Initialization of mixed-signal systems in VHDL-AMS

Complex systems, such as telecommunication systems, control systems, etc., most often consist of both analog circuitry and digital circuitry. It is more efficient in simulation to represent such complex systems as mixed signal systems that both analog description and digital description are adopted. VHDL-AMS is an IEEE standard which is designed to describe multi-discipline, mixed-signal systems. Three port types are supported in VHDL-AMS including terminal, quantity and signal. Properties of a port vary significantly with port type. It is crucial to have a good understanding of these properties which have significant impact on the behavior in simulation. DC analysis is the most essential analysis for mixed signal systems that leads the system to an appropriate initial point for transient analysis. This paper first discusses the port properties of all three port types. Then, the important language features in VHDL-AMS for mixed-signal system description are discussed. A generic model for mixed-signal systems and the criteria that a DC solution of a mixed-signal system is reached is presented. Then, analysis of the interaction between different signal types in DC analysis is presented to explore the common issues in mixed-signal system simulation. Finally, with an interface model as an example, DC simulation results of a mixed-signal system are presented and analyzed. The techniques for modeling mixed signal systems in VHDL-AMS are summarized.