madhu

1. A Dual-Pulse-Clock Double Edge Triggered Flip-Flop
for Low Voltage and High Speed Application
Kuo-Hsing Cheng and Yung-Hsiang Lin
Department of Electrical Engineering
TamkangUniversity
Taiwan, R.0.C
Email: cheng@ee.t!a.edu.tw
ABSTRACT
In this paper, a low voltage dual-pulse-clock double
edge triggered D'flip-flop (DPDET) is proposed. The
DPDET flip-flop uses a split output latch clocked by a
short pulse train. Compared to the previously reported
double edge triggered flip-flops, the DPDET flip-flop
uses only six transistors with two transistors being
clocked, operating correctly under low supply voltage.
The total transistors count is reduced to improve speed
and power dissipation in flip-flop. The number of
transistors is reduced by 40% to 70% compared to other
double edge triggered flip-flops. Based on 0.35um single-
poly quad-metal' CMOS technology, the HSPICE
simulation results show that the operating speed of the
DPDET flip-flop is 2.7 GHz at a 3.3V supply voltage.
The operating speed of the DPDET flip-flop is increased
about 41% and 49% in compared with others for 3.3V and
2.5V supply voltage, respectively. The power dissipation
is reduced about 36% and 29% in compared with others
for 3.3V and 2.5V supply voltage, respectively. Moreover,
the DPDET flip-flop can be used in a 0.9V supply voltage
with 224 MHz operating speed. Therefore the proposed
DPDET flip-flop is suitable for low supply voltage and
high speed CMOS applications.
1. INTRODUCTION
In recent years there has been an increasing demand for
high-speed digital circuits at low power consumption.
Double edge triggered (DET) flip-flop, which can
maintain the same data rate at half clock frequency in
compare with conventional single edge triggered (SET)
flip-flop. As a result, the half clock frequency significantly
reduces the power dissipation. True single phase clocking
(TSPC) [1][2] has been demonstrated to be an efficient
methodology to achieve very high-speed digital VLSI
design. Double edge triggered flip-flop based on the TSPC
technique will he suitable design for high-speed and low
power VLSI applications.
In this paper, a new dual-pulse-clock double edge
triggered D-flip-flop (DPDET) is proposed. The total
transistors count is lower than previously published DET
flip-flops. The dual-pulse train generation circuit is
extemal to the flip-flop and therefore can be used to feed
multiple double edge triggered flip-flop. It can achieve the
reduction of total capacitances. The operating speed of the
proposed DPDET flip-flop is faster than conventional
DET flip-flops [3][4]. Moreover, the important advantage
of the DPDET flip-flop is that it can operate correctly
under very low supply voltage, 0.9 volt, and the
previously proposed DET flip-flops can not operate
correctly under the same supply voltage.
The HSPICE simulations, based on 0.3Sum single-poly
quad-metal CMOS technology, are made to determine
speed and power dissipation with various supply voltage.
This paper is constructed as follows. In Section 2, four
well-known double edge triggered flip-flops are presented.
In Section 3, the operation of the proposed flip-flop is
given. In Section 4, simulations of the flip-flop are
performed and the comparison of the proposed DPDET
flip-flop with the existing structures. Finally, a conclusion
is given in Section 5.
2. CONVENTIONAL DET FLIP-FLOP
In this paragraph, four well-known DET flip-flops
[3][4][5] will be briefly presented. Figure 1 shows two
double edge triggered flip-flops proposed by M. Afghahi
and I. Yuan [3]. which use twenty transistors to implement
two complementary latches in parallel. The main
difference of the two flip-flops is the merging structure. In
Fig.l(b), the merging structure uses a NAND gate to
reduce the number of clocked transistors. The third type
DET flip-flop proposed by J. S. Wang [4] is shown in
Fig.2, which has eighteen transistors with fewer clock
capacitive loading. In Figures 1 and 2, those three flip-
flops have the same character, using two complementary
latches in parallel. One latch must react to the positive
edge of the clock pulse and the other must react to the
negative edge. Then a merging circuit is used to merge the
outputs of two latches into a single output.
