on the
output at all.
For this reason Synopsys does not recommend using the +old_ntc
compile-time option. It exists only for unforeseen circumstances.
Checking Conditions
VCS evaluates the expressions in the timestamp_cond and
timecheck_cond arguments either when there is a value change
on the original reference and data signals at the cell boundary, or
when the value changes propagate from the delayed versions of
these signals at the sequential device inside the cell. It decides when
to evaluate the expressions depending on which signals are the
operands in these expressions. Note the following:
• If the operands in these expressions are neither the original or
delayed versions of the reference or data signals, and if these
operands are signals that do not change value between value
changes on the original reference and data signals and their
delayed versions, then it does not matter when VCS evaluates
these expressions.
• If the operands in these expressions are delayed versions of the
original reference and data signals, then you want VCS to
evaluate these expressions when there are value changes on the
delayed versions of the reference and data signals. VCS does
this by default.