Reducing switching activity on datapath buses with control-signal gating

This paper presents a technique for saving power dissipation in large datapaths by reducing unnecessary switching activity on buses. The focus of the technique is on achieving effective power savings with minimal overhead. When a bus is not going to be used in a datapath, it is held in a quiescent state by stopping the propagation of switching activity through the module(s) driving the bus, The "observability don't-care condition" of a bus is defined to detect unnecessary switching activity on the bus. This condition is used to gate control signals going to the bus-driver modules so that switching activity on the module inputs does not propagate to the bus. A methodology for automatically synthesizing gated control signals from the register-transfer-level description of a design is presented. The technique has very low area, delay, power, and designer effort overhead, It was applied to one of the integer execution units of a 64-bit, two-way superscalar RISC microprocessor. Experimental results from running various application programs on the microprocessor show an average of 26.6% reduction in dynamic switching power in the execution unit, with no increase in critical path delay and negligible area overhead.