TABLE OF CONTENTS Thinking that you're doing it right does not count.
If you can't measure it, it's just an opinion.
Mark Goodridge
An important part of any design process is verifying that the design meets its specifications. To do this, specific observation hooks must be built into the design. Properly designed hooks allow test equipment such as oscilloscopes and logic analyzers to attach to the system without significantly degrading the probed signals. This chapter explains how to select and build proper test hooks into the system as well as how to take the basic electrical measurements of the transmitter and receiver eye.
One of the most basic problems in trying to perform electrical validation of PCI Express is determining the optimum data patterns to use for stimulating the interconnect. Ideally, the data patterns will maximize jitter and inter-symbol interference (ISI) on an individual lane while at the same time producing maximum crosstalk on adjacent lanes. Furthermore, there is the practical problem of programming a device to generate the pattern(s) in the proper sequence on a continuous basis without running into such bus features as data scrambling or buffer full conditions that limit bus performance.
Fortunately, the authors of the PCI Express specification anticipated this problem and provided a special test state called compliance measurement mode (CMM). When a link is in CMM, the transmitters on that link send a repeated data pattern called the compliance pattern across the bus. The compliance pattern is a set...
