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Description:
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Analyzing circuit leakage and minimizing leakage during the standby mode of oper -
ation of a circuit are important problems faced during contemporary circuit design .
Analysis of the leakage profiles of an implementation would enable a designer to
select between several implementations in a leakage optimal way . Once such an im -
plementation is selected , minimizing leakage during standby operation (by finding
the minimum leakage state over all input vector states ) allows further power reduc -
tions . However , both these problems are NP -hard . Since leakage power is currently
approaching about half the total circuit power , these two problems are of prime rel -
evance .
This thesis addresses these NP -hard problems . An Algebraic Decision Diagram
(ADD ) based approach to determine and implicitly represent the leakage value for all
input vectors of a combinational circuit is presented . In its exact form , this technique
can compute the leakage value of each input vector , by storing these leakage values
implicitly in an ADD structure . To broaden the applicability of this technique , an
approximate version of the algorithm is presented as well . The approximation is done
by limiting the total number of discriminant nodes in any ADD . It is experimentally
demonstrated that these approximate techniques produce results with quantifiable
errors . In particular , it is shown that limiting the number of discriminants to a value between 12 and 16 is practical , allowing for good accuracy and lowered memory
utilization .
In addition , a heuristic approach to determine the input vector which minimizes
leakage for a combinational design is presented . Approximate signal probabilities of
internal nodes are used as a guide in finding the minimum leakage vector . Probabilistic
heuristics are used to select the next gate to be processed , as well as to select the
best state of the selected gate . A fast satisfiability solver is employed to ensure the
consistency of the assignments that are made in this process . Experimental results
indicate that this method has very low run -times , with excellent accuracy , compared
to existing approaches . |