3 Mixed-Signal Simulators
Finally, it is important to recognize that Verilog-AMS is primarily used for verifica-
tion. Unlike the digital languages, the AMS languages will not be used for synthesis
in the foreseeable future because the only synthesis that is available for analog circuits
is very narrowly focused.
3
Mixed-Signal Simulators
Mixed-signal simulators, by their very nature, combine two different methods of sim-
ulation: event-driven simulation as found in logic simulators, and continuous-time
simulation as found in circuit simulators. As such, they are said to have two kernels; a
discrete-event kernel and a continuous-time kernel. These two kernels are an essential
feature of any mixed-signal simulator. Indeed, it is what separates mixed-signal simu-
lation from other types of simulation. Within these constraints, mixed-signal simula-
tors have changed considerably through the years.
Mixed-signal simulators first established themselves in the early 1990’s. At this time
there were two basic approaches, as shown in Figure 3. In one, a mixed-signal kernel
was added to an established circuit simulator. Analogy’s Saber and Georgia Tech’s
XSPICE are examples. These simulators offered relatively simple and low-level mech-
anisms to support event-driven simulation. They were quite different from, and
incompatible with, the standard logic simulators of the day, such as Verilog-XL. As a
result, while useful, these capabilities never gained wide acceptance. This lack of
acceptance was addressed in the other approach, which simply glued together an
established circuit simulator, generally some form of SPICE, and an established logic
simulator, usually Verilog-XL. An example of this type of simulator is Spectre/Ver-
ilog-XL from Cadence. This approach addressed the lack of acceptance issue, but cre-
ate
d
ease-of-use and performance problems. The ease-of-use problems stem from the
complexity of getting two simulators with very different use models to operate
together. Generally, some form of mixed-signal design environment is required to
manage the process of splitting the netlist between SPICE and Verilog, inserting the
interface components, setting up and running the simulation, and accessing and dis-
playing the results. Even with the environment, glued simulators developed a reputa-
tion of being difficult to use. The performance issues stem from the distant separation
between the analog and digital parts of the circuit, and the overhead in the communi-
cation between the two simulators.
The AMS languages, Verilog-AMS and VHDL-AMS, address many of these issues.
They provide a single standard input language that both supports mixed-signal
descriptions, and is based on the standard languages used by logic simulators. As
such, they provide the advantages of both types of early mixed-signal simulators,
5