Proceedings of the Third International Conference on Digital Information Processing, E-Business and Cloud Computing, Reduit, Mauritius 2015
Design and Evaluation of Pulse Triggered Flip-Flop Based on Split Output
TSPC Latch for Low Power High Performance Digital Circuit
Sandeep Singh Gill1 and Gurinderjit Kaur2
1, 2
Guru Nanak Dev Engineering College, Ludhiana, India
1
ssg@gndec.ac.in
ABSTRACT
In this paper a Pulse Triggered Flip-Flop based on
Split Output TSPC Latch suitable for low power
high performance application is proposed. The
Pulse Triggered Flip-Flop is constructed using a
split output TSPC latch with embedded logic.
Proposed flip-flop has the advantages of simple
structure, less number of transistors, low
dissipation power and lower transistor area.
Proposed circuit is simulated in cadence analog
design environment with 0.25µm CMOS
technology. Simulation results show that by using
the proposed circuit, dissipation power can be
reduced by 40%, number of transistors by 40%,
current drawn by 48% and transistor area can be
reduced by 68%.
KEYWORDS:
Pulse triggered flip-flops
(PTFF), Split Output TSPC latch, Semi dynamic
flip-flop (SDFF), Hybrid latch flip-flop (HLFF),
Embedding logic, Low power.
1. INTRODUCTION
Most digital circuits today are constructed
using static CMOS logic and edge- triggered
flip-flops. Although such techniques have
been adequate in the past and will remain
adequate in the future for low performance
design, they will become inefficient for highperformance components as the number of
transistors are increasing resulting in increase
of area. Some conventional high performance
flip-flops like Hybrid Latch Flip-Flop (HDFF)
and Semi Dynamic Flip-Flop (SDFF) have
the disadvantage that they have large amount
of power dissipation due to redundant
ISBN: 978-1-941968-14-7 ©2015 SDIWC
transistors at internal nodes, including the
issue of increase in the number of transistors.
Designers will therefore need to adopt new
techniques and circuits that can improve the
performance by lowering power dissipation
and area of the design for low power digital
circuits with less number of transistors.
2. REVIEW OF EXISTING FLIP-FLOPS
A design was developed by Sun
Microsystems Inc. used in UltraSPARC-III
micro-processors using a Semi Dynamic FlipFlop, proposed by Fabian Klass with 0.25 µm
CMOS technology. This design had short
latency, Small clock load with low power
consumption. This design was bulky having
large number of transistors and required
larger area on die. This design was improved
by Arnab Ghosh from university of Idaho
with same Semi Dynamic Flip-Flop (SDFF)
by scaling and embedded logic [1], [2]. The
flip-flop on a 1 bit adder consisting carry and
sum functions was implemented and by the
use of scaling and embedded logic timing
parameters and area were improved. These
improvements were achieved with the penalty
of bulk in circuit, large delay, large setup and
hold time. This design is considered as a
benchmark for this paper [1]. This section
will discuss the used structure of Flip-flop i.e.
Semi-Dynamic Flip-Flop (SDFF) and the
conventional high speed Flip-Flop i.e.
Hybrid-Latch Flip-Flop (HLFF). The circuit
diagram of a Semi-Dynamic Flip-Flop
(SDFF) is shown in Fig. 1. The circuit is
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Proceedings of the Third International Conference on Digital Information Processing, E-Business and Cloud Computing, Reduit, Mauritius 2015
faster than TSPC but still has some
shortcomings. First, internal node X is truly
dynamic, i.e. it is not actively driven by any
device during most of the evaluation phase.
Second, output Q is high impedance when the
clock signal is low. The circuit is composed
of a dynamic front-end and a static back-end.
The flop samples input D and produces output
QB, which is the logic complement of D. The
circuit operates as follows. On the falling
edge of clock CLK, the flop enters the
precharge phase.
Fig.1. Semi Dynamic Flip-Flop (SDFF)
Node X is precharged high, cutting off node
Q from the input stage. The evaluation phase
begins with the rising edge of clock CLK. If
input D is low, node X would remain high.
Node Q would either remain low or will be
discharged through transistors. The circuit
diagram of Hybrid Latch Flip-Flop is shown
in Fig.2.
having negative setup time in generating
pulse which gives small D-Q delay. It also has
small logic-embedding with small penalty. As
mentioned earlier this Flip-flop has the
disadvantage of large amount of power
dissipation [3-13].
