Differences between DDR and SDRAM

Differences between DDR and SDRAM

Double Data Rate (DDR) SDRAM is a type of synchronous dynamic random-access memory that succeeded Single Data Rate (SDR) SDRAM. Both are types of SDRAM, but the primary difference is that DDR memory transfers data on both the rising and falling edges of the clock signal, effectively doubling the data transfer rate without increasing the clock frequency. This improvement allows for significantly higher bandwidth compared to its predecessor. SDR SDRAM, which transfers data only once per clock cycle, was common in computers before 2002, after which DDR SDRAM became the mainstream technology.

Due to architectural and physical differences, DDR SDRAM and SDR SDRAM are not forward or backward compatible. Motherboards and controllers are designed to support only one type of memory. The memory modules are physically keyed with notches to prevent incorrect installation in an incompatible slot.

Comparison Table

Feature	SDR SDRAM	DDR SDRAM
Full Name	Single Data Rate Synchronous Dynamic Random-Access Memory	Double Data Rate Synchronous Dynamic Random-Access Memory
Data Transfers per Clock Cycle	1	2
Clock Signal Edge Used	Rising edge only	Rising and falling edges
Prefetch Buffer Size	1n (1 data word per access)	2n (2 data words per access)
Operating Voltage	3.3 V	2.5 V / 2.6 V
DIMM Module Pins	168 pins	184 pins
DIMM Notches	Two	One

Venn diagram comparing Differences between DDR and SDRAM

Technical Architecture

The key to DDR SDRAM's increased performance is its 2n prefetch architecture. The internal memory bus fetches two words of data from the memory array during a single clock cycle and places them in an I/O buffer. The memory module then transfers these two words of data consecutively on the rising and falling edges of the clock signal. In contrast, SDR SDRAM uses a 1n prefetch, transferring only one word of data per clock cycle.

This "double pumping" technique allows a DDR module to achieve twice the bandwidth of an SDR module running at the same core clock speed. For example, an SDR SDRAM module on a 133 MHz bus has a transfer rate of 133 MT/s (megatransfers per second), while a DDR SDRAM module on the same 133 MHz bus can achieve 266 MT/s.

Physical Characteristics

SDR SDRAM and DDR SDRAM modules have distinct physical differences that prevent them from being interchangeable. Standard desktop modules, known as DIMMs, have different pin counts and notch locations. An SDR SDRAM DIMM has 168 pins and two notches on its connecting edge. A DDR SDRAM DIMM has 184 pins and only a single notch.

Additionally, DDR memory operates at a lower voltage than SDR memory. Standard DDR SDRAM modules run at 2.5 volts, compared to the 3.3 volts required for SDR SDRAM. This reduction in voltage results in lower power consumption.