I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 479

11.9.3 Advanced Design Considerations

The examples in Section 11.9.1 are intended to illustrate the concepts, and repre-
sent only two of a wide range of choices for high-performance receiver designs. In
practice, designers have multiple options for improving the various performance
aspects of the receiver, such as common-mode range and input offset voltage.
We refer interested readers to Dally and Poulton [1997] for more information on
receiver design options and techniques.

11.10 IBIS MODELS

As we mentioned in Section 11.1, silicon suppliers do not like to provide transistor
models for their I/O circuits. However, simple linear models often do not satisfy
the accuracy requirements of high-speed signaling links. To meet this need, the
industry has developed the I/O Buffer Information Specification (IBIS). As the
name suggests, IBIS is a format for specifying I/O circuit information. Created
in the early 1990s, it is now an industry standard owned by the IBIS Forum
and is supported by approximately 60 companies. The diverse membership has
allowed IBIS to evolve to meet the changing needs of signal integrity and I/O
design engineers. In this section we give a brief overview of the IBIS standard,
highlighting the major components and providing a high-level description of the
model development process. The major features of IBIS include:

• Nonlinear current versus voltage curves for transmitters, ESD devices, and
on-chip termination

Separate nonlinear voltage versus time curves for transmitter pull-up and
pull-down devices

•

Pad capacitance for I/O circuits

•

Models for minimum, typical, and maximum cases within a single model

•

Description for multiple types of I/O, including differential pins, open-drain
output, tristate outputs, and receivers with hysteresis

•

Inclusion of signal quality specifications, including input logic thresholds,
overshoots, and so on

•

The Golden Parser, a tool that checks model syntax for conformance to the
standard

•

Backward compatibility with models created under previous revision of the
standard

•

For further details we refer readers to the IBIS specification [IBIS, 2006] and
model development “cookbook” [IBIS, 2005].

11.10.1 Model Structure and Development Process
The basic structure of an IBIS-compliant model is shown in Figure 11-35. The
i - v and v - t curves are specified in table format, with columns for the minimum,