1. INTERNATIONAL JOURNAL OF ELECTRONICS AND
International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 4, Issue 2, March – April, 2013, pp. 257-263
IJECET
© IAEME: www.iaeme.com/ijecet.asp
Journal Impact Factor (2013): 5.8896 (Calculated by GISI) ©IAEME
www.jifactor.com
LOW POWER SRAM
Sapna1, Prof. B. P. Singh2
1
(M. Tech. Student, Electronics and Communication Engineering,
Mody Institute of Technology & Science, Lakhshmangarh (Sikar), India,
2
(H.O.D., Electronics and Communication Engineering, Mody Institute of Technology &
Science, Lakhshmangarh (Sikar), India,
ABSTRACT
Over the years, power requirement reduction in VLSI circuits has undergone tremendous
advancement. Low power device design is now a vital field of research on account of increasing
demand of portable devices. Sub-threshold operation holds promise for ultra low energy
operation in emerging applications. This paper proposes a low power Schmitt Trigger based
SRAM cell. Experimental results reveal that proposed design has reduced power consumption
and has temperature sustainability. The simulation work has been carried out on Tanner EDA
tool at 45nm technology.
Keywords: Power consumption, SRAM, Schmitt Trigger, Sub-threshold, Tanner EDA.
1. INTRODUCTION
The ongoing demand for reduction in energy consumption has motivated the design of
sub-threshold digital circuits, which were shown to be reliable even for complex systems and
memories. [1]. Supply voltage scaling has significant impact on the overall power consumption
[2]. Aggressive scaling of transistor dimensions with each technology generation has resulted in
increased integration density and improved device performance. Increased integration density
along with the increased leakage necessities ultra low power operation in the present power
constrained design environment. The power requirement for battery operated devices such as cell
phones and medical devices are even more stringent [3]. Supply voltage scaling has remained the
major focus of low power design. This has resulted in circuits operating at a supply voltage
lower than the threshold voltage of a transistor.
257

2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
Sub-threshold current of a MOSFET device occurs when the gate-to-source voltage (VGS)
of a device is lower than its threshold voltage (Vt). The reason for a growing importance of sub-
threshold conduction is that the supply voltage has been continually scaled down, both to reduce
the dynamic average power consumption of integrated circuits (the power that is consumed when
the transistor is switching from an on-state to an off-state, which depends on the square of the
supply voltage), and to keep electric fields inside small devices low, to maintain device
reliability. [4].
The amount of memory required in a particular system depends on the type of the
application. The semiconductor memory is generally classified according to the types of data
storage and data access. The read/write memory is commonly called Random Access Memory
(RAM) [5]. RAMs are classified into SRAM (Static RAM) and DRAM (Dynamic RAM).
SRAM and DRAM hold data but in different ways. DRAM requires the data to be refreshed
periodically in order to retain the data. SRAM does not need to be refreshed as the transistors
inside would continue to hold the data as long as the power supply is not cut off. The additional
circuitry and timing needed to introduce the refresh creates some complications that make
DRAM memory slower and less desirable than SRAM. In human terms, memory is an
organism's ability to store, retain, and recall information and experiences. Similarly memory
refers to physical devices is used to store data on a temporary or permanent basis. Semiconductor
memory is capable of storing large quantities of digital information that are essential to all digital
system. Read/Write (R/W) memory must permit the modification (writing) of data bits stored in
the memory array, as well as their retrieval (reading).
2. EXISTING SRAM CELLS
The memory cell is a fundamental element in the design of low power high density
SRAMs because a significant portion of the memory’s size is taken by the cell area. The memory
chip array contains the memory cells in which the data bits are already stored or in which they
are about to be stored. SRAM utilizes a flip-flop mechanism, which uses static latches and these
operate in a manner similar to the way in which memory cells work. The electrical feedback
ensures that voltage levels are sustained so that the data may retain active indefinitely for as long
as power supply continues [5].
2.1. Existing 6T SRAM Cell
The 6T memory cell implementation in its simplest form is shown in Fig. 1 [5]. The cell
is made up of two access transistors (M5 and M6) and a latch formed by two cross coupled
inverters (M1, M3 and M2, M4). The pass transistors which are connected to two
complementary bit lines BL and BLB are controlled by the word line signal WL. They act as
transmission gates providing the bidirectional access between the latch and the bit lines.
Before the read operation, the voltages at both bit lines get precharged to an equalized
potential. When this particular memory cell is selected by asserting signal either BL or BLB will
be discharged to the ground terminal via M5 and M6. As a result a small potential
258

