Static Timing Analysis - Definitions

Definitions

• The critical path is defined as the path between an input and an output with the maximum delay. Once the circuit timing has been computed by one of the techniques below, the critical path can easily be found by using a traceback method.

• The arrival time of a signal is the time elapsed for a signal to arrive at a certain point. The reference, or time 0.0, is often taken as the arrival time of a clock signal. To calculate the arrival time, delay calculation of all the components in the path will be required. Arrival times, and indeed almost all times in timing analysis, are normally kept as a pair of values - the earliest possible time at which a signal can change, and the latest.

• Another useful concept is required time. This is the latest time at which a signal can arrive without making the clock cycle longer than desired. The computation of the required time proceeds as follows. At each primary output, the required times for rise/fall are set according to the specifications provided to the circuit. Next, a backward topological traversal is carried out, processing each gate when the required times at all of its fanouts are known.

• The slack associated with each connection is the difference between the required time and the arrival time. A positive slack s at a node implies that the arrival time at that node may be increased by s without affecting the overall delay of the circuit. Conversely, negative slack implies that a path is too slow, and the path must be sped up (or the reference signal delayed) if the whole circuit is to work at the desired speed.