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Encyclopedia > Static random access memory
Computer memory types
Volatile
Non-Volatile

Static random access memory (SRAM) is a type of semiconductor memory. The word "static" indicates that the memory retains its contents as long as power remains applied, unlike dynamic RAM (DRAM) that needs to be periodically refreshed (nevertheless, SRAM should not be confused with read-only memory and flash memory, since it is volatile memory and preserves data only while power is continuously applied). SRAM should not be confused with SDRAM, which stands for synchronous DRAM and is entirely different from SRAM, or with pseudostatic RAM (PSRAM), which is DRAM disguised as SRAM. The terms storage (U.K.) or memory (U.S.) refer to the parts of a digital computer that retain physical state (data) for some interval of time, possibly even after electrical power to the computer is turned off. ... Volatile memory refers to computer memory that must be powered to maintain its data. ... Dynamic random access memory (DRAM) is a type of random access memory that stores each bit of data in a separate capacitor within an integrated circuit. ... eDRAM stands for embedded DRAM, a capacitor-based dynamic random access memory usually integrated on the same die or in the same package as the main ASIC or processor, as opposed to external DRAM modules and transistor-based SRAM typically used for caches. ... 1T-SRAM is MoSyss implementation of embedded-DRAM on a conventional digital-logic (standard-cell) ASIC process. ... Z-RAM, short for zero capacitor DRAM is a new type of computer memory in development by Innovative Silicon Inc. ... TTRAM, short for Twin Transistor RAM is new type of computer memory in development by Renesas. ... The Williams tube or (more accurately) the Williams-Kilburn tube (after Freddie Williams and coworker Tom Kilburn), developed about 1946 or 1947, was a cathode ray tube used to store electronic data. ... Mercury memory of UNIVAC I (1951) Delay line memory was a form of computer memory used on some of the earliest digital computers, such as the EDSAC and UNIVAC I. The basic concept of the delay line originated with World War II radar research, specifically to reduce clutter from reflections... The Selectron was an early form of computer memory developed by RCA. Development started in 1946 with a planned production of 200 by the end of the year, but production problems meant that they were still not available by the middle of 1948. ... It has been suggested that this article or section be merged with NVRAM. (Discuss) Non-volatile memory, nonvolatile memory, NVM or non-volatile storage, is computer memory that can retain the stored information even when not powered. ... Read-only memory (usually known by its acronym, ROM) is a class of storage media used in computers and other electronic devices. ... D23128C PROM on the board of ZX Spectrum A programmable read-only memory (PROM) or field programmable read-only memory (FPROM) is a form of digital memory where the setting of each byte is locked by a fuse or antifuse. ... Read-only memory (ROM) is used as a storage medium in computers. ... EPROM. The small quartz window admits UV light during erasure. ... An EEPROM (also called an E2PROM)[] or Electronically Erasable Programmable Read-Only Memory, is a non-volatile storage chip used in computers and other devices to store small amounts of volatile (configuration) data. ... A USB flash drive. ... Ferroelectric RAM (FRAM or FeRAM) is a type of non-volatile computer memory, similar to EEPROM but based on electric field orientation and with near-unlimited number (exceeding 1010 for 5V devices and even more for 3. ... Magnetoresistive Random Access Memory (MRAM) is a non-volatile computer memory (NVRAM) technology, which has been in development since the 1990s. ... Phase-change memory (also known as PCM, PRAM, Ovonic Unified Memory and Chalcogenide RAM [C-RAM]) is a type of non-volatile computer memory. ... Wikipedia does not have an article with this exact name. ... RRAM or Resistive Random Access Memory is a new non-volatile memory type begin developed by Sharp, Samsung, Fujitsu, Spansion, Macronix, Winbond and other companies. ... Nano-RAM, is a proprietary computer memory technology from the company Nantero. ... hi i am cool xbox is all most as cool as me hi again ... A 16×16 cm area core memory plane of 128×128 bits, i. ... Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles, which each store one bit of data. ... A semiconductor is a fuckin solid whose electrical conductivity is in between that of a metal and that of an insulator, and can be controlled over a wide range, either permanently or dynamically. ... Dynamic random access memory (DRAM) is a type of random access memory that stores each bit of data in a separate capacitor within an integrated circuit. ... Memory refresh is the process of periodically reading information from an area of computer memory, and immediately rewriting the read information to the same area with no modifications. ... Read-only memory (usually known by its acronym, ROM) is a class of storage media used in computers and other electronic devices. ... A USB flash drive. ... Volatile memory refers to computer memory that must be powered to maintain its data. ... This article or section does not cite any references or sources. ... DRAM is a type of random access memory that stores each bit of data in a separate capacitor. ...

