Optimum loading dispersion for high-speed tree-type decision circuitry

Abstract

With increasing density and capacity due to technology scaling, augmenting data (especially in semiconductor memories) burden selection circuitry with exponentially growing capacitive loads. This tendency violates stringent timing requirements. This work ameliorates the situation for k-stage tree-type decision circuitry. We show that for a k-stage binary decision tree, there always exists an optimum solution such that, after the select-signal arrangement, the worst case loading among select signals equals a lower bound. Our proposed procedure not only provides an optimum solution but also minimizes the loading variance. The worst case loading can be reduced up to nearly k/2 times, thus speeding up and saving power up to W2 times or so for the select signal with the heaviest loading. In contrast, excluding one unit-loading select signal, the empirical variance of the remaining (k-1) signals is always less than 1 instead of diverging. Hence, our approach, for timing-driven layout synthesis, is competent to design high-performance tree-type decision circuitry with more accurate timing and power prediction. In addition, by the presented approach, we can have the alternative of optimizing either for k-stage or for (k-1)-stage, meanwhile possibly minimizing the other. Our algorithm, also, can easily be extended for a general k-stage decision tree with r descendants per node, not restricted to a binary tree; the resultant worst case loading could be quite close to the lower bound and reduced up to nearly k(r-1)/r times.