3. PROPOSED FLIP-FLOP
To overcome the disadvantages of Design
using Semi Dynamic Flip-Flop and
Conventional high speed flip flops we
proposed a Pulse Triggered Flip- Flop based
on split output TSPC latch. It has a simpler
structure composed of five transistors and
back to back inverters. It is a positive latch if
it is triggered by the rising edge of the clock.
Back to back inverters enhance the robustness
of its output operation. Proposed flip-flop can
reduce the power dissipation, current drawn,
transistor count, total transistor width and
estimated transistor area. Fig.3 Shows the
Circuit diagram of Pulse Triggered Flip-Flop
based on split output TSPC latch. In proposed
circuit clock is applied to pull up circuit to
fulfill the need to have a mirror circuit of pull
down to implement the embedded logic.
Clock is also applied to NMOS at the middle
to trigger the circuit, data is applied to pull
down circuit.
Fig.2. Hybrid Latch Flip-Flop (HLFF)
The circuit of Hybrid Latch Flip-Flop consists
of two stages: the front end function as pulse
generator and the back end is to capture the
pulse as a latch. It has the advantage of
ISBN: 978-1-941968-14-7 ©2015 SDIWC
Fig.3. PTFF based on Split output TSPC latch
This circuit will also avoid bulk in the design
as compared to Semi Dynamic Flip-Flops
(SDFF) as it do not consists of any redundant
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Proceedings of the Third International Conference on Digital Information Processing, E-Business and Cloud Computing, Reduit, Mauritius 2015
transistors at internal node. An inverted clock
is applied to PMOS connected at X and Y
node to remove the potential difference
between both the nodes. This gate here
behaves like a pass gate. At the back end we
have cross coupled inverters which is a basic
storing element also, it increases the
robustness of the circuit. In this cross coupled
inverters, width of feedback inverter should
be greater than the width of feed through
inverter.
4. EMBEDDING LOGIC FUNCTION
Fig.4. PTFF based on split output TSPC latch with
embedded logic
Embedding logic function is an important
technique using which we can easily
incorporate most logic functions into flop,
such as wide OR functions, multiplexers and
complex gates. Fig.4. shows the Pulse
Triggered Flip-Flop based on split output
TSPC latch with embedded Sum and Carry
functions. It will reduce the number of
combinational stages and clock cycles, which
will provide high throughput of the design.
5. SIMULATION RESULTS
In this experiment we implemented the design
of 1-bit registered adder having sum and carry
function using Semi Dynamic Flip-flop [1]
discussed in this paper and the proposed Pulse
Triggered Flip-flop based on split output
TSPC latch. We also simulated and analyzed
ISBN: 978-1-941968-14-7 ©2015 SDIWC
both the designs using Cadence analog design
environment. We compare the transistor
count, total transistor width, estimated
transistor area, current drawn, power
dissipation, C-Q, C-Q̅̅ , D-Q. D-Q̅̅, Set up and
hold times and transparency pulse width of
circuit. The experiment conditions are shown
in Table.1. Simulation results are shown in
Table.2 to Table.10. In this experiment all the
flip-flops have the same data rate and all
transistor sizes are optimized to achieve the
desired results. The rise time and fall time of
the input signal are 100ps. We can see that
Transistor count of the proposed flip-flop is
15, with embedded logic it consists of 19
transistors and for 1 bit registered full adder
transistor count is 90 in Table.2. In Table.3,
we compare the Total Transistor Width which
is 116.04µm for 1-bit registered full adder.
We also compare Estimated Transistor Area
which is 172.203µm2 for proposed circuit and
250.901µm2 for benchmark [1] in Table.4. In
Table.5 transparency pulse width is given,
which is 205ps and 180ps for Semi Dynamic
Flip-Flip (SDFF) and Pulse Triggered FlipFlop Based on split output TSPC latch
respectively. In Table.6 we compare the Set
up and Hold time of the proposed and
benchmark [1]. We can see at the fall edge the
C-Q for benchmark circuit is 120.3ps and for
the proposed circuit it is 189.3ps, at the rise
edge the C-Q for benchmark circuit is 276.5ps
and for the proposed circuit it is 215ps given
in Table.7. In Table.8 we can see at fall edge
the D-Q for benchmark circuit is146ps and for
the proposed circuit it is 207.9ps, at the rise
edge the D-Q for benchmark circuit is
293.1ps and for the proposed circuit it is
228.3ps. Table.9 shows the comparison
between Current drawn of both the designs
which is 0.7nA and 0.23nA for benchmark
and the proposed circuit respectively. Power
dissipation is also compared in Table.10
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Proceedings of the Third International Conference on Digital Information Processing, E-Business and Cloud Computing, Reduit, Mauritius 2015
which is 1.4nW for benchmark circuit and
0.445nW for the proposed circuit.
