Energy-Efficient Dual-Voltage Design Using Topological Constraints

We propose a method for dual supply voltage digital design to reduce energy consumption without violating the given performance requirement. Although the basic idea of placing low voltage gates on non-critical paths is well known, a new two-step procedures does it so more efficiently. First, given a circuit and its nominal single supply voltage, we find a suitable value for a lower second supply voltage that is likely to give the best advantage in power reduction. Besides, using the critical path timing constraint and a linear-time gate slack calculation we also classify gates into three groups. All gates in Group 1 can be simultaneously assigned the lower voltage. Any gate in Group 2 can be assigned the lower voltage but then gate slacks must be recalculated because the group classifications may change. No gate in Group 3 can be assigned the lower voltage. A second step then assigns the lower voltage to the largest possible number of gates using the gate classifications and imposing a topological constraint, preventing any low voltage gate from feeding into a higher voltage gate, thus avoiding the use of level converters. SPICE simulation of dual-voltage ISCAS’85 benchmark circuits using the 90nm bulk CMOS PTM (predictive technology model) shows energy savings of up to 60% with no increase in the original critical path delay and up to 70% with relaxed critical path delay.