January JEDEC. STANDARD. DDR2 SDRAM SPECIFICATION JEDEC organization there are procedures whereby a JEDEC standard or publication. JEDEC-standard V I/O (SSTL_compatible). • Differential data strobe (DQS, DQS#) option. • 4n-bit prefetch architecture. • Duplicate output strobe (RDQS). VDDSPD = –V. • JEDEC-standard V I/O (SSTL_compatible). • Differential data strobe (DQS, DQS#) option. • 4n-bit prefetch architecture. • Dual rank.
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In addition to double pumping the data bus as in DDR SDRAM transferring data on the rising and falling edges of the bus clock signalDDR2 allows higher bus speed and requires lower power by running the internal clock at half the speed of the data bus. The two factors combine to produce a total of four data transfers per internal clock cycle.
DDR2 SDRAM STANDARD
Alternatively, DDR2 memory operating at twice the external data bus clock rate as DDR may provide twice the bandwidth with the same latency. During an access, four bits were read or written to or from a four-bit-deep prefetch queue.
This queue received or transmitted its data over the data bus in two data bus clock cycles each clock cycle transferred two bits of data. DDR2’s bus frequency is boosted by electrical interface improvements, on-die terminationprefetch buffers and off-chip drivers.
However, latency is greatly increased as a trade-off.
Thus, DDR2 memory must be operated at twice the data rate to achieve the same latency. This packaging change was necessary to maintain signal integrity at higher bus speeds.
Power savings are achieved primarily due to an improved manufacturing process through die shrinkage, resulting in a drop in operating voltage 1.
The lower memory clock frequency may also enable power reductions in applications that do not require the highest available data rates.
DIMMs are identified by their peak transfer capacity often called bandwidth. standarx
At least one manufacturer has reported this reflects successful testing at a higher-than-standard data rate  whilst others simply round up for the name. Bandwidth is calculated by taking transfers per second and multiplying by eight.
This is because DDR2 memory modules transfer data on a bus that is 64 data bits wide, and since a byte comprises 8 bits, this equates to 8 bytes of data per transfer.
DDR2 was introduced in the second quarter of at two initial clock rates: Both performed worse than the original DDR specification due to higher latency, which made total access times longer. These chips are mostly standard DDR chips that have been tested and rated to be capable of operation at higher clock rates by the manufacturer.
Such chips draw significantly more power than slower-clocked chips, but usually offered little or no improvement in real-world performance. DDR2 started to become competitive against the older DDR standard by the end ofas modules with lower latencies became available.
It had severe overheating issues due to the nominal DDR voltages. However, further confusion has been added to the mix with the appearance of budget and mid-range graphics cards which claim to use “GDDR2”. These cards actually use standard DDR2 chips designed for use as main system memory although operating with wtandard latencies to achieve higher clockrates.
These chips cannot achieve the clock rates of GDDR3 but are inexpensive and fast enough to be used as memory on mid-range cards. From Wikipedia, the free encyclopedia.
DDR2 SDRAM – Wikipedia