What is SDRAM Memory: DDR, DDR2, DDR3, DDR4, DDR5

SDRAM or synchronous DRAM is the name for a form of 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    

Memory types:   Memory types & technologies     DRAM     EEPROM     Flash     FRAM     MRAM     Phase change memory     SDRAM     SRAM    

SDRAM, or Synchronous Dynamic Random Access Memory is a form of DRAM semiconductor memory can run at faster speeds than conventional DRAM, and it is widely used as the random access memory in a computer, etc.

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, DDR4 and DDR5 . . . .

Printed circuit board containing computer memory: many later updates of SDRAM exist including DDR, DDR2, DDR3, DDR4, DDR5, etc
Printed circuit board containing computer memory

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 as the main form of computer memory because of its greater speed of operation.

Nowadays SDRAM based memory is the major type of dynamic RAM used across the computing spectrum, and in particular for computer random access memory.

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 once the idea was established.

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 developments 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 SDRAM, 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 was DDR4 SDRAM and this was followed by DDR5.

Advantages & disadvantages of SDRAM

It is always wise to be aware of both the advantages as well as the disadvantages of using any technology before using it in a new electronic circuit design or project.

That said, SDRAM has virtually cornered the market for many computer memory situations and in particular for RAM.

SDRAM offers a number of significant advantages:

  • Simple design
  • Low cost
  • Speed
  • Complex manufacturing process required
  • DDR versions of SDRAM double the data rate of basic SDRAM by using both edges of the clock cycle

In most applications SDRAM has very few disadvantages

  • High power consumption
  • It is a volatile form of memory, i.e. it loses its data when the power is removed
  • Data requires refreshing
  • Slower than SRAM

When selecting any form of computer memory whether it is random access memory or any other function, it is necessary to carefully weight up the advantages and disadvantages.

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.

The main development associated with SDRAM was that a new approach was introduced called DDR - double data rate. Today the different versions of SDRAM are classes according to their DDR version: DDR2, DDR3, DDR4, DDR5, etc.

This form of SDRAM doubles the data rate by transferring data on both edges of the clock, i.e. rising and falling edges. This provides twice the number of opportunities to transfer the data synchronously.

In view of the vast usage of SDRAM chips, the industry organised the standardisation of the operation and interfaces of the chips under the auspice of JEDEC, the oint Electron Device Engineering Council.

  • SDR SDRAM:   This is the basic type of SDRAM that was first introduced and it appeared in 1993. It has now been superseded by the other types below. It is referred to as single data rate SDR SDRAM, or just SDRAM.
  • DDR SDRAM:   DDR SDRAM, also known as DDR1 SDRAM was introduced in 2000 and it gained 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.
    Read more about . . . . DDR / DDR1 SDRAM

  • DDR2 SDRAM:   DDR2 SDRAM was introduced in 2003 and it operated the external bus twice as fast as its predecessor.
    Read more about . . . . DDR2 SDRAM

  • DDR3 SDRAM:   DDR3 SDRAM was introduced in 2007 and it was a further development of the double data rate type of SDRAM. It provides further improvements in overall performance and speed.
    Read more about . . . . DDR3 SDRAM

  • 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.
    Read more about . . . . DDR4 SDRAM

  • 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.

There is always the possibility that someone might try to use different versions of DDR memory together. These would not work because of the offering requirements for the electronic circuit design. To overcome this each DDR generation has a notch cut in the pins at different locations

Summary of the main DDR characteristics

It is worth looking at the different performance characteristics and interface figures for the various versions of SDRAM computer memory.

Standard Supply Voltage Amount of Data Transferred (Words per Clock Cycle) Associated RAM Clock Rates Data Transfer Rate Range
SDRAM) 3.3 V 1 66 – 133 MHz 100 – 166  MT/s
DDR SDRAM 2.6 V 2 100 – 200 MHz 200 – 400  MT/s
DDR2 SDRAM 1.8 V 4 200 – 400 MHz 400 – 1066  MT/s
DDR3 SDRAM 1.5 V 8 400 MHz – 1066 MHz 800 – 2133 MT/s
DDR4 SDRAM 1.2 V 8 1066 – 1600 MHz 1600 – 3200 MT/s

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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