Bounded potential slack: Enabling time budgeting for dual-Vt allocation of hierarchical design

Time budgeting, which assigns timing assertion at block boundary, is a crucial step in hierarchical design. The proportion of high- and low-Vt gates of each block, which determines overall leakage power consumption, is dictated by timing assertion, yet dual-Vt allocation is not taken into account during conventional time budgeting. Bounded potential slack is introduced as a measure of dual-Vt allocation, and is experimentally shown to be strongly correlated with the percentage of high-Vt gates. A new time budgeting is proposed with objective of achieving bounded potential slack, which is formulated as a linear programming problem. In experiments with example hierarchical designs implemented in 45-nm commercial technology, the proposed time budgeting reduced leakage power by 32% on average compared to conventional time budgeting, when both are followed by the same dual-Vt allocation. The time budgeting is also applied to voltage island design, where each block can have its own Vdd with mix of high- and low-Vt gates.