Contents

Design

A six-transistor CMOS SRAM cell.
A six-transistor CMOS SRAM cell.

Random access means that locations in the memory can be written to or read from in any order, regardless of the memory location that was last accessed. Image File history File links Download high resolution version (925x788, 20 KB) Description: Diagram of a 6 transistor SRAM cell. ... Image File history File links Download high resolution version (925x788, 20 KB) Description: Diagram of a 6 transistor SRAM cell. ... In computer science, random access is the ability to access a random element of a group in equal time. ...


Each bit in an SRAM is stored on four transistors 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. It thus typically takes six MOSFETs to store one memory bit. This article is about the unit of information. ... Assorted discrete transistors A transistor is a semiconductor device, commonly used as an amplifier or an electrically controlled switch. ... Negation, in its most basic sense, changes the truth value of a statement to its opposite. ... The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is by far the most common field-effect transistor in both digital and analog circuits. ...


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. While it's not strictly necessary to have two bit lines, both the signal and its inverse are typically provided since it improves noise margins. In electrical engineering, noise margin is the amount by which a signal exceeds the minimum amount for proper operation. ...


During read accesses, the bit lines are actively driven high and low by the inverters in the SRAM cell. This improves SRAM speed compared to DRAMs—in a DRAM, the bit line is connected to storage capacitors and charge sharing causes the bitline to swing upwards or downwards. The symmetric structure of SRAMs also allows for differential signalling, 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. Dynamic random access memory (DRAM) is a type of random access memory that stores each bit of data in a separate capacitor within an integrated circuit. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ... It has been suggested that High-voltage differential signalling be merged into this article or section. ...


The size of an SRAM with m address lines and n data lines is 2m words, or 2m × n bits.


SRAM operation

A SRAM cell has three different states it can be in: standby where the circuit is idle, reading when the data has been requested and writing when updating the contents. 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 M1 – M4 will continue to reinforce each other as long as they are disconnected from the outside world.


Reading

Assume that the content of the memory is a 1, stored at Q. The read cycle is started by precharging both the bit lines to a logical 1, then asserting the word line WL, enabling both the access transistors. The second step occurs when the values stored in Q and Q are transferred to the bit lines by leaving BL at its precharged value and discharging BL through M1 and M5 to a logical 0. On the BL side, the transistors M4 and M6 pull the bit line toward VDD, a logical 1. If the content of the memory was a 0, the opposite would happen and BL would be pulled toward 1 and BL toward 0.


Writing

The start of a 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 a 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. Note that the reason this works is that the bit line input-drivers are designed to be much stronger than the relatively weak transistors in the cell itself, so that they can easily override the previous state of the cross-coupled inverters. Careful sizing of the transistors in a SRAM cell is needed to ensure proper operation. In electronics, a latch is data storage device used to store information in asynchronous sequential logic systems. ...


Bus behaviour

A ram memory with an access time of 70 ns will output valid data within 70 ns from the time that the address lines are valid. But the data will remain for a hold time aswell (5-10ns). Rise and fall time also affect (~5ns). By reading the lower part of an address range bits in sequence (page cycle) one can read with significantly shorter access time (30ns). [1]


Applications and Use

Characteristics

SRAM is a little more expensive, but faster and significantly less power hungry (especially idle) than DRAM. It is therefore used where either speed or low power, or both, are of prime interest. SRAM is also easier to control (interface to) and generally more truly random access than modern types of DRAM. Due to a more complex internal structure, SRAM is less dense than DRAM and is therefore not used for high-capacity, low-cost applications such as the main memory in personal computers. Dram can mean several things: Dram (unit), an imperial unit of volume Dram, an imperial unit of weight or mass, see avoirdupois and apothecaries system Ottoman dram, a unit of weight, see dirhem Armenian dram, a monetary unit DRAM, a type of RAM Category: ... The terms storage (U.K.) or memory (U.S.) refer to the parts of a digital computer that retain physical state (data) for some interval of time, possibly even after electrical power to the computer is turned off. ...


