Timing Events

 

      A signal can become stable at different times for various reasons, for example the transition may be caused by different logic paths, or it may be affected by physical variations that affect circuit behavior. Such factors can cause the signal to transition at different points of time. Therefore a signal can become stable at latest time (a late event) and unstable at earliest time (an early event).

Late Event:

     

When you want to check whether a signal is sure to arrive in time to meet a timing requirement coming from a sampling element, you need to check the signal’s latest possible transition to a stable value. This is the late event. After this the signal is guaranteed to be stable, and you can be sure it will be stable at sampling time. The late event value is the maximal delay value, or max delay. Timing analysis based on max delays is called max delay analysis. Max delay analysis is also called setup analysis, because it is used to determine whether signals get "set up" (in the sense of "become ready") in time to be sampled correctly by sampling elements.

Early event:

When you want check whether a signal remains stable long enough to be sampled correctly, you need to check the earliest possible time when the signal can become unstable. This is the early event. The early event value is the minimal delay value, or min delay. Timing analysis based on min delays is called min delay analysis. Min delay analysis is also called hold analysis, because it is used to determine whether a signal "holds" its value long enough to be sampled correctly.