What is the difference between SDRAM, DDR1, DDR2, DDR3 and DDR4?

In the realm of computer memory, Synchronous Dynamic Random Access Memory, or SDRAM, stands as a pivotal advancement that emerged in the late 1990s. The “synchronous” attribute defines its behavior, indicating its ability to synchronize with the timing of the Central Processing Unit (CPU). Unlike its predecessors, SDRAM is crafted to align itself with the CPU’s clock cycle, eliminating the need for the CPU to wait between memory accesses. For instance, PC66 SDRAM operates at 66 million transfers per second (MT/s), PC100 SDRAM at 100 MT/s, PC133 SDRAM at 133 MT/s, and so forth.

SDRAM can be further categorized as Single Data Rate SDRAM (SDR SDRAM), where the Input/Output (I/O), internal clock, and bus clock operate at the same frequency. In the case of PC133, all these elements run at 133 MHz. The “Single Data Rate” denotes that SDR SDRAM can perform only one read/write operation in a clock cycle, necessitating a wait for the completion of the preceding command before initiating another read/write operation.

The evolution continues with DDR SDRAM (Double Data Rate SDRAM), representing the next generation after SDR. DDR achieves higher bandwidth by transferring data on both the rising and falling edges of the clock signal, effectively doubling the transfer rate without increasing the clock frequency. DDR SDRAM has a prefetch buffer of 2 bits (double that of SDR SDRAM), and its transfer rate ranges from 266 to 400 MT/s.

Moving forward, DDR2 SDRAM (Double Data Rate Two SDRAM) brings further enhancements, particularly in the ability to operate the external data bus twice as fast as DDR. Achieved through improved bus signals, DDR2 boasts a 4-bit prefetch buffer (double that of DDR SDRAM). While maintaining the same internal clock speed as DDR (133-200 MHz), DDR2 achieves transfer rates between 533 and 800 MT/s.

The march of progress leads to DDR3 SDRAM (Double Data Rate Three SDRAM), which addresses power consumption concerns by reducing it by 40% compared to DDR2. This reduction is facilitated by lower operating currents and voltages (1.5V, compared to DDR2’s 1.8V or DDR’s 2.5V). DDR3’s prefetch buffer width is 8 bits, doubling that of DDR2. Additionally, DDR3 introduces features like ASR (Automatic Self-Refresh) and SRT (Self-Refresh Temperature) to control refresh rates based on temperature variations.

The latest frontier is DDR4 SDRAM (Double Data Rate Fourth SDRAM), offering a lower operating voltage (1.2V) and higher transfer rates. Operating at speeds ranging from 2133 to 3200 MT/s, DDR4 introduces innovations like Bank Groups technology, enabling single-handed operation within each bank group. DDR4’s efficiency is enhanced by processing four data within a clock cycle. It introduces functions such as DBI (Data Bus Inversion), CRC (Cyclic Redundancy Check), and CA parity to improve signal integrity and data transmission/access stability.

In the ever-advancing landscape of memory technology, each iteration of SDRAM brings about improvements in performance, efficiency, and functionality, catering to the evolving needs of modern computing systems.

 DDR1, DDR2, DDR3 and DDR4
DDR1, DDR2, DDR3 and DDR4

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