Static random-access memory
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|Computer memory types|
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Static random-access memory (SRAM) is a type of semiconductor memory that uses bistable latching circuitry to store each bit. The term static differentiates it from dynamic RAM (DRAM) which must be periodically refreshed. SRAM exhibits data remanence, but it is still volatile in the conventional sense that data is eventually lost when the memory is not powered.
- 1 Applications and uses
- 2 Types of SRAM
- 3 Design
- 4 SRAM operation
- 5 See also
- 6 References
Applications and uses
- SRAM is more expensive and less dense than DRAM and is therefore not used for high-capacity, low-cost applications such as the main memory in personal computers.
Clock rate 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 bandwidth. On the other hand, static RAM used at a somewhat slower pace, such as in applications with moderately clocked microprocessors, draws very little power and can have a nearly negligible power consumption when sitting idle — in the region of a few micro-watts.
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 micro-controllers (usually from around 32 bytes up to 128 kilobytes)
- as the primary caches in powerful microprocessors, such as the x86 family, and many others (from 8 kB, up to several megabytes)
- to store the registers and parts of the state-machines used in some microprocessors (see register file)
- on application specific ICs, or ASICs (usually in the order of kilobytes)
- in FPGAs and CPLDs
- Many categories of industrial and scientific subsystems, automotive electronics, and similar, contain static RAM.
- Some amount (kilobytes or less) is also embedded in practically all modern appliances, toys, etc. that implement an electronic user interface.
- Several megabytes may be used in complex products such as digital cameras, cell phones, synthesizers, etc.
SRAM is also used in personal computers, workstations, routers and peripheral equipment: CPU register files, 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).
Hobbyists, specifically homebuilt 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 directly accessible rather than multiplexed. 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 SRAMs, or nvSRAMs, have 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 situations—networking, aerospace, and medical, among many others —where the preservation of data is critical and where batteries are impractical.
Asynchronous SRAM are available from 4 Kb to 64 Mb. The fast access time of SRAM makes asynchronous SRAM appropriate as main memory for small cache-less embedded processors used in everything from industrial electronics and measurement systems to hard disks and networking equipment, among many other applications. They are used in various applications like switches and routers, IP-Phones, IC-Testers, DSLAM Cards, to Automotive Electronics.
By transistor type
- Bipolar junction transistor (used in TTL and ECL) — very fast but consumes a lot of power
- MOSFET (used in CMOS) — low power and very common today
- 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
- 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 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 & write ports, quadruple data rate I/O
By flip-flop type
- Binary SRAM
- Ternary SRAM
A typical SRAM cell is made up of six MOSFETs. Each bit in an SRAM is stored on four transistors (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 such six-transistor (6T) SRAM, other kinds of SRAM chips use 4T, 8T, 10T, 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, allowing for very high-resistance pull-up resistors.  Drawback of using 4T SRAM is that current flows continuously from side where 1 is stored (because NMOS of that side is ON). so power dissipation is very high for 4T than 6T.
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.
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 margins.
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 causes the bitline to swing upwards or downwards. The symmetric structure of SRAMs also allows for differential signaling, 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 m address lines and n data lines is 2m words, or 2m × n bits.
An SRAM cell has three different states. It can be in: standby (the circuit is idle), reading (the data has been requested) and writing (updating the contents). The SRAM to operate in read mode and write mode should have "readability" and "write stability" respectively. The three different states work as follows:
- 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 connected to the supply.
- 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 (i. e. eventually discharging through the transistor M1 as it is turned on because the Q is logically set to 1). On the BL side, the transistors M4 and M6 pull the bit line toward VDD, a logical 1 (i. e. eventually being charged by the transistor M4 as it is turned on because Q is logically set to 0). If the content of the memory was a 0, the opposite would happen and BL would be pulled toward 1 and BL toward 0. Then these BL and BL will have a small difference of delta between them and then these lines reach a sense amplifier, which will sense which line has higher voltage and thus will tell whether there was 1 stored or 0. The higher the sensitivity of sense amplifier, the faster the speed of read operation.
- 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 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. 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 an SRAM cell is needed to ensure proper operation.
RAM 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 as well (5-10 ns). Rise and fall times also influence valid timeslots with approximately ~5 ns. By reading the lower part of an address range bits in sequence (page cycle) one can read with significantly shorter access time (30 ns). 
- Sergei Skorobogatov (June 2002). Low temperature data remanence in static RAM. University of Cambridge, Computer Laboratory. Retrieved 2008-02-27.
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- A 160 mV Robust Schmitt Trigger Based Subthreshold SRAM
- United States Patent 6975532: Quasi-static random access memory
- Preston, Ronald P. (2001). "14: Register Files and Caches". The Design of High Performance Microprocessor Circuits. IEEE Press. p. 290.
- United States Patent 6975531: 6F2 3-transistor DRAM gain cell
- 3T-iRAM(r) Technology
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