In This Article
What is Static Random Access Memory (SRAM)?
Static Random Access Memory refers to a special type of volatile RAM that stores every bit by using a latching circuitry. It stores data in static form which stays till it has the power and does not need to be refreshed.
Technically, a Static RAM holds a bit of data on 4 transistors by using two cross-coupled inverters.
KEY TAKEAWAYS
- The Static RAM does not need to be refreshed continuously like the Dynamic RAM.
- The Static RAM stores each bit of data on four transistors with two cross-coupled inverters. The stable states of the SRAM are characterized by 0 and 1.
- There are another two access transistors used during the read and write operations for managing the availability to the memory cells.
Understanding SRAM (Static Random Access Memory
SRAM, pronounced as S-RAM and not ‘sram,’ is a special type of RAM that stores data and information in a static form.
This means that it is stored in the memory as long as it has power.
An SRAM or Static Random Access Memory in a computer is usually meant to meet two specific requirements. These are:
- To offer a direct interface with the Central Processing Unit and that too at a speed which is usually not achievable by the Dynamic RAM and
- To replace the DRAM in the computer to ensure low power consumption.
To play the first role comprehensively, the SRAM acts as a cache memory which interfaces between the memory and the CPU.
As for the second role played by the SRAM, its primary driving force is the low power applications.
It is for this low power consumption of the Static RAM technology that makes it so useful for the portable equipment.
It is very easy to use Static RAM in comparison to a Dynamic RAM because it typically needs only three controls such as:
- Chip Enable or CE
- Write Enable or WE and
- Output Enable or OE.
If it is a synchronous SRAM, then you also have the usage benefit of the Clock (CLK).
Also, the refresh current of the Dynamic RAM is higher than the low power standby current of the Static RAM by several orders of magnitude.
Since the Static RAM does not need to be refreshed or paused in between accesses for that, it reduces its cycle time significantly in comparison to the cycle time of the DRAM.
And, the relatively low access time of the Static RAM in comparison to the DRAM is also another reason for its high utility.
Typically, the access time of SRAM is about 10 nanoseconds as opposed to about 60 nanoseconds of the DRAM.
There are six MOSFETs or Metal Oxide Semiconductor Field Effect Transistors required by the SRAM to hold each memory bit in it.
The chips with MOSFETs are one of the two types of Static RAMs while the other one is with the bipolar junction transistor.
The bipolar junction transistor is pretty fast in operation but it consumes a lot of power which is why the MOSFET is a more favored SRAM type.
Uses
Ideally, most of the computers today use a Dynamic RAM because it supports higher densities and offers a low cost per megabyte.
Still, the Static RAM is used for other specific purposes.
For example, an SRAM can be a part of the RAMDAC or the RAM Digital to Analog Converter on the graphics or video card of a computer.
An SRAM may also have other specific uses such as:
- It can also be used in the disk drive like a buffer cache
- It can be used in a computer peripheral such as the LCD monitor or the printer and
- It can also be used in a network device such as a switch, modem or a router.
In addition to that, you will find a Static RAM being used in other devices as well such as:
- In cell phones
- In wearables
- In toys
- In other consumer electronics and appliances
- In CDs or Compact Discs
- In DVDs or Digital Versatile Discs
- In digital cameras
- In automobiles
- In industrial equipment and
- In a wide range of Internet of Things or IoT devices.
That is not all. An SRAM is also fund embedded in several different medical products which include and are not limited to:
- The hearing aids
- Body area networks and more.
Therefore, though the SRAM is not suitable for use as a main memory due to its high cost, complicated internal architecture, and size in spite of its higher speed than a DRAM, it is best suited for secondary operations such as storing registers and cache memory.
History
Different types of Static RAMs were invented by different people in different times.
Ideally, this particular memory was the primary driving force behind the new Complementary Metal Oxide Semiconductor or CMOS-based fabrication process and technology since 1959.
This is when the CMOS was invented.
The semiconductor bipolar Static RAM was invented by Robert Norman in 1963 at Fairchild Semiconductor.
Metal Oxide Semiconductor or MOS Static RAM was invented by John Schmidt in 1964 at Fairchild Semiconductor.
This particular model is known as the 64-bit MOS p-channel SRAM.
The hard wired memory cell was invented by Arnold Farber and Eugene Schlig in 1965 while they were working for IBM.
They used a transistor gate, a tunnel diode latch and substituted the latch with two resistors and two transistors.
This configuration was later known as the Farber-Schlig cell.
And, a team headed by Benjamin Agusta at IBM created a 16-bit silicon memory in 1965.
The configuration was based on the Farber-Schlig cell and it constituted 64 resistors, 80 transistors, and 4 diodes.
