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Static random-access memory (static RAM or SRAM) is a type of
random-access memory Random-access memory (RAM; ) is a form of computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the ...
(RAM) that uses latching circuitry (flip-flop) to store each bit. SRAM is volatile memory; data is lost when power is removed. The term ''static'' differentiates SRAM from
DRAM Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxid ...
(''dynamic'' random-access memory) — SRAM will hold its data permanently in the presence of power, while data in DRAM decays in seconds and thus must be periodically refreshed. SRAM is faster than DRAM but it is more expensive in terms of silicon area and cost; it is typically used for the cache and internal registers of a
CPU A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...
while DRAM is used for a computer's
main memory Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a comput ...
.


History

Semiconductor bipolar SRAM was invented in 1963 by Robert Norman at Fairchild Semiconductor. MOS SRAM was invented in 1964 by John Schmidt at Fairchild Semiconductor. It was a 64-bit MOS p-channel SRAM. The SRAM was the main driver behind any new
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSF ...
-based technology fabrication process since 1959 when CMOS was invented. In 1965, Arnold Farber and Eugene Schlig, working for IBM, created a hard-wired memory cell, using a
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
gate and
tunnel diode A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively " negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suz ...
latch A latch or catch (called sneck in Northern England and Scotland) is a type of mechanical fastener that joins two (or more) objects or surfaces while allowing for their regular separation. A latch typically engages another piece of hardware on t ...
. They replaced the latch with two transistors and two
resistor A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...
s, a configuration that became known as the Farber-Schlig cell. In 1965, Benjamin Agusta and his team at IBM created a 16-bit silicon memory chip based on the Farber-Schlig cell, with 80 transistors, 64 resistors, and 4 diodes.


Characteristics

Though it can be characterized as volatile memory, SRAM exhibits data remanence. SRAM offers a simple data access model and does not require a refresh circuit. Performance and reliability are good and power consumption is low when idle. Since SRAM requires more transistors per bit to implement, it is less dense and more expensive than DRAM and also has a higher power consumption during read or write access. The power consumption of SRAM varies widely depending on how frequently it is accessed.


Applications and uses


Embedded use

Many categories of industrial and scientific subsystems, automotive electronics, and similar
embedded system An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded ...
s, contain SRAM which, in this context, may be referred to as ESRAM. Some amount (kilobytes or less) is also embedded in practically all modern appliances, toys, etc. that implement an electronic user interface. SRAM in its dual-ported form is sometimes used for real-time
digital signal processing Digital signal processing (DSP) is the use of digital processing, such as by computers or more specialized digital signal processors, to perform a wide variety of signal processing operations. The digital signals processed in this manner are ...
circuits.


In computers

SRAM is also used in personal computers, workstations, routers and peripheral equipment: CPU
register file A register file is an array of processor registers in a central processing unit (CPU). Register banking is the method of using a single name to access multiple different physical registers depending on the operating mode. Modern integrated circuit- ...
s, internal
CPU cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, whi ...
s and external burst mode SRAM caches,
hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magn ...
buffers, router buffers, etc. LCD screens and printers also normally employ SRAM to hold the image displayed (or to be printed). SRAM was used for the main memory of most early personal computers such as the
ZX80 The Sinclair ZX80 is a home computer launched on 29 January 1980 by Science of Cambridge Ltd. (later to be better known as Sinclair Research). It is notable for being one of the first computers available in the United Kingdom for less than a ...
, TRS-80 Model 100, and
VIC-20 The VIC-20 (known as the VC-20 in Germany and the VIC-1001 in Japan) is an 8-bit home computer that was sold by Commodore Business Machines. The VIC-20 was announced in 1980, roughly three years after Commodore's first personal computer, the ...
.


Hobbyists

Hobbyists, specifically home-built processor enthusiasts, often prefer SRAM due to the ease of interfacing. It is much easier to work with than DRAM as there are no refresh cycles and the address and data buses are often directly accessible. In addition to buses and power connections, SRAM usually requires only three controls: Chip Enable (CE), Write Enable (WE) and Output Enable (OE). In synchronous SRAM, Clock (CLK) is also included.


Types of SRAM


Non-volatile SRAM

Non-volatile SRAM (nvSRAM) has standard SRAM functionality, but they save the data when the power supply is lost, ensuring preservation of critical information. nvSRAMs are used in a wide range of situationsnetworking, aerospace, and medical, among many otherswhere the preservation of data is critical and where batteries are impractical.


