SD, SM, MS, etc. sockets

We have just done exactly this - micro-SD card in a socket on the board, to hold calibration data. The card and socket contacts are both gold-plated, so I'm not expecting any problems. Other contact materials, I'd be a bit wary.

Lots of embedded-PC-based systems (including ours) boot from a Compact Flash card. These usually have gold-plated contacts on both sides; our oldest machines have been in the field for about 5 years with no problems in this area.

HTH

Very true.

Once we did exactly that for an industrial device. Even if you secure the card and socket with additional straps, the socket + card assy remains one of the least reliable places. It is also prone to manufacturing problems, such as loose soldering. So, in the next revisions of the project we changed to the bare NAND flash soldered directly on the board, and that was heck of a lot better.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Several years ago I was working on a product that looked like it was going to need a NAND. We didn't go down that path, but I looked into things long enough to know that there didn't seem to be any commercial solutions to using NAND in all it's glory, and worse there weren't even white papers (in English, at least) that detailed any manufacturer's recommendations on using the redundant bits for error checking and correction.

Has that changed? Did you use the redundant bits for FEC or did you ignore them ore use them for other things? Did you roll your own solution, or did you find something off the shelf?

--
www.wescottdesign.com

Not exactly true, there's the SSFDC specification (thank God that's dead). Since an SSFDC is just an encapsulated, ESD-protected NAND chip, it was handy in our products to have a SSFDC socket as well as a soldered NAND chip - and a switch to toggle the select lines. Flip one way, we boot off internal NAND; the other way, we boot off the SSFDC card. Great for development (no JTAG on the processor, and no ICE available - it was burn and pray).

Nowadays we have the MTD drivers in Linux, at least. Lots of buyware solutions too. You'd have to be out of your mind to roll your own FS and physical sector handler code these days.

You can get some documentation and appnotes at Samung, Sandisk, Micron, etc, however AFAIK you won't find the detailed NAND flash datasheets without signing an NDA.

The NAND flash that I used had eMMC interface, so the very low level stuff like ECC is been handled transparently and independently from the host. The eMMC also solves compatibility problems between the ICs of the different vendors.

The POSIX compliant filesystem over FLASH was developed at home. I looked into so-called "free opensource" as well as into some paid solutions; found them rather inconvenient and unsatisfactory.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Though there *are* times when this is necessary -- sometimes even required! (products I've designed that ended up in third world countries where "support" was little more than mailing replacement parts -- or, smuggling them through customs -- which could replace socketed components :< )

OTOH, when you get a board back from someone with bits of

*toothpick* wedged in/under a socketed PLCC you *really* have to wonder if there are (un?)trained gorillas working on repairing these things... :<

Which technology did you use? I.e., I would expect SD and SM to have different issues as a consequence of their physical characteristics.

How was the "media card" used by the application? I.e.,

*booting* off it but otherwise executing out of RAM places infrequent demands on it. OTOH, using it in a datalogger would place *continuous* demands on it.

Did you use any special selection criteria when picking the socket? Or, just some run-of-the-mill part for consumer devices?

Yes. But, what sorts of failures did you encounter? Problems because folks "played" with the parts (because they *looked* like they were removable)? Failures of the connections over time (oxidation, biological agents, etc.)? Intermittent due to things like vibration? I.e., are there mitigating factors that *could* be addressed to better your experience in that case (a "no user serviceable parts inside" enclosure to keep out prying hands; conformal coat for atmosphere-born contaminants; mount on something that *doesn't* vibrate, etc.)

"Loose soldering" because the component was fastened to the board in a secondary operation? Or, because of mechanical stresses placed on the socket by the (repeated?) insertion and removal of the card?

That's what I would expect. Though I'm not sure how to put a "figure" on "heck of a lot better". :< Removable media offers some advantages (i.e., it can be replaced/upgraded over time so you don't end up replacing the entire device)

How often is the data updated or referenced? I.e., this isn't XIP-able technology so it inherently needs to be cached...

Agreed.

Yes, but how often are they rebooted? And, how tenacious/finnicky is the bootstrap loader?

What environment are you operating in? Relatively benign (office, etc.) or more aggressive (industrial, outdoor, etc.)? What sorts of environmental extremes do you see (shake 'n' bake)?

Many cell phones use micro SD cards for music and picture storage. They often put the music on through a USB connection. The pictures are often taken and sent out via the phone. I've never removed the 2GB sd micro in my phone, but it still works fine after more than a year.

A lot of digital cameras use SD memory and have the capability to upload the photos via high-speed USB. The card may stay in the camera for many months at a time.

