Low power and high performance detff using common feedback inverter logic

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 02 Issue: 10 | Oct-2013, Available @ http://www.ijret.org 317
LOW POWER AND HIGH PERFORMANCE DETFF USING COMMON
FEEDBACK INVERTER LOGIC
Y.Yaswanth Kumar1
, A.Varalakshmi2
1
II M. Tech VLSI System Design, SITAMS, JNTU Anantapur, Andhra Pradesh, India, yaswanth.yadalam@gmail.com
2
Asst. Professor, Department of ECE, SITAMS, JNTU Anantapur, Andhra Pradesh, India, varalakshmi.a@gmail.com
Abstract
The power consumption of a system is crucial parameter in modern VLSI circuits especially for low power applications. In this
project, a low power Double Edge Triggered D-Flip Flop (DETFF) design is proposed in 65nm CMOS technology. The proposed
DETFF is having less number of transistors than earlier designs. Simulations are carried out using HSPICE tool with different
clock frequencies ranging from 400MHz to 2GHz and with different supply voltages ranging from 0.8V to 1.2V. In general, a
power delay product (PDP)-based comparison is appropriate for low power portable systems. At nominal condition, the PDP of
the proposed DETFF is improved by 65.48% and 44.85% over earlier designs DETFF1 and DETFF2 respectively. Simulation
results show lowest power dissipation and least delay than existing designs, which claims that the proposed DETFF is suitable for
low power and high speed applications.
Keywords: CMOS, flip-flops, Double-edge triggered, power dissipation, delay and PDP.
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1. INTRODUCTION
In recent years, consumer's attitudes are gearing towards
better accessibility and mobility that caused a demand for an
ever-increasing number of portable applications, requiring
low-power dissipation and high throughput. For example,
notebook, hand-held, palm-top computers are portable
versions of microprocessors which require low power
dissipation designs. In these applications, not only voice, but
data as well as video are transmitted via wireless links. The
weight and size of these portable devices is determined by
the amount of power required. Low power design is very
important to support these devices, particularly with respect
to the integrated circuits, in which excessive power
consumption will make the chip over heat and cause error or
even permanent damage. The seriousness of this problem
increases with the level of integration and thus we need to
consider the power consumption of the chips for every
component in the device. Hence, the major concerns of the
VLSI circuit designer were low power dissipation, high
speed, small silicon area, low cost and reliability. The
battery lifetime for these products is crucial; hence, a well-
planned low-energy consumption design strategy must be
necessary.
The design for low power issues can’t be overcome without
précised power prediction and optimization tools. Therefore,
there is a critical need for certain tools to calculate power
dissipation during the design to meet the power constraints
to ignore the costly redesign effort. Power dissipation is
given by the equation, P = α CV2
f. Voltage scaling is the
most effective way to decrease power consumption, since
power is proportional to the square of the voltage. However,
voltage scaling is associated with threshold voltage scaling
which can cause the leakage power to increase
exponentially. In order to reduce the complexity of circuit
design, a large proportion of digital circuits are designed to
be synchronous circuits. The power dissipation in
synchronous VLSI circuits is contributed by several factors:
i.e. I/O, logic and clock power dissipation. Clock power
dissipation is divided into three major contributions: clock
wires, clock buffers and flip-flops. Studies show that 40-
45% of total power dissipated in integrated system is due to
clock distribution network and 90% of which is consumed
by the flip-flops and the last branches of the clock
distribution network. D-type flip-flop’s (DFF’s) are one of
the most fundamental building blocks in modern VLSI
systems and it contributes a significant part of the total
power dissipation of the system.
The most popular synchronous digital circuits are edge
triggered flip-flops. Single-edge triggered (SET) flip-flops
and Double-edge triggered (DET) are edge-sensitive
devices, that is, data storage in these flip-flops occurs at
specific edges of the clock signal. During each clock period,
single-edge triggered flip-flops latch data at only one edge,
either rising or falling edge of the clock signal. Double-edge
triggered flip-flops latch data at both rising and falling edges
of the clock signal during each period of the clock signal.
