Now a day’s data storage is gaining
more importance in human life. All electronic and digital devices need memory
for reducing the power consumption. The concept of “more data in less space” is
useful for increasing the system performance and overall system efficiency.
Generally we used semiconductor memory as “SRAM”. SRAM can be abbreviated
“Static Random Access Memory”. Many VLSI chip can have SRAM memory because of
their large storage capacity and fast accessing time. Where the word static
indicates that it does not need to be periodically refreshed but the DRAM need
periodically refreshed. DRAM can be abbreviated as “Dynamic Random Access
Memory” which is another type of memory. Both the memories can be classified
from “Random Access Memory: (RAM). In
this paper the power analysis of 6 transistor SRAM is compared with 7
transistor SRAM. As a result the power dissipation
of 7 transistors is high when compared with 6 transistors. The power
dissipation of 6T SRAM is about 2.991mW and the power dissipation of 7T SRAM is
about 3.183Mw. SRAM are mostly used for mobile applications, because of their
ease of use and low leakage of power. In this paper the schematic of 6T SRAM
and 7T SRAM are drawn using DSCH software and the layouts are drawn using
MICROWIND software.1. INTRODUCTION
recent days, Static Random Access Memory has become the major role in digital
world. Because which occupies the largest part of SOC (system-on-chip). The major
device need SRAM memory mainly for device dissipate small amount of power. But
the dynamic power dissipation causes problems in digital circuits because the dynamic power
depends on supply voltage, switching frequency and output voltage swing.
Dynamic power dissipation can be minimized by reducing the supply voltage. At
the same time low supply voltage leads to performance degradation and also
decreases the threshold voltage which in turn increases the sub threshold
current hence the static power dissipation increases. This paper discuss about
the power dissipation of 6 transistors and 7 transistors SRAM. It also includes
the functional view of 6T and 7T SRAM cells.
2. 6T STATIC RANDOM ACCESS MEMORY
A conventional 6T SRAM
consists 6 transistors which form two cross coupled inverters. This bit cell
can be read and write single bit data. When a bit is stored in memory the 6T
SRAM behave like a latch. The cross coupled inverter pattern which causes large
area consumption which is a drawback of 6T SRAM when compared to resistive load.
Conventional SRAM with 6 transistors is shown in figure 1 and 6T SRAM have
three states they are read, write and hold states.
2.1. Hold State
write operation (WL=0) the accessible transistor M1 and M2 disconnect the cell
from bit lines. The leakage current can be drawn from vdd.
2.2. Read State
accessible transistor M3 and M6 should be ON when pre-charging bit and bit bar
line to high.
2.3 Write state
When the WL=0, the value to be written to
the bit and bit bar line. Hence we write a data value is “0”, we take bit value
is “0” and bit bar value is “1”.and the data value is “1”, we take bit value
should be “1” and the bit bar value is “0”.
7T STATIC RANDOM ACCESS MEMORY
As like 6T SRAM, the 7T SRAM circuit also
consists of two CMOS cross coupled to each other. In this circuit we
additionally connect NMOS transistor to write line. And it also have two pass
NMOS transistor connected to the bit and bit bar line. The access transistor N3
and N4 which are correspondingly connected to the write and read line to
perform the write and read operation. Before write operation the 7T SRAM cell
depends upon feedback connection. These feedback connection and disconnection
can be performed by N5 transistor.
3.1. Write Operation
write operation can be start by turning off the N5 transistor to this cut off
feedback connection. When N3 is on and N4 is off the bit line bar carries
complement of input data. The N5 is turned on and WL is turned off for
reconnect the feedback connection to store new data. The bit line bar is
discharged to “0” for storing “1” in the cell. And there is no need to
discharge bit line for storing “0” in the cell.
When performing the read operation
both read and word line are turned to on and also the transistor N5 is kept on.
4. RESULTS AND
Here we analysis and discuss about the 6T and 7T SRAM cell during read
and write operation. And the schematic view of SRAM cell is designed and
implemented by using DSCH and MICROWIND software.