Static Random Access Memories (SRAMs)
Random Access Memory (RAM) refers to a read/write
memory device that can read data from or write data to any of its memory
addresses, regardless of what memory address was last accessed for reading
or writing. RAM comes in two major classifications:
and Dynamic RAM,
SRAMs store data in flip-flops, which retain data as long as the SRAM is
powered up. DRAMs store data in cells that depend on capacitors,
which need to be 'refreshed' continuously since they are not able to
retain data indefinitely even if the device is continuously powered up.
A typical SRAM
IC has address lines, data lines, and control lines. The
are used to identify the location of the memory storage element(s) or
cell(s) to be read from or written to. The
the value of the data read or being written into the memory cells
are used to direct the sequence of steps needed for the read or write
operations of the SRAM.
elements of an SRAM are arranged in an
of rows and columns. Each row of memory cells share a common
while each column of cells share a common
line. The number of columns of such a memory array is known as the bit
width of each word.
storage element of an
is a circuit that consists of 4 to 6 transistors. This multi-transistor
circuit usually forms cross-coupled inverters that can hold a '1' or '0'
state as long as the circuit is powered up. A pair of
have the output of one inverter going into the input of the other and vice
versa, such that the output (and input) of one inverter is the
of that of the other.
doesn't need periodic refreshing or clocking in order to hold its data,
making the SRAM faster than a DRAM (which needs data refreshing).
All it needs is a constant supply of power. However, since the memory cell
of an SRAM is more complex than that of a DRAM, it eats up more space on
the chip (which means that you get less memory per given area), making
SRAM's more expensive than DRAM's.
into an SRAM's storage element by setting the 'bit' line (usually referred
to as Dataj) to the data value to be written and then enabling the
element's corresponding word line. Asserting the 'Word Enable' line while
driving the data bit and its complement into the cross-coupled inverters
of the storage element causes the data bit to overwrite the previous state
of the element. If a word consists of several bits, then the whole word
may be written with new data in one step if the new values are provided to
all the bit lines at the same time before the 'Word Enable' line is
the content of an SRAM's storage element also requires the 'Word Enable'
line to be asserted. This time, however, the SRAM uses sense
amplifiers to detect the voltage difference between the voltage at Dataj
and that of Dataj's complement at the outputs of the cross-coupled
inverters. If the former is greater than the latter, then the cell
contains a logic '1'. Otherwise, the cell contains a logic '0'.
operating parameter of an SRAM is its memory access time, which is the
time needed for new data to appear at the data outputs after the new data
address becomes valid. Fast SRAMs have shorter memory access times.
For more details on SRAM parameters, please see:
SRAM test parameters.
are seldom used as a system's main RAM because of its high cost and lower
memory density. They are, however, widely used in
for the microprocessor, since these are speed-sensitive applications that
the SRAM is better suited for.
What is a
SRAM Test Parameters; DRAMs;
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