What is SRAM Memory: Static RAM

Static RAM or SRAM is a form of computer memory often used for caches and particularly in microcontrollers and microprocessors


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SRAM or Static Random Access Memory is a form of semiconductor memory widely used in electronics, microprocessor and general computing applications.

This form of computer memory gains its name from the fact that data is held in the memory chip in a static fashion, and does not need to be dynamically updated as in the case of DRAM memory.

While the data in the SRAM memory does not need to be refreshed dynamically, it is still volatile, meaning that when the power is removed from the memory device, the data is not held, and will disappear.

S-RAM has the advantage that it offers better performance than DRAM because DRAM needs to be refreshed periodically when in use, while SRAM does not. However, SRAM is more expensive and less dense than DRAM, so SRAM sizes are orders of magnitude lower than DRAM.

SRAM basics

There are two key features to SRAM - Static Random Access Memory, and these set it out against other types of memory that are available:

  • The data is held statically:   This means that the data is held in the semiconductor memory without the need to be refreshed as long as the power is applied to the memory.
  • SRAM memory is a form of random access memory:   A random access memory is one in which the locations in the semiconductor memory can be written to or read from in any order, regardless of the last memory location that was accessed.

The circuit for an individual SRAM memory cell comprises typically four transistors configured as two cross coupled inverters. In this format the circuit has two stable states, and these equate to the logical "0" and "1" states.

The transistors are MOSFETs because the amount of power consumed by MOS circuit is considerably less than that of bipolar transistor technology which is the other feasible option, but it will consume much more.

Using bipolar technology limits the level fo integration because the issue of heat removal becomes a major issue. In view of this bipolar technology is rarely used.

The four transistor version of the SRAM computer memory cell using four FETs and two resistors
Four transistor version of the SRAM cell

In addition to the four transistors in the basic memory cell, and additional two transistors are required to control the access to the memory cell during the read and write operations.

This makes a total of six transistors, making what is termed a 6T memory cell, although more complex in terms of the number of components it has a number of advantages.

The main advantage of the six transistor SRAM circuit is the reduced static power. In the four transistor version, there is a constant current flow through one or other of the pull down resistors and this increases the overall power consumption of the chip. This can limit the level of integration as well as increasing the circuit design issues as a result of increased power dissipation.

It is also worth mentioning that the four transistor SRAM computer memory cell provides some advantages in terms of its density but this comes at the cost of manufacturing complexity as resistors need to be fabricated and this requires additional processing. Also, the resistors must have small dimensions and large values to meet the requirements for the cell.

The six transistor version of the SRAM computer memory cell using six FETs
Six transistor version of the SRAM cell

Sometimes further transistors are used to give either 8T or 10T memory cells. These additional transistors are used for functions such as implementing additional ports in a register file, etc for the SRAM memory.

Although any three terminal switch device can be used in an SRAM, MOSFETs and in particular CMOS technology is normally used to ensure that very low levels of power consumption are achieved.

With semiconductor memories extending to very large dimensions, each cell must achieve a very low levels of power consumption to ensure that the overall chip does not dissipate too much power.

SRAM memory cell operation

The operation of the SRAM memory cell is relatively straightforward. When the cell is selected, the value to be written is stored in the cross-coupled flip-flops.

The cells are arranged in a matrix, with each cell individually addressable.

Most SRAM memories select an entire row of cells at a time, and read out the contents of all the cells in the row along the column lines.

While it is not necessary to have two bit lines, using the signal and its inverse, this is normal practice which improves the noise margins and improves the data integrity. If the two lines are the same, then the system will understand there is an issue and re-interrogate the cell.

The two bit lines are passed to two input ports on a comparator to enable the advantages of the differential data mode to be accessed, and the small voltage swings that are present can be more accurately detected.

Access to the SRAM memory cell is enabled by the Word Line. This controls the two access control transistors which control whether the cell should be connected to the bit lines. These two lines are used to transfer data for both read and write operations.

SRAM vs DRAM

There is often a debate about the differences and similarities between static RAM and Dynamic RAM. Both are used as forms of computer memory, but their applications and operation is very different.

Comparison of SRAM vs DRAM
 
Attribute Static RAM, SRAM Dynamic RAM
General SRAM uses bistable latching circuitry to store each bit. The term static differentiates it from dynamic RAM or DRAM which must be periodically refreshed. DRAM requires periodic refreshing to retain the data
Applications Generally used for Caches Used for main computer memory
Typical sizes Often up to 16 MB Up to 16GB and beyond
Cell complexity The six transistor version of the SRAM computer memory cell using six FETs Basic dynamic RAM, DRAM memory cell
Cell density Lower than DRAM because of more complex circuitry Much higher than SRAM because only one device is used in the cell
Speed ~x10 that os DRAM Possibly around one tenth that of SRAM

SRAM memory applications

There are many different types of computer memory that are available these days. Choices need to be made regarding the correct memory type for a given application.

Possibly two of the most widely used types are DRAM and SRAM memory, both of which are used as microcontroller / microprocessor and computer memory. Of these two SRAM is more expensive than DRAM. However SRAM is faster and consumes less power especially when idle.

In addition to this SRAM memory is easier to control than DRAM as the refresh cycles do not need to be taken into account, and in addition to this the way SRAM can be accessed is more exactly random access. A further advantage if SRAM is that it is more dense than DRAM.

As a result of these parameters, SRAM memory is used where speed or low power are considerations. Its higher density and less complicated structure also lend it to use in semiconductor memory scenarios where high capacity memory is used, as in the case of the working memory within computers.

SRAM is widely used in microcontrollers where the less complicated operation and low power are needed. Also the higher memory capability of DRAM is not normally needed with microcontrollers.

Microcontrollers which incorporate battery-backed, nonvolatile SRAM are widely used in the embedded computing arena. Unlike Flash memory or EEPROM technology, nonvolatile SRAM has no write limitations, i.e. no limitations on the number of write / read cycles which makes it ideal for real-time data logging applications.



Although SRAM does not have the capabilities of some forms of computer memory, it is nevertheless widely used for applications like handling caches in computers and especially in microcontrollers where is speed, low power and simpler operation lend it to these situations far more than DRAM, Flash and other forms of computer memory.


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