Minimizing leakage power in sequential circuits by using mixed V_t flip-flops

Dual V_t has been widely used to control leakage, while, at the same time, satisfying circuit performance. However, current approaches target the combinational circuits even though sequential elements, such as flip-flops and latches, contribute an appreciable proportion of the total leakage. The use of dual V_t flip-flops is limited to circuits of large timing slack, because introducing high V_t flip-flops in place of low V_t ones yields abrupt change in timing. We propose mixed V_t flip-flops, which are designed by using both low and high V_t, but in different transistors. Compared to low V_t flip-flop, the mixed V_t flip-flops exhibit increased delay, but either on setup time or on clock-to-Q delay but not on both, while their leakage is greatly reduced. We extend the conventional sensitivity-based dual V_t allocation algorithm to incorporate mixed V_t flip-flops together with dual V_t combinational gates. Experimental results show that an average leakage saving of 31% is achieved, compared to the use of dual V_t on combinational subcircuits alone. The leakage of the flip-flops themselves is cut by 57% on average.