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Introduction to DDR-2: The DDR Memory Replacement

Introduction to DDR-2: The DDR Memory Replacement - PCSTATS
Abstract: In this article, PCstats will look at the features and specifications of DDR2 memory, and take a sneak peek at some of its future applications.
Filed under: Memory Published:  Author: 
External Mfg. Website: Various May 07 2004   Mike Dowler  
Home > Reviews > Memory > Various

New DDR-2 Technologies

The DDR2 standard also includes some improvements over DDR which help make the new memory more efficient, both at transferring data and at saving power:

On Die Termination (ODT) is built into each DDR2 memory chip on a module. Instead of having terminating resistors built into the motherboard to eliminate excess electrical 'noise' in the signal of the memory bus, DDR2 builds a resistor into each chip, deactivating it when the chip is in use and reactivating it when the chip is in standby. This apparently decreases signal interference while transferring data to and from the memory by moving the termination closer to the source, preventing interference within the module itself. This isn't something the end user has to worry about, but for electrical engineers, ensuring signal interference is lessened, allows the memory to function more reliably.

Posted CAS and additive latency are two new technologies which work hand in hand to eliminate data collisions within memory. In conventional DDR memory it is possible for a bank activation and a read command to occur on the same clock cycle, causing a collision and forcing the activation command to be shifted by one clock cycle. This results in a read command from the shifted activation command being shifted one clock cycle, and so on. This creates a gap in memory transfers when it happens, reducing bandwidth.

With posted CAS and additive latency, a read command is issued immediately after the activate command, then this read command is delayed internally by a predetermined amount of clock cycles (hence additive latency) before being executed. As this action is done without need for an additional command, no collision will occur. This holds true even if another activate command is issued on the same clock cycle the read command eventually executes on.

What about Latency issues?

Latency is one area where the DDR2 specs fall short of regular DDR memory, at least on paper. The JEDEC definition of DDR2 allows for CAS latencies of 3,4 or 5, as compared to DDR's 1.5,2 and 2.5. While some manufacturers have released 400MHz DDR2 at CAS latency 3, all 533MHz DDR2 chips currently shipping use a Latency timing of 4-4-4. As 533MHz DDR2 is likely to be the first widely used speed for the new standard, these are the timings that will be the default standard.

Write latency is also considerably greater with DDR2. While DDR1 allows a single cycle for write latency (1T), DDR2 defines write latency as read latency-1. This means that with a CAS latency of 4, DDR2-533 should have an effective write latency of 3T. Again this seems like bad news on paper.

To be fair, it may be difficult to compare DDR2 to DDR memory directly in terms of latency and performance. The various new features that DDR2 implements may make the difference in memory latency less significant. For more information, check out PCstats' great article on the benefits of low memory latency versus memory speed with respect to DDR, and for a (much) more technical examination of the latency issues of DDR2 memory, look here.

As it stands, it seems unlikely that DDR2 memory will have any performance advantage over DDR of an equivalent speed. The slower speed of the DDR2 core is cancelled out by its ability to move more data each clock cycle. Manufacturers claim that the various new signal enhancement features that DDR2 possesses will add to effective bandwidth by reducing interference, but then DDR2 takes more time to perform data actions (higher latency). Only proper lab tests will decide this. Stay tuned.

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Contents of Article: Various
 Pg 1.  Introduction to DDR-2: The DDR Memory Replacement
 Pg 2.  Speeds of DDR2 Memory
 Pg 3.  DDR 2 Inside and Out
 Pg 4.  — New DDR-2 Technologies
 Pg 5.  i915, i925 chipsets and DDR-2

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