Compare DDR2 and DDR3
Considering the ever growing need of speed and efficiency, understanding Double-Data-Rate (DDR) memory is important to system developers.
DDR memory's primary advantage is the ability to fetch data on both the rising and falling-edge of a clock cycle. DDR1, DDR2, DDR3 memories are powered up with 2.5, 1.8 and 1.5V supply voltage respectively. Temporization is another characteristic of of DDR memories, memory temporization is given in a series of numbers, such as 2-3-2-6-T1, 3-4-4-8 or 2-2-2-5 (CL-tRCD-tRP-tRAS-CMD) for DDR1. These numbers indicate the number of clock pulses that it takes to perform a certain operation- smaller the number faster the memory.
CL - Column Address Strobe, tRCD - Row Access Strobe to CAS delay
Type of DDR Memories
DDR1 with a maximum rated clock of 400MHz and a 64bit(8 bytes) data bus ( Now it is obsolete) .
DDR2 with a maximum rated clock of 800Mhz and a 64bit(8 bytes) data bus. DDR2 memory is physically incompatible ith previous generation of DDR memories.
DDR3 starts where DDR2 left off(800 MHz) and bring the speed up to 1.6 GHz. The 1.5V DDR3 voltage also save power( it will work in 1.36v also)
DDR2 Vs DDR3
DDR2 memories include 400Mhz, 533Mhz, 667Mhz and 800MHz versions, while DDR3 includes 800MHz, 1066Mhz, 1333MHz, and 1666Mhz versions.
Besides the enhanced bandwidth, DDR3 also uses less power than DDR2 by operating on 1.5V(16.3% reduction)
A newly introduced automatic calibration feature for the output data buffer enhances the ability to control the system timing during variations in voltage and temparature.
DDR3 devices introduce an interrupt reset for system flexibility.
Difference in the CL ( Column Address Strobe)
The controller in DDR2 memories works by preloading 4 data bits from the storage are(prefetch) while DD# works by loading 8 bits in advance.
For more details read ECN August 2008, Understanding RAM timings (article) www.hardwaresecrets.com/article/26/1 and powerelectronics.com/mag/power_ddr_memories_require/