Sum Addressed Decoder - Further Optimizations: Predecode

Further Optimizations: Predecode

Many decoder designs avoid high-Fan-In AND gates in the decode line itself by employing a predecode stage. For instance, an 11 bit decoder might be predecoded into three groups of 4, 4, and 3 bits each. Each 3 bit group would drive 8 wires up the main decode array, each 4 bit group would drive 16 wires. The decoder line then becomes a 3 input AND gate. This reorganization can save significant implementation area and some power.

This same reorganization can be applied to the sum-addressed decoder. Each bit in the non-predecoded formulation above can be viewed as a local two-bit add. With predecoding, each predecode group is a local three, four, or even five bit add, with the predecode groups overlapping by one bit.

Predecoding generally increases the number of wires traversing the decoder, and sum-addressed decoders generally have about twice as many wires as the equivalent simple decoder. These wires can be the limiting factor on the amount of feasible predecoding.