Trade-offs between Performance and Robustness for Ultra Low Power/Low Energy Subthreshold D flip-flops in 65nm CMOS

Abstract

The need for Ultra Low Power systems has increased with increasing number of portable devices. The maintenance costs of battery powered systems can be greatly reduced by improving the battery time, especially in places where battery replacement is hard or impossible. Implementation of subthreshold D flip-flops in layout is one step closer to having a subthreshold building block library. The task for this thesis is to implement D flip-flop blocks, which are highly suitable for subthreshold operation in layout. These are the PowerPC 603, C$^2$MOS, a Classic NAND-based D flip-flop, and two Minority3-based D flip-flops. The D flip-flops are first custom designed for $250mV$ in schematic at transistor level, and then implemented in layout. The implementation in layout focuses on high robustness against process variations, by using high regularity for the cost of area.The D flip-flops are simulated in both schematic and layout, and the results are compared to each other and earlier results found in papers. The results show that the PowerPC 603 has the lowest PDP, the lowest power consumption, very low propagation delay, and an average relative standard deviation for delay. The C$^2$MOS has the lowest propagation delay, low power consumption and low PDP results. However, it has the highest relative standard deviation on delay. The Minority3-based D flip-flops have a very low relative standard deviation for delay, which makes them the most robust against process variations in this sense. However, they have the highest propagation delay, highest power consumption and PDP, and consumes the highest chip area. The Classic NAND-based D flip-flop has good PDP and power consumption results, but a high delay and average standard deviation for delay.Earlier papers show similar results for the C$^2$MOS and the PowerPC 603, but no results are found for the rest. Future work consists of implementing and testing forced-stacked blocks, body biasing, high threshold voltage transistors, and tape-out measurements.