DDR2

Memory technology standard

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DDR2/DDR II（ Double Data Rate 2 ） SDRAM Is created by JEDEC (Joint Commission on Electronic Equipment Engineering) technical standard , which is different from the previous generation DDR memory The biggest difference between the technical standards is that although the clock rise is adopted/ Falling edge Simultaneous data transmission But the DDR2 memory has more than twice the pre read capacity of the first generation DDR memory (that is, 4bit data read prefetch). In other words, each clock of DDR2 memory can External bus And can read/write data internally Control bus Run at 4 times the speed.

As the DDR2 standard stipulates that all DDR2 memories use FBGA package Form, but different from the widely used TSOP /TSOP-Ⅱ Packaging form ， FBGA package Can provide better Electrical performance With heat dissipation, it provides a solid foundation for the stable operation of DDR2 memory and the development of future frequency. Recall the development of DDR, from the first generation application to personal computer DDR200 of DDR266 、 DDR333 With the dual channel DDR400 technology, the development of the first generation of DDR has reached the technical limit, and it is difficult to improve the working speed of memory through conventional methods; With the development of Intel's latest processor technology, Front end bus yes Memory bandwidth The requirements of Operating frequency DDR2 memory will be the general trend.^[1]

Foreign name: Double Data Rate 2
Abbreviation: DDR2

R&D institutions: Electronic equipment Joint Engineering Committee
application area: Computer field

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Delay problem

Frequency of DDR2 memory

It can be seen from the above table that, in the same Core frequency Next, the actual DDR2 working frequency yes DDR Twice as much. This is because DDR2 memory has twice the standard DDR memory 4 BIT pre read capability. In other words, although DDR2 and DDR both use the basic method of data transmission at the same time when the clock goes up and down, DDR2 has twice the ability of pre reading system command data as DDR. That is to say, in the same 100MHz working frequency Next, DDR Actual frequency 200MHz, while DDR2 can reach 400MHz.

Then another problem arises: DDR In DDR2 memory and DDR2 memory, the memory latency of the latter is slower than that of the former. For example, DDR 400 and DDR2-400 have the same latency, while the latter has twice the bandwidth. In fact, DDR2-400 and DDR 400 have the same bandwidth, which is 3.2GB/s, but DDR400 The core operating frequency of DDR2-400 is 200MHz, while the core operating frequency of DDR2-400 is 100MHz, which means that the delay of DDR2-400 is higher than that of DDR400.

encapsulation

DDR2

The biggest breakthrough of DDR2 memory technology is not that users think the transmission capacity is twice that of DDR, but that it is lower Calorific value Under the condition of lower power consumption, DDR2 can achieve faster frequency increase, breaking the 400MHZ limit of standard DDR.

DDR memory usually adopts TSOP chip Packaging form This packaging form can work well on 200MHz. When the frequency is higher, its too long pin will produce high impedance and Parasitic capacitance , which will affect its stability and the difficulty of frequency increase. This is why the core frequency of DDR is difficult to break through 275MHZ. DDR2 memory uses FBGA package Form. Unlike the widely used TSOP packaging, FBGA packaging provides better Electrical performance With heat dissipation, it provides a good guarantee for the stable operation of DDR2 memory and the development of future frequency.

The DDR2 memory uses 1.8V voltage, which is much lower than the DDR standard of 2.5V, thus providing significantly smaller power consumption and heating capacity. This change is significant.

Dual channel memory

INTEL required Chipset The memory CAS latency and capacity need to be the same.

However, INTEL's Elastic dual channel The appearance of makes the conditions for the formation of dual channels more relaxed. Memory with different capacities can even form dual channels

In addition to the above differences, DDR2 also introduces three new technologies, namely OCD, ODT and Post CAS 。

Offline drive

OCD (Off Chip Driver): also known as offline drive adjustment, DDR II can improve signal integrity through OCD. DDR Ⅱ makes the voltage of the two equal by adjusting the resistance value of pull up/pull down. Use OCD to improve signal integrity by reducing the tilt of DQ-DQS; adopt control voltage To improve the signal quality.

