“Signal Integrity requires engineering judgment…you will consistently be asked to craft answers from insufficient data based on ill-defined measures of success.” This just about summarizes Don Telian’s new book, Signal Integrity in Practice.
Don has more than forty years of experience as a master of signal integrity (SI), focusing most of this time on writing specifications, designing high-speed serial links, and helping clients optimize (when he gets in early enough) or debugging (when he gets called in after the fact) their systems. His book is not a textbook on principles of signal integrity; it is a narrative of what is left out in most textbooks. It is the stuff you would learn if you worked in a company with your desk next to a guru who has been doing this for forty years.
High-speed serial links started proliferating twenty years ago because, as Don explains, Intel’s Jim Pappas declared, “We’re now at the point where it’s getting cheaper to put more gates behind a fast serial line than to lay on copper traces.” Now most of signal integrity is about serial links. This book transfers, from a pundit, engineering judgment on what it takes to create successful, high-speed serial links.
Don boils down all the recommended principles for high-speed serial links into seven steps:
- Minimize discontinuities
- Manage loss
- Route using best practices
- Route using double-digit data rate best practices
- Remove unacceptable stubs
- Prevent fabrication problems
- Engage the firmware team
From this list, he further boils them down to their fundamental essence, “manage loss and discontinuities.” As he illustrates with numerous examples, sometimes loss can be your friend. A high-speed link that is short with not much loss may have reflection noise dominating the intersymbol interference (ISI) and collapsing an eye. A little loss will reduce the amplitude of the reflections and, if not excessive, open the eye.
This book assumes you have the textbook knowledge of the fundamental principles. Don focuses on the nuances. For example, watch out for your fab vendor changing the line width of traces on you if you specify a controlled impedance, which may have a ripple impact on losses. It is sometimes more important to use a consistent characteristic impedance throughout your interconnect than to use a specified impedance in some sections, only to have variations due to vias and connectors in others. One of the biggest sources of manufacturing variations is the thickness of pre-preg layers which can vary as much as 1 mil.
Do you really need to simulate all your links? Maybe it is not so critical if your technology is not new, if your design is similar to what you’ve already done before, and if you are using the devices the way the manufacture recommended; focus on the more important problems.
I found the most valuable sections covering the importance of software solutions to SI problems. As Don says, “…while there are a few important things to know and do in hardware, the dominant factor affecting performance is the software, or the SES.”
The Serdes equalization settings (SES) are the register settings in the firmware of the field-programmable gate array (FPGA) or application-specific integrated circuit (ASIC) which determine the values for the decision feedback equalization (DFE) at the receiver and the feed-forward equalization (FFE) at the transmitter. Efficient equalization that balances the eye-opening margin, robustness for handling short and long interconnects and power consumption, requires careful attention to the SES. While the default values are a starting place, many links can be dramatically improved by finding the optimal settings. Often this means backing off from the FFE to let the DFE correct for some losses and reflection noise.
Sometimes it’s not so easy to get the SES correct. This is firmware. It requires not only finding the information about what registers to load with what bit of information but having confidence you got it right. While signal integrity engineers are designing the interconnects, it is sometimes a completely different group of engineers responsible for the firmware.
This is from the guy who is one of the founders of the input output buffer information specification IBIS spec and its follow-on algorithmic modeling interface (AMI) executables.
If you’ve just started on the path of designing high-speed serial links or have designed dozens of your own, this is the book that will accelerate your engineering judgment and possibly save you from multiple design spins. If you are already a long-time expert, this book will provide a pleasant read down memory lane, nodding your head through each chapter as you relive your experiences of learning these hard-fought lessons Don offers up to the next generation of engineers.
With the book’s publication, Telian celebrates four decades of work in the field of signal integrity. You can purchase ‘Signal Integrity, In Practice’ now on Amazon.