Pseudostatic RAM

Pseudostatic RAM (PSRAM) is DRAM combined with a self-refresh circuit. It appears externally as slower SRAM, albeit with a density/cost advantage over true SRAM, and without the access complexity of DRAM.


By transistor type

*
Bipolar junction transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipola ...
(used in TTL and ECL) very fast but with high power consumption *
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(used in
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSF ...
) low power and very common today


By flip-flop type

* Binary SRAM * Ternary SRAM


By function

*
Asynchronous Asynchrony is the state of not being in synchronization. Asynchrony or asynchronous may refer to: Electronics and computing * Asynchrony (computer programming), the occurrence of events independent of the main program flow, and ways to deal wit ...
independent of clock frequency; data in and data out are controlled by address transition. Examples include the ubiquitous 28-pin 8K × 8 and 32K × 8 chips (often but not always named something along the lines of 6264 and 62C256 respectively), as well as similar products up to 16 Mbit per chip. * Synchronous all timings are initiated by the clock edges. Address, data in and other control signals are associated with the clock signals. In the 1990s, asynchronous SRAM used to be employed for fast access time. Asynchronous SRAM was used as
main memory Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a comput ...
for small cache-less embedded processors used in everything from
industrial electronics Power electronics is the application of electronics to the control and conversion of electric power. The first high-power electronic devices were made using mercury-arc valves. In modern systems, the conversion is performed with semiconducto ...
and
measurement system A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement i ...
s to
hard disk A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magn ...
s and networking equipment, among many other applications. Nowadays, synchronous SRAM (e.g. DDR SRAM) is rather employed similarly to synchronous DRAM
DDR SDRAM Double Data Rate Synchronous Dynamic Random-Access Memory (DDR SDRAM) is a double data rate (DDR) synchronous dynamic random-access memory (SDRAM) class of memory integrated circuits used in computers. DDR SDRAM, also retroactively called DDR1 ...
memory is rather used than asynchronous DRAM. Synchronous memory interface is much faster as access time can be significantly reduced by employing pipeline architecture. Furthermore, as DRAM is much cheaper than SRAM, SRAM is often replaced by DRAM, especially in the case when a large volume of data is required. SRAM memory is, however, much faster for random (not block / burst) access. Therefore, SRAM memory is mainly used for
CPU cache A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, whi ...
, small on-chip memory, FIFOs or other small buffers.


By feature

*
Zero bus turnaround 0 (zero) is a number representing an empty quantity. In place-value notation such as the Hindu–Arabic numeral system, 0 also serves as a placeholder numerical digit, which works by multiplying digits to the left of 0 by the radix, u ...
(ZBT) the turnaround is the number of clock cycles it takes to change access to the SRAM from ''write'' to ''read'' and vice versa. The turnaround for ZBT SRAMs or the latency between read and write cycle is zero. * syncBurst (syncBurst SRAM or synchronous-burst SRAM) features synchronous burst write access to the SRAM to increase write operation to the SRAM. * DDR SRAM synchronous, single read/write port, double data rate I/O. * Quad Data Rate SRAM synchronous, separate read and write ports, quadruple data rate I/O.


Integrated on chip

SRAM may be integrated as RAM or cache memory in micro-controllers (usually from around 32 bytes up to 128 
kilobyte The kilobyte is a multiple of the unit byte for digital information. The International System of Units (SI) defines the prefix '' kilo'' as 1000 (103); per this definition, one kilobyte is 1000 bytes.International Standard IEC 80000-13 Quant ...
s), as the primary caches in powerful microprocessors, such as the x86 family, and many others (from 8  KB, up to many megabytes), to store the registers and parts of the state-machines used in some microprocessors (see
register file A register file is an array of processor registers in a central processing unit (CPU). Register banking is the method of using a single name to access multiple different physical registers depending on the operating mode. Modern integrated circuit- ...
), on
application-specific integrated circuit An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-effici ...
s (ASICs) (usually in the order of kilobytes) and in
field-programmable gate array A field-programmable gate array (FPGA) is an integrated circuit designed to be configured by a customer or a designer after manufacturinghence the term ''Field-programmability, field-programmable''. The FPGA configuration is generally specifi ...
s (FPGAs) and complex programmable logic devices (CPLDs).