Terminating resistor

ODT: ODT is the end of the built-in core resistor 。 We know that the motherboard with DDR SDRAM is used to prevent data line Terminal reflection signals require a large number of terminating resistors. It greatly increases the Manufacturing cost 。 In fact, different Memory module The requirements for the termination circuit are different. The size of the termination resistance determines the signal ratio and reflectivity If the terminal resistance is small, the signal reflection of the data line is low, but Signal-to-noise ratio Also lower; If the termination resistance is high, the signal to noise ratio of the data line is high, but the signal reflection will also increase. Therefore, the terminating resistor on the motherboard cannot match the memory module very well, and it will also affect the signal quality to a certain extent. DDR2 can build proper terminating resistance according to its own characteristics, which can ensure the best signal waveform. Using DDR2 can not only reduce the cost of motherboard, but also get the best signal quality, which is incomparable to DDR.

utilization efficiency

Post CAS: It is set to improve the utilization efficiency of DDR Ⅱ memory. In the Post CAS operation, the CAS signal (read/write/command) can be inserted behind the RAS signal Clock cycle , the CAS command can remain valid after the additive latency. customary tRCD (RAS to CAS and delay) is replaced by AL (Additive Latency), which can be set in 0, 1, 2, 3, 4. Since the CAS signal is placed one clock cycle after the RAS signal, the ACT and CAS signals will never collide. Dual channel operation is adopted, and the speed is twice that of DDR. In general, DDR2 has adopted many new technologies and improved many shortcomings of DDR. Although it has many shortcomings such as high cost, slow latency, it is believed that with the continuous improvement and perfection of technology, these problems will eventually be solved.

Technical characteristics

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DDR2 introduces three new technologies, namely OCD, ODT and PostCAS.

OCD (Off ChipDriver): also known as offline drive adjustment.

ODT: ODT is the termination resistor of the built-in core.

PostCAS: It is set to improve the utilization efficiency of DDR2 memory.

Purchasing strategy

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cost performance

When purchasing DDR2 memory, you should still buy mainstream frequency products. Don't blindly pursue high-frequency memory. If the price difference is not large, you may want to consider high frequency when purchasing DDR2 memory. After all, frequency improvement is helpful to the overall performance. However, the premise is that the motherboard supports high external frequencies, otherwise it will be overqualified. The market of the memory market has changed rapidly, especially for high-end products. Some high-frequency memory products are not as expensive as they were when they first came into the market Enthusiast It is worth buying.

brand

Individual users are paying more and more attention Memory module The quality of bulk memory modules has become less and less. Brand memory has gradually become the mainstream of purchase. After all, its quality, workmanship, performance and stability are obvious to all consumers, and many of them have been providing for up to three years defects liability period Lifetime warranty and after-sales service are incomparable. Be careful not to confuse the brand memory with the so-called "Samsung" and "Hyundai" memory in the market. Those just use Samsung or HY chips, while the chips are assembled by some small workshops, so the quality cannot be guaranteed. In order to distinguish from these inferior products, the names of genuine products are generally different, such as South Korea Samsung Electronics All original and genuine memory modules sold in China are called Three star gold bar 。

development history

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past times

DDR Ⅱ memory can have so many technological innovations, thanks to many years of research by researchers. In fact, as early as 1998, JEDEC (Electronic engineering design Development Association) began to study DDR2 technology and passed JEDEC specification standardization in 2003. Introducing 915/925 at Intel Chipset Previously, compared with DDR400, DDR2 Technical performance In addition to being more excellent, there is no advantage in practicality or cost performance. In front of most Intel Pentium 4 processors with 800MHz FSB, the 6.4GB/S bandwidth provided by DDR400 in dual channel mode can already meet the requirements, and the price of DDR2 at the beginning of its launch cannot compete with DDR, so whether from Performance requirements From the perspective of cost performance, DDR2 has been low for a long time market share Wandering under the circumstances.

Until the beginning of 2005, the new LGA775 Pentium4 The 800MHz front-end bus is still used, and the old DDR400 can still meet the bandwidth requirements. DDR2 is increasing Memory bandwidth Has also increased Memory Delay time, that is, the cost of each operation Clock cycle Will increase. DDR2 memory requires 4 clock cycles or 5 clock cycles, and tRAS may also increase to at least 8 clock cycles, while tRAS of high-speed DDR memory can reach 5 clock cycles. Therefore, under the 800MHz front-end bus, DDR2 does not bring much improvement to the system performance. Only after the front-end bus of the CPU is improved again, can DDR2 truly play its advantages in high bandwidth.

In addition, Intel 915/925 Chipset It also supports DDR memory. Even if the CPU front-end bus is upgraded to 1066MHz or higher 1200MHz in the future, some high specification DDR566 and DDR600 can still meet the bandwidth requirements. Therefore, in the transition period when DDR2 replaces DDR, DDR can still show strong vitality.

at present

Although the high frequency DDR566 and DDR600 can meet the needs of Intel 915/925 platform (1066MHz), the high specification DDR Yield As the cost is not easy to control, in the Chinese market, which is very sensitive to the cost performance ratio, high specification DDR is bound to be difficult to become. On the 915/925 platform, most users who are chasing price performance will be more optimistic about DDR2 memory.

The latest 945/955X has officially abandoned its support for DDR memory, and its memory specification has also been upgraded to the level of dual channel DDR2-667. Intel's practice is undoubtedly a death sentence for DDR. If the 915/925 period is still the coexistence and transition period of DDR2 and DDR, 945/955 is the heyday when DDR2 completely replaced DDR.

a main board Chipset The emerging force n VI DIA also launched nForce4 SLI Intel Edition, which supports DDR2-667 memory. Other chipset manufacturers certainly follow suit. Even the more stubborn AMD And decided that the next generation Athlon DDR2-667 will be integrated on 64/Opteron processor Memory controller This indicates that DDR2-667 will replace the existing DDR2-533 as a new standard. Although DR2 still has shortcomings in long delay time, with the advantage of high frequency, the actual efficiency of DDR2-667 system will significantly exceed that of DDR system. Memory manufacturers also showed an optimistic attitude towards DDR-2.

DDR2 memory specifications include DDR2-400, DDR2-533, DDR2-667, and DDR2-800. stay Memory capacity Like DDR, DDR2 also has a variety of specifications. The common specifications range from 256MB to 2GB.

DDR2-800 is the mainstream of DDR2. DDR2 is gradually being DDR3 replace.

future

The next generation successor of DDR2

Out of compatibility In consideration of, the DDR2 standard seems to be a little timid at the beginning of formulation, which directly leads to its performance in all aspects has not made significant progress compared with DDR. The new generation of DDR3 adopts ODT (core integration Terminator ）Technology and EMRS technology used to optimize performance, while also allowing the input clock to be asynchronous. stay Pin In terms of definition, DDR3 shows strong independence, and even dares to completely abandon TSOP II and mBGA packaging forms and adopt more advanced FBGA packaging. DDR3 memory 0.08 microns used manufacturing process Manufactured, will work at 1.5V.

In the long run Single chip Bit width And the frequency and power advantages of DDR3 are encouraging.

DDR3 is gradually accelerating to replace DDR2, DDR3 Has become install equipment The mainstream and standard configuration of.

performance analysis

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DDR2 SDRAM Get servers, workstations, and personal computer OEM Broad support from manufacturers, DDR2 storage have High data rate 、 low power consumption as well as high-density Features, which are also suitable for current figures Consumer Electronics Application requirements, such as set-top boxes and Digital camera Etc. This paper compares and analyzes the performance characteristics of DDR2 compared with traditional memory, and introduces the application opportunities of DDR2 in digital consumer electronics products.

The DRAM market is characterized by continuous improvement of technology and continuous growth of demand. For these applications, DDR2 SDRAM is a natural choice because of its speed and bandwidth ratio DDR SDRAM Much higher, 1.8V of DDR2 working voltage It can reduce the power consumption of its previous generation products by 50%.

However, the advantages of DDR2 are by no means limited to these applications. The high density, high efficiency and improved thermal characteristics of DDR2 are Desktop computer 、 Notebook computer And small shapes Consumer Electronics Has brought great advantages. The utilization of these advantages will depend on the continuous development of packaging and module technology, especially in the field of consumer electronics. This emerging market represents a new cutting-edge application of DRAM industry, which will bring a lot of opportunities for businesses willing to accept challenges to meet new requirements.

Convert to DDR2

Traditional DRAM markets such as servers, workstations and personal computers are rapidly transitioning to DDR2. Intel The company has announced all its future Chipset DDR2 will be supported, and other major chipset vendors seem to follow Intel's lead. The activities of Intel Developers Forum and Storage Manufacturers Forum this spring encouraged those who want to accelerate the transformation to DDR2. Most of the major suppliers in DRAM market provide DDR2 products that have been verified by Intel. semiconductor Production equipment From 8 inches to 12 inches wafer process The transformation of helps improve the product Yield To increase the output of DRAM. For 1Gb DRAM devices, the circuit technology that supports both DDR1 and DDR2 architecture on a single chip is critical, which makes the conversion to DDR2 easier.

