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Type of Document	Dissertation
Author	Riedel, Marcus
Author's Email Address	riedel AT caltech.edu
URN	etd-05032004-153842
Persistent URL	http://resolver.caltech.edu/CaltechETD:etd-05032004-153842
Title	Cyclic combinational circuits
Degree	PhD
Option	Electrical Engineering
Advisory Committee	Advisor Name Title Professor Jehoshua Bruck Committee Chair Professor Alain Martin Committee Member Professor Ali Hajimiri Committee Member Professor Andrew Viterbi Committee Member Professor Erik Winfree Committee Member Professor Yaser Abu-Mostafa Committee Member
Keywords	logic synthesis combinational logic loops combinational circuits cycles feedback
Date of Defense	2003-11-17
Availability	unrestricted
Abstract A collection of logic gates forms a combinational circuit if the outputs can be described as Boolean functions of the current input values only. Optimizing combinational circuitry, for instance, by reducing the number of gates (the area) or by reducing the length of the signal paths (the delay), is an overriding concern in the design of digital integrated circuits. The accepted wisdom is that combinational circuits must have acyclic (i.e., loop-free or feed-forward) topologies. In fact, the idea that "combinational" and "acyclic" are synonymous terms is so thoroughly ingrained that many textbooks provide the latter as a definition of the former. And yet simple examples suggest that this is incorrect. In this dissertation, we advocate the design of cyclic combinational circuits (i.e., circuits with loops or feedback paths). We demonstrate that circuits can be optimized effectively for area and for delay by introducing cycles. On the theoretical front, we discuss lower bounds and we show that certain cyclic circuits are one-half the size of the best possible equivalent acyclic implementations. On the practical front, we describe an efficient approach for analyzing cyclic circuits, and we provide a general framework for synthesizing such circuits. On trials with industry-accepted benchmark circuits, we obtained significant improvements in area and delay in nearly all cases. Based on these results, we suggest that it is time to re-write the definition: combinational might well mean cyclic.
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