Table 4. Estimated Transistor Area
Table 1. Experiment Conditions
Circuit
Design
Specification
Technology
MOSFET
Model
Benchmark Proposed Work
0.25um
TSMC deep
submicron
0.25um
Nominal
2V
25 degree C
Conditions
Supply Voltage
Temperature
Rise time of
100ps
input signal
Fall time of
100ps
input signal
Clock frequency 100 MHz
Clock duty
50%
cycle
Delay
Between 50%
calculations
points
0.25um
TSMC deep
submicron
0.25um
Nominal
2V
25 degree C
Flip-Flop
Flip-Flop
with
embedded
logic
1-bit
registered
full adder
Benchmark
design (µm2)
38.115
Proposed
design (µm2)
23.607
53.615
35.480
250.901
172.203
100ps
100ps
Table 5. Transparency Pulse Width of the Circuit
100 MHz
50%
Circuit
Between 50%
points
Flip-Flop
Flip-Flop
with
embedded
logic
Benchmark
design (ps)
193
Proposed
design (ps)
180
205
180
Table 2. Transistor Count
Transistor count
Circuit
Flip-Flop
Flip-Flop
with
embedded
logic
1-bit
registered
full adder
Table 6. Set up and Hold times
Benchmark
23
Proposed
15
Circuit
27
227
19
90
Table 3. Total Transistor Width
Circuit
Flip-Flop
Flip-Flop
with
embedded
logic
1-bit
registered full
adder
25.695
Proposed
design
(µm)
15.9
35.995
23.82
168.39
116.04
Benchmark
design (µm)
ISBN: 978-1-941968-14-7 ©2015 SDIWC
Setup time
Flip-Flop
Flip-Flop
with
embedded
logic
Hold time
Flip-Flop
Flip-Flop
with
embedded
logic
Benchmark
design
Virtual Real
(ps)
(ps)
Proposed
design
Virtual Real
(ps)
(ps)
-55
237
-58.82
210.3
-30
263
-55.05
223.8
160.3
-133
179
-90
160
-133
126
-154
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Proceedings of the Third International Conference on Digital Information Processing, E-Business and Cloud Computing, Reduit, Mauritius 2015
Table 7. Comparison of C-Q
Clk – Q (ps)
(LH)
(HL)
Benchmark design
Flip-Flop
253.5
118
Flip-Flop
with
276.5 120.3
embedded
logic
Proposed design
Flip-Flop
226.6
147
Flip-Flop
with
215
189.3
embedded
logic
Circuit
Table 10. Power Dissipation
Clk – (ps)
(LH)
(HL)
190.2
305.1
190.1
330
231.5
283.1
280
272.7
Table 8. Comparison of D-Q
D – Q (ps)
(LH)
(HL)
Benchmark design
Flip-Flop
250.5
220.9
Flip-Flop
with
293.1
146
embedded
logic
Proposed design
Flip-Flop
210.8
207.2
Flip-Flop
with
228.5
207.9
embedded
logic
Circuit
– (ps)
(LH)
(HL)
291.4
299.8
231.7
344.7
301.6
266.1
299.9
283.6
Circuit
Flip-Flop
Flip-Flop with
embedded
logic
1-bit registered
full adder
Benchmark
design (nW)
1.4
Proposed
design (nW)
0.445
1.4
0.456
11.7
4.63
CONCLUSION
A Design using Pulse triggered Flip-Flop
based on split output TSPC latch is proposed
and simulated in a 0.25µm process. Our
simulation results justify our analysis that we
reduced power dissipation by 40%, transistor
area by 68%, transistor count by 40% and
current drawn by 48% without affecting the
high performance of the circuit.
ACKNOWLEDGEMENT
The authors thank Guru Nanak Dev
Engineering College, Gill Road, Ludhiana,
for technical support for implementation and
simulation.
Table 9. Current Drawn
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Benchmark
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0.7
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0.23
0.7
0.23
6.1
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