DDR SDRAM

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DDR SDRAM
Double Data Rate Synchronous Dynamic Random-Access Memory
Desktop DDR Memory Comparison.svg
Comparison of DDR modules for desktop PCs (DIMM).
Developer Samsung [1] [2] [3]
JEDEC
Type Synchronous dynamic random-access memory
Generations
Release date
  • DDR: 1998
  • DDR2: 2003
  • DDR3: 2007
  • DDR4: 2014
  • DDR5: 2020 (estimated)
Specifications
Voltage
  • DDR: 2.5/2.6
  • DDR2: 1.8
  • DDR3: 1.5/1.35
  • DDR4: 1.2/1.05

Double Data Rate Synchronous Dynamic Random-Access Memory, officially abbreviated as DDR SDRAM, is a double data rate (DDR) synchronous dynamic random-access memory (SDRAM) class of memory integrated circuits used in computers. DDR SDRAM, also retroactively called DDR1 SDRAM, has been superseded by DDR2 SDRAM, DDR3 SDRAM and DDR4 SDRAM. None of its successors are forward or backward compatible with DDR1 SDRAM, meaning DDR2, DDR3, and DDR4 memory modules will not work in DDR1-equipped motherboards, and vice versa.

Contents

Compared to single data rate (SDR) SDRAM, the DDR SDRAM interface makes higher transfer rates possible by more strict control of the timing of the electrical data and clock signals. Implementations often have to use schemes such as phase-locked loops and self-calibration to reach the required timing accuracy. [4] [5] The interface uses double pumping (transferring data on both the rising and falling edges of the clock signal) to double data bus bandwidth without a corresponding increase in clock frequency. One advantage of keeping the clock frequency down is that it reduces the signal integrity requirements on the circuit board connecting the memory to the controller. The name "double data rate" refers to the fact that a DDR SDRAM with a certain clock frequency achieves nearly twice the bandwidth of a SDR SDRAM running at the same clock frequency, due to this double pumping.

With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate (in bytes/s) 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.

History

Samsung demonstrated the first DDR memory prototype in 1997, [1] and released the first commercial DDR SDRAM chip (64  Mb) in June 1998, [6] [2] [3] followed soon after by Hyundai Electronics (now SK Hynix) the same year. [7] The development of DDR began in 1996, before its specification was finalized by JEDEC in June 2000 (JESD79). [8] JEDEC has set standards for data rates of DDR SDRAM, divided into two parts. The first specification is for memory chips, and the second is for memory modules. The first retail PC motherboard using DDR SDRAM was released in August 2000. [9]

Specification

Generic DDR Memory (Xytram).jpg
4 DDR slots 4 slots DDR.JPG
4 DDR slots
Corsair DDR-400 memory with heat spreaders Corsair CMX512-3200C2PT 20080602.jpg
Corsair DDR-400 memory with heat spreaders
Physical DDR layout DDR layout sketch.png
Physical DDR layout
Comparison of memory modules for portable/mobile PCs (SO-DIMM). Laptop SODIMM DDR Memory Comparison V2.svg
Comparison of memory modules for portable/mobile PCs (SO-DIMM).

Modules

To increase memory capacity and bandwidth, chips are combined on a module. For instance, the 64-bit data bus for DIMM requires eight 8-bit chips, addressed in parallel. Multiple chips with the common address lines are called a memory rank. The term was introduced to avoid confusion with chip internal rows and banks. A memory module may bear more than one rank. The term sides would also be confusing because it incorrectly suggests the physical placement of chips on the module. All ranks are connected to the same memory bus (address + data). The chip select signal is used to issue commands to specific rank.

Adding modules to the single memory bus creates additional electrical load on its drivers. To mitigate the resulting bus signaling rate drop and overcome the memory bottleneck, new chipsets employ the multi-channel architecture.

Comparison of DDR SDRAM standards
Name Chip Bus Timings Voltage (V)
StandardTypeModule Clock rate (MHz)Cycle time (ns) [10] Clock rate (MHz) Transfer rate (MT/s) Bandwidth (MB/s)CL-TRCD-TRP CAS latency (ns)
DDR-200PC-16001001010020016002.5±0.2
DDR-266PC-2100133⅓7.5133⅓266.672133⅓2.5-3-3
DDR-333PC-2700166⅔6166⅔333⅓2666⅔2.5
DDR-400APC-3200200520040032002.5-3-332.6±0.1
B3-3-32.5
C3-4-42

Note: All above listed are specified by JEDEC as JESD79F. [11] All RAM data rates in-between or above these listed specifications are not standardized by JEDEC—often they are simply manufacturer optimizations using tighter-tolerance or overvolted chips. The package sizes in which DDR SDRAM is manufactured are also standardized by JEDEC.