Hence by using double-edge triggered flip-flops (DETFFs),
the clock frequency can be significantly reduced - ideally, to
half while preserving the rate of data processing. Using
lower clock frequency may translate into considerable
power savings for the clocked portions of a circuit.
The DETFF design aims at saving energy both on the clock
distribution network (by halving the clock frequency) and
flip-flops. It is preferable to reduce circuits’ clock loads by
minimizing the number of clocked transistors. A simple
DETFF is implemented with about 50% extra transistors
than the traditional SET flip flop, however, this issue is

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being resolved in recent intensive researches. It motivates to
use DETFF for larger and complicated designs, especially
for application specific integrated circuits (ASIC) design.
Several DET flop-flops have been proposed in the literature.
The main attention has been in improving the performance
of the circuits.
This paper is organized as follows: Section II explains the
DETFF structures consist of conventional and proposed
DETFF circuits. The nominal simulation conditions along
with transient analysis and optimization performed during
simulation are discussed in Section III. Section IV consists
of Results and performance of proposed design and
conventional designs comparisons in terms of average
power dissipation, delay and PDP. Paper ends with the
conclusion.
2. DOUBLE EDGE TRIGGERED FLIP FLOP
STRUCTURES
There are several ways to implement a DETFF; In general
they can be categorized into two ways. The first idea is to
insert additional circuitry to generate internal pulse signals
on each clock edge. The second idea is to duplicate the
pathway to enable the flip-flop to sample data on every
clock edge. The same data throughput can be achieved with
half of the clock frequency by using DETFF. In other words
double edge clocking can be used to save half of the power
on the clock distribution network.
2.1 Conventional Double-Edge Triggered Flip-Flops
The DET flip flop given in [1] is shown in figure 1. This
flip- flop is basically a Master Slave flip-flop structure and
has two data paths. The upper data path consists of a Single
Edge Triggered flip-flop (SETFF) implemented using
transmission gates. It employs positive edge. The lower data
path consists of a negative edge triggered flip-flop
implemented using transmission gates. Both the data paths
have feedback loops connected such that whenever the clock
is stopped, the logic level at the output is retained. This flip-
flop has 22 transistors excluding clock driver. In these 22
transistors, 12 transistors are clocked transistors.
Fig -1: DETFF1 given in [1]
The figure 1 shows a pair of parallel loops. When the clock
pulse changes from low to high, the upper loop holds data
and the down loop samples data. But when the clock pulse
changes from high to low, the top loop switches to sample
data and then the down loop switches to hold data.
The DET flip flop given in [2] is shown in figure 2. This
flip-flop is also basically a Master Slave flip-flop structure
and consists of two data paths. The transmission gates (TG)
in both the data path are clocked such that the upper data
path works as positive edge triggered flip flop and lower
data path works as negative edge triggered flip flop. The
upper data path consists of transmission gates TG1, TG2
and inverter I1. The lower data path consists of transmission
gates TG3, TG4 and inverter I3.
The input data is connected to TG1 and TG3 and the output
is taken from inverter I5 whose input is connected with TG2
and TG4. Both the data paths have feedback loops
connected such that whenever the clock is stopped, the logic
level at the output is retained so as to maintain the static
functionality. The feedback in upper data path consists of an
inverter I2 and a pass transistor P1. The feedback in lower
data path consists of an inverter I4 and a pass transistor P2.
This design is identical to figure 1 except feedback path.
The feedback transmission gates of figure 1 are not on
critical paths and replaced with pass transistors. This flip-
flop has 20 transistors excluding clock driver. In these 20
transistors, 10 transistors are clocked transistors.
Fig -2: DETFF2 given in [2]
2.2 Proposed Double-Edge Triggered Flip-Flop
The proposed DET Flip-Flop design is shown in figure 3.
