SDRAM, or Synchronous Dynamic Random Access Memory is a form of DRAM semiconductor memory can run at faster speeds than conventional DRAM.
SDRAM memory is widely used in computers and other computing related technology. After SDRAM was introduced, further generations of double data rate RAM have entered the mass market – DDR which is also known as DDR1, DDR2, DDR3 and DDR4.
The use of SDRAM was so effective 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.
Nowadays SDRAM based memory is the major type of dynamic RAM used across the computing spectrum.
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.
What is 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: DDR versions, etc
SDRAM technology underwent a huge amount of development. As a result several successive families of the memory were introduced, each with improved performance over the previous generation.
- 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, also known as DDR1 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.
- DDR4 SDRAM: DDR4 SDRAM was the next generation of DDR SDRAM It provided enhanced performance to meet the demands of the day. It was introduced in the latter half of 2014.
- DDR5 SDRAM: Development of SDRAM technology is moving forwards and the next generation of SDRAM, labelled DDR5 is currently under development. The specification was launched in 2016 with expected first production in 2020. DDR5 will reduce power consumption while doubling bandwidth and capacity.
Further details of each type of DDR SDRAM are given on successive pages.
In view of it large volume use of SDRAM, development is always on-going to ensure that performance stays up with the requirements. SDRAM DDR4 was latest that has been launched, and development is on-going as there is a huge need for ever more effective forms of semiconductor memory.