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    DDR2 SDRAM, an abbreviation for double data rate two synchronous dynamic random access memory, is a type of a random access memory (RAM) commonly used in personal computers and various digital electronics today. This offspring from the SDRAM (synchronous dynamic random access memory), a part of DRAM (dynamic random access memory), is like an evolution from the DDR SDRAM; it can operate the external data bus twice as fast as its predecessor. This was possible by abandoning the original clock rate of the DDR, and operating the memory cells at half the rate. If the DDR2 was clocked at the same rate, the performance would be worse.


    The DDR2 was first introduced in 2003 at 200 MHz (PC2-3200) and 266 MHz (PC2-4200), but it failed to outperform the original DDR due to a latency problem. Nonetheless, a combination of the original DDR technology being at a speed of about 266 MHz (but 533 MHz effective), the JEDEC (Joint Electron Device Engineering Council) Solid State Technology Association not standardizing the faster DDR chips, and a release of newer DDR2 modules with lower latencies, the DDR2 eventually began the better solution. By the end of 2004, the DDR2 really began to compete against the standard DDR.

    Behind the DDR2 Technology

    As DDR2 is a type of an SDRAM, it stores its memory in memory cells. These cells are then activated at according times to correctly operate with an external data bus. Although the DDR2 shares many similarities with the original DDR, as in the case with data transfer happening with the rise and fall at the edge of the clock, it is different in that the DDR2's bus is clocked at twice the speed. This basically allows the DDR2 to be twice as more efficient than the original DDR.

    The key behind the high bus frequency with DDR2s (although the latency is augmented as a trade off) is the coalition of electrical interface improvements, prefetch buffers, and various off-chip drivers. This is because DDR's prefetch buffer is twice of the DDR, which is 2bits deep. Also, its read latencies are between four and six cycles; again this is twice that of the DDR. Thus, twice as much bus speed is necessary to achieve an equal latency.


    Also, the DDR2 is packaged relatively more expensively and more complicatedly than the DDR SDRAM or the SDR SDRAM, in exchange for a more blazing speed.

    Backward Compatibility

    Backward compatibility with DDR2 DIMMs are quite limited. In fact, they are not designed to be compatible with DDRM DIMMs at all. This is because firstly, the notch positions in the DDR2 and the DDR are different along with the pin density slightly higher on the DDR DIMMs. Although not much of a "backward compatibility," but faster DDR2 DIMMs can be used with slower DDR2 DIMMs. However, in such a situation, the faster one would run at the speed of the slower DDR2.

    Standard DDR2 SDRAM DIMM's

    DDR2 SDRAM is normally packaged in DIMM modules.

    DIMM Module Chip Type Clock Speed Data Rate Transfer Rate
    PC2-3200 DDR2-400 200 400 3,200
    PC2-4200 DDR2-533 266 533 4,266
    PC2-5300 DDR2-667 333 667 5,333
    PC2-6400 DDR2-800 400 800 6,400
    PC2-7400 DDR2-933 466 933 7,460
    PC2-8500 DDR2-1066 533 1066 8,530
    PC2-9600 DDR2-1200 600 1200 9,600
    PC2-10600 DDR2-1333 667 1333 10,660
    PC2-11700 DDR2-1466 733 1466 11,730
    PC2-12800 DDR2-1600 800 1600 12,800

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    1. Frank

      9 September, 2011 at 9:53 pm

      OK My laptop has 2X2GB Hynix Sodimm 667mhz, CL5  modules as standard. I’m thinking of going to Kingston HyperX modules and have a choice of 800mhz CL5, or 667mhz CL4 modules. The MB is ok with 800mhz so I’m told, (HP pavillion dv2940se). So which is a better choice for me?  CPU is an AMD Turion TL62 …I heard the hyper transport of  the  AMD’s like low latencies..Is this even a worthwhile upgrade, I already upgraded the laptop with Seagate’s Momentus XT hybrid HDD and windows 7.  Any advice anyone?

    2. Pir Ahsan Jan

      19 February, 2011 at 8:08 am

      I don’t Understand ddr ram how many pin
      and sdrm how many pin

      • memenode

        20 February, 2011 at 2:25 pm

        Old SDRAM memory has 168 pins.

        DDR has 184, DDR for laptops (SO-DIMM) has 200.

        DDR2 has 240 pins, for laptops 200.

        DDR3 has 240 pins as well, but for laptops 204.

        Neither non-DRM SDRAM nor DDR, DDR2 or DDR3 are compatible with each other though. If your motherboard supports DDR2 you can use only 240 pin DDR2 RAM, for example.


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