I'm not conversant with SD card requirements from a circuit design point
yet, but in order to understand more, I'm looking at 2 devices that seem
to have flaws. The first is a Kodak camera. It's several years old, with
no spec on max capacity, but Kodak sells up to a 2GB card on its site
for it. While I haven't tested all combinations, it seems that 128 MB
and 256 MB cards format fine, but a Sandisk or Kodak 1GB card, as well
as a no-label store brand all cause the camera to power down partway
through the format without a clue. An interesting note is that they say
to use their cards or 'bigger name' cards as they have tighter
tolerances and cheaper ones may not work. Well, the 1GB ones from
anywhere don't work.

Next is a tablet/pad that specs no limit on SD card cap. I've seen
several units that don't operate with a 1GB card (Sandisk or house
brand) but will do fine with a smaller card. Some other units seem fine.
On further testing, it appears that working units may fail to work with
the bigger cards if the batteries are not very freshly charged. However,
the smaller cards are fine anytime. They too say to only use quality
cards and recommend Sandisk and they may not work with off-brands.

The camera uses 2 AA cells for 3v nom (2.4v for typical NimH), and the
tablet uses 4 AAA NimH cells, possibly in series or as 2 sets of 3v
(I've not taken it apart and they install as 2 banks). Using alkalines
to increase the voltage doesn't do it for the camera, but can help the
marginal tablets.

Other, more expensive devices I've seen using SD cards don't seem to
have this problem. It seems like a supply design problem to me, and
smells like maybe there isn't enough overhead on whatever regulator they
use, and if bigger cards draw more, the supply drops below spec.
Assuming the interface is on a chip and dropped into the design, are
there some interface chips that are known to have this problem? Do
bigger cards draw more in use? And how do the cheap cards differ from
"better" ones and how does one test this?

Any ideas from experience on what's going on? I'd like to not design in
these problems when my time comes...

Thanks,
Skip



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