I/O CIRCUITS AND MODELS - Advanced Signal Integrity for High-Speed Digital Design - page 440

11.9.3 Advanced design considerations 483

11.10 IBIS models 483

11.10.1 Model structure and development process 483

11.10.2 Generating model data 485

11.10.3 Differential I/O models 488

11.10.4 Example of an IBIS file 490

11.11 Summary 492

References 492

Problems 494

So far we have discussed the behavior of high-speed interconnects and have
provided techniques for analyzing and modeling the key physical phenomena
that affect signal quality at multi-Gb/s data rates. To fully analyze and understand
the behavior of high-speed signaling links, we must include the I/O circuits that
transmit and receive the digital data. Design of high-performance links demands
that the circuits and interconnects be jointly optimized as a unified system. To do
so successfully, the signal integrity engineer must be able to communicate with
the circuit designer.
In this chapter we describe the operation and modeling of contemporary
high-speed I/O circuits, including transmitters, receivers, and on-die terminations.
We do not attempt to provide a complete treatment on how to design high-speed
I/O; instead, we wish to give the SI engineer insight into the behavior and sen-
sitivities of modern transceivers. This insight is fundamental to developing a
sufficient understanding of the interactions between I/O circuits and physical
interconnects in order to optimize a signaling system. In this chapter we identify
the design parameters of the circuits for use in analyzing and optimizing designs,
and describe techniques for modeling I/O circuits. In addition, we introduce the
Bergeron diagram, a useful tool analyzing the time-domain behavior of a com-
plete signaling system. Finally, we acknowledge that transceivers can be designed
using either bipolar or MOSFET devices, although throughout this chapter we
focus on MOSFET-based circuits.

11.1 I/O DESIGN CONSIDERATIONS

The function of a transmitter is to launch a signal representing digital data onto an
interconnect for propagation to a receiver circuit. To maximize performance, engi-
neers typically must use design techniques to provide controlled output impedance
and rise and fall times. In addition, transmitters (often abbreviated as Tx) can
be designed for either singled-ended or differential transmission, and to operate
either as a voltage or a current source. In this section we use the term transmitter
interchangeably with driver and output buffer , all of which are commonly used
in the industry.