This flip-flop is also basically Master Slave flip-flop
structure and it consists of two data paths. The upper data
path consists of transmission gate TG1, pass transistors (P1,
P2) and an inverter I1. The lower data path consists of
transmission gate TG2, pass transistors (P3, P4) and an
inverter I2. The transmission gate and pass transistors in
both the data paths are clocked such that the upper data path
works as positive edge triggered flip flop and lower data
path works as negative edge triggered flip flop. The inverter

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I3 serves as a common feedback inverter and the output of
the inverter I4 forms the output of the present invention.
This design is identical to figure 1 except feedback path has
been changed and the number of clocked transistors is
reduced by using n-type pass transistors instead of
transmission gates. The transmission gates TG2, TG3, TG5,
and TG6 of figure 1 are replaced with pass transistors P1,
P2, P3 and P4 respectively. Basically n-type pass transistors
give weak high but in figure 3, the n-type pass transistors
are followed by an inverter, which results in strong high.
Thus the proposed DETFF in figure 3 is free from threshold
voltage loss problem of pass transistors. The feedback
network of figure 2 is altered by placing the n-type pass
transistor instead of p-type pass transistor. Since, the area
incurred by NMOS is less than that of PMOS transistor in
order to compensate the mobility constraint of NMOS and
PMOS transistors.
According to the DETFF1 and/or DETFF2, one of two loops
always samples the data and the other loop always holds the
data. Therefore, only a single feedback loop is required to
hold the data. The proposed DETFF design eliminates one
feedback loop and reduces the number of transistors
compared to the existing designs. The two inverters I2 and
I4 of figure 1 are used for two feedback paths, but here a
single feedback inverter I3 is used as a common feedback
inverter for two feedback paths as shown in figure 3.
The input data is supplied to one end each of two
transmission gates (TG1, TG2). The other ends of the two
transmission gates (TG1, TG2) are separately connected to
the input of two inverters (I1, I2). The outputs of the two
inverters (I1, I2) are separately connected to the inputs of
another two inverters (I3, I4) via the pass transistors (P1,
P3). The output of inverter I3 goes through two pass
transistors (P2, P4) to connect to one end of the above
mentioned two transmission gates (TG1, TG2) separately,
and the output of the inverter I4 forms the output of the
present invention. Both the data paths have feedback loops
connected such that whenever the clock is stopped, the logic
level at the output is retained so as to maintain the static
functionality.
Fig -3: Proposed DETFF
The novelty of the proposed flip flop lies in the feedback
strategy using common feedback inverter and pass
transistors to make the design static. This improves the
power efficiency of the proposed flip flop. This flip flop has
16 transistors excluding clock driver. In these 16 transistors,
8 transistors are clocked transistors. It is preferred to reduce
number of clocked transistors to achieve low power
dissipation. So, the main advantages of the proposed design
are low power dissipation and increased performance with
low transistor count. Thus the proposed Double Edge
Triggered Flip-Flop (DETFF) has become more efficient in
terms of area, power and speed which claim for better
performance than conventional designs.
3. SIMULATIONS AND TRANSIENT ANALYSIS
Simulation parameters used for comparison are shown in
Table 1. For a fair comparison we simulated all flip-flops on
same simulation parameters.
Table -1: CMOS Simulation parameters
Technology 65nm CMOS
MOSFET Model BSIM 4
Nominal Conditions Vdd =1V, T=25° C
Duty Cycle 50 %
Nominal Clock Frequency 400MHz
Data Rate at Nominal
Clock Frequency
800Mbps
All simulations are carried out using HSPICE simulation
tool at nominal condition with different range of frequencies
from 400MHz to 2GHz and with different range of supply
voltages from 0.8V to 1.2V. The simulated waveform for
the proposed DET flip-flop at nominal condition is shown in
figure 4. This waveform shows that the DETFF is storing
the data in both rising edge and falling edge of the clock
signal.