There is no architectural difference between DDR SDRAM modules. Modules are instead designed to run at different clock frequencies: for example, a PC-1600 module is designed to run at  , and a PC-2100 is designed to run at  . A module's clock speed designates the data rate at which it is guaranteed to perform, hence it is guaranteed to run at lower ( underclocking ) and can possibly run at higher ( overclocking ) clock rates than those for which it was made. [12]

DDR SDRAM modules for desktop computers, dual in-line memory modules (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 SDRAM 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 if unsure of a correct match. Most 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 V.

JEDEC Standard No. 21–C defines three possible operating voltages for 184 pin DDR, as identified by the key notch position relative to its centreline. Page 4.5.10-7 defines 2.5V (left), 1.8V (centre), TBD (right), while page 4.20.5–40 nominates 3.3V for the right notch position. The orientation of the module for determining the key notch position is with 52 contact positions to the left and 40 contact positions to the right.

Increasing operating voltage slightly can increase maximum speed, at the cost of higher power dissipation and heating, and at the risk of malfunctioning or damage.

Capacity
Number of DRAM devices
The number of chips is a multiple of 8 for non-ECC modules and a multiple of 9 for ECC modules. Chips can occupy one side (single sided) or both sides (dual sided) of the module. The maximal number of chips per DDR module is 36 (9×4) for ECC and 32 (8x4) for non-ECC.
ECC vs non-ECC
Modules that have error-correcting code are labeled as ECC. Modules without error correcting code are labeled non-ECC.
Timings
CAS latency (CL), clock cycle time (tCK), row cycle time (tRC), refresh row cycle time (tRFC), row active time (tRAS).
Buffering
registered (or buffered) vs unbuffered.
Packaging
Typically DIMM or SO-DIMM.
Power consumption
A test with DDR and DDR2 RAM in 2005 found that average power consumption appeared to be of the order of 1–3 W per 512 MB module; this increases with clock rate and when in use rather than idling. [13] A manufacturer has produced calculators to estimate the power used by various types of RAM. [14]

Module and chip characteristics are inherently linked.

Total module capacity is a product of one chip's capacity and the number of chips. ECC modules multiply it by 8/9 because they use 1 bit per byte (8 bits) for error correction. A module of any particular size can therefore be assembled either from 32 small chips (36 for ECC memory), or 16(18) or 8(9) bigger ones.

DDR memory bus width per channel is 64 bits (72 for ECC memory). Total module bit width is a product of bits per chip and number of chips. It also equals number of ranks (rows) multiplied by DDR memory bus width. Consequently, a module with a greater number of chips or using ×8 chips instead of ×4 will have more ranks.

Example: Variations of 1 GB PC2100 registered DDR SDRAM module with ECC
Module size (GB)Number of chipsChip size (Mbit)Chip organizationNumber of ranks
13625664M×42
11851264M×82
118512128M×41

This example compares different real-world server memory modules with a common size of 1 GB. One should definitely be careful buying 1 GB memory modules, because all these variations can be sold under one price position without stating whether they are ×4 or ×8, single- or dual-ranked.

There is a common belief that number of module ranks equals number of sides. As above data shows, this is not true. One can also find 2-side/1-rank modules. One can even think of a 1-side/2-rank memory module having 16(18) chips on single side ×8 each, but it's unlikely such a module was ever produced.

Chip characteristics

DRAM density
Size of the chip is measured in megabits. Most motherboards recognize only 1 GB modules if they contain 64M×8 chips (low density). If 128M×4 (high density) 1 GB modules are used, they most likely will not work. The JEDEC standard allows 128M×4 only for slower buffered/registered modules designed specifically for some servers, but some generic manufacturers do not comply. [15] [ verification needed ]
Organization
The notation like 64M×4 means that the memory matrix has 64 million (the product of banks x rows x columns) 4-bit storage locations. There are ×4, ×8, and ×16 DDR chips. The ×4 chips allow the use of advanced error correction features like Chipkill, memory scrubbing and Intel SDDC in server environments, while the ×8 and ×16 chips are somewhat less expensive. x8 chips are mainly used in desktops/notebooks but are making entry into the server market. There are normally 4 banks and only one row can be active in each bank.

Double data rate (DDR) SDRAM specification

From Ballot JCB-99-70, and modified by numerous other Board Ballots, formulated under the cognizance of Committee JC-42.3 on DRAM Parametrics.

