Memory Cards

Static RAM

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 principle considerations. 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.

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.

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 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 (usually in the order of a few kilobytes or less)

Embedded use

Many categories of industrial and scientific subsystems, automotive electronics, and similar, contains static RAM. Some amounts (kilobytes 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.

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 kB 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.

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).

26.06.2008. 10:50

This article hasn't been commented yet.