SIMPLIS has two digital-source models, which implement a single-bit stimulus such as a clock generator and a multi-bit stimulus such as a digital-bus driver.
When driving advanced digital models with either single-bit clocks or multi-bit digital buses, use one of these sources instead of a waveform generator. Since most advanced digital applications do not require a ground pin, using one of these sources to drive the advanced digital blocks will result in faster simulations because the analog-to-digital boundary is not crossed by a waveform generator that always generates an analog voltage.
For more information, see Maximizing Simulation Speed in Mixed-Mode Simulations.
For information on when to use a digital source with a ground reference, see When is Ground Ref Required?