Standard No. 79 Revision Log:

  • Release 1, June 2000
  • Release 2, May 2002
  • Release C, March 2003 – JEDEC Standard No. 79C. [16]

"This comprehensive standard defines all required aspects of 64Mb through 1Gb DDR SDRAMs with X4/X8/X16 data interfaces, including features, functionality, ac and dc parametrics, packages and pin assignments. This scope will subsequently be expanded to formally apply to x32 devices, and higher density devices as well."

Organization

PC3200 is DDR SDRAM designed to operate at 200 MHz using DDR-400 chips with a bandwidth of 3,200 MB/s. Because PC3200 memory transfers data on both the rising and falling clock edges, its effective clock rate is 400 MHz.

1 GB PC3200 non-ECC modules are usually made with 16 512 Mbit chips, 8 on each side (512 Mbits × 16 chips) / (8 bits (per byte)) = 1,024 MB. The individual chips making up a 1 GB memory module are usually organized as 226 8-bit words, commonly expressed as 64M×8. Memory manufactured in this way is low-density RAM and is usually compatible with any motherboard specifying PC3200 DDR-400 memory. [17] [ citation needed ]

High-density RAM

In the context of the 1 GB non-ECC PC3200 SDRAM module, there is very little visually to differentiate low-density from high-density RAM. High-density DDR RAM modules will, like their low-density counterparts, usually be double-sided with eight 512 Mbit chips per side. The difference is that each chip, instead of being organized as 64M×8, is organized as 227 4-bit words, or 128M×4.

High-density memory modules are assembled using chips from multiple manufacturers. These chips come in both the familiar 22 × 10 mm (approx.) TSOP2 and smaller squarer 12 × 9 mm (approx.) FBGA package sizes. High-density chips can be identified by the numbers on each chip.

High-density RAM devices were designed to be used in registered memory modules for servers. JEDEC standards do not apply to high-density DDR RAM in desktop implementations.[ citation needed ] JEDEC's technical documentation, however, supports 128M×4 semiconductors as such that contradicts 128×4 being classified as high-density[ clarify ]. As such, high density is a relative term, which can be used to describe memory that is not supported by a particular motherboard's memory controller.[ citation needed ]

Generations

DDR (DDR1) was superseded by DDR2 SDRAM, which had modifications for higher clock frequency and again doubled throughput, but operates on the same principle as DDR. Competing with DDR2 was Rambus XDR DRAM. DDR2 dominated due to cost and support factors. DDR2 was in turn superseded by DDR3 SDRAM, which offered higher performance for increased bus speeds and new features. DDR3 has been superseded by DDR4 SDRAM, which was first produced in 2011 and whose standards were still in flux (2012) with significant architectural changes.

DDR's prefetch buffer depth is 2 (bits), while DDR2 uses 4. Although the effective clock rates of DDR2 are higher than DDR, the overall performance was not greater in the early implementations, primarily due to the high latencies of the first DDR2 modules. DDR2 started to be effective by the end of 2004, as modules with lower latencies became available. [18]

Memory manufacturers stated that it was impractical to mass-produce DDR1 memory with effective transfer rates in excess of 400 MHz (i.e. 400 MT/s and 200 MHz external clock) due to internal speed limitations. DDR2 picks up where DDR1 leaves off, utilizing internal clock rates similar to DDR1, but is available at effective transfer rates of 400 MHz and higher. DDR3 advances extended the ability to preserve internal clock rates while providing higher effective transfer rates by again doubling the prefetch depth.

The DDR4 SDRAM is a high-speed dynamic random-access memory internally configured as 16 banks, 4 bank groups with 4 banks for each bank group for x4/x8 and 8 banks, 2 bank groups with 4 banks for each bank group for x16 DRAM. The DDR4 SDRAM uses an 8n prefetch architecture to achieve high-speed operation. The 8n prefetch architecture is combined with an interface designed to transfer two data words per clock cycle at the I/O pins. A single read or write operation for the DDR4 SDRAM consists of a single 8n-bit-wide 4-clock data transfer at the internal DRAM core and 8 corresponding n-bit-wide half-clock-cycle data transfers at the I/O pins. [19]

RDRAM was a particularly expensive alternative to DDR SDRAM, and most manufacturers dropped its support from their chipsets. DDR1 memory's prices substantially increased since Q2 2008, while DDR2 prices declined. In January 2009, 1 GB DDR1 was 2–3 times more expensive than 1 GB DDR2. High-density DDR RAM suit about 10% of PC motherboards on the market, while low-density suit almost all motherboards on the PC Desktop market.[ citation needed ]