Fig -4: Output waveform for proposed DETFF
We have optimized the transistor sizes. The transistors, that
are not located on critical path, are implemented with
minimum size. We have improved the feedback path in our
design. We simulated the design to achieve minimum
power- delay (clk-q) product (PDP). Transistor count is also

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included to maintain a fair level of comparisons. In DETFF1
there are 12 clocked transistors, in DETFF2 there are 10
clocked transistors while in our design there are only 8
clocked transistors. We have reduced the number of clocked
transistors. Thus we have reduced the power dissipation.
The switching power, which is dominant power, is
proportional to the square of the supply voltage. We have
reduced supply voltage to 1V or even less voltages for the
reduction of the power.
4. PERFORMANCE COMPARISON
The performance of the proposed DETFF is evaluated by
comparing the average power dissipation, delay and PDP
with DETFF 1 and DETFF 2. Comparison of simulation
results at nominal condition for the three FFs is summarized
in Table 2.
Table -2: Comparison of simulation results for the three FFs
at nominal condition
Specification DETFF 1 DETFF 2 Proposed
DETFF
Average Power
Dissipation (µW)
1.471 1.066 0.881
Clk-Q Delay (ps) 26.47 22.86 15.26
PDP (10-18
J) 38.94 24.37 13.44
PDP
Improvement %
over 1
---- 37.42 65.48
No. of Transistors
(excluding clock
driver)
22 20 16
Since D to Q delay depends on when the data transition
occurs, here we measured clock to Q delay. The clock-to-Q
delay is the delay from the active clock input to the new
value of the output. We simulated all DETFFs with different
clock frequencies ranging from 400MHz to 2GHz and with
different supply voltages ranging from 0.8V to 1.2V. Each
flip-flop is optimized for power delay product. In general, a
PDP-based comparison is appropriate for low power
portable systems in which the battery life is the primary
index of energy efficiency.
Table 3 shows the PDP of all three flip flops at different
supply voltages, this indicates that the proposed design has
an average improvement of 65.28% and 45.98% in terms of
PDP as compared to DETFF1 and DETFF2 respectively.
Table -3: PDP of 3 FFs with the variation of supply voltage
VDD
(V)
DET
FF 1
(10-
18
J)
DET
FF 2
(10-
18
J)
Proposed
DETFF
(10-18
J)
Improv
ement
% Over
1
Improv
ement
% Over
2
0.8 37.84 20.81 11.06 70.77 46.85
0.9 36.8 22.09 12.04 67.28 45.5
1.0 38.94 24.37 13.44 65.48 44.85
1.1 41.3 27.85 15.25 63.07 45.24
1.2 43.54 33.32 17.51 59.78 47.45
Table 4 shows the PDP of all three flip flops at different
clock frequencies, this indicates that the proposed design has
an average improvement of 70.82% and 44.25% in terms of
PDP as compared to DETFF1 and DETFF2 respectively.
Chart 1 and chart 2 show the statistical variation of PDP for
all the 3 flip-flops on varying supply voltage and clock
frequency respectively. This shows our design is suitable for
low power dissipation and high performance applications.
Table-4: PDP of 3 FFs with the variation of clock frequency
Clk
Freq
(GH
z)
DET
FF 1
(10-
18
J)
DET
FF 2
(10-
18
J)
Proposed
DETFF
(10-18
J)
Improv
ement
% Over
1
Improv
ement
% Over
2
0.4 38.94 24.37 13.44 65.48 44.85
0.5 43.84 26.92 15.52 64.6 43.46
1.0 67.47 33.89 19.2 71.54 43.35
1.5 95.09 41.54 22.94 75.87 44.78
2.0 114.69 48.55 26.8 76.63 44.8
Chart -1: Statistical variation of PDP for the 3 FFs on
varying supply voltage

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Chart -2: Statistical variation of PDP for the 3 FFs on
varying clock frequency
CONCLUSION
The proposed Double-edge-triggered D flip-flop is a low
power, low voltage, high speed and low transistor count flip-
flop designed in 65nm CMOS technology. This is static in
nature. The proposed DETFF design eliminates one
feedback loop by using common feedback inverter logic
thereby reducing the number of transistors and also by using
pass transistors the number of clocked transistors is reduced
compared to the existing DET flip flops. This improves the
power efficiency of the proposed DETFF. The three DET
flip-flops are simulated with different supply voltages
ranging from 0.8V to 1.2V and with different clock
frequencies ranging from 400MHz to 2GHz. At nominal
condition, the PDP of the proposed DETFF is improved by
65.48% and 44.85% over earlier designs DETFF1 and
DETFF2 respectively. Simulation results show that the
proposed design has lowest power dissipation and least
delay thereby it has lowest PDP than existing designs.