Advantages of DDR2 SDRAM

DDR2 SDRAM Data transmission rate The maximum is 533Mbps, which is DDR266 Twice as much. In addition to some improvements in the original bandwidth, it also improves the performance and efficiency of the system, and facilitates system design 。 These improvements can be divided into the following four categories:

4-bit prefetching architecture adopts 4-bit prefetching of DDR2（ Prefetch ）Architecture, DDR2 SDRAM as External bus Each clock reads/writes four times more data from the memory unit array, and it works Frequency ratio inside Bus frequency Four times faster.

On chip Termination Other features of DDR2 bring benefits to motherboard design engineers, such as using DDR2 on-chip termination（ ODT ）To simplify the DQ bus design. In DDR2 SDRAM, the termination register is implemented in the DRAM chip instead of being installed on the motherboard. DRAM controller can set termination for each signal register On or off, these signals include data I/O Difference data Strobe signal And writing Data shielding 。 Using ODT eliminates the need for Vtt Generator Or Rtt resistance, and can reduce Multiple reflection , improve Signal integrity And increase the timing margin.

Off chip drive（ OCD ）Calibration OCD calibration improves the signal integrity of DDR2 SDRAM. The method is to set the I/O Driver To adjust the voltage and compensate for pull-up/ Pull down resistance ； Improve signal integrity by minimizing DQ-DQS offset; control Overshoot And undershoot to improve signal quality; The process difference between different DRAM suppliers can be corrected through I/O driver voltage calibration. Pre CAS and additional delay In a pre CAS operation, a CAS signal (read/write command) can become the input of the next clock after the RAS signal is input. The CAS instruction can be held on the DRAM side and executed after additional delays (0, 1, 2, 3, and 4). This simplifies controller design because it avoids Command bus Conflict on. Moreover, a simple instruction is adopted sequence It can also improve the command and data bus Efficiency. The actual Memory bandwidth It has also been improved. Finally, DDR2 uses a fine pitch ball grid array（ FBGA ）Encapsulation can reduce system size and improve signal integrity. A variant of this technology is the new stacked FBGA (sFBGA), which increases the air between modules Flow space Thereby improving Thermal performance And reliability. This kind of compliance Industry standards , compatible JEDEC Innovation is the key to optimizing the advantages of DDR2.

Memory module

For developers who have fully prepared to switch to DDR2, 1GB and 512MB DDR2 dual in-line with registers for server platforms Memory module （registered DIMM ）It is available in batch, and samples of 2GB DDR2 DIMMs with registers are available. These 2GB modules will greatly increase the density of server products, and can achieve up to 4.3GB per second Data transmission rate 。 Full buffered DIMMs (FB DIMMs) are also under development, which can adapt to new and higher speed buses technical requirement , such as PCI Express. FB DIMMs will be very important to the server market because servers need high bandwidth and high density. Although DDR2 significantly increases the speed and bandwidth, the tape is placed on the motherboard register The number of DIMMs is limited, otherwise the core logic Chipset Overload. By adding Buffer The chipset can be connected to the first DIMM, the first DIMM can be connected to the second DIMM, and then the third DIMM can be connected, At the same time, the buffer transmits the signal to the next DIMM. Each bus is segmented so that more DIMMs can be added to the motherboard and the load on the chipset is minimized. The FB DIMM module for servers is estimated to be available in late 2005.

system performance

In addition to the memory architecture, the system using DDR2 will also use the advanced kernel logic technology required by the new memory architecture. For example, Intel's DDR2 for desktops has four interfaces: to CPU Of Front end bus (FSB) interface, graphics to GPU Bus interface , Peripherals I/O bus And main memory bus. To make system performance Optimization, FSB, graphics bus and Main memory The bus should work on approximately the same bandwidth. Here, the FSB transmission of the processor data rate 800MHz × 8B=6.4 GBps; use PCI-Express The transmission data rate of graphics bus (× 16) is 8GBps; Has a Dual channel The transmission data rate of DDR2 DIMMs (533MHz × 8B × 2 channels) is 8.5GBps. These three interfaces are relatively balanced, and none of them will obviously become the bottleneck of other interfaces.

