DDR SDRAM : Notes

01/10/2020 DDR SDRAM - Wikipedia
https://en.wikipedia.org/wiki/DDR_SDRAM 1/5
DDR SDRAM
Double Data Rate Synchronous
Dynamic Random-Access
Memory
Comparison of DDR modules for
desktop PCs (DIMM).
Developer Samsung[1][2][3]
JEDEC
Type Synchronous
dynamic random-
access memory
Generations DDR2
DDR3
DDR4
DDR5
Release
date
DDR: 1998
DDR2: 2003
DDR3: 2007
DDR4: 2014
DDR5: 2020
Specifications
Voltage DDR: 2.5/2.6
DDR2: 1.8
DDR3: 1.5/1.35
DDR4: 1.2/1.05
DDR SDRAM
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been superseded by DDR2 SDRAM, DDR3 SDRAM, and DDR4 SDRAM, and soon will be superseded by
DDR5 SDRAM. None of its successors are forward or backward compatible with DDR1 SDRAM, meaning
DDR2, DDR3, DDR4 and DDR5 memory modules will not work in DDR1-equipped motherboards, and
vice versa.
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
Specification
Modules
Chip characteristics
Double data rate (DDR) SDRAM specification
Organization
High-density RAM
Generations
Mobile DDR
See also
References
External links
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]
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.
Contents
History
Specification
Modules

4 DDR slots
Corsair DDR-400 memory with heat
spreaders
Physical DDR layout
Comparison of memory modules for
portable/mobile PCs (SO-DIMM).
Comparison of DDR SDRAM standards
Name Chip Bus Timings
Voltage
(V)
Standard Type Module
Clock
rate
(MHz)
Cycle
time
(ns)[10]
Clock
rate
(MHz)
Transfer
rate
(MT/s)
Bandwidth
(MB/s)
CL-
TRCD-
TRP
CAS
latency
(ns)
DDR-200
PC-
1600
100 10 100 200 1600
2.5±0.2DDR-266
PC-
2100
133⅓ 7.5 133⅓ 266.67 2133⅓
2.5-3-
3
DDR-333
PC-
2700
166⅔ 6 166⅔ 333⅓ 2666⅔ 2.5
DDR-400
A
PC-
3200
200 5 200 400 3200
2.5-3-
3
3
2.6±0.1
B 3-3-3 2.5
C 3-4-4 2
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 100 MHz, and a PC-2100
is designed to run at 133 MHz. 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 chips Chip size (Mbit) Chip organization Number of ranks
1 36 256 64M×4 2
1 18 512 64M×8 2
1 18 512 128M×4 1
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.
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]
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.
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."
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]
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. JEDEC's technical documentation, however, supports 128M×4 semiconductors as such that contradicts
128×4 being classified as high-density. As such, high density is a relative term, which can be used to describe memory that is not supported by a
Chip characteristics
Double data rate (DDR) SDRAM specification
Organization
High-density RAM

particular motherboard's memory controller.
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 suits about 10% of PC motherboards on the market, while low-density DDR RAM suits almost all
motherboards on the desktop PC market.
Comparison of DDR SDRAM generations
Name
Release
year
Chip Bus
Voltage
(V)
Pins
Gen Standard
Clock
rate
(MHz)
Cycle
time
(ns)
Pre-
fetch
Clock
rate
(MHz)
Transfer
rate
(MT/s)
Bandwidth
(MB/s)
DIMM
SO-
DIMM
Micro-
DIMM
DDR
DDR-200
2000
100 10
2n
100 200 1600
2.5
184 200 172
DDR-266 133 7.5 133 266 2133
DDR-333 166⅔ 6 166⅔ 333 2666⅔
DDR-400 200 5 200 400 3200 2.6
DDR2
DDR2-400
2003
100 10
4n
200 400 3200
1.8 240 200 214
DDR2-533 133⅓ 7.5 266⅔ 533⅓ 4266⅔
DDR2-667 166⅔ 6 333⅓ 666⅔ 5333⅓
DDR2-800 200 5 400 800 6400
DDR2-1066 266⅔ 3.75 533⅓ 1066⅔ 8533⅓
DDR3
DDR3-800
2007
100 10
8n
400 800 6400
1.5/1.35 240 204 214
DDR3-1066 133⅓ 7.5 533⅓ 1066⅔ 8533⅓
DDR3-1333 166⅔ 6 666⅔ 1333⅓ 10666⅔
DDR3-1600 200 5 800 1600 12800
DDR3-1866 233⅓ 4.29 933⅓ 1866⅔ 14933⅓
DDR3-2133 266⅔ 3.75 1066⅔ 2133⅓ 17066⅔
DDR4
DDR4-1600
2014
200 5
8n
800 1600 12800
1.2/1.05 288 260
DDR4-1866 233⅓ 4.29 933⅓ 1866⅔ 14933⅓
DDR4-2133 266⅔ 3.75 1066⅔ 2133⅓ 17066⅔
DDR4-2400 300 3⅓ 1200 2400 19200
DDR4-2666 333⅓ 3 1333⅓ 2666⅔ 21333⅓
DDR4-2933 366⅔ 2.73 1466⅔ 2933⅓ 23466⅔
DDR4-3200 400 2.5 1600 3200 25600
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.
Generations
Mobile DDR

Fully buffered DIMM
ECC memory, a type of computer data storage
List of device bandwidths
Serial presence detect
Official JEDEC website (http://www.jedec.org/)
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See also
References
External links

DDR SDRAM : Notes

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DDR SDRAM : Notes