SI

In the good old days of 10-MHz clock frequencies, the chief design challenges in circuit boards or packages were how to route all the signals in a two-layer board and how to get packages that wouldn't crack during assembly. The electrical properties of the interconnects were not important because they didn't affect system performance. In this sense, we say that "the interconnects were transparent to the signals."

A device would output a signal with a rise time of roughly 10 nsec and a clock frequency of 10 MHz, for example, and the circuits would work with the crudest of interconnects. Prototypes fabricated with wire-wrapped boards worked as well as final products with printed circuit boards and engineering change wires.

But clock frequencies have increased and rise times of signals have decreased. For most electronic products, signal-integrity effects begin to be important at clock frequencies above about 100 MHz or rise times shorter than about 1 nsec. This is sometimes called the high-frequency or high-speed regime. These terms refer to products and systems where the interconnects are no longer transparent to the signals and, if you are not careful, one or more signal integrity problems arise.

Signal integrity refers, in its broadest sense, to all the problems that arise in high-speed products due to the interconnects. It is about how the electrical properties of the interconnects, interacting with the digital signal's voltage and current waveforms, can affect performance.

All of these problems fall into one of the following categories:

1.Timing

2.Noise

3.Electromagnetic interference (EMI)

Timing is a complicated field of study by itself. In one cycle of a clock, a certain number of operations must happen. This short amount of time must be divided up and allocated, in a budget, to all the various operations. For example, some time is allocated for gate switching, for propagating the signal to the output gate, for waiting for the clock to get to the next gate, and for waiting for the gate to read the data at the input. Though the interconnects affect the timing budget, timing is not covered in this book. We refer any interested readers to a number of other books listed in the reference section for more information on this topic. Instead, this book concentrates on the effects of the interconnects on the other generic high-speed problem, too much noise.

We hear about a lot of signal-integrity noise problems, such as ringing, ground bounce, reflections, near-end cross talk, switching noise, non-monotonicity, power bounce, attenuation, capacitive loading. All of these relate to the electrical properties of the interconnects and how the electrical properties affect the waveforms of the digital signals.