Comparison of DDR SDRAM generations
NameRelease
year
Chip Bus Voltage
(V)
Pins
GenStandard Clock rate
(MHz)
Cycle time
(ns)
Pre-
fetch
Clock rate
(MHz)
Transfer rate
(MT/s)
Bandwidth
(MB/s)
DIMM SO-
DIMM
Micro-
DIMM
DDRDDR-2002000100102n10020016002.5184200172
DDR-2661337.51332662133
DDR-333166⅔6166⅔3332666⅔
DDR-400200520040032002.6
DDR2 DDR2-4002003100104n20040032001.8240200214
DDR2-533133⅓7.5266⅔533⅓4266⅔
DDR2-667166⅔6333⅓666⅔5333⅓
DDR2-80020054008006400
DDR2-1066266⅔3.75533⅓1066⅔8533⅓
DDR3 DDR3-8002007100108n40080064001.5/1.35240204214
DDR3-1066133⅓7.5533⅓1066⅔8533⅓
DDR3-1333166⅔6666⅔1333⅓10666⅔
DDR3-16002005800160012800
DDR3-1866233⅓4.29933⅓1866⅔14933⅓
DDR3-2133266⅔3.751066⅔2133⅓17066⅔
DDR4 DDR4-1600201420058n8001600128001.2/1.05288260
DDR4-1866233⅓4.29933⅓1866⅔14933⅓
DDR4-2133266⅔3.751066⅔2133⅓17066⅔
DDR4-24003003⅓1200240019200
DDR4-2666333⅓31333⅓2666⅔21333⅓
DDR4-2933366⅔2.731466⅔2933⅓23466⅔
DDR4-32004002.51600320025600

Mobile DDR

MDDR is an acronym that some enterprises use for Mobile DDR SDRAM, a type of memory used in some portable electronic devices, like mobile phones, handhelds, and digital audio players. Through techniques including reduced voltage supply and advanced refresh options, Mobile DDR can achieve greater power efficiency.

See also

Related Research Articles

Synchronous dynamic random-access memory (SDRAM) is any dynamic random-access memory (DRAM) where the operation of its external pin interface is coordinated by an externally supplied clock signal.

DIMM computer memory module that has separate electrical contacts on each side of the module and a 64-bit data path

A DIMM or dual in-line memory module comprises a series of dynamic random-access memory integrated circuits. These modules are mounted on a printed circuit board and designed for use in personal computers, workstations and servers. DIMMs began to replace SIMMs as the predominant type of memory module as Intel P5-based Pentium processors began to gain market share.

Rambus DRAM (RDRAM), and its successors Concurrent Rambus DRAM (CRDRAM) and Direct Rambus DRAM (DRDRAM), are types of synchronous dynamic random-access memory (SDRAM) developed by Rambus from the 1990s through to the early-2000s. The third-generation of Rambus DRAM, DRDRAM was replaced by XDR DRAM. Rambus DRAM was developed for high-bandwidth applications, and was positioned by Rambus as replacement for various types of contemporary memories, such as SDRAM.

DDR2 SDRAM second generation of double-data-rate synchronous dynamic random-access memory

Double Data Rate 2 Synchronous Dynamic Random-Access Memory, officially abbreviated as DDR2 SDRAM, is a double data rate synchronous dynamic random-access memory interface. It superseded the original DDR SDRAM specification, and is superseded by DDR3 SDRAM. DDR2 DIMMs are neither forward compatible with DDR3 nor backward compatible with DDR.

SO-DIMM variant of DIMM in a smaller form factor

A SO-DIMM, SODIMM, or small outline dual in-line memory module, is a type of computer memory built using integrated circuits. SO-DIMMs are a smaller alternative to a DIMM, being roughly half the size of regular DIMMs.

PC100 is a standard for internal removable computer random access memory, defined by the JEDEC. PC100 refers to Synchronous DRAM operating at a clock frequency of 100 MHz, on a 64-bit-wide bus, at a voltage of 3.3 V. PC100 is available in 168-pin DIMM and 144-pin SO-DIMM form factors. PC100 is backward compatible with PC66 and was superseded by the PC133 standard.

PC133 is a computer memory standard defined by the JEDEC. PC133 refers to SDR SDRAM operating at a clock frequency of 133 MHz, on a 64-bit-wide bus, at a voltage of 3.3 V. PC133 is available in 168 pin DIMM and 144 pin SO-DIMM form factors. PC133 is the fastest and final SDR SDRAM standard ever approved by the JEDEC, and delivers a bandwidth of 1066 MB per second. PC133 is backward compatible with PC100 and PC66.

Double data rate

In computing, a computer bus operating with double data rate (DDR) transfers data on both the rising and falling edges of the clock signal. This is also known as double pumped, dual-pumped, and double transition. The term toggle mode is used in the context of NAND flash memory.

