It is important to identify the type of circuit because our timing calculation approach differs accordingly. Combinational circuits timing analysis deals primarily with propagation delay issues. Sequential circuits have additional specific timing characteristics that must be satisfied in order to prevent metastability, including setup time, hold time, and minimum clock period. Designers of sequential devices must specify these important timing characteristics in order to allow the device to be used without error.

Combinational Circuits

Propagation Delay (tpd)

Propagation Delay indicates the amount of time needed for a change in the logic input to result in a permanent change at an output.

Combinational propagation delays are additive.  It is possible to  determine the propagation delay of a larger combinatinal circuit by adding the propagation delays of the circuit components along the longest path.

Contamination Delay (tcd)

Contamination Delay indicates the amount of time needed for a change in the logic input to result in an initial change at an output.

The determination of the comtamination delay fo the combined circuit requires identifying the shortest path of contamination delays from input to output

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