Fast Low-Power Decoders for RAMs

Decoder design involves choosing the optimal circuit style and figuring out their sizing, including adding buffers if necessary. The problem of sizing a simple chain of logic gates has an elegant analytical solution, though there have been no corresponding analytical results until now which include the resistive effects of the interconnect. Using simple RC models, we analyze the problem of optimally sizing the decoder chain with RC interconnect and find the optimum fan-out to be about 4, just as in the case of a simple buffer chain. As in the simple buffer chain, supporting a fan-out of 4 often requires noninteger number of stages in the chain. Nevertheless, this result is used to arrive at a tight lower bound on the delay of a decoder. Two simple heuristics for sizing of real decoder with integer stages are examined. We evaluate a simple technique to reduce power, namely, reducing the sizes of the inputs of the word drivers, while sizing each of the subchains for maximum speed, and find that it provides for an efficient mechanism to trade off speed and power. We then use the RC models to compare different circuit techniques in use today and find that decoders with two input gates for all stages after the predecoder and pulse mode circuit techniques with skewed N to P ratios have the best performance.