Clock speed and power

The power consumption of SRAM varies widely depending on how frequently it is accessed; it can be as power-hungry as dynamic RAM, when used at high frequencies, and some ICs can consume many watts at full speed. On the other hand, static RAM used at a somewhat slower pace, such as in applications with moderately clocked microprocessors, draw very little power and can have a nearly negligible power consumption when sitting idle — in the region of a few microwatts. For delivered electrical power, see Electrical power industry. ... Integrated circuit of Atmel Diopsis 740 System on Chip showing memory blocks, logic and input/output pads around the periphery Microchips with a transparent window, showing the integrated circuit inside. ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ...


Static RAM exists primarily as:

  • general purpose products
    • with asynchronous interface, such as the 28 pin 32Kx8 chips (usually named XXC256), and similar products up to 16 Mbit per chip
    • with synchronous interface, usually used for caches and other applications requiring burst transfers, up to 18 Mbit (256Kx72) per chip
  • integrated on chip
    • as RAM or cache memory in microcontrollers (usually from around 32 bytes up to 128 kibibytes)
    • as the primary caches in powerful microprocessors, such as the x86 family, and many others (from 8 KiB, up to several mebibytes)
    • on application specific ICs, or ASICs (usually in the order of kibibytes)
    • in FPGAs and CPLDs (usually in the order of a few kilobytes or less)

It may be noted that cpu registers and parts of the state-machines used in microprocessors are also often (not always!) built around static RAM. A kibibyte (a contraction of kilo binary byte) is a unit of information or computer storage, commonly abbreviated KiB (never kiB). 1 kibibyte = 210 bytes = 1,024 bytes The kibibyte is closely related to the kilobyte, which can be used either as a synonym for kibibyte or to refer to... x86 or 80x86 is the generic name of a microprocessor architecture first developed and manufactured by Intel. ... A kibibyte (a contraction of kilo binary byte) is a unit of information or computer storage, commonly abbreviated KiB (never kiB). 1 kibibyte = 210 bytes = 1,024 bytes The kibibyte is closely related to the kilobyte, which can be used either as a synonym for kibibyte or to refer to... The acronym ASIC, depending on context, may stand for: Application-specific integrated circuit ASIC programming language Australian Securities and Investments Commission This is a disambiguation page — a navigational aid which lists pages that might otherwise share the same title. ... A field-programmable gate array or FPGA is a gate array that can be reprogrammed after it is manufactured, rather than having its programming fixed during the manufacturing — a programmable logic device. ... An Altera MAX 7000-series CPLD with 2500 gates. ...


Embedded use

Many categories of industrial and scientific subsystems, automotive electronics, and similar, contains static RAM. Some amounts (kibibytes or less) is also embedded in practically all modern appliances, toys, etc that implements an electronic user interface. Several mebibytes may be used in complex products such as digital cameras, cell phones, synthesizers, etc.


SRAM in its dual-ported form is sometimes used for realtime digital signal processing circuits. Dual-ported RAM (DPRAM) is a type of Random Access Memory that allows multiple reads or writes to occur at the same time, or nearly the same time, unlike single-ported RAM which only allows one access at a time. ... Digital signal processing (DSP) is the study of signals in a digital representation and the processing methods of these signals. ... An electronic circuit is an electrical circuit that also contains active electronic devices such as transistors or vacuum tubes. ...


