SRAM - STATIC RAM

A computer has two types of memory devices: static and dynamic. SRAM (static random access memory) is a type of volatile memory that retains its input values as long as the power is ON. Data in the SRAM need not be refreshed constantly. It is different from RAM and DRAM. It is typically used in the system’s cache memory.

Working of SRAM

SRAM uses a flip-flop to hold bits of memory. Each flip-flop has four transistors that are cross-coupled to form inverters. SRAM uses the 0 and 1 concept to store and release data. Since SRAM accepts input data at the same time, it is fast.

Types of SRAM

Classification is done based on their functioning and the type of transistors used, according to memory size, single density or DDR if SRAMs are used in packs (a 4 pack is a quad).

Functioning: There are two types: synchronous and asynchronous. Asynchronous SRAM has a sequential pattern of READ and WRITE operation, while the synchronous SRAM has an overlapping READ and WRITE operation.
Type of transistor used: CMOS (complementary metal oxide semiconductor) and bi-polar. The bipolar type consumes a lot a power and is very fast. However, it has become obsolete now.

Selecting a SRAM:

The following parameters are important:

§ Memory capacity: memory size in megabytes. SRAMs are available in sizes of 8, 16, 32 and 128 MB.

§ SRAM read cycle: time required to read data (e.g. 40 nanoseconds).

§ SRAM write cycle: time required to write data (e.g. 17 nanoseconds).

Applications of SRAM

SRAM is faster than other RAMs and does not have memory leaks. However, it consumes relatively more power then a DRAM. Some of the areas where they are used are:

§ Level1 cache and Level2 cache memory of CPUs, digital signal processing circuits, signal rangers, mobile devices such as cellular phones and PDAs.

§ They are also used in high-speed data transfer, network equipment and testers, RISC microcontroller.