SDRAM Memory Basics & Tutorial

- overview, tutorial about the basics of what is SDRAM, Synchronous Dynamic Random Access Memory, its operation & technology.

SDRAM, or Synchronous Dynamic Random Access Memory is a form of semiconductor memory can run at faster speeds than conventional DRAM and is therefore the use of SDRAM is becoming more widespread.

So effective is SDRAM, that it only took about four years after its introduction in 1996/7 before its use had exceeded that of DRAM in PCs because of its greater speed of operation.

Now SDRAM based memory is the major type of dynamic RAM used across the computing spectrum.

SDRAM development

The basic idea behind SDRAM has been in existence for many years. The first ideas appeared as early as the 1970s. The SDRAM concept was also used in some early Intel processors.

One of the first commercial SDRAM offerings was the KM48SL2000 which was introduced by Samsung in 1993. Although this did not gain universal acceptance immediately, the uptake was relatively quick. The improved speed of SDRAM meant that by about the turn of the century, i.e. 2000 SDRAM had virtually replaced the standard DRAM technology in most computer applications.

In order to ensure that SDRAM technology is interchangeable, JEDEC, the industry body for semiconductor standards, adopted its first SDRAM standard in 1993. This facilitated an open common standard for developing SDRAM. It also enabled developers to be able to have the facility of utilising product from more than one manufacturer and having a viable second source option.

With the basic SDRAM established, further develops took place. A form of SDRAM known as double data rate, DDR SDRAM appeared in 2000 with JEDEC Release 1 of their standard 79C which was updated to Release 2 in May 2002 and then Release C in March 2003.

DDR SDRM was followed by the next version named DDR2 SDRAM. It was first introduced in mid 2003 when two clock rates were available: 200 MHz (referred to as PC2-3200) and 266 MHz (PC2-4200). The first offerings of DDR2 SDRAM were inferior to the previous DDR SDRA, but by the end of 2004 its performance had been improved making its performance exceed that of DDR formats.

Later the next version of SDRAM was launched. Known as DDR3 SDRAM, the first prototypes were announced in early 2005. However it took until mid-2007 before the first computer motherboards using DDR3 became available.

Further developments include the next phase of SDRAM which will be DDR4 SDRAM.

SDRAM basics

Traditional forms of memory including DRAM operate in an asynchronous manner. They react to changes as the control inputs change, and also they are only able to operate as the requests are presented to them, dealing with one at a time.

SDRAM is able to operate more efficiently. It is synchronised to the clock of the processor and hence to the bus

With SDRAM having a synchronous interface, it has an internal finite state machine that pipelines incoming instructions. This enables the SDRAM to operate in a more complex fashion than an asynchronous DRAM. This enables it to operate at much higher speeds.

As a result of this SDRAM is capable of keeping two sets of memory addresses open simultaneously. By transferring data alternately from one set of addresses, and then the other, SDRAM cuts down on the delays associated with asynchronous RAM, which must close one address bank before opening the next.

The term pipelining is used to describe the process whereby the SDRAM can accept a new instruction before it has finished processing he previous one. In other words, it can effectively process two instructions at once.

For writing, one write command can be immediately followed by another without waiting for the original data to be stored within the SDRAM memory itself.

For reading the requested data appears a fixed number of clock pulses after the read instruction was presented. It is possible to send additional instructions during the delay period which is termed the latency of the SDRAM.

SDRAM types and development

Since SDRAM was introduced, it has been developed to make it faster and more effective. As a result there are a number of different types of SDRAM that are available.

  • SDR SDRAM:   This is the basic type of SDRAM that was first introduced. It has now been superseded by the other types below. It is referred to as single data rate SDRAM, or just SDRAM.
  • DDR SDRAM:   DDR SDRAM gains its name from the fact that it is Double Data Rate SDRAM. This type of SDRAM provides data transfer at twice the speed of the traditional type of SDRAM memory. This is achieved by transferring data twice per cycle.
  • DDR2 SDRAM:   DDR2 SDRAM can operate the external bus twice as fast as its predecessor and it was first introduced in 2003.
  • DDR3 SDRAM:   DDR3 SDRAM is a further development of the double data rate type of SDRAM. It provides further improvements in overall performance and speed. As a result its use is becoming more widespread.
  • DDR4 SDRAM:   This is a further type of SDRAM being developed and anticipated to be available in 2012.

The various types of SDRAM are now widely used and have taken over from some other types of memory.

By Ian Poole

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