In computers

SRAM is also used in personal computers, workstations, routers and peripheral equipment: internal CPU caches and external burst mode SRAM caches, hard disk buffers, router buffers, etc. LCD screens and printers also normally employ static RAM to hold the image displayed (or to be printed). Small SRAM buffers are also found in CDROM and CDRW drives; usually 256 KiB or more are used to buffer track data, which is transferred in blocks instead of as single values. The same applies to cable modems and similar equipment connected to computers. The so called "CMOS RAM" on PC motherboards was originally a battery-powered SRAM chip, but is today more often implemented using EEPROM or Flash. Diagram of a CPU memory cache A CPU cache is a cache used by the central processing unit of a computer to reduce the average time to access memory. ... Burst mode (alternatively burst-mode, with an hyphen) is a generic computing term referring to any situation in which a device is transmitting data repeatedly without waiting for input from another device or waiting for an internal process to terminate before continuing the transfer of data. ... Typical hard drives of the mid-1990s. ... Cisco 1800 Router ERS-8600 In simple layman terms, a router is a device that determines the proper path for data to travel between different networks. ... ... This does not cite any references or sources. ... The CD-ROM (an abbreviation for Compact Disc Read-Only Memory (ROM)) is a non-volatile optical data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive. ... Compact Disc ReWritable (CD-RW) is a rewritable optical disc format. ... A cable modem is a special type of modem that is designed to modulate a data signal over cable television infrastructure. ... Symbols representing a single Cell (top) and Battery (bottom), used in circuit diagrams. ... An EEPROM (also called an E2PROM)[] or Electronically Erasable Programmable Read-Only Memory, is a non-volatile storage chip used in computers and other devices to store small amounts of volatile (configuration) data. ... A USB flash drive. ...


Hobbyists

Hobbyists 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 directly accessible rather than multiplexed. In addition to buses and power connections, SRAM usually require only three controls: Chip Enable (CE), Write Enable (WE) and Output Enable (OE). Dram can mean several things: Dram (unit), an imperial unit of volume Dram, an imperial unit of weight or mass, see avoirdupois and apothecaries system Ottoman dram, a unit of weight, see dirhem Armenian dram, a monetary unit DRAM, a type of RAM Category: ...


Types of SRAM

By transistor type

A bipolar junction transistor (BJT) is a type of transistor. ... A Motorola 68000-based computer with various TTL chips. ... In electronics, emitter coupled logic, or ECL, is a logic family in which current is steered through bipolar transistors to compute logical functions. ... The metal-oxide-semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is by far the most common field-effect transistor in both digital and analog circuits. ... Static CMOS Inverter Complementary metal–oxide–semiconductor (CMOS) (see-moss, IPA: ), is a major class of integrated circuits. ...

By function

  • Asynchronous — independent of clock frequency; data in and data out are controlled by address transition
  • Synchronous — all timings are initiated by the clock edge(s). Address, data in and other control signals are associated with the clock signals

Asynchrony is the state of not being synchronized. ... Synchronization is coordination with respect to time. ...

By feature

  • ZBT (ZBT stands for zero bus turnaround) — 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 speed up write operation to the SRAM.

See also

Dram can mean several things: Dram (unit), an imperial unit of volume Dram, an imperial unit of weight or mass, see avoirdupois and apothecaries system Ottoman dram, a unit of weight, see dirhem Armenian dram, a monetary unit DRAM, a type of RAM Category: ... Two types of DIMMs: a 168-pin SDRAM module (top) and a 184-pin DDR SDRAM module (bottom). ... A USB flash drive. ...

External links

  • SRAM vs. Other Memory Types

References

  1. ^ Tentative Toshiba mos digital integrated circuit silicon gate cmos 4,194,304-word by 16-bit cmos pseudo static RAM. 070731 toshiba.com
  • Digital Integrated Circuits - a design perspective. J. M. Rabaey, A. Chandrakasan, B. Nikolić. Prentice Hall, 2003. ISBN 0-13-120764-4.

  Results from FactBites:
 
What is SRAM? - a definition from Whatis.com - see also: static random access memory (179 words)
SRAM (static RAM) is random access memory (RAM) that retains data bits in its memory as long as power is being supplied.
Unlike dynamic RAM (DRAM), which stores bits in cells consisting of a capacitor and a transistor, SRAM does not have to be periodically refreshed.
SRAM is used for a computer's cache memory and as part of the random access memory digital-to-analog converter on a video card.
Quasi-static random access memory - Patent 6975532 (3898 words)
A quasi-static random access memory cell as set forth in claim 1, further comprising a bitline conditioning circuit for conditioning the bitline and bitline complement.
A quasi-static random access memory cell as set forth in claim 5, where said bitline conditioning circuit precharges the bitline and the bitline complement.
A further object of the present invention is the provision of a quasi-static random access memory which eliminates READ mode operation instability caused by random mismatches of threshold voltages between devices comprising the memory.
  More results at FactBites »

 
 

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