Another scheme for double edge triggered flip-flop
design without merging circuit is shown in Fig.3. The
PDET flip-flop proposed by B. Pontikakis and M. Nekili
V-4250-7803-7761-31031517.0002003 IEEE

2. [SI does not use a merging circuit. As shown in Fig.3(a)
and (h), the PDET flip-flop consists two parts, the double
edge triggered pulsed clock generator and pulse-clock
PDET flip-flop. The PDET flip-flop has fewer MOS
transistors as compare with previous work. However, the
operation of the PDET flip-flop has some serious problem
as following. When pulse is high and D-in is low, nodes
A and B are charged to high, MN3 is turned on, then node
C is low and output is high. When pulse changes from
high to low, the output keeps its previous state. If the pulse
keeps low and D-in changes from low to high, MPI turns
off and MN2 tuns on. Because the node C is low, MP2 is
turned on. Then the voltage of node A is dropped to turn
on MP3, through MP2 and MN2. Then node C is charged
to high and output changes high to low. As a result, at the
hold time of the PDET, it cannot keep the previous data at
the flip-flop output. It will make a mistaken output data.
Q
(a) (b)
Fig. 1ConventionalDET Flip-Flop
(a)M. Afghahi9la,(b)M. Afghahi9lb [3]
Q
--
Fig2 The DET Flip-Flop, J. S. Wang97 [4]
"dd
T
* (b)
Fig.3 The pulse-clockedDET flip-flop [5]
(a) Pulse-ClockGenerator,(b)PDETFlip-Flop
3. THE PROPOSED DPDET FLIP-FLOP
DESIGN
As shown in Fig.4(a) and (h), the proposed DPDET
flip-flop circuit includes two parts, dual-pulse-clock
generator and split DPDET latch, respectively. The design
of the dual-pulse-clock generator is based on the pass-
transistor logic design scheme. Therefore, the proposed
new XOR / XNOR gate for dual-pulse-clock generator has
low power and high speed properties. The proposed split
DPDET latch uses a PMOS transmission gate to connect
nodes A and B. However, this makes the new circuit has
higher operating speed and can he operated in low voltage
applications. The detail operations of those circuits are
described as follows.
As shown in Figures 4 and 5, the new dual-pulse-clock
generator uses a dual rails XOR / XNOR gate to generate
clkgulse and clkhgulse, respectively, on the positive and
negative edge of the input clock signal. When the input
clock signal clk is low, as a result, clkb is high, clkd is low.
Therefore, MNI and MPI are tumed on, and MN2, MN3,
MN4, MP2 are turned off. Then the output clkjulse is
low and clkbqulse is high. On the rising edge of clk,
MN1 still turns on for a short duration of inverter gate
delay. On this short time duration, clkqulse generates a
positive narrow pulse to sample input data value. On the
same rising edge, MN4 is tumed on and clkd still keeps at
low for a short duration of inverter gate delay, clkhgulse
generates a negative narrow pulse to sample input data
value. Comparatively, on the falling edge of clk, clkgulse
and clkhgulse have the same previous results,
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3. respectively. Because the generator circuit uses four
NMOS transistors as pass-transistors, clkgulse and
clkbgulse are weak high. In order to improve this
drawback, adding two PMOS transistors, MP1 and MP2,
driven by clkgulse and clkbgulse, respectively.
Therefore, the proposed circuit can be used for low supply
voltage.
clk clkb
I -Ielkgulse clkbgulse
Vdd
T
Vdd
T
Fig.4 The Proposed PDPET Flip-Flop
(a)Dual-Pulse-Clockgenerator,(b)SplitDPDET latch
clk
clkb
clkd
clkgulse n n n
clkbgulse U UFig.5 Pulse waveforms of the Dual-Pulse-Clockgenerator
As the previous mention, the four DET flip-flops have
the same disadvantage. They are not suitable for low
supply voltage. The proposed new circuit can improve this
drawback. Another advantage of the proposed circuit is
that it uses only six transistors. The number count of
transistors is reduced by 70% in compared with twenty
transistors used in Fig.1, 67% reduction in compared with
Fig.2, and 25% reduction in compared with the split-
output latch in Fig.3, respectively. The dual-pulse train
generation circuit is extemal to the flip-flop, and therefore,
can be used to feed multiple double edge triggered flip-
flop. It can achieve the reduction of total capacitances and
the power dissipation. The operating speed of our
proposed circuit is faster than conventional DET flip-flop.
