DDR SDRAM And DDR2 SDRAM - A Comparison

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DDR SDRAM And DDR2 SDRAM - A Comparison

DDR SDRAM And DDR2 SDRAM - A Comparison

The DDR SDRAM or the Double Data Rate synchronous dynamic random access memory can be defined as a class of memory integrated circuits' that are commonly being used in computers. The DDR SDRAM interface definitely makes higher transfer rates achievable as compared to SDR (Single data rate) SDRAM. This is possible through a stricter control of the electrical data and the clock signal timing. In order to reach the timing accuracy, phase locked loops and self calibration schemes have to be used. For lowering of the clock frequency the interface uses a technique called double pumping'. Double pumping is the transferring of data both on the rising and falling edges of the clock signal. By keeping the clock frequency down, the signal integrity requirements on the circuit board that connects memory to controller, get reduced.

The DDR RAM is termed as double data rate' as it is able to achieve, with a certain clock frequency, almost twice the bandwidth of a single data rate' the (SDR) SDRAM, which is running at the same clock frequency. On the transfer of data of 64 bits at one time, the DDR SDRAM gives the following transfer rate:

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(memory bus clock rate) x 2(for dual rate) x 64(number of bits transferred)/8(number of bits/byte)

Thus the DDR SDRAM is able to give a maximum transfer rate of 1600 MB/s with a bus frequency of 100 Mhz.

The DDR SDRAM got superseded by a newer DDR2 SDRAM. Therefore it is sometimes also referred to a DDR1 SDRAM.

The DDR2 SDRAM has an additional feature in addition to double pumping of data bus as in DDR SDRAM. It allows higher bus speed and it needs lower power. This is possible by running the internal clock at one quarter of the speed of the data bus. Both these factors put together, require a total of 4 data transfers per internal clock.

The DDR2 SDRAM is able to give the following transfer rate, when data is transferred at 64 bits at a time:

(memory clock rate) x 2(for bus clock multiplier) x 2 (for dual rate) x 64(number of bits Transferred)/8(number of bits/byte).
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Thus DDR2 SDRAM can give a maximum transfer rate of 3200MB/s with a memory clock frequency of 100 MHz. Compared to the best rated DDR memory modules, the best rated DDR2 memory modules work atleast twice as fast.

In both DDR and DDR2 the memory is stored in memory cells that get activated with a clock signal that aligns its operation with an external data bus. In both, the double pumping' technique takes place i.e the I/O buffer transfers data on the rising as well as the falling edge of the clock. The principal difference between the DDR and DDR2 is that in case of DDR2 the memory cells are clocked at one quarter the rate of the bus rather than half. This needs a 4 bit deep prefetch queue. DDR2 is able to operate most efficiently at twice the bus speed of DDR, and that too without changing the memory cells. The electrical improvements, prefetch buffers, on-die termination and off-chip drivers boost up the bus frequency. But latency is greatly increased. The prefetch buffer is 4 bits deep for DDR2, 2 bits deep for DDR and 8 bits deep for DDR3. Latencies are between 2 and 3 bus cycles for DDR SDRAM and between 4 and 6 cycles for DDR2. Hence to achieve the same latency, the DDR2 memory needs to be operated at twice the data rate.

The DDR2 chips are packaged in a BGA package that is more expensive and more difficult to assemble, leading to an increased bandwidth cost. As the manufacturing process is improved because of die shrinkage, power savings are being achieved.

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