6.915 Digital Systems Engineering

how to make computers go real fast, reliably.

Summary

Logistics

Schedule

Lecture Notes

Assignments

Readings

Signalling Conventions

To transport a bit, it is encoded as a current or a voltage on a wire. A good encoding method or signalling convention allows high bit rates (gigabits/s) at low power by isolating signals from noise. Most conventional signalling conventions, such as those implemented by most off-the-shelf TTL and CMOS parts, on the other are limited to 100Mb/s or less and consume large amounts of power. We will discuss the fundamentals of signalling with examples drawn from a number of high-performance signalling conventions.

Timing and Synchronization

A timing convention governs when symbols are placed on wires and when they are sampled off. Most digital systems today use synchronous timing conventions that present challenging problems in clock distribution. Passing information between parts of a system that employ different timing conventions requires synchronization. In some cases, a system may operate completely asynchronously. Very often, the choice of timing conventions and synchronization methods makes a very large difference in the speed and reliability of a system. In particular we will see that, with the proper choice of timing convention, the throughput of a system is limited only by the timing noise (skew and jitter), not by delay. We will discuss the fundamentals of timing and synchronization and look at examples of succesful and unsuccessful approaches to timing.

Noise in Digital Systems

Noise is the dominant concern in the design of both signalling and timing conventions. The dominant noise sources in digital systems, such as power supply noise, crosstalk, and inter-symbol interference, are actually interference generated by the system itself. Other sources, such as thermal noise and alpha particles are truly noise. In either case, understanding noise and managing it is essential to the reliable high-speed operation of a digital system. We will survey the most important sources of noise in a digital system and present a methodology of budgeting for noise, both amplitude and timing noise.

Power Distribution

Digital systems demand large amounts of AC current (kA are not unusual) while requiring a stable DC voltage (to within a few hundred mV). The current may switch from zero to peak and back to zero in a few ns giving peak derivatives of 10^12 A/s or more. Off-chip bypass capacitors, which look more like inductors over 20MHz, are of little help in smoothing this current demand. We will examine this problem in detail and look at a number of approaches for solving it.

Instructor

Professor William J. Dally

Teaching Assistant

To Be Determined

Time

Tuesday and Thursday 11:00 to 12:30

Location

36-144

Text

Digital Systems Engineering
William J. Dally and John Poulton
(Draft chapters of this book will be handed out as course notes)

Prerequisite

6.002 Required, 6.004 Recommended

Grading

Two quizes (50%)
Problem sets (20%)
Class project (20%)
Participation (10%)

• Lecture 1: Introduction to Digital Systems Engineering
• Lecture 5: Noise Sources in Digital Systems
• Lecture 12: Signalling over Lossy Lines