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Crystal is a timing verification program for digital nMOS and CMOS circuits. Using the circuit extracted from a mask set, the program determines the length of each clock phase and pinpoints the longest paths. Crystal can process circuits with about 40 000 transistors in about 20-30 min of VAX-11/780 CPU time. The program uses a switch-level approach in which the circuit is decomposed into chains of switches called stages. A depth-first search, with pruning, is used to trace out stages and locate the critical paths. Bidirectional pass transistor arrays are handled by having the designer tag such structures with flow control information, which is used by Crystal to avoid endless searches. Delays are computed on a stage-by-stage basis, using a simple resistor-switch model based on rise-time ratios (a measure of how fully turned-on the transistors in the stage are). The delay modeler executes 10 000 times as fast as SPICE, yet produces delay estimates that are typically within 10 percent of SPICE for digital circuits.