3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
Figure 1: Schematic of existing 6T SRAM cell
Difference appears at the bit lines. A sense amplifier detects this potential difference and
amplifies it to a full swing signal at the bit lines. During the write operation, the data bit to be
written gets transferred to BL whereas its complement gets transferred to BLB. When the cell is
selected by WL, the access transistors will store the data bit in a latch formed by two cross
coupled inverters. During hold operation, access transistors are turned off and the two cross
coupled inverters holds the data as long as the power supply is applied.
2.2. Existing 7T SRAM Cell
The 7T SRAM cell uses a novel write mechanism. In this cell, as shown in Fig. 2, the
feedback between the two inverters is cut off during the write operation [6]. The write
mechanism depends only on one of the two bit lines, which reduces the activity factor of
discharging the bit line pair.
Figure 2: Schematic of existing 7T SRAM cell
259

4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
3. SCHMITT TRIGGER
In the previously reported SRAM cell, the basic element for the data storage is a cross
coupled inverter pair. Extra transistors are added to decouple the read and write operations. The
cross coupled inverter pair of an SRAM cell operating at ultra low supply voltage consumes high
power. So to improve the inverter characteristics, Schmitt Trigger configuration is used.
3.1. Existing Schmitt Trigger
The existing Schmitt Trigger (ST) circuit consists of three pMOS devices P1, P2, P3 and
three nMOS devices N1, N2, N3 as shown in Fig. 3 [1]. ST circuit has two different high-to-low
(Vୌ ) and low-to-high (V୐ ) transition threshold voltages. When the threshold voltage of the
existing ST circuit is high, input signal goes up to Vୈୈ from ground. In other words, output
signal is pulled low as input signal exceeds Vୌ . Similarly, when the threshold voltage of the
existing ST circuit is low, input signal goes down to ground fromVୈୈ . In other words, output
signal is pulled up as input signal is lower than V୐ [7].
Figure 3: Schematic of existing Schmitt Trigger
3.2. Modified Schmitt Trigger
The modified Schmitt Trigger consists of four transistors P1, P2, P3, and N1 unlike the
existing ST, shown in Fig. 4 [7]. Thus the area and the leakage current are reduced as the
transistors N2 and N3 are removed.
260

5. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
Figure 4: Schematic of modified Schmitt Trigger
4. PROPOSED 11T SRAM CELL
For a stable SRAM cell operating at lower supply voltages, power consumption of the
cell should be reduced. Therefore a Schmitt Trigger based 11T SRAM cell has been proposed
having built in feedback mechanism for reduced power consumption. The proposed 11T SRAM
cell requires no architectural change compared to the existing 7T cell architecture except that the
cross coupled inverter pair is replaced by the proposed Schmitt Trigger pair as shown in Fig. 5.
Transistors M1, M3, M5 and M7 form one ST inverter while transistors M2, M4, M6 and M8
form another ST inverter. M9 and M10 are access transistors.
Figure 5: Schematic of proposed 11T SRAM cell
261

6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
5. SIMULATION AND ANALYSIS
In low power applications area, average power consumption and delay introduced by the
device are the main technological aspects to prefer a design over its other counterparts. The
existing and the proposed SRAM cells have been simulated using Tanner EDA Tools version
13.0. The cells are tested for power consumption, delay and power-delay product at various
temperatures. While working in sub-threshold region the voltage value is taken below the
threshold value that is 0.40V at all temperatures. For fair comparison, the aspect ratio of all
transistors is taken to be 1. The circuits are compared at 45nm technology.
7.00E-09
Power Consumption (watt)
6.00E-09
5.00E-09
4.00E-09
Existing 6T cell
3.00E-09
Existing 7T cell
2.00E-09
Proposed 11T cell
1.00E-09
0.00E+00
25 35 45 55 65 75
Temperature (°C)
Figure 6: Power consumption at various temperatures
Power consumption of the proposed cell is remarkably reduced as compared to the
existing cells, shown in Fig. 6. Also the power consumption decreases with increase in
temperature. The obtained results of delay are shown in Table I. It clearly shows that delay for
the proposed cell is less than the existing cells.
Table I: Delay at Various Temperatures
Temperature Delay
(°C) Existing 6T Existing 7T Proposed 11T
cell cell cell
25 9.8692e-9 9.8713e-9 9.8693e-9
35 9.8682e-9 9.8703e-9 9.8685e-9
45 9.8672e-9 9.8694e-9 9.8678e-9
55 9.8663e-9 9.8686e-9 9.8671e-9
65 9.8654e-9 9.8678e-9 9.8666e-9
75 9.8646e-9 9.8670e-9 9.8660e-9
262

7. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN 0976 –
6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 2, March – April (2013), © IAEME
Table II: PDP at Various Temperatures
Temperature PDP
(°C) Existing 6T cell Existing 7T cell Proposed 11T cell
25 5.6565e-17 5.4519e-17 2.9605e-17
35 5.6256e-17 5.1785e-17 2.9512e-17
45 5.5433e-17 4.8765e-17 2.9376e-17
55 5.4726e-17 4.5689e-17 2.9152e-17
65 5.3871e-17 4.3316e-17 2.8766e-17
75 5.3564e-17 4.1002e-17 2.8280e-17
The quantitative approach showing the variations of the Power-Delay Product (PDP) at different
temperatures is presented in Table II. It depicts that PDP for the proposed cell is much lower than existing
cells. Thus the proposed 11T SRAM cell is preferable for low power devices.
6. CONCLUSION
As the need of low power consumption and improvement in efficiency in the digital systems is
increasing day by day, the demand of low voltage, low power SRAM is escalating. The proposed 11T
SRAM cell is compared with the existing 6T and 7T SRAM cells in sub-threshold region. The reason for
a growing importance of sub-threshold conduction is that the supply voltage has continually scaled down,
to reduce the power consumption. The proposed cell consumes less power and has reduced delay as
compared to the existing cells. The tremendous decrease in power-delay product reported in the proposed
11T SRAM cell during the simulation make it better than the existing 6T and 7T SRAM cells. Thus the
proposed cell is better viable options for low power and high performance applications.
REFERENCES
[1] N. Lotze, and Y. Manoli, A 62 mV 0.13 µm CMOS standard-cell-based design technique using
Schmitt-trigger Logic, IEEE J. Solid State Circuits, 47(1), 2012, 47-60.
[2] J. P. Kulkarni and K. Roy, Ultralow-voltage process-variation-tolerant Schmitt-trigger-based
SRAM design, IEEE Trans. Very Large Scale Integration (VLSI) Syst., 20(2), 2012, 319-332.
[3] J. P. Kulkarni, K. Kim, and K. Roy, A 160 mV robust Schmitt trigger based subthreshold SRAM,
IEEE J. Solid State Circuits, 42(10), 2007, 2303–2313.
[4] H. Soeleman, K. Roy, and B. Paul, Robust sub-threshold logic for ultra-low power operation,
IEEE Trans. Very Large Scale Integration (VLSI) Syst., 9(1), 2001, 90-99.
[5] K. Roy and K.-S. Yeo, Low-voltage, low-power VLSI Subsystems (New Delhi: Tata McGraw-Hill
International, 2009).
[6] R. Aly, M. Faisal, and A. Bayoumi, Novel 7T SRAM cell for low power cache design, Proc.
IEEE SOC Conf., 2005, 171-174.
[7] Sapna and B. P. Singh, Low power schmitt trigger in sub-threshold region, International Conf. on
Recent Trends in Computing and Communication Engineering, Hamirpur, India, 2013.
[8] S. S. Khot, P. W. Wani, M. S. Sutaone and S.K.Bhise, “A Low Power 2.5 V, 5-Bit, 555-Mhz Flash
Adc In 0.25µ Digital CMOS”, International Journal of Computer Engineering & Technology (IJCET)
Volume 3, Issue 2, pp. 533 - 542 ISSN 0976 – 6367(Print), ISSN 0976 – 6375(Online) Published by
IAEME.
263