[Discuss-gnuradio] The [hopefully] full story SD Cards

[Matt Ettus]

I'll attempt to summarize the SD card situation as I see it.

There are 3 main reasons why you might have trouble with SD cards in 
your USRP2 -

- You aren't actually using an SD card.  SD Cards have been made in 
sizes from 8MB up to 2GB.  Any card of more than 2 GB is NOT an SD card, 
it is an SDHC card.  They share the same connector, but they do not 
support the same access modes and will not work with the USRP2. 
Similarly, MMC cards look the same, but are very different inside and 
will also not work.

- You aren't programming it properly.  You need root privileges to 
properly program the card, for one thing.  Make sure the filesystem is 
not mounted in Linux, follow the directions here:

        http://gnuradio.org/redmine/wiki/gnuradio/USRP2UserFAQ

- You have a bad SD card.  If you were part of the unