优秀设计十大戒律.pdf

The mandments of Excellent Design
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Peter Chambers
Engineering Fellow
VLSI Technology
This report will give you some pointers that
will help you design synchronous circuits
that work first time. mandments
that shouldalways be followed!
Using Synchronous Circuits
Synchronous digital systems are pervasive in today’s designs. Engineers create
clocked circuits for every conceivable application, with frequencies from DC to
GHz. Every synchronous system employs mon characteristics, and
is prone to a group mon faults. These faults can cause instability and
unreliability, and may not be uncovered in the typical design process.
result is a poor product that fails to meet the design criteria, and the engineer
has to go through the suffering of design modification and revision. This is time-
consuming and costly. However, by applying a few simple rules, you can avoid
synchronous design faults in your designs and achieve consistent first-pass
ess. In this article you’ll learn the sources of the mon problems
and their solutions, and how to apply these ideas to your designs.
© 1997, VLSI Technology 1
Digital Systems 101
Digital Systems 101
We’ll begin by describing a typical synchronous circuit. Many variations are pos-
sible, but a simple example will be adequate to illustrate the sources of error.
Figure 1 shows the circuit and timing for one clocked element of the example.
One issue that deserves mention is this: Why use synchronous logic at all?
Wouldn’t asynchronous logic be faster? The answers to these questions could
take a book, but here are some reasons to use synchronous designs:
• Synchronous designs eliminate the problems associated with speed varia-
tions through different paths of logic. By sampling signals at well-defined
time intervals, fast paths and slow paths can be handled in a simple manner.
• Synchronous designs work well under variations of temperature, voltage and
process.