Milxed-mode simulation with spice-like circuit simulators

An extension to the popular SPICE circuit simulator is described that analyses analog circuits with digital components represented a t the gate, functional or behavioral levels. The analogto-digital conversion is performed by (multilevel) “thresholders” which assign digital signals to subranges of voltages determined by a set of voltage thresholds. The digital-toandog conversion is implemented by voltage sources with “controlled“ characteristics that approximate (discrete) digital signals by continuous piecewise linear functions. The paper outlines internal organization of mixed, analog/digital simulation, presents modification of analyses and extensions to the input language needed for analog-digital interactions, and describes the specification of digital circuits. 1. I N T R O D U C T I O N The phrase “mixed-mode” or “mixed analog/digitd” simula~ tion hits been used to refer to the simulation of electrical networks consisting of both analog and digital parts, regardless of the level of design abstraction. Design abstraction, in this context, means the level at which the network is specified for simulation; it can be dcvice, circuit, functional or behavioral level [MAR]. The popularity of mired-mode simulation [CoWi,MAR,SaDi, Samp] is due to analog circuitry that exists iii virtually all digital systems. Some of that circuitry is now showing up in applicatianspecific ICs. It is expected that by 1990, half of all semicustom circuitry, and one third of standard-cell designs will be more than 10 percent analog [Goer]. Roughly 80 percent of all printed circuit boards contain some analog components today. It appears, I~owever, t h t even sophisticated users of design automation tools prefer t o separate the analog and digital portions of their Systems; the analog circuitry is generally verified using some derivation of the papular SPICE circuit simulator [Coh, Vlad], and there are many commercially amilable logic simulators t ha t can bc tised for simulation of digital circuits [VLSI]. Unfortunately, it is rather difficult to correlate these two distinct simulations, and to analyze how analog and digital parts aflcct each other. Clearly, a n “integrated” approach is needed which can handle both analog aird digital simulation within one, coflsistent simulation environment. Three basic approaches have k e n takcn toward implementatioiiS of mixed analog-digitd simulation. The first method uses a n analog silnulator t o perform botb analog and digital sirnulation [Getr, ORoo]; the second uses a digital simulator to perform both digital and analog simulation [SaDi]; in the third method two Simulators, a digital aird analog one, are coupled together [CoWi]. In the first method, the digital elements are usually analyzed by the same mechanisms its the analog ones, which results in too accurate but also too inefficient simulation. The second approach extends the digital (discrete) methods to analog el* ments; this usually provides quite efficient but rather inaccurate simulation. Only the coupled approach combines the advantages of analog (accuracy) and digital (efficiency) simulations but this depends upon the level of coupling. Loosely coupled simulators basically execute two independent simulation programs (analog and digital) that “communicate” whenever they need information from the other part of the circuit; they are relatively simple to design but perform simulation of mixed amlog-digital circuits neither really accurately nor efficiently. Tightly coupled or integrated simulatort “synchronize” the two simulatioii mecharisms a t the level of internal timestops and time event control, so they can easily avoid any redundant emhat ions without any loss of aK“raCy. Any implementation of trully integrated mixed-mode simulation must solve two basic questions [Samp], (i) conversion of analog to digital and digital t o analog information on interfaces of analog and digital components, and (ii) synchronization of the (usually variable) timesteps of the analog simulation [McC, Pedc] with the event list that drives the (event-driven) digital simulation [MME]. The analog-to-digital conversion can be handled by establishing voltage thresholds and corresponding digital Signals (the conversion is periorined by elements called “threslrolders” [MAR]). The digital-to-analog conversion is more difficult because it must generate a. continuous analog waveform on the basis of discrete digital values. Two popular simple solutions assume that the converted waveforms are piecewise linear and piecewise exponential. A more sophisticated conversion hits been implemented for example in SAMSON [SaDi]. This paper describes an approach to integrated mixed-mode simulation in which analog simulation is provided by SPICE-PAC [Zubl], amodified SPICE simulator. SPICGPAC is a simulation package that is upward compatible with the popular SPICE simulation program [Col,, Vlad]. It means tha t SPICE-PAC accepts the same circuit description and performs all the analyses which are available in the SPICE programs, but also provides a number of features that do not exist in SPICE, for example an I~CCCSS to internal d u e s of circuit elements, hierarchical naming scheme, parameterized subcircuit expansion. dynamic definitions of parameters and outputs, and also “enhanced” circuit simulation in which USEIS can extend or modify some standard simulation c a ~ pabilities by their own routines in ordcr to increue efficiency, accuracy or applicability of simulators. Digital simulation capability is an example of such enhancements.