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SEMINAR ON
Gate Diffusion Input(GDI):A Power Efficient Method For
Digital Combinational Circuits
Presented By:
Vijaya Shekhawat
M.Tech (VLSI Design)
IInd Year
Enrollment No:080619
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CONTENTS
MotivationCircuit Design Style Gate Diffusion Input (GDI)Modified GDI TechniqueConclusionReferences
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Market demands high performance low power portable electronic
devices powered by batteries
Higher performance devices → Higher integration of transistor→
Higher power dissipation/unit area
Higher energy densities → can become explosive
Reliability issues → Every 10 ْ C increase in temperature roughly
doubles electronic component failure rate
Expensive packaging and cooling systems cost increase with
increase in power dissipation
Lower the power then delay increase
Motivation
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CMOS Logic Style
Pass Transistor Logic Style (PTL)
Transmission Gate Logic Style (TG)
Pseudo nMOS Logic Style
Complementary Pass Transistor Logic(CPL)
Double Pass Transistor Logic (DPL)
Circuit Design Style
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Most commonly used logic in
VLSI design
Ease of use, well developed
synthesis methods
High noise margins
Low power consumption
No static power dissipation
Good current driving capabilities
CMOS Logic Style
CMOS INVERTER
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XNOR Gate using PTL
Widely used alternative to
complementary CMOS
Fewer transistors are required for a
given function
Reduced number of transistors
means there is lower capacitance
Dedicated buffers need to be
inserted to boost driving strength
Pass Transistor Logic Style (PTL)
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Symbol of Transmission Gate
It solves the problem of low logic
level swing by using pMOS as well
as nMOS
It act as Bidirectional switch.
But, it require large no transistors
Design is much complex because
control signal requires both true and
complementary form
Transmission Gate Logic Style (TG)
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Logic Network of Pseudo nMOS Style
Used where majority of outputs are
high, viz address decoder in memory
No. of transistors for N-input= N+1
Less no of transistor required
High Speed
Swing degradation
Non –Zero static power dissipation
Ratio pMOS
Pseudo nMOS Logic Style
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Every signal and its complement
is generated
Modular design
Useful for modular (array)
circuits like adders, multipliers,
barrel shifter, etc.
Swing degradation
Buffer required
Complementary Pass Transistor Logic(CPL)
And and Nand Gate using CPL
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XNOR and XOR gate using DPL
Double pass-transistor logic
(DPL) uses complementary
transistors to keep full swing
operation
This eliminates the need for
restoration circuitry
One disadvantage of DPL is the
large area used due to the
presence of PMOS transistors
Double Pass Transistor Logic (DPL)
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The problem of existing PTL is top-down logic design complexity
One of the main reasons for this is that no simple and universal
cell library is available for PTL-based design
The new low-power design technique that allows solving most of
the problems mentioned above—gate diffusion input (GDI)
technique
This method is suitable for design of fast, low-power circuits,
using a reduced number of transistors, while improving logic level
swing and static power characteristics and allowing simple top-
down design by using small cell library
Gate Diffusion Input (GDI)
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• The GDI method is based on the use
of a simple cell as shown in Figure. At
first glance, the basic cell reminds one
of the standard CMOS inverter, but
there are some important differences
• 1) The GDI cell contains three input, P
(input to the source/drain of pMOS),
and N (input to the source/drain of
nMOS)
• 2) Bulks of both nMOS and pMOS are
connected to N or P
Basic GDI Function
Basic GDI Cell
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FUNCTIONS IMPLEMENTED USING GDI CELL AND THE TRANSISTORS REQUIRED USING STANDARD CMOS PROCESS
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GDI CMOS
Comparison of logic Function1 implemented by GDI and CMOS
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GDI CMOS
Comparison of XNOR Gate implemented by GDI and CMOS
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Operational Analysis of GDI Cell
Fan-in and Fan-out Bounds in GDI
Analysis of Swing-Restoring Buffers
Analysis of GDI circuits
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To understand the effects of the low swing problem in a GDI cell, I suggest the analysis, based on the example of Function1 and can be easily extended to use in other GDI functions
Table presents a full set of logic states and related functionality modes of Function1
The fact that demands special emphasis is that in about 50% of the cases (for B=1), the GDI cell operates as a regular CMOS inverter, which is widely used as a digital buffer for logic-level restoration
Operational Analysis of GDI Cell
A B Functionality Output
0 0 PMOS Transmission Gate Vtp
0 1 CMOS Inverter 1
1 0 NMOS Transmission Gate 0
1 1 CMOS Inverter 0
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Fan-out: GDI approach allows definition of fan-out bounds by using the
logic-effort concept. The logic effort is directly related to the fan-out .
The effort delay of the logic gate is the product of these two factors
f=g.h
Fan-in: Fan-in: The addition of diffusion inputs in GDI for the same
structure results in an improved fan-in (i.e. (n+2)) where n input are in CMOS.
Note that for F1 and F2 functions, where only one additional input applied to diffusion, the fan-in will increase by one compared to CMOS.
Fan-in and Fan-out Bounds in GDI
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The simplest method of swing restoration is to add a buffer stage after every GDI cell.
Finally, several points have to be emphasized concerning the buffer insertion topology in GDI.
