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  • Writer's pictureRakhi Kundu

DDR Memories Comparison and overview

We realize that one of the most important aspects of a computer is its capability to store large amounts of information in what we normally call “memory.” Specifically, it’s random access memory (RAM), and it holds volatile information that can be accessed quickly and directly. And considering the ever-growing system need for speed and efficiency, understanding double-data-rate (DDR) memory is important to system developers.

DDR Memory Characteristics

DDR memory’s primary advantage is the ability to fetch data on both the rising and falling edge of a clock cycle, doubling the data rate for a given clock frequency. For example, in a DDR200 device, the data transfer frequency is 200 MHz, but the bus speed is 100 MHz.

DDR1, DDR2, and DDR3 memories are powered up with 2.5, 1.8, and 1.5V supply voltages respectively, thus producing less heat and providing more efficiency in power management than normal SDRAM chipsets, which use 3.3V.

Types of DDR Memories

There are presently three generations of DDR memories:

1. DDR1 memory, with a maximum rated clock of 400 MHz and a 64-bit (8 bytes) data bus is now becoming obsolete and is not being produced in massive quantities. Technology is adopting new ways to achieve faster speeds/data rates for RAM memories.

2. DDR2 technology is replacing DDR with data rates from 400 MHz to 800 MHz and a data bus of 64 bits (8 bytes). Widely produced by RAM manufacturers, DDR2 memory is physically incompatible with the previous generation of DDR memories.

3. DDR3 technology picks up where DDR2 left off (800 Mbps bandwidth) and brings the speed up to 1.6 Gbps. One of the chips already announced by ELPIDA contains up to 512 megabits of DDR3 SDRAM, with a column access time of 8.75 ns (CL7 latency) and a data transfer rate of 1.6 Gbps at 1.6 GHz. The 1.5V DDR3 voltage level also saves some power compared to DDR2 memory. What is more interesting is that at an even lower 1.36V, the DDR3 RAM runs fine at 1.333 GHz (DDR3-1333) with a CL6 latency (8.4 ns total CAS time), which matches the CAS time of the fastest current DDR2 memory.

4. DDR4 SDRAM provides the lower operating voltage (1.2V) and higher transfer rate. The transfer rate of DDR4 is 2133~3200 MT/s. DDR4 adds four new Bank Groups technology. Each bank group has the feature of singlehanded operation. DDR4 can process 4 data within a clock cycle, so DDR4's efficiency is better than DDR3 obviously. DDR4 also adds some functions, such as DBI (Data Bus Inversion), CRC (Cyclic Redundancy Check), and CA parity. They can enhance DDR4 memory's signal integrity, and improve the stability of data transmission/access.

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