Characteristics
Apparently, a Static RAM can be characterized as a volatile memory since it needs power to retain the contents but it still shows data remanence.
Apart from that, an SRAM offers a much simpler data access model and most importantly, it does not need periodic refreshing.
It consumes less power when idle but offers higher reliability and performance than DRAM.
However, during the read and write operations, the power consumption may be high and will vary according to the frequency of accessing.
And, it needs more transistors for implementing each bit which makes it more complex and costlier.
The four transistors in the circuit for each separate SRAM memory cell are configured as double cross-coupled inverters.
This format typically has two stable states namely:
- The logical 0 state and
- The logical 1 state.
The MOSFET structure offers a higher level of integration because there is no major issue of heat removal as it is in bipolar transistors.
The two additional access control transistors in the SRAM make a total of six transistors. This makes it a 6T memory cell.
Advantages
Though SRAM is more complex in design and in number of components, it offers some significant advantages. These are:
- Lower static power due to the six transistors which eliminates continuous current flow from any of the pull down resistors and reduces overall power consumption by the chip and
- Higher level of integration and lower circuit design issues due to low power dissipation by the chip.
Though the four transistors Static RAM provides density benefits, the 8T or 10T memory cells may perform additional functions such as using as extra ports in a register file
Applications and Uses
There are several different applications and uses of Static RAM over and above those mentioned earlier such as:
- SRAM is used extensively in embedded systems such as scientific and industrial subsystems as well as in those modern appliances that use an electronic user interface.
- SRAM is also sometimes used in the dual-ported form in the digital and real-time signal processing circuits.
- SRAM is also used in computers, workstations, peripheral equipment, internal CPU caches, CPU register files, and external burst mode caches.
- SRAM is also preferred by hobbyists, especially those home-built processor enthusiasts due to its easy interfacing and direct accessibility of data and address buses.
Typically, an SRAM is used in those areas where low power consumption and high speed are the primary requirements.
Functions of SRAM
The main function of the Static RAM is to allow faster access by storing all data in the flip-flop which contains each bit of this RAM.
The multiple transistors in it allow storing each bit of data by using a bi-stable latching circuitry.
The Static RAM stores every bit of data on four transistors with 2 cross-coupled inverters in two stable states such as 0 and 1, as said earlier.
There are also three different input ports such as:
- BL
- WL and
- BLbar.
Apart from that, there are two output ports namely:
- Q and
- Qbar.
With all these components, the Static RAM has three primary functions which are described as under:
Hold Function
In this specific operation, the two access transistors M5 and M6 are turned OFF or in 0 state. The RAM can hold this state because it has a latching element in it.
Read Function
During this specific operation, the two bit lines namely BL and BLbar are pre-charged to supply voltage. Also, the access transistors are turned ON, or in 1 state.
The sense amplifier detects the difference in voltage between the two bit lines and uses only one of them based on the data value store. This allows detecting the content of the memory.
Write Function
The data to be written in the memory is provided to the bit line in this stage and the access transistors are turned ON. This allows writing the data on the memory.
Where is SRAM in a Computer?
The Static RAM is mainly used as cache memory and is typically found in the hard drives as disc cache.
It is therefore called as a RAM cache or a cache store as well and, as said earlier, it is also found in other place such as in:
- CDs or Compact Discs
- DVDs or Digital Versatile Discs
- Printers
- Modems
- Routers and
- Digital cameras.
The main reason that the Static RAM is used as a cache memory is that it is a lot faster as compared to the Dynamic RAM.
It is also used to make memory caching more effective because in most of the times the programs typically access the similar type of instructions or data over and over.
What is Bit Cell in SRAM?
According to the dictionary, a bit cell refers to a container where a bit or a single binary digit is held either temporarily or permanently.
In a Static RAM the bit cell acts like the fundamental building block of the caches in it.
Every bit cell of a memory is actually a small circuit that has two main elements such as:
- The memory element and
- The selector element.
Here the memory element is used to store data in binary form either a 1 or 0 and the selector element is responsible for activating the particular cell as and when it is accessed.
The characteristics of the bit cell of the Static RAM can be summarized as follows:
- It stores a single bit of information
- It ensures a high memory output
- It meets design provisions for performance
- It takes care of important aspects such as area, stability and leakage.
Depending on the design of the SRAM and its bit cell, each bit in the cell can be stored in four different transistors to create two cross-coupled inverters which eventually form a storage cell with dual stable states.
Conclusion
With all that said and explained about Static Random Access Memory in this article now you know that it is an important component that helps the computer system to work properly.
The best part of this specific type of RAM is that it does not need to be refreshed frequently like a Dynamic RAM.