What is SDRAM? Explaining Synchronous Dynamic Random Access Memory

What is SDRAM?

Synchronous Dynamic Random Access Memory, commonly abbreviated as SDRAM, is a type of Dynamic Random Access Memory (DRAM). It is more prevalent in most computers compared to regular DRAM and offers faster operating speeds.

SDRAM (synchronous dynamic random-access memory), which first became available in the 1990s, featured an interface that was synchronous with the processor, thereby eliminating the delay of control signals propagating through semiconductor paths. In contrast, SDRAM ICs used an external clock signal to synchronize the operation of their external pin interfaces.

SDRAM (Synchronous Dynamic Random Access Memory) has an interface that synchronizes with the system bus, transferring information from the CPU to a memory controller hub. This results in a swift response in sync with the system bus, which is the primary reason for SDRAM's widespread popularity and use in most computer systems.

SDRAM offers higher data transfer rates and concurrency compared to traditional asynchronous DRAM. Additionally, it provides a simpler and less expensive design, which confers significant benefits to manufacturers. These advantages have made SDRAM a widely popular and preferred choice in the computer memory market, particularly in the realm of RAM.

## Evolution of SDRAM

Over time, SDRAM has gone through several generations, each with its specific feature set. Here's a brief overview of how SDRAM has evolved over the decades to its latest iteration:

DDR (first generation)

The first generation of SDRAM, called DDR (Double Data Rate) SDRAM, preceded SDR. DDR provided stricter control over the phase of the clock signal, enabling higher transfer rates.

Another notable feature of the DDR interface is double pumping, which allows data to be transferred on both edges of the clock cycle, effectively doubling the bandwidth of the data bus. This also helps avoid the need for excessively high clock frequencies, reducing the demands on signal integrity.

DDR operates at clock frequencies between 133 and 200 MHz, with a prefetch buffer size of 2n (two data words per memory access). It has a transfer rate of 2.1–3.2 Gbit/s and an operating voltage of 2.5/2.6 V.

DDR2 (second generation)

Double Data Rate 2 Synchronous Dynamic Random-Access Memory (DDR2 SDRAM), first introduced by Samsung in 2001, succeeded the original DDR SDRAM. Later versions of DDR2 ran at either 200 or 266 MHz. DDR2 had an increased prefetch length, with four bits prefetched per byte.

By increasing the prefetch length, DDR2 can provide almost twice the data transfer rate across the data bus. The main idea behind DDR2 is to achieve this without increasing processor power consumption. However, DDR2 does increase memory latency by three to nine clock cycles.

DDR2 memory modules are specified to run at a maximum voltage of 1.9V and bus clock frequencies of 266 to 400 MHz. They support data transfer rates of 4.2 to 6.4 GB/s, but because they have a pin density of 240 (versus 184 for DDR), they are not backward compatible with DDR modules.

DDR3 (Third Generation)

First appearing in devices in 2007, DDR3 SDRAM (Double Data Rate Third Generation Synchronous Dynamic Random Access Memory) has unique signal voltages and timing, and so is not forward- or backward-compatible with any prior generation of RAM.

DDR3 has twice the prefetch buffer size of DDR2, at 8n, allowing for data transfer rates up to eight times faster. This enables DDR3 to deliver higher data transfer rates and bandwidth.

Operating at voltages between 1.35 and 1.5V, DDR3 modules transfer data at rates of up to 8.5 to 14.9 GB/s.

DDR4 (fourth generation)

DDR3 was succeeded in 2014 by the fourth generation of Double Data Rate SDRAM, DDR4, which differs significantly from earlier types of RAM in its physical interface and signal voltage. Thus, DDR4 is not compatible with its predecessors, and it offers data transfer rates of 8.5-14.9 GB/s.

DDR4 offers higher module densities and data transfer rates at lower voltages (up to 1.2V for frequencies of 800 MHz to 1600 MHz) than previous generations of DDR SDRAM. The prefetch buffer length of DDR4 is the same as that of DDR3 (8n), but DRAM banks have been divided into two or four groups for increased bandwidth.

DDR5 stands for Fifth Generation Dynamic Random-Access Memory.

DDR5 SDRAM is the latest generation of Double Data Rate memory, released in 2020. It has the same prefetch buffer size and latency as DDR3 and DDR4, but DDR5 supports a bandwidth of up to 4.8 GB/s. It also achieves further voltage reduction, with a maximum of 1.1V.

SK Hynix's highest-frequency DDR5 module, launched in 2019, runs at 6,400 MT/s; in 2021, Samsung announced a 512 GB DDR5 DIMM running at 7,200 MHz. DDR5 allows for up to eight bank groups, with up to four banks per group, enabling greater bandwidth.

Differences Between SDRAM and DDR

By now, you should have a good grasp of the basics of SDRAM. After looking at each generation, you might be interested in exploring the key differences that set these memory types apart from one another. We'll use SDRAM and DDR as an example and present a clear table outlining their main distinctions:

Conclusions

SDRAM (Synchronous Dynamic Random-Access Memory) is a widely used type of memory in integrated circuits, having gained significant popularity since its introduction. With its fast speed and synchronous interface, SDRAM has largely replaced DRAM in most computer systems. The latest generation, DDR5 SDRAM, offers increased bandwidth while significantly reducing power consumption.