Recovery Time, Removal Time

Recovery Time:
Recovery time is the minmium time that as asynchronous control signal must be stable before the clock active- edge transition. In other words, this check ensures that after the asynchronous signal become inactive, there is adequate time to recover so that the next active clock edge can be effective.

Consider the time as show in below figure, between an asynchronous reset becoming inactive and the clock active edge of a flip-flop. If the active clock edge occurs too soon after the release of reset, in this case the state of the flip-flop may be unknow. Therefore it is required to have minimum time for asynshronus control signal to become stable. Recovery time is similar to setup time.

Removal Time:
Removal time is the minimum length of time that an asynchronous control must be stable after the clock active edge transition. This check ensures that the active clock edge has no effect because the asynchronus control signal remains active until removal time after the active clock edge.

Consider the asynchronous control signal is released(becomes inactive) well after the active clock edge so that the clock edge can have no effect. Similar to hold check, it is minimum path check except that it is on an asynchronous pin of flip flop.