The problem with assertions about the
$past() is their value prior to the
first clock tick. For this reason, we add the check for
That’s not enough, however, since the reset might’ve been active in the past
as well. In the case of a reset,
o_REQUEST should then be false. We might
therefore rewrite this assertion as,
As the slide above points out, you could also do this with a concurrent
assertion instead of with immediate assertions. The following concurrent
assertion (without the
f_past_valid would (just about) check the same thing.
The slide above recommends using
disable iff. This would work as well,
although there is a subtle difference when using it. Can you tell what that
difference would be? If not, we’ll come back to the topic in Quiz 19.