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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.
| DDR SDRAM | DDR2 SDRAM | DDR3 SDRAM | |
| 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 PC-2700/PC-3200 |
PC2-3200/PC2-4200 PC2-5300/PC2/6400 |
PC3-6400/PC3-8500 PC3-10600/PC3-12800 |
| 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 |
