User`s guide

7
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30 Di
g
ital Device Modelin
g
hold timing from the data sheet. Then the simulator can verify
that these conditions are met during the simulation.
Di
g
ital Primitives
Primitives in the simulator are devices or functions which are
compiled directly into the code. The primitives serve as
fundamental building blocks for more complex macro models.
There are two types of primitives in the simulator: gate level and
behavioral. A gate level primitive normally refers to an actual
physical device (such as buffers, AND gates, inverters). A
behavioral primitive is not an actual physical device, but rather
helps to define parameters of a higher level model. Just like gate
level primitives, behavioral primitives are intrinsic functions in
the simulator and are treated in much the same manner. They are
included in the gate count for circuit size and cannot be
described by any lower level model.
In our 74160 example (see The TTL Data Book from Texas
Instruments for schematic and description), the four J-K flip-
flops are the four digital gate level primitives. While flip-flops
are physically more complex than gates in terms of modeling,
they are defined on the same level as a gate (for example, flip-
flops are a basic device in the simulator). Since all four share a
common Reset, Clear, and Clock signal, they can be combined
into one statement as an array of flip-flops. They could just as
easily have been written separately, but the array method is
more compact. See the Digital Devices chapter in the online
MicroSim PSpice A/D Reference Manual for more information.
The Lo
g
ic Expression
(LOGICEXP Primitive)
Looking at the listing in The 74160 Example on page 7-37 and
at the schematic representation of the 74160 subcircuit, you can
see that there are three main parts to the subcircuit. Following
the usual header information, .SUBCKT keyword, subcircuit