User`s guide

Input/Output Characteristics 7
-17
Input/Output
Characteristics
A digital device model’s input/output characteristics are defined
by the I/O Model that it references. Some characteristics, such
as output drive resistance and loading capacitances, apply to
digital simulation. Others, such as the interface subcircuits and
the power supplies, apply only to mixed analog/digital
simulation.
This section describes in detail:
• the I/O Model
• the relationship between drive resistances and output
strengths
• charge storage on digital nets
• the format of the interface subcircuits
Input/Output Model
I/O Models are common to entire logic families. For example, in
the model libraries, there are only four I/O Models for the entire
74LS family: IO_LS, for standard inputs and outputs;
IO_LS_OC, for standard inputs and open-collector outputs;
IO_LS_ST, for Schmitt trigger inputs and standard outputs; and
IO_LS_OC_ST, for Schmitt trigger inputs and open-collector
outputs. In contrast, Timing Models are unique to each device.
I/O Models are specified as
.MODEL <I/O model name> UIO [model parameters]*
where valid model parameters are described in Table 7-2
.