Design

A typical SRAM cell is made up of six
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
s, and is often called a SRAM cell. Each bit in the cell is stored on four
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch ...
s (M1, M2, M3, M4) that form two cross-coupled inverters. This storage cell has two stable states which are used to denote 0 and 1. Two additional ''access'' transistors serve to control the access to a storage cell during read and write operations. In addition to 6T SRAM, other kinds of SRAM chips use 4, 8, 10 (4T, 8T, 10T SRAM), or more transistors per bit. Four-transistor SRAM is quite common in stand-alone SRAM devices (as opposed to SRAM used for CPU caches), implemented in special processes with an extra layer of
polysilicon Polycrystalline silicon, or multicrystalline silicon, also called polysilicon, poly-Si, or mc-Si, is a high purity, polycrystalline form of silicon, used as a raw material by the solar photovoltaic and electronics industry. Polysilicon is produce ...
, allowing for very high-resistance pull-up resistors. The principal drawback of using 4T SRAM is increased
static power Static may refer to: Places *Static Nunatak, a nunatak in Antarctica United States *Static, Kentucky and Tennessee *Static Peak, a mountain in Wyoming ** Static Peak Divide, a mountain pass near the peak Science and technology Physics *Static el ...
due to the constant current flow through one of the pull-down transistors (M1 or M2). This is sometimes used to implement more than one (read and/or write) port, which may be useful in certain types of
video memory Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxid ...
and
register file A register file is an array of processor registers in a central processing unit (CPU). Register banking is the method of using a single name to access multiple different physical registers depending on the operating mode. Modern integrated circuit- ...
s implemented with multi-ported SRAM circuitry. Generally, the fewer transistors needed per cell, the smaller each cell can be. Since the cost of processing a silicon wafer is relatively fixed, using smaller cells and so packing more bits on one wafer reduces the cost per bit of memory. Memory cells that use fewer than four transistors are possible; however, such 3T or 1T cells are DRAM, not SRAM (even the so-called
1T-SRAM 1T-SRAM is a pseudo-static random-access memory (PSRAM) technology introduced by MoSys, Inc. in September 1998, which offers a high-density alternative to traditional static random-access memory (SRAM) in embedded memory applications. Mosys use ...
). Access to the cell is enabled by the word line (WL in figure) which controls the two ''access'' transistors M5 and M6 which, in turn, control whether the cell should be connected to the bit lines: BL and BL. They are used to transfer data for both read and write operations. Although it is not strictly necessary to have two bit lines, both the signal and its inverse are typically provided in order to improve
noise margin In electrical engineering, noise margin is the maximum voltage amplitude of extraneous signal that can be algebraically added to the noise-free worst-case input level without causing the output voltage to deviate from the allowable logic voltage le ...
s. During read accesses, the bit lines are actively driven high and low by the inverters in the SRAM cell. This improves SRAM bandwidth compared to DRAMs in a DRAM, the bit line is connected to storage capacitors and
charge sharing Charge sharing is an effect of signal degradation through transfer of charges from one electronic domain to another. Charge sharing in semiconductor radiation detectors In pixelated semiconductor radiation detectors - such as photon-counting or ...
causes the bit line to swing upwards or downwards. The symmetric structure of SRAMs also allows for
differential signaling Differential signalling is a method for electrically transmitting information using two complementary signals. The technique sends the same electrical signal as a differential pair of signals, each in its own conductor. The pair of conduc ...
, which makes small voltage swings more easily detectable. Another difference with DRAM that contributes to making SRAM faster is that commercial chips accept all address bits at a time. By comparison, commodity DRAMs have the address multiplexed in two halves, i.e. higher bits followed by lower bits, over the same package pins in order to keep their size and cost down. The size of an SRAM with address lines and data lines is words, or bits. The most common word size is 8 bits, meaning that a single byte can be read or written to each of different words within the SRAM chip. Several common SRAM chips have 11 address lines (thus a capacity of 2 k words) and an 8-bit word, so they are referred to as "2k × 8 SRAM". The dimensions of an SRAM cell on an IC is determined by the
minimum feature size Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) chips such as modern computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are pr ...
of the process used to make the IC.


SRAM operation

An SRAM cell has three different states: ''standby'' (the circuit is idle), ''reading'' (the data has been requested) or ''writing'' (updating the contents). SRAM operating in read and write modes should have "readability" and "write stability", respectively. The three different states work as follows:


Standby

If the word line is not asserted, the ''access'' transistors M5 and M6 disconnect the cell from the bit lines. The two cross-coupled inverters formed by M1M4 will continue to reinforce each other as long as they are connected to the supply.


