What is SDRAM Memory

SDRAM is the name for any dynamic random-access memory DRAM where the operation the external interface is synchronised by an external clock signal - hence the name synchronous DRAM.

SDRAM Memory Tutorial Includes:
What is SDRAM memory     SDRAM architecture     SDRAM timing & control     DDR / DDR1 SDRAM     DDR2 SDRAM     DDR3 SDRAM     DDR4 SDRAM     JEDEC 79 Standard    

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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.

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.

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