What is DDR?

Strictly speaking, DDR should be called DDR SDRAM, and people are used to calling it DDR. Some beginners often see DDR SDRAM and think it is SDRAM. DDR SDRAM is the abbreviation of double data rate SDRAM, which means double rate synchronous dynamic random access memory. DDR memory is developed on the basis of SDRAM memory and still uses SDRAM production system. Therefore, for memory manufacturers, DDR memory can be produced only by slightly improving the equipment for manufacturing ordinary SDRAM, which can effectively reduce the cost.

SDRAM only transmits data once in a clock cycle, and it transmits data in the rising cycle of the clock. DDR memory, on the other hand, transmits data twice in a clock cycle and can transmit data once in the rising and falling periods of the clock, so it is called dual-rate synchronous dynamic random access memory. DDR memory can achieve higher data transfer rate at the same bus frequency as SDRAM.

Compared with SDRAM, DDR adopts a more advanced synchronization circuit, which makes the main steps of specifying address and data transmission and output be executed independently and keep complete synchronization with CPU. DDR uses DLL (delay locked loop) technology. When the data is valid, the memory controller can use the data filter signal to accurately locate the data, output it every 16 times, and resynchronize the data from different memory modules. DDR can double the speed of SDRAM without increasing the clock frequency. It allows reading data on the rising and falling edges of clock pulses, so it is twice as fast as standard SDRA.

There is not much difference between DDR and SDRAM in appearance and volume. They have the same size and the same pin distance. However, DDR has 184 pins, which is 16 more than SDRAM, and mainly contains new signals such as control, clock, power supply and grounding. DDR memory adopts SSTL2 standard supporting 2.5V voltage instead of LVTTL standard supporting 3.3V voltage used by SDRAM.