DIGITAL CIRCUIT SIMULATION

A primary circuit model using a primary simulator 40' operates in conjunction with a plurality of secondary circuit models 10, 20, 30 using a secondary simulator 100. Only signals external to the secondary circuit models are passed between the primary simulator and the secondary simulator. The primary simulator and the secondary simulator independently increment their simulated time when they are active. The secondary simulator is invoked when a signal to which one of the secondary circuit models is responsive is changed by the primary simulator. The secondary simulator is not exited until no further signal changes are being driven by the secondary circuit models at which stage the last asserted values of signals changed by the secondary circuit models are returned to the primary model. If there are pending signal changes within the secondary simulation when it is entered in response to a signal from the primary simulator, then that primary simulator signal is buffered until those pending signal changes have taken place.