User`s guide

Creatin
g
a Di
g
ital Model Usin
g
the PINDLY and LOGICEXP Primitives 7
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Creatin
g
a Di
g
ital Model
Usin
g
the PINDLY and
LOGICEXP Primitives
Unlike the majority of analog device types, the bulk of digital
devices are not primitives that are compiled into the simulator.
Instead, most digital models are macro models or subcircuits
that are built from a few primitive devices.
These subcircuits reference interface and timing models to
handle the D-to-A and A-to-D interfaces and the overall timing
parameters of the physical device. For most families of digital
components, the interface models are already defined and
available in the
dig_io.lib library, which is supplied with all
digital and mixed-signal packages. If you are unsure of the exact
name of the interface model you need to use, use a text editor to
look in
dig_io.lib.
For instance, if you are trying to model a 74LS component that
is not already in a library, open
dig_io.lib with your text editor
and search for 74LS to get the interface models for the 74LS
family. You can also read the information at the beginning of the
file which explains many of the terms and uses for the I/O
models.
In the past, the timing model has presented the greatest
challenge when trying to model a digital component. This was
due to the delays of a component being distributed among the
various gates. Recently, the ability to model digital components
using logic expressions (LOGICEXP) and pin-to-pin delays
(PINDLY) has been added to the simulator. Using the
LOGICEXP and PINDLY digital primitives, you can describe
the logic of the device with zero delay and then enter the timing
parameters for the pin-to-pin delays directly from the
manufacturer’s data sheet. Digital primitives still must reference
a standard timing model, but when the PINDLY device is used,
the timing models are simply zero-delay models that are
supplied in
dig_io.lib. The default timing models can be
found in the same manner as the standard I/O models. The
PINDLY primitive also incorporates constraint checking which
allows you to enter device data such as pulse width and setup/
not
included
in: