[En-Nut-Discussion] PHATFS 4GB SD card

Nathan Moore nategoose at gmail.com
Tue Nov 6 17:07:04 CET 2012

> We had been using 2GB cards with our device without problems, but we got a
> recent lot of 4GB cards and the OS fails to mount them. I think the OS
> needs to be told to use Fat32, is there some configuration option or MACRO
> to handle this?

The original SD protocol only worked up to 2GB.  It used a 32 bit value for
something, but only
31 of the bits were address bits.  There were some SD readers and cards
that used the other bit
for address as well, but this didn't meet the standard.  I'm not sure how
the larger format standard
works for 4GB cards and readers that used that hack, and there may have
been multiple
incompatible hacks that were used.
Any cards over 4GB should use the newer standard and be SDHC, but I think
that cards that are
exactly 4GB could possibly be non-standard.

I'm no expert on this, but I do remember this being an issue years ago on a
different system where
uBoot on our boards could only use 2GB SD cards to load new embedded Linux
firmware, though
the Linux firmware could use bigger cards.  I had to go out and buy up a
bunch of 2GB SD cards
so that our team would each have a card to do updates with.

I'm not sure if any of this is actually relevant, but 2GB and 4GB are
special in the SD card world.


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