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Choosing a DRAM supplier for embedded server applications

by Zsolt Kerekes, editor - November 2014

Choosing a DRAM supplier for new embedded server applications used to be much simpler than it is today - involving factors like:-
  • capacity - Although there was always a market for memory makers who could pack more into the same sockets - either by qualifying new memory sooner than traditional oems - or by mechanical packaging techniques which were able to incorporate more.
  • speed - When you look into the details of each memory generation - it's surprising how much variation there has been in products from different suppliers - even if competing products started out with the idea of aiming at the same standard generic speed.
  • reliability - Aside from factors which the memory buyer can control - such as testing and burn-in - the biggest determinator of memory (and most other complex semiconductor devices) is how hot they get.

    So a big part of the reliability question is about power consumption.

    But the actual design of the chips (layout and details of the cells) also determine things like the operating margins on refresh. And the only safe way to know these - is to stress sample products and vary the timing parameters at different temperatures - and see at what point they lead to failure. Another factor in how hot do the memories get is airflow and conduction cooling. Are some modules better than others by accident or by design?
  • price and availability - Buyers will often say - these are the most important factors. It may be so - depending when they enter the conversation. But having cheap memory which fails - under condistions when your system is still expeced to work - is an overriding technical consideration.

    Dependability of supply is probably more important than spot market pricing. Will you still be able to get devices when there are worldwide shortages of the raw memory chips? And will you still be able to get devices in 2, 3 , or 7 years time - if you still need them?
  • experience - Anyone who has been designing electronic systems for several product generations learns things which are important to their business - which are rarely discussed in the technical press.

    Sometimes - even if you can't identify exactly what it is that makes apparently identical products from some suppliers more dependable than others - you know that it's true.

    For example - in the earliest days of DRAM it was known (or strongly suspected) that the designs from some Japanese companies had been based on copying (or reverse engineering) the original designs of other competitors.

    That introduced problems due to mask registrations errors and different process tolerances.

    It also used to be the case with leading US memory companies - such as Intel - that the same product produced by different fabs would have different failure characteristics - due to subtle undocumented differences in what was happening in the production plants during different shifts.

    Those variations are better managed nowadays - but you can still see differences in the "same" product from batch to batch - depending on whether the original chip maker has changed their production process - something which may not matter to most of their customers - but which may make a difference to you.
And that was - when memory was simple - and the applications were simpler too.

For modern day embedded mamory designers there are additional variables to consider.
  • there's a much wider range of design variation in DRAM chips from different suppliers and different product ranges. Obvious examples being 2D vs 3D.
  • there's a much wider range of operational temperatures which embedded servers can expect to experience. Not only at the cool end of the specturm (as servers get out into places they wouldn't have fitted before) but at the hot end too - as some users decide that a good way of saving money on electrical power is to have less cooling in their racks. The high cost of rack space - also means that servers are packed more tightly than they used to be. All of which means that "standard" operating temperatures for memory devices - is a different to what it was before.
  • physical variations in DIMMs - in the past it used to be known that some standard DIMMs occupied less height than others. Many of these differences have now been standardized - so if you need low profile or ultra low profile modules - you can now depend on these characteristics as part of the sepcification of the product. (Much better than someone in manufacturing coming to tell you in a panic that the memory you designed in last year doesn't fit any more.)
  • value added tricky features:- in the market today we're seeing a lot of application dependent value added features being offered in DRAM DIMM footprints. The obvious examples are NVDIMMs. But some DRAM DIMMs have integrated data integrity and ECC - while other DIMMS which look like RAM have no internal RAM at all.
All of these variations mean that it really pays to get to know your memory supplier, understand what drives their thinking, try to assess in your own mind how much effort they have expended on understanding and characterizing memory - and then see how well - all that aligns with your own needs.

And if no one does exactly what you want - then find a supplier whose product ideas you like the sound of - and don't be afraid to ask about customization or BOM control so that you get the memory you need.
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Memory designers can mitigate heat using a range of simple but powerful thermal concepts.

The more DRAM in standby mode, the less power the module consumes - frequently achieved by using DRAM with the widest data bus.

For example, a 36 - chip 4 rank x8 DIMM uses less power than a 36 - chip 2 rank x4 D IMM.

Thermal sensors are critical tools for memory designers and enable the system's BIOS to adapt the refresh rate at different temperatures and / or control fans.

Designers should choose a module with a DRAM placement that does not allow all the DRAM devices to be active on the same side at the same time.
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