Wednesday, September 25, 2002
By: Cecil Ho, CST, Inc.
Significance of JEDEC DIMM Module
Non-JEDEC compliance memory modules have been flooding the market resulting in unstable computer systems and confusion. System integrators and consumer are easily tempted by the short-term cost saving without knowing its long-term consequence. This article investigates the design process in JEDEC Raw cards. It highlights the details on proper memory module design. Whether it is clock net, stub length, termination resistor, or impedance control, they all involve thousands of hours of simulation and engineering. The resulting Gerbers are offered free of charge to you. It might make you think differently next time you pick up a memory module.
We are seeing DIMM modules with only half of the number of bypass capacitors. We see DIMM modules with screened on resistors with value vary 30% across the same board. We see DIMM modules made out of 4 layer printed circuit boards instead of 6 layer boards. We see DIMM module that has the correct circuitry but fail to work in the computer system. We see deceptive memory module manufacturers cut all corners to reduce cost and to gain sales. On the other hand, we are frustrated at the many memory modules that do not work properly in our computer system. That is when JEDEC, the semiconductor standard setting committee, gets into the play. A unified standard on memory modules is needed to make sure every memory module work in every computer. With a unified standard, we can therefore, enjoy the benefit of state-of-the-art and at the same time attending the lowest price.
Why is JEDEC Important?
JEDEC is an organization made up of 300 plus member companies from all over the world. They consist of memory vendor companies like Micron, Samsung, Infineon, Hynix and Nanya. Memory module manufacturers like Kingston, Smart Modular, and Wintec. Connector manufacturers like Molex, and Foxconn. Memory user companies like Sun Micro, Silicon Graphics, and HP. Tester companies like Tektronix, Agilent and CST. Chipset companies like Nvidia, Via and SIS.
All these companies realize that the only way to have the best price for memory and its associated system is to promote one mainstream technology for the entire industry. Therefore, these companies get together 4 times a year all into one room with their difference set aside and to concentrate on formulating a standard memory that the industry can accept. The result is robust design memory devices and modules that will work reliably in the computer systems. This standard becomes the minimum requirement for the memory market. It is recognized by all OEM computer manufacturers and is supported by the entire industry.
Member companies worked together on specification, simulation, prototyping, example design, test parameters and system validations. A guideline is set down so that all JEDEC compatible memories will be functionally inter-changeable with similar performances
Developing a JEDEC Memory Specification
JEDEC has been looking ahead for the needs of the semiconductor memory industry in advance of many years. It started the work on DDR memory back in 1997. Through its effort, DDR is now the mainstream memory in the computer industry. JEDEC has just now completed the specification for DDR-II which should fill the market by 2005. The next task is DDR-III which is expected to materialize by 2007
JEDECís design procedure starts out with series of survey to users and semiconductor vendors. The user companies are asked what they would want while vendor companies are asked on what they can practically produced economically not now but several years down the line. The user companies have to look at the memory bandwidth required to support their applications down the line. Whether it is video streaming, 4G handset or fiber-to-home networking, system companies have to forecast their memory requirements in speed, feature, and in density. The semiconductor vendors are asked to look at their process technology down the line. The challenge is placed before them for tighter line width technologies. Memory cores working at 2.5V power supply today will have to go to 1.8V and 1.2V to gain the extra speed and bandwidth in a few years.
One of the major tasks for JEDEC is to assign pin-out for memory device and memory modules. Whether it is for the memory chip or the memory module, pin-out assignment is no small task. Trial layouts of the motherboard and the DIMM have to be done to find the optimum routing. Special attention is applied to minimize lead inductance and to accommodate a universal pin-out for all different die sizes in the future. Not to mention mechanical support and reliability standards
While users are eager to use memories that would give them the most timing margin workable timing tolerance) for systems, vendors are fighting for uniform specifications that every one can agree and produce. Either it is the hold time, latency, or clock duty cycle tolerance, semiconductor vendors want to achieve maximum yield with their process. JEDEC, the standardization committee gets to be the mediator when "wants" and "needs" comes into conflict.
Simulation Plays a Key Role in JEDEC Memory Design
In digital circuit design, we used to say, "Itís only ones and zeros, will work either way you hook it up". Well, in a 333MHz memory circuit, the statement is no longer true. Every line segment (stub) becomes a transmission line that can get echoes back to distort the original waveform. Therefore, a transmission line model must be simulated and examined for echoes and waveform distortions. Series resistor and termination resistors must be examined for the best value and performance. That is why you see stub lengths on a memory DIMM fully defined in the JEDEC design specifications.
Before the engineer has a chance to build the system, he must also figure out how much timing margin his system has before hand. He takes the worst cases tolerance on hold time, clock jittering, and a lot of other factors into the calculation and comes up with an "eye pattern" simulation that tells how much margin the system has when incorporated with the memories.
After all these simulation and calculations, the engineer might decide on one particular impedance value for the PC board to give best performance. He has, therefore, layout his PC board for a fixed impedance (for example 60 ohm). He then use line width, dielectric, and layer stack specification to set his PCB impedance. Formulae and application software are available to do these calculations. The result is constant and stable impedance on the PCB to avoid signal distortion and to work at the highest frequency possible.
Engineers also like to add measurement coupons (blank lines) on their PC board for practical measurement of the actual impedance. In such case, a TDR (Termination Dependent Return Loss) measurement is taken at the PC board manufacturing to determine the accuracy of the board impedance. The impedance control can also be examined with a cross-section of the PC board measured under a microscope.
Memory Module Prototyping
JEDEC members provide a large number of example designs of PC board for memory modules. It is usually called the Raw Cards as they are supplied in the common PCB layout format call the Gerber format. Any engineer can take this "Gerber" file to a PCB fabrication shop and have a PCB made.
These "Raw Cards", contributed by the JEDEC member companies are simulated, analyzed, and carefully designed to provide robust operations in the computer. These Gerber files are generally open for downloading to everybody in the industry
Before these Gerbers are released, actual memory modules are usually built and verified against the design and simulation results. Any disagreement are studied and corrected before final release.
Memory Module System Validation
JEDEC members put these Raw Cards to test in different motherboards to verify their functionality in the system. The motherboard memory sockets are usually fully populated. Functional test is performed at 4 temperature/voltage extreme corners (low and high) to uncover any problem in the actual operational environment. Clock arrival at the memory chip is also checked for skews and delays with the help of a JEDEC Standard Clock Reference Board.
JEDEC Standard Clock Reference Board
JEDEC members have determined that if common design modules are to work with every motherboard, the module clock skew is a very critical factor. Therefore, JEDEC has designed a Clock Reference Board to be used for an industry wide uniform calibration. Many versions of the Clock Reference Boards are made over time for the different generations of memory technologies. There is the 133MHz CRB for 133Mhz SDRAM modules. There are also CRB for the PC2100 DIMM as well as a new version for the PC2700 DIMM
The Clock Reference Board is a standard clock generator split into different clock paths on the CRB. One path goes to a standard clock termination load while the other clock paths end into DRAM chips. This provides the engineer with a mechanism to measure and to calibrate the clock arrival
time to the chips on the module with an oscilloscope. Any unwanted skew can, therefore, be corrected by adding capacitors to the feedback of the clock PLL (phase lock loop).
These CRBís are manufactured in one batch of PCB and one batch of uniform component to minimize the variations. The result is the best way to unify the industry on clock arrival time and thus the assurance of modules will work in different systems.
Proper Memory Module Manufacturing, Meeting JEDEC Standard
A well design JEDEC module cannot realize its full potential if it is not manufactured properly. As the DIMM operational frequency goes up, so as the need for tighter manufacturing process. A good memory module manufacturer usually have a solder paste screening machine that gives him good control on the amount of solder paste dispensed. The process engineer is aware of the importance of freshness and the quality of his soldering paste. A good module manufacturer also has a precision Chip-Mounter (Pick-n-Place) machine for automatic placement of his DRAM chips and associated components. The loaded boards are then placed into a reflow oven to complete the solder joints.
Proper DIMM Module Testing, Key to Success
Module manufacturers also need testers to test their module for assembly problems and process damages. There are tester companies among JEDEC members that design testers based on JEDEC memory module specifications. As a result, the JEDEC quality can be pass-on to the final user.
There are basically two classes of memory module testers that are made for JEDEC DIMM testing. The ATE (automatic test equipment) class tester allows virtually any arbitrary waveform generated to simulate almost any conditions that the memory modules can encounter in the computer system. These machines are infinitely flexible and programmable. They are good for production as well as fault analysis. The only drawback is the cost that can run into millions of US dollars.
Another type of memory module tester straightly made for memory module production test. It takes the assumption that the DRAMs have been tested in the semiconductor manufacturing process. Therefore, only functional test have to be applied to catch assembly errors and process damages during the module manufacturing process. This class of tester is most popular in the module manufacturing industry.
On top of that, a good test process control record is needed to satisfy ISO9000 requirements. Robotic mechanical handlers are usually used with memory module testers to realize good test process control. Robotic test handlers would separate the good module from the bad module automatically and confidently. It would also keep an account and record of the module faults detected. It can also trace and track modules to the manufacturing date, DRAM batch, and the machine it was tested on even months after the DIMM has left the factory.
In summary, good working memory modules have been designed for you by JEDEC, the industry standard setting committee. It is free for your access. Through JEDEC, unified DRAM and DIMM specifications have been generated and are available to memory and memory module manufacturers. All you have to do is to follow JEDEC guideline in choosing memory modules and not fall for the cost cutters. You will be able to build your good computer with stable and reliable memories that would last for a long long time.
By: Cecil Ho, CST Inc
Copyright © 2002 CST, Inc. All Rights Reserved