Ngspice implements XSPICE extensions for behavioral and mixed-mode (analog and digital) modeling. In the XSPICE framework this is referred to as code level modeling. Behavioral modeling may benefit dramatically because XSPICE offers a means to add analog functionality programmed in C. Many examples (amplifiers, oscillators, filters ...) are presented in the following. Even more flexibility is available because you may define your own models and use them in addition and in combination with all the already existing ngspice functionality. Digital and mixed mode simulation is speeded up significantly by simulating the digital part in an event driven manner, in that state equations use only a few allowed states and are evaluated only during switching, and not continuously in time and signal as in a pure analog simulator.

This chapter describes the predefined models available in ngspice, stemming from the original XSPICE simulator or being added to enhance the usability. The instructions for writing new code models are given in Chapt. 28.

To make use of the XSPICE extensions, you need to compile them in. Linux, CYGWIN, MINGW and other users may add the flag --enable-xspice to their ./configure command and then recompile. The pre-built ngspice for Windows distribution has XSPICE already enabled. For detailed compiling instructions see Chapt. 32.1.