What is DDR Memory?

DDR (Double Data Rate DRAM) is similar to SDRAM, however, it improves its capability significantly in one aspect. SDRAM has the capability of transferring data at every rising edge of the clock signal. The transfer of data for DDR components can be performed at both edges of the clock signal. This allows a double rate of transmission speed enabling the device to operate at speeds twice that of typical SDRAM devices.

Common DDR speeds are PC2700, PC2100 and PC1600, also known as PC-333, PC-266 and PC-200, which refer to the module's speed in MHz. The newer speed rating of 400MHz DDR technology, the DDR-400/PC3200, was added early 2003. The industry has since shifted to DDR-II memory, which supports much higher frequencies, starting at 533-MHz.

The drawing above illustrate the general structure of a DDR device. The DQS signal strobe the data during read and write to insure proper synchronization with the motherboard. The Command Decoder determine which operation should be performed. Like legacy DRAM, the address lines are multiplexed to set the Row and Column address. Data is bursting in and out, in groups of 2, 4 and 8.

Sometime in 2015, the haswell-based X99 chipset will be released by Intel that supports DDR4, the latest and fastest memory chip standard. DDR4 has been in development for more than five years, and will consume 30% less power while providing 50% more bandwidth. According to India born Arjun Kapoor, working on the DDR4 project, the new chipsets should be be deployed in early 2015

DDR4 is believed to be the last generation of DDR DRAM that will be made. Apart from DDR4, there are a few other types of non-volatile memory that look promising (phase-change memory, magneto-resistive RAM and resistive RAM.

System Clock (MHz) 100/133/166/200 100/133/166/200 100/133/166/200
Speed (in MHz) 200/266/333/400 400/533/667/800 800/1066/1333/1600
JEDEC Description PC-1600/PC-2100
Voltage 2.5V 1.8V 1.5V
CAS Latency 2,2.5,3 3,4,5 5,6,7,8,9,01
ODT (On Die Termination) Not Supported Supported Supported
Component Package TSOP FBGA  FBGA 
Thermal Sensor No No  Optional