SDRAM Memory Basics

- an overview, tutorial about the basics of what is SDRAM, Synchronous Dynamic Random Access Memory and the DDR, DDR2 and DDR3 SDRAM technology that gives further improvements in operating speed.

This overview of the different types of semiconductor memory is split into several pages, each addressing a different semiconductor memory type or 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 memory is the major type of dynamic RAM used across the computing spectrum.

SDRAM operation

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 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.

DDR SDRAM

A further advance in SDRAM memory technology is known as DDR SDRAM, or double data rate SDRAM. This form of SDRAM provides data transfer at twice the speed of traditional SDRAM memory. This is achieved by transferring data twice per cycle, i.e. on both the rising and then the falling edge of the clock signal.

DDR2 SDRAM

A further evolution of DDR SRAM is known as DDR2 SRAM. This can operate the external bus twice as fast as its predecessor. This gives considerable improvements in overall system speed and therefore this form of semiconductor memory was adopted very quickly.

The new DDR2 form of SDRAM memory was first introduced in 2003. At its launch the memory did not outperform the previous DDR SRAM as a result of a latency problem. As a result of standardisation and a resolution of the latency problem new DDR2 memories were able to outperform the existing DDR SRAM memories available and at this point, around 2004, they started to make significant inroads into the semiconductor memory markets.

DDR2 memory is more complicated than its predecessor. The memory cells are activated in a way that enables them to operate with an external bus. As with DDR, DDR2 transfers data at twice the clock speed by transferring data on the rising and falling clock edges, but the bus is clocked at twice the speed of that for DDR. This increase in clock speed is achieved by using a number of interface improvements including what are termed pre-fetch buffers and off-chip drivers. The problem with DDR2 is that the buffers introduce a latency which is twice that of DDR, requiring a doubling of the bus speed to counteract the latency.

The improvements provided by DDR2 come at a cost. As a result of the additional circuitry and the more exacting packaging requirements, DDR2 chips are more expensive than their DDR, or straight SDRAM predecessors.

DDR3 SDRAM

DDR3 is the third generation of DDR SDRAM technology. It brings with it further improvements in overall performance and as a result its use is becoming more widespread.

The new DDR3 SDRAM memories provide a higher bandwidth as a result of an increase in clock rate. They have also been migrated to small fabrication technologies with lower operating voltages (reduced from 1.8 to 1.5 volts) and this reduces their power consumption as well as allowing more memory for a given size of wafer. Thirdly the pre-fetch buffer in the DDR3 technology has been increased to 8 bits thereby increasing their speed of operation.

Further pages from this tutorial
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