As well as an analog simulator, SIMetrix incorporates an event driven digital simulator tightly coupled to the analog portion. This system can rapidly and accurately simulate mixed signal circuits containing both analog and digital components. Of course, an analog only simulator can simulate a mixed signal circuit using digital models constructed from analog components, but this approach is slow. The advantage of this mixed-mode approach is that it is dramatically faster, typically in the order of 100 times for pure digital circuits.

The SIMetrix mixed mode simulator is based on the XSPICE system developed by the Georgia Technical Research Institute. Although based on XSPICE, SIMetrix features many enhancements over the original system. See Enhancements over XSPICE for details of these improvements.

If you only use digital models supplied in the device library, then you don't need to know much about the digital simulator in order to use it. Just select the devices you need from the parts browser and simulate in the normal way. This chapter describes some of the inner workings of the simulator including how it interfaces to the analog system. More importantly, perhaps, this chapter also describes how you can design your own digital models.