User`s guide

7
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32 Di
g
ital Device Modelin
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primitives, see the Digital Devices chapter in the online
MicroSim PSpice A/D Reference Manual.
IO_STD, shown in the listing, is the standard I/O model. This
determines the A-to-D and D-to-A interface characteristics for
the subcircuit. The device contains family-specific information,
but the models have been created for nearly all of the stock
families. The various I/O models can be found in the library file
dig_io.lib.
The logic expressions themselves are straightforward. The first
nine are buffering the input signals from outside the subcircuit.
The rest describe the logic of the actual device up to the flip-
flops. By tracing the various paths in the schematic, you can
derive each of the logic equations.
The D0_EFF timing model, shown in the listing, is a zero-delay
default model already defined in
dig_io.lib for use with flip-
flops. All of the delays for the device are defined in the PINDLY
section. The I/O model is IO_STD as identified previously. We
have not specified a MNTYMXDLY or IO_LEVEL parameter,
so the default values are used. For a more detailed description of
the general digital primitives MNTYMXDLY and IO_LEVEL,
see the Digital Devices chapter in the online MicroSim PSpice
A/D Reference Manual.
The primitive MNTYMXDLY specifies whether to use the
minimum, typical, maximum, or digital worst-case timing
values from the device’s timing model (in this case the PINDLY
device). For the 74160, MNTYMXDLY is set to 0. This means
that it takes on the current value of the DIGMNTYMX
parameter. DIGMNTYMX defaults to 2 (typical timing) unless
specifically changed using the .OPTIONS command.
The primitive IO_LEVEL selects one of four possible A-to-D
and D-to-A interface subcircuits from the device’s I/O model. In
the header of this subcircuit, IO_LEVEL is set to 0. This means
that it takes on the value of the DIGIOLVL parameter.
DIGIOLVL defaults to 1 unless specifically changed using the
.OPTIONS command.