DDR SDRAM

DDR SDRAM (double data rate synchronous dynamic random access memory) is a class of memory integrated circuits used in computers. It achieves nearly twice the bandwidth of the preceding [single data rate] SDRAM by double pumping (transferring data on the rising and falling edges of the clock signal) without increasing the clock frequency

With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte). Thus with a bus frequency of 100 MHz, DDR SDRAM gives a maximum transfer rate of 1600 MB/s.

DR SDRAM for desktop computers DIMMs have 184 pins (as opposed to 168 pins on SDRAM, or 240 pins on DDR2 SDRAM), and can be differentiated from SDRAM DIMMs by the number of notches (DDR SDRAM has one, SDRAM has two).

DDR for notebook computers SO-DIMMs have 200 pins which is the same number of pins as DDR2 SO-DIMMs. These two specifications are notched very similarly and care must be taken during insertion when you are unsure of a correct match.

DDR SDRAM operates at a voltage of 2.5 V, compared to 3.3 V for SDRAM. This can significantly reduce power consumption. Chips and modules with DDR-400/PC-3200 standard have a nominal voltage of 2.6 Volt.

Like the ordinary SDRAM, DDR can be also classified into two types: 200MHz (100MHz DDR) PC1600 SDRAM and 266MHz (133MHz DDR) PC2100 SDRAM. The table below shows all the memory types available in the today's memory market:

Memory	Frequency, MHz	Memory Bus, bit	Memory Bus Bandwidth, MB/sec	Supported Platforms
PC100 SDRAM	100	64	800	Pentium III (i440BX, VIA Apollo Pro 133A, i815) Athlon (AMD 750, VIA KT133)
PC133 SDRAM	133	64	1064	Pentium III (VIA Apollo Pro 133A, i815) Athlon (VIA KT133)
PC800 RDRAM	800	16	1600	Pentium III (i820)
Dual Channel PC800 RDRAM	800	32	3200	Pentium 4 (i850)
PC1600 DDR SDRAM	200	64	1600	Pentium III (VIA Apollo Pro266, ALi Aladdin Pro 5) Athlon (AMD 760, VIA KT266, ALi MAGiK 1)
PC2100 DDR SDRAM	266	64	2128	Pentium III (VIA Apollo Pro266, ALi Aladdin Pro 5) Athlon (AMD 760, VIA KT266, ALi MAGiK 1)

Chips and modules

Standard name	Memory clock	Cycle time	I/O Bus clock	Data transfers per second	JEDEC standard VDDQ voltage	Module name	Peak transfer rate
DDR-200	100 MHz	10 ns [1]	100 MHz	200 Million	2.5v +/- 0.2v	PC-1600	1600 MB/s
DDR-266	133 MHz	7.5 ns	133 MHz	266 Million	2.5v +/- 0.2v	PC-2100	2100 MB/s
DDR-333	166 MHz	6 ns	166 MHz	333 Million	2.5v +/- 0.2v	PC-2700	2700 MB/s
DDR-400	200 MHz	5 ns	200 MHz	400 Million	2.6v +/- 0.1v	PC-3200	3200 MB/s

The memory modules exterior was also a bit modified. Although the DDR modules size remained the same, they have more pins - 184, while the regular PC100/PC133 SDRAM DIMM modules have only 168 pins.

Besides a larger number of pins, new DDR SDRAM DIMM modules feature lower voltage, 2.5V, which helps to reduce EMI (compare with 3.3V by the regular SDRAM). As for the latency, it is almost the same as by PC100/PC133 SDRAM modules and much lower than by RDRAM.

In addition to speed, DDR2 and DDR3 were developed to run at lower voltages than regular DDR RAM, resulting in less power needed to run memory. DDR used 2.5v. DDR2 runs off 1.8v and DDR3 1.5v.

A 184-pin DDR memory module

DDR Compared to DDR2

Chips and modules

For use in computers, DDR2 SDRAM is supplied in DIMMs with 240 pins and a single locating notch. DIMMs are identified by their peak transfer capacity (often called bandwidth).

Standard name	Memory clock	Cycle time	I/O Bus clock	Data transfers per second	Module name	Peak transfer rate
DDR2-400	100 MHz	10 ns	200 MHz	400 Million	PC2-3200	3200 MB/s
DDR2-533	133 MHz	7.5 ns	266 MHz	533 Million	PC2-4200 PC2-43001	4266 MB/s
DDR2-667	166 MHz	6 ns	333 MHz	667 Million	PC2-5300 PC2-54001	5333 MB/s
DDR2-800	200 MHz	5 ns	400 MHz	800 Million	PC2-6400	6400 MB/s
DDR2-1066	266 MHz	3.75 ns	533 MHz	1066 Million	PC2-8500 PC2-86001	8533 MB/s

JEDEC standard modules

Standard name	Memory clock	Cycle time	I/O Bus clock	Data transfers per second	Module name	Peak transfer rate
DDR3-800	100 MHz	10 ns	400 MHz	800 Million	PC3-6400	6400 MB/s
DDR3-1066	133 MHz	7.5 ns	535 MHz	1066 Million	PC3-8500	8533 MB/s
DDR3-1333	166 MHz	6 ns	667 MHz	1333 Million	PC3-10600	10667 MB/s[1]
DDR3-1600	200 MHz	5 ns	800 MHz	1600 Million	PC3-12800	12800 MB/s