4. SIMULATION AND COMPARISON
RESULTS
The simulation of various conventional DET flip-flops
and new proposed DPDET flip-flop are based on 0.35 um
single-poly quad-metal CMOS process with an output load
capacitor of O.lpF. Because the previous proposed flip-
flop [5] which is shown in Fig.3, causes some problems
described in Section 3. The simulation of the new
proposed DPDET flip-flop only compares with three
previous proposed DET flip-flops in [3][4]. Analyzing the
lowest supply voltage for the flip-flops, the supply voltage
is gradually decreased down from 3.3V to a point where
the flip-flop fails to operate. The lowest operating voltage
is 1.3V, 1.3V, 1.7V and 0.9V for Fig.l(a), Fig.l@), Fig.2,
and the new DPDET flip-flop, respectively. As shown in
Fig.6, in order to measure the speed for all flip-flops, the
Q output !?om the DET flip-flop is fed to an odd nunher
of inverter chains and connected back to the flip-flipinput
in a ring oscillator formed [3]. Then, to measure the
operating frequency of the flip-flop, the frequency of the
input clock is gradually increased up to a point where the
oscillator fails to operate. However, the inverter chains
delay must be subtracted from the clock period for the
calculation of the maximum operating frequency of the
flip-flips.
c l k d I
Fig.6 Schematicdiagram of the simulation for operatingspeed
The maximum operating frequency of conventional and
proposed flip-flops are shown in Fig.7 for various supply
voltage. It is clear that the operating speed of the new
proposed DPDET flip-flop is increased by 35% to 41%
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4. and 27% to 49% in compared with others for 3.3V and
2.5V supply voltage, respectively. The operating speed of
the DPDET flip-flop can achieve 2.7 GHz at 3.3V supply
voltage and 224 MHz at 0.9V supply voltage. The power
dissipation of various DET flip-flop is shown in Fig.8. The
power dissipation of the new proposed DPDET flip-flop is
reduced by 21% to 36% and 18% to 29% in compared
with others for 3.3V and 2.5V supply voltage, respectively.
Because the dual-pulse trains generation circuit is external
to the flip-flop and therefore can he used to feed multiple
double edge triggered flip-flop, we need only considering
the power dissipation of the OUT proposed split DPDET
latch in Figd(b).
Fig.7 The comparison of the operating speed
160
140
I20
iw
80
60
40
UW
Supply voltage
Fig.8The comparison of the power dissipation
Comparisons of simulation results for the four flip-flops
indicate significantly that OUT proposed DPDET flip-flop
offers advantages in terms of supply voltage, speed, power
and transistors count.
5. CONCLUSION
triggered flip-flops, the DPDET flip-flop uses only six
transistors, operated correctly under the low supply
voltage. The total transistors count is reduced to improve
speed and power dissipation. The number of transistors is
reduced by 40% to 70% compared to other double edge
triggered flip-flops. The operating speed of the DPDET
flip-flop is 2.7 GHz at 3.3V supply voltage. The operating
speed is increased by 35% to 41% and 27% to 49% in
compared with others for 3.3V and 2.5V supply voltage,
respectively. Moreover, the DPDET flip-flop can he used
in a 0.9V supply voltage with 224 MHz operating speed.
The power dissipation is reduced by 21% to 36% and 18%
to 29% in compared with others for 3.3V and 2.W supply
voltage, respectively. Simulations are performed using
HSPICE in 0.35um single-poly quad-metal CMOS
technology. The proposed circuit is suitable for low supply
voltage, 0.9 volt, and high speed CMOS applications.
REFERENCES
[l] J. Yuan and C. Svensson, “High-speed CMOS Circuit
Technique,” IEEE J. Solid-Slats Circuits, vo1.24, no.I ,
pp.62-71, Feb. 1989.
[Z] M. Afghahi and C. Svensson, “A Unified Single-
Phase Clocking Scheme for VLSI Systems,” IEEE J.
Solid-Sluts Circuits, vol.SC-25, no.1, pp.225-233, Feb.
1990.
[3] M. Afghahi and I. Yuan, “Double Edge-Triggered D-
Flip-Flop for High-speed CMOS Circuits,’’ IEEE J.
Solid-Stuts Circuits,vol. SC-26, no.8, pp.1168-1170,
Aug. 1991.
[4] Jinn-Shyan Wang, “A New True-Single-phase-
Clocked Double-Edge-Triggered Flip-Flop for Low -
Power VLSI Designs,” IEEE Infernufionu/
Symposium on Circuits and Systems, Hong Kong,
pp.1896-1899, June 9-12, 1997.
[5] B. Pontikakis and M. Nekili, “A Novel Double Edge-
Triggered Pulse-Clocked TSPC D Flip-Flop for High-
Performance and Low-Power VLSI Design
Applications,” IEEE International Symposium on
Circuit andSystems, Arizona, vo1.5, pp.101-104, May
26-29,2002.
In this paper, a low voltage dual-pulse-clock double
edge triggered D-flip-flop is proposed. The DPDET flip-
flop uses a split output latch clocked by a short pulse train.
Compared to the previously reported double edge
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