I. INTRODUCTION

Low-power design is progressing because of the growing interest in high energy-constrained mobile applications, such as wearable devices and the internet of things. In the applications, operating speed is no longer the main concern issue. Instead, power is the main issue of the design aspects. Digital designs often employ extensive FF for data buffering or pipelining, and the circuit efficiency of FF designs largely affects the overall area and power consumption. New FF designs evolve constantly with the advances of new process technology as well as the target applications ^[1-4]. In this paper, we revisit the basic FF problem. A transmission gate based FF (TGFF) as shown in Fig. 1(a), is the most widely used one. One possible drawback of this design is the excessive work load to the clock signal which leads to larger dynamic power even when the data switching activity is lower. Recently, true-single-phase clock operation FFs are presented targeting low power applications ^[5-8]. The major idea is to reduce clock signal loading via circuit reduction. In this paper, we present a novel SR-latch based FF design. It also features a true-single-phase clock operation, which greatly alleviates the clock loading. The design also exhibits better energy consumption when compared with previous FF designs.

II. PROPOSED DESIGN

The FF circuit design idea initiates with a low power SR-latch based FF named ACFF ^[5] as shown in Fig. 1(b). In this design, the data contention problem encountered by slave-latch and the advantage of power saving diminishes as the rise of the switching activity. The p-MOS type pass-transistor logic based master latch also causes a longer setup time performance especially operates at lower voltage. Moreover, this design suffers from a power leaking problem (floating) when certain input and internal node combinations occur ^[7].

In view of these possible drawbacks, an enhanced design applying to the Set-Reset latch based design is developed as shown in Fig. 2. For pull-down network (n1 and n2), one CK controlled nMOS transistor can be shared by the two discharging paths. For pull-up network (p1 and p2), nodes x2 and x2-bar are always complementary to each other, implying that either the x3 or x4 controlled pMOS transistor will turn on as indicated in the small figure in Fig. 2(b). The drain node of the turned on pMOS transistor corresponds to a virtual $V_{DD}$.

Thus a clock-driven transistor can be removed without affecting the function with fully static operation. The resultant circuit design is shown in Fig. 2 (b).The transistor-count is reduced to 24 and only 2 transistors (1pMOS and 1nMOS) are driven directly by the clock signal. All these factors contribute to a significant power consumption and energy saving of the design ^[5]. Moreover, the height of nMOS transistors of proposed design in series is only 2 while design ^[6-7] are pMOS transistors in series is 3-transistor, smaller sized transistors can thus be adopted to achieve the FF design. The speed performance, power consumption, and layout area are all better than previous designs. Simulation waveforms depict the details of each node within our design, as shown in Fig. 3(a). All internal nodes assume a full voltage swing.

We also noticed a potential internal node floating problem associated with ACFF design. When the clock signal was frozen at logic 1 and the input data changed, both internal nodes $X$ and $XB$ were determined to be in a floating state shown in Fig. 3(b).This means the design may suffer from the static power problem when the FF design is clock gated. Post-layout simulation results also show that, the power consumption of ACFF is over 3.5 times that of our design in this particular case i.e. 1MHz/0.5V, which should not be overlooked when the ACFF is employed in the low power designs.

III. SIMULATION RESULTS

Four FF designs, i.e., the TGFF, ACFF, sense- amplifier FF (SAFF) ^[9] and our design are compared. The target technologies are the TSMC 180nm CMOS process. Transistor sizing is subject to the optimization of power-delay-product ($PDP_{CQ}$) with the same high on layout schematic. The operating conditions are set as 1MHz/0.5V to emphasize the low voltage and low power applications. Five test patterns, each corresponding to a different data switching probability are applied.

The simulation results are summarized in Table 1. In terms of the power consumption behavior, our design is the most power efficient. The power consumption (at 25% switching) of the proposed design is 68.7% less than TGFF. In terms of the setup time performance, ACFF plays higher than TGFF design due to weak pMOS type pass transistor logic. The setup time of our design is 63.7% smaller compared to ACFF design. As for the hold time, the all designs are showing a negative number expect the SAFF. Our design also leads in $PDP_{CQ}$. When the switching activity is 25%, the PDP saving against the TGFF, SAFF and ACFF can reach 68.7%, 35.8% and 52.4%, respectively.

Fig. 4 shows the average power at different data switching activity and $PDP_{CQ}$ performance of these FF designs at different process corners. All designs function properly subject to process variations expect the ACFF design failed at SF corner. Note that, for each process corner, the setup time and hold time were scanned to obtain the best PDP number as shown in Fig. 5. All FF designs were determined to function properly under process variations. Our design maintained its lead in most of cases. Therefore, this evaluation also verifies the performance consistency of the proposed design.

VI. CONCLUSIONS

A novel Set-Reset latch based FF supporting low voltage and low power operations is presented. Our design is successfully removed 2-transistor controlled by clock signal to achieve power and energy performance. Conducted evaluations show that our design is the most power economical in all compared designs.