I. Introduction
The ever-decreasing feature size of the silicon technology, combined with the convergence of computer, multimedia, and telecommunication, results in single-chip system that require both digital blocks as well as analog blocks. Integrating both analog and digital functionality is known as mixed-signal design. Mixed-signal design requires analog and digital simulation (i.e, continuous time analog simulation and digital event-driven simulation) for both design and verification. Mixed-signal modeling and simulation is an important step for the design integration of an entire system into a single IC[1]. SPICE is one of the most widely used simu-lation programs for analog circuits design and is used to characterize gate-level models for simu-lation of complex digital systems. But simulation via SPICE reached its limits, because circuits containing thousands of nodes could not be si-mulated within reasonable amounts of time and available memory provided by CPU; hence the advent of HDLs. A good HDL can describe cir-cuits at various abstraction levels. So top-down designs do. The lower the models with more de-tails are, the more accurate the simulation results are, and the longer the simulation time are need-ed. Because behavioral modeling is only involved in high-level aspect such as differential algebraic functions, it is an efficient way to simulate ICs. As the scale of IC becomes larger, behavioral modeling is becoming increasingly important[2]. What's more, traditionally the analog and digital portions of a mixed-signal circuit were designed by analog and digital experts. The analog design was verified using SPICE-like circuit simulators while the digital section was simulated using various digital logic simulators. However it is extremely difficult to correlate the results of the two distinct simulations and analyze how analog and digital elements affect each other. To describe mixed-signal behavioral models, AHDL (analog HDL) and mixed-signal simulator are developed. Mixed-signal HDLs can describe analog and digital elements in the same model. And mixed-signal simulator uses many types of algorithms controlling the time step synchronization between the analog and digital.