4 Sub-/near-threshold flip-flops with application to frequency dividers

Four different flip-flops dimensioned for subthreshold operation have been designed and implemented in layouts. The four full custom, race-free, D-flip-flops were implemented in a standard 65 nm CMOS process and verified by measurements, when used in 2 divide-by-3 circuits. The first frequency divider, using standard topologies, demonstrated functionality down to a supply voltage of 132 mV, while the second variant, based on a recently proposed “`slice-based'” approach, was functional for a supply voltage down to 137 mV. The frequency divider using traditional 4-transistor NAND and NOR topologies had lower energy per operation than the alternative 8-transistor NAND and NOR implementation. At 0.1 MHz, the figures were about 2.1 fJ and 3.5 fJ, respectively. For supply voltages from 0.2 to 1.2 V, a static flip-flop using 8-transistor NOR-gates plus one inverter had the lowest static power consumption among the 4 flip-flops.