SDRAM stands for Synchronous DRAM. It runs in sync with the memory bus. It delivers data in very high-speed bursts using high speed clock. Since the signals are synchronized with the motherboard bus, SDRAM eliminates the latency involved in asynchronous DRAM. There are different generations of SDRAM: DDR. DDR2 and DDR3.
Double Data Rate (DDR) SDRAM is an evolutionary upgrade of SDRAM released in 2001. In DDR, data is transferred twice as quickly as generic SDRAM. DDR memory does not double the actual clock rate to improve the performance; instead, it doubles its performance by transferring twice per transfer cycle:
DDR SDRAM user is DIMM (Dual Inline Memory Module) module design (we will cover that later in the session) with 184 pins to eliminate ambiguity with DDR, standard SDRAM is often known as single data rate (SDR)
DDR2 is the second generation of SDRAM which is a faster category of DDR memory. It uses differential pairs of signal wires to allow faster signaling without noise and interference problems and in this way, it achieves higher throughput. DDR2 is the modified signaling method enabling higher clock seeds to be achieved with more noise immunity. 400MHz is the lowest speed of DDR2 which is the fastest in case of DDR. Though DDR performs better than DDR2 at 400 MHz, at the higher levels, DDR2 is better.
DDR3 is the third generation of SDRAM with higher level of performance, lower power consumption and higher reliability than DDR2. JEDEC started developing DDR3 in 2002 released the first DDR3 memory modules in 2007.DDR3 modules are incompatible with DDR2 circuits: however 240-pin DDR3 modules are similar in count and shape to DDR2 modules.
Besides DDR, DDR2 and DDR3, the fourth generation of DDR memory has also been developed but syllabus include up to three generation.