I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 441

A range of complexity for modeling transmitters is possible. At its simplest,
the model can be a simple transient voltage in series with an output resistance
or a current source in parallel with an output resistance. Since they use simple
circuit elements, variation of linear model parameters for the purposes of
identifying a working solution and understanding sensitivity to design and
process variation is extremely easy. In addition, they are easy to analyze and
provide the fastest simulation times. As a result, linear models are very useful
for the initial stage of the design process, when large numbers of simulations
are performed in order to identify the potential solution space.
More complex nonlinear behavioral transmitter models provide improved
accuracy over linear models by comprehending the nonlinear relationship
between the output voltage and output current, staged switching of the output
devices to control rise and fall times, and parasitic capacitances. Parametric model
variation is more difficult than with linear models, but is still possible. Nonlinear
behavioral models are widely used (via IBIS, the I/O Buffer Information Spec-
ification) because they allow component suppliers to provide accurate models
without divulging the specifics of the circuit design and manufacturing process.
Finally, achieving maximum accuracy may require the use of full transistor
models. These models are typically used only as a final check of the design,
since they are more complex to construct and they require significantly more
simulation time. In addition, suppliers are often reluctant to provide transistor
models because they can divulge proprietary design and process information. As
a result, in this chapter we focus on linear and nonlinear behavioral modeling
rather than transistor-level modeling.
Options for modeling receiver circuits follow the same progression as for
transmitters. Simple receiver models include only the termination resistors and
input capacitance. Nonlinear behavioral models include the voltage versus current
characteristics of the ESD protection circuitry and of terminations that are imple-
mented using transistor devices. Full transistor models incorporate all device
effects. Requirements and trade-offs for the various model types are summarized
in Table 11-1.
In our modeling discussions in this chapter we focus heavily on linear models
due to their extreme usefulness in the early design stages. We also discuss the

TABLE 11-1. Summary of Modeling Approaches and Trade-offs
Model Type

Elements Intellectual Property Speed Sweepability

Linear Voltage and/or current

sources and resistors

None Fastest Most

Nonlinear Current vs. voltage curves,

voltage vs. time curves

Little Fast Some

Full transistor All devices, including
pre-driver, compensation
circuits, etc.

Design and process Slow Very limited