In computing, serial presence detect (SPD) is a standardized way to automatically access information about a memory module. Earlier 72-pin SIMMs included five pins that provided five bits of parallel presence detect (PPD) data, but the 168-pin DIMM standard changed to a serial presence detect to encode much more information.

Double Data Rate 3 Synchronous Dynamic Random-Access Memory, officially abbreviated as DDR3 SDRAM, is a type of synchronous dynamic random-access memory (SDRAM) with a high bandwidth interface, and has been in use since 2007. It is the higher-speed successor to DDR and DDR2 and predecessor to DDR4 synchronous dynamic random-access memory (SDRAM) chips. DDR3 SDRAM is neither forward nor backward compatible with any earlier type of random-access memory (RAM) because of different signaling voltages, timings, and other factors.

Fully Buffered DIMM memory technology

Fully Buffered DIMM is a memory technology that can be used to increase reliability and density of memory systems. Conventionally, data lines from the memory controller have to be connected to data lines in every DRAM module, i.e. via multidrop buses. As the memory width increases together with the access speed, the signal degrades at the interface between the bus and the device. This limits the speed and memory density, so FB-DIMMs take a different approach to solve the problem.

GDDR4 SGRAM, an abbreviation for double data rate type four synchronous graphics random access memory, is a type of graphics card memory specified by the JEDEC Semiconductor Memory Standard. It is a rival medium to Rambus's XDR DRAM. GDDR4 is based on DDR3 SDRAM technology and was intended to replace the DDR2-based GDDR3, but it ended up being replaced by GDDR5 within a year.

GDDR5, an abbreviation for graphics double data rate type five synchronous dynamic random-access memory, is a modern type of synchronous graphics random-access memory (SGRAM) with a high bandwidth interface designed for use in graphics cards, game consoles, and high-performance computing. It is a type of GDDR SDRAM.

Memory module discrete printed circuit board on which memory chips are mounted

In computing, a memory module is a printed circuit board on which memory integrated circuits are mounted. Memory modules permit easy installation and replacement in electronic systems, especially computers such as personal computers, workstations, and servers. The first memory modules were proprietary designs that were specific to a model of computer from a specific manufacturer. Later, memory modules were standardized by organizations such as JEDEC and could be used in any system designed to use them.

The JEDEC memory standards are the specifications for semiconductor memory circuits and similar storage devices promulgated by the Joint Electron Device Engineering Council (JEDEC) Solid State Technology Association, a semiconductor trade and engineering standardization organization.

Double Data Rate 4 Synchronous Dynamic Random-Access Memory, officially abbreviated as DDR4 SDRAM, is a type of synchronous dynamic random-access memory with a high bandwidth interface.

LPDDR computer hardware

Low-Power Double Data Rate Synchronous Dynamic Random Access Memory, commonly abbreviated as Low-Power DDR SDRAM or LPDDR SDRAM, is a type of double data rate synchronous dynamic random-access memory that consumes less power and is targeted for mobile computers. It is also known as Mobile DDR, and abbreviated as mDDR.

HyperCloud Memory (HCDIMM) is a DDR3 SDRAM Dual In-Line Memory Module (DIMM) used in server applications requiring a great deal of memory. It was initially launched in 2009 at the International Supercomputing Conference by Irvine, California based company, Netlist Inc. It was never a JEDEC standard, and the main server vendors supporting it were IBM and Hewlett Packard Enterprise.

UniDIMM specification for DIMMs that can be populated with either DDR3 or DDR4 chips, with no support for any additional memory control logic; created by Intel for Skylake microarchitecture

UniDIMM is a specification for dual in-line memory modules (DIMMs), which are printed circuit boards (PCBs) designed to carry dynamic random-access memory (DRAM) chips. UniDIMMs can be populated with either DDR3 or DDR4 chips, with no support for any additional memory control logic; as a result, the computer's memory controller must support both DDR3 and DDR4 memory standards. The UniDIMM specification was created by Intel for its Skylake microarchitecture, whose integrated memory controller (IMC) supports both DDR3 and DDR4 memory technologies.

High Bandwidth Memory high-performance RAM interface for 3D-stacked DRAM from AMD and Hynix

High Bandwidth Memory (HBM) is a high-performance RAM interface for 3D-stacked SDRAM from Samsung, AMD and SK Hynix. It is used in conjunction with high-performance graphics accelerators and network devices. The first HBM memory chip was produced by SK Hynix in 2013, and the first devices to use HBM were the AMD Fiji GPUs in 2015.

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