1) Buffer insertion has to be considered only in the case of linking GDI cells through diffusion inputs. No buffers are needed before gate inputs of GDI cells.
2) The “mixed path” topology can be used as an efficient method for buffer insertion. It allows one to reduce the number of buffers by intermittently involving diffusion and gate inputs in a given signal path. The designer should check the tradeoff between buffer insertion and delay, area, and power consumption to achieve an efficient swing restoration.
Analysis of Swing-Restoring Buffers
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Comparison between GDI and CMOS with respect to Power
MUX OR AND Function1 Function20
10
20
30
40
50
60
GDI Power(µW)CMOS Power(µW)
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Comparison between GDI and CMOS with respect to Delay
MUX OR AND Function1 Function20
0.5
1
1.5
2
2.5
GDI Delay (nsec)CMOS Delay (nsec)
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Gate Diffusion Input (GDI) logic style suffers from some practical
limitations like
swing degradation
fabrication complexity in standard CMOS process
bulk connections
These limitations can be overcome by modified gate diffusion input
(Mod-GDI) logic style.
Modified GDI Technique
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Modified GDI Cell
The Mod-GDI cell uses standard four
terminal NMOS and PMOS transistor.
Modified-GDI [Mod- GDI] cell contains
a low-voltage terminal SP configured to
be connected to a high constant voltage
and a high-voltage terminal SN
configured to be connected to a low
constant voltage .
In favor of improving logic level swings
and static power characteristics and
allowing simple top-down design
methodology a small cell library.
Basic Modified GDI Cell
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high-speed
low power circuits
using reduced number of transistors
even as improving swing degradation
allowing easy top-down design by using a small cell library
The Mod-GDI cell be capable of as well implemented in all kinds
of non-standard technologies, like twin-well CMOS technology,
Silicon on Insulator (SOI) technology and Silicon on Sapphire (SOS)
technology
Important feature of Modified GDI Technique
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VARIOUS LOGIC FUNCTIONS
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Power Delay
Comparison Modified GDI and CMOS
ORAND
Functi
on1
Functi
on2MUX
0
10
20
30
40
50
60
MOD-GDI Power(µW)CMOS Power(µW)
ORAND
Functi
on1
Functi
on2MUX
0
0.5
1
1.5
2
2.5
MOD-GDI Delay (n sec)CMOS Delay (n sec)
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Simulation results shows up to 45% reduction in power-delay product in
Mod-GDI.
Mod-GDI gates lower the transistor count
The leakage power and switching power of Mod-GDI gates is lower than
the traditional logic styles.
The problem of fabrication of GDI gates in standard nano-scale CMOS
process is overcome by connecting the sources of PMOS and NMOS to
VDD and GND respectively in Mod-GDI logic style.
The problem of threshold drop is not a very serious issue in deep sub-nm
regions. The Mod-GDI logic style based design adopts interruption of
inverter to alleviate the problem of signal degradation during
propagation.
This proposed logic style is analyzed to exploit the high speed potential
and low power feature of Mod-GDI based circuit applications.
In short, the proposed Mod-GDI logic style based designs can be taken a
better alternative in future.
Conclusion
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• [1]Arkadiy Morgenshtein, Alexander Fish, and Israel A. Wagner, “Gate-Diffusion Input (GDI): A Power-
Efficient Method for Digital Combinatorial Circuits,” IEEE Transactions on Very Large Scale Integration
(VLSI) systems, Vol. 10, No. 5, October 2002, pp. 566-581.
• [2]Arkadiy Morgenshtein, Alexander Fish and Israel A. Wagner, “Gate Diffusion Input (GDI) - A technique
for low power design of digital circuits: Analysis and characterization,” Prod. IEEE 2002, pp. 477-480.
• [3]Padmanabhan Balsubramanian, Johince John, “Low Power Digital Design using modified GDI ,” Prod.
IEEE 2006.
• [4]Arkadiy Morgenshtein, Idan Shwartz and Alexander Fish, “Gate Diffusion Input (GDI) Logic in
Standard CMOS Nanoscale Process,” IEEE 26-th Convention of Electrical and Electronics Engineers in
Israel 2010, pp. 776-780.
• [5]Madhusudhan Dangeti, S.N.Singh, “Minimization of Transistors Count and Power in an Embedded
System using GDI Technique: A realization with digital circuits,” International Journal of Electronics and
Electrical Engineerin, ISSN : 2277-7040, Volume 2 Issue 9, September 2012, pp. 21-30.
• [6]Kunal and Nidhi Kedia, “GDI Technique: A Power-Efficient Method for Digital Circuit,”International
Journal of Advanced Electrical and Electronics Engineering, (IJAEEE), ISSN (Print): 2278-8948, Volume-
1, Issue-3, 2012, pp 87-93.
• [7]Pankaj Verma, Ruchi Singh and Y. K. Mishra,, “Modified GDI Technique - A Power EfficientMethod
For Digital Circuit Design,” International Journal of Electronics and Computer Science Engineering,
ISSN- 2277-1956, Volume2, Number 4, 2013, pp 1071-1080.
• [8]http://www.tanner.com/EDA/product/index.html
• [9]http://www.tanner.com/EDA/product/Tools_SchematicCapture.html
References
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THANK YOU