Mobile applications

With 1GB DDR2 small form factor dual in-line memory module（ SO-DIMM ）The laptop will also take advantage of the low power, high density, high performance and small size of DDR2 shape factor And other advantages. Thanks to the development of the Internet and wireless communication Performance improvement, cellular phone, PDA and others Mobile applications And gradually began to process more data, sound and Video stream These applications may also be applied to DDR2 devices to improve performance in the future.

D in this market RAM The products are based on SDR and DDR device architecture, and provide mobile RAM specific functions, such as: some arrays refresh themselves, that is, only refresh some specific memory cell arrays to reduce the self refresh current; temperature compensation Self refresh, i.e. by adjusting refresh frequency To adapt to the change of temperature, so as to play a similar role in reducing the self refresh current; Deep Power Down, which cuts off the internal voltage to achieve the lowest power consumption.

Multiple packages

As the density, speed and efficiency of DRAM continue to improve, consumer products that need DRAM to handle increasingly complex functions will also increase accordingly. Flexibility and diversity are the key to such applications. A strategy with a specification suitable for all applications is well applied in the field of traditional computing systems, but consumer centered digital cameras, digital televisions Digital Video Recorder And personal videos video recorder And other products need some new changes.

Consumer Electronics The market will probably take the last bus to transform to DDR2. DRAM suppliers have been exploring ways to develop these markets. According to consumption Product market Features, it is obviously necessary to provide a variety of packaging options, such as TSOP , FBGA and LQFP 。 Suitable for customer's own Multichip packaging (MCP) or System level packaging (SiP) design implementation“ Bare film ”Product is an important design consideration. stay emerging market It is also very important to be able to provide products of various densities, such as 64Mb, 128Mb and 256Mb, and have 16 bit and 32-bit structures.

Diversified needs

The needs of consumer applications are diversified to Car navigation system For example, these systems need not only broadband data transmission Ability, but also must be in a very wide ambient temperature Work under. This requires memory Scope of work - 40 to 85 ° C, while the working range of standard DRAM is 0 to 70 ° C. and Digital broadcasting , set-top box and digital TV are all broadband applications. Conventional DDR SDRAM devices can reach 3.2GBps Data transmission rate Therefore, it can meet the requirements of standard definition digital TV and HDTV Needs.

Consumer products such as DVD/HD video recorders are fundamentally changing the role of home televisions. These products can benefit from higher DDR2 storage Capacity and speed. The correct application of DDR and DDR2 technologies will contribute to the growth of this market, and the DRAM market has the ability to easily adapt to the evolving needs of these applications.

image quality The improvement and miniaturization of Digital camera Development of. In digital video camera video processing It can be optimized with × 32-bit I/O SDRAM or DDR SDRAM, which is“ Bare film ”The design is used in another important area of the customer's own SiP solution.

Digital STB

Worldwide, the transition to digital TV is slower than expected, because many consumers are still unwilling to rush to buy a number tuner and monitor 。 STB is a feasible alternative, and STB application will be another growth area of DRAM.

Advanced Graphic Technology

Finally, to provide advanced film image quality Graphic technology Maybe it is another area where DRAM can exert its superior performance. Because of the Internet sustained development and Communication line Performance improvements, digital Consumer Electronics Start processing more data and sound. These environments increase the demand for ultra fast, high-capacity DRAM, such as DDR2 RDRAM And XDR.

Routing rules

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Clock line includes MEM_CLKOUT # 0 MEM_CLKOUT0、MEM_CLKOUT#1、MEM_CLKOUT1，MEM_CLKOUT#2、MEM_CLKOUT2；　MEM_CLKOUT#3、 MEM_CLKOUT3、MEM_CLKOUT#4、MEM_CLKOUT4、MEM_CLKOUT#5、MEM_CLKOUT5。 DDR2 clock cable routing rule points Linepair The distance from the pair is 20mil min; The distance between DDR clock line and other lines is 20mil min; The north bridge breaks out for 4mil, the internal distance of the difference line is 6mil min, and the length is controlled within 1000mil. Come out again line width 6.5mil, differential pair^[2] The spacing between the two inner wires is 5mils, serpentine Line spacing 20 mils; DDR2 clock line routing length constraint rule Two lines in the differential line pair ± 10mils; Three pairs of differential lines of each DIMM are matched within 50mils, that is Maximum reduce minimum value Not more than 50mils; Impedance control of all lines between 2850mils and 6500mils: 70 Ω± 10% (differential line)

Discoid structure

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Disk domain receptor 2

discoidin domain receptor 2 cell membrane A protein tyrosine kinase on the.

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