Reading

In theory, reading only requires asserting the word line WL and reading the SRAM cell state by a single access transistor and bit line, e.g. M6, BL. However, bit lines are relatively long and have large
parasitic capacitance Parasitic capacitance is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other. When two electrical conductors at different voltages ...
. To speed up reading, a more complex process is used in practice: The read cycle is started by precharging both bit lines BL and BL, to high (logic 1) voltage. Then asserting the word line WL enables both the access transistors M5 and M6, which causes one bit line BL voltage to slightly drop. Then the BL and BL lines will have a small voltage difference between them. A sense amplifier will sense which line has the higher voltage and thus determine whether there was 1 or 0 stored. The higher the sensitivity of the sense amplifier, the faster the read operation. As the NMOS is more powerful, the pull-down is easier. Therefore, bit lines are traditionally precharged to high voltage. Many researchers are also trying to precharge at a slightly low voltage to reduce the power consumption.


Writing

The write cycle begins by applying the value to be written to the bit lines. If we wish to write a 0, we would apply a 0 to the bit lines, i.e. setting BL to 1 and BL to 0. This is similar to applying a reset pulse to an SR-latch, which causes the flip flop to change state. A 1 is written by inverting the values of the bit lines. WL is then asserted and the value that is to be stored is latched in. This works because the bit line input-drivers are designed to be much stronger than the relatively weak transistors in the cell itself so they can easily override the previous state of the cross-coupled inverters. In practice, access NMOS transistors M5 and M6 have to be stronger than either bottom NMOS (M1, M3) or top PMOS (M2, M4) transistors. This is easily obtained as PMOS transistors are much weaker than NMOS when same sized. Consequently, when one transistor pair (e.g. M3 and M4) is only slightly overridden by the write process, the opposite transistors pair (M1 and M2) gate voltage is also changed. This means that the M1 and M2 transistors can be easier overridden, and so on. Thus, cross-coupled inverters magnify the writing process.


Bus behavior

RAM with an access time of 70 ns will output valid data within 70 ns from the time that the address lines are valid. Some SRAM cells have a "page mode", where words of a page (256, 512, or 1024 words) can be read sequentially with a significantly shorter access time (typically approximately 30 ns). The page is selected by setting the upper address lines and then words are sequentially read by stepping through the lower address lines.


Production challenges

With the introduction of the
FinFET A fin field-effect transistor (FinFET) is a multigate device, a MOSFET (metal-oxide-semiconductor field-effect transistor) built on a substrate where the gate is placed on two, three, or four sides of the channel or wrapped around the channel, ...
transistor implementation of SRAM cells, they started to suffer from increasing inefficiencies in cell sizes. Over the last 30 years (from 1987 to 2017) with a steadily decreasing transistor size (node size) the footprint-shrinking of the SRAM cell topology itself slowed down, making it harder to pack the cells more densely. Besides issues with size a significant challenge of modern SRAM cells is a static current leakage. The current, that flows from positive supply (Vdd), through the cell, and to the ground, increases exponentially when the cell's temperature rises. The cell power drain occurs in both active and idle states, thus wasting useful energy without any useful work done. Even though in the last 20 years the issue was partially addressed by the Data Retention Voltage technique (DRV) with reduction rates ranging from 5 to 10, the decrease in node size caused reduction rates to fall to about 2. With these two issues it became more challenging to develop energy-efficient and dense SRAM memories, prompting semiconductor industry to look for alternatives such as STT-MRAM and
F-RAM Ferroelectric RAM (FeRAM, F-RAM or FRAM) is a random-access memory similar in construction to DRAM but using a ferroelectric layer instead of a dielectric layer to achieve non-volatility. FeRAM is one of a growing number of alternative non- ...
.


Research

In 2019 a French institute reported on a research of an
IoT The Internet of things (IoT) describes physical objects (or groups of such objects) with sensors, processing ability, software and other technologies that connect and exchange data with other devices and systems over the Internet or other comm ...
-purposed 28nm fabricated IC. It was based on fully depleted silicon on insulator-transistors (FD-SOI), had two-ported SRAM memory rail for synchronous/asynchronous accesses, and selective
virtual ground In electronics, a virtual ground (or virtual earth) is a node of a circuit that is maintained at a steady reference potential, without being connected directly to the reference potential. In some cases the reference potential is considered to be th ...
(SVGND). The study claimed reaching an ultra-low SVGND current in a "sleep" and read modes by finely tuning its voltage.


See also

*
Flash memory Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both use ...
*
Miniature Card The Miniature Card or MiniCard is a flash or SRAM memory card standard first promoted by Intel in 1995. The card was backed by Advanced Micro Devices, Fujitsu and Sharp Electronics. They are no longer manufactured. The Miniature Card Implementer ...
, a discontinued SRAM memory card standard * In-memory processing


References

{{DEFAULTSORT:Static Random Access Memory Computer memory Types of RAM