Therefore the proposed design is very well suited for low
power and high speed applications operating at low
voltages.
REFERENCES
[1] M. Pedram, Q. Wu, and X. Wu, “A New Design of
Double Edge Triggered Flip-Flops,” Proceedings
of the Asia and South Pacific Design Automation
Conference (ASP-DAC), pp. 417–421, 1998.
[2] Imran Ahmed Khan, Danish Shaikh and Mirza
Tariq Beg,“ 2 GHz Low Power Double Edge
Triggered flip-flop in 65nm CMOS Technology”
IEEE Conference, pp 978-1-4673-1318-6/12, 2012.
[3] Xiaowen Wang, and William H. Robinson, “A
Low-Power Double Edge-Triggered Flip-Flop with
Transmission Gates and Clock Gating” IEEE
Conference , pp 205-208, 2010.
[4] Hossein Karimiyan Alidash, Sayed Masoud Sayedi
and Hossein Saidi, “Low-Power State-Retention
Dual Edge-Triggered Pulsed Latch,” Proceedings
of ICEE 2010, May 11-13, IEEE 2010.
[5] Peiyi Zhao, Jason McNeely, Pradeep Golconda and
Jianping Hu, ”Low Power Design of Double-Edge
Triggered Flip-Flop by Reducing the Number of
Clocked Transistors,” IEEE Conference, 2008.
[6] Yu Chien-Cheng ,“Design of Low-Power Double
Edge-Triggered Flip- Flop Circuit", Second IEEE
Conference on Industrial Electronics and
Applications, pp 2054-2057, 2007.
[7] Ahmed Sayed and Hussain Al-Asaad,“A New Low
Power High Performance Flip-Flop” IEEE
Conference, pp 723-726, 2006.
[8] Nikola Nedovic, Mark Aleksic and Vojin G.
Oklobdzija, “Comparative Analysis of Double-
Edge Versus Single-Edge Triggered Clocked
Storage Elements” IEEE Conference, pp V-105 to
V-108, 2002.
[9] Chandrakasan, W.Bowhill, and F. Fox, “Design of
High-Performance Microprocessor Circuits”, 1st
ed. Piscataway, NJ: IEEE, 2001.
[10]G. E. Tellez, A. Farrahi, and M. Sarafzadeh,
“Activity-Driven Clock Design for Low Power
Circuits,” Proceedings of the IEEWACM
International Conference on Computer-Aided
Design (ICCAD), pp. 62- 65, 1995.
BIOGRAPHIES
A.Varalakshmi, received her Bachelor
Degree in Electronics &
Communication Engeering from
Jawaharlal Nehru Technological
University Hyderabad in the year 2004,
Master’s Degree in VLSI System
Design from Jawaharlal Nehru
Technological University Hyderabad in
the year 2008. Presently working as an Assistant Professor
in ECE Department In SITAMS at Chittoor. Her interested
areas of research are Nano Electronics, VLSI System Design
and Low Power VLSI Design

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Y.Yaswanth Kumar, received his
Bachelor Degree in Electronics and
Commuication Engineering from
Jawaharlal Nehru Technological
University Anantapur in the year 2011.
Currently he is doing his Master’s
Degree in VLSI System Design in
Jawaharlal Nehru Technological University Anantapur. His
interested areas are Low Power VLSI Design, Digital
Circuits Design and VLSI Technology

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Low power and high performance detff using common feedback inverter logic