I came to the conclusion a few years ago that SD and SD micro sockets were OK for data storage, and have incorporated them in a number of data loggers. Before that, I had only used CF cards for storage---but they had worked out OK.

I'll add the disclaimer that I'm using the SD and SD micro cards primarily in oceanographic data loggers. The loggers are in sealed pressure cases, so dust and humidity are not an issue. (If anything gets wet, it's generally a catastrophic failure!).

Mark Borgerson

I used a similar chip on a recent project. Just one word of caution: MMC flash chips do not fit under Linux's MTD subsystem (for NAND and NOR) that Lewin mentioned. That has implications for file system support (no JFFS, for example).

Steve

Compact Flash.

BTW, there is a considerable difference in the reliability of the cards depending on particular make.

Cards were used for raw data storage. Hundreds of files, gigabytes of data, sustained write ~several MB/sec. Environmental: shock and vibration, temperature variations. Unsupervised operation in outdoor conditions.

I.e.,

Yes.

Tried several TH and SMT sockets, looking for best quality of contacts and overall mechanical robustness.

Random problems in connection cardsocket and socketboard. Cured by replacement of card and socket.

Card failure was one of the common problems; no such issue with ICs soldered on the board.

:< Removable media

Removable media works fine for tabletop conditions; however it is difficult to design a removable media that should work reliably in the field.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

Check. Solder "plating" on PCB fingers and tin socket contacts last roughly

2 years before a likely failure - which can be fixed by unplugging and re-insertion.

One of the new Freescale ARM CPUs has an SD interface for flash, and enough logic in it to be able to boot from SD. So you can have (micro) SD as the only nonvolatile storage on the board.

I know one consumer device (the Chumby One) that does just this. It has hidden microSD inside - accessible if you unscrew it from the board, but otherwise intended to be hidden (the idea is easier user fixing of problems... brick your box? Just pop the firmware card in a Linux box and run 'dd')

Theo

--
Adrian C

Hi Vladimir,

[attributions elided]

Wow! And you had problems with that? I would have thought the CF connector to be more reliable than SD, MS, SM, etc. Though CF's tend to also be more massive which might make vibration more of an issue...

So a pretty hostile environment? I might be able to win, there, as these *should* see nothing more than temperature extremes (~60C ambient). If I open source the design, then folks may actually *welcome* the "memory card" approach. :-/ I'll have to think hard about the consequences for *my* uses...

But, this was a datalogger so the card was (frequently?) removed and reinserted (?). I.e., what if it had been plugged and left?

And this wasn't related to insertion/removal fatigue?

Yes. I think cameras are probably a more "abused" device in this regard. E.g., I do a remove/insert cycle each time I use one of my cameras. But, they are SD, MS (I've since discarded the SM camera). I think my phone has a mini SD (not micro)

I have found it easier to remove the card.

But, even if the card sits in the camera for ages, it *can* be removed to "freshen" the connection. What if removing the card is complicated (e.g., imagine your oceanographic probe spending a few

*years* under water)

But, they aren't filled with inert gases, are they? (i.e., there still is *some* atmosphere that they are exposed to)

A wee bit of a leak... :>

But, what if they solvent welded the case shut?

E.g., you can find wireless routers that have PCMCIA wireless cards buried within. You *know* they aren't expecting anyone to remove that device but just opted to take that approach for whatever manufacturing/availability reason...

Mini-PCI WiFi cards seem popular in routers too -- even in brands like Netgear where they're going after very high volume markets and you'd think it'd be cheaper for them to just solder down the chips.

Yes. This sort of thing is counterintuitive to me. Which makes me second guess my initial assumptions.

Of course, with wireless stuff, they may simply be "buying the certification". Especially on the short term.

Dunno. The economics and the practical aspects are often out of sync...

The CF versions have spent up to 9 months underwater--but only collected data for about 6 months because of battery life.

The SD card versions are heading for moorings on the equator this summer. I'm hoping that gold-on-gold contacts, and a dry temperature-stable environment will prove reliable.

Many instruments and loggers are flushed with dry nitrogen or argon before they are sealed to avoid condensation. They generally get assembled at about

20C, but spend most of their logging time at 5 to 10C.

It doesn't take much salt water when there are a few volts across the pins.

The worst situation is when there is enough water in the case with the batteries to generate some gases.

Back in the 70's a leaky case generated enough hydrogen that a friend was injured when opening the pressure case. I once opened an instrument that had been on a mooring for several months and it started hissing as I loosened the screws. I set it down and left the room for 24 hours. After that we added some pressure relief capability to the design.

Two pounds of lithium batteries and 3 ounces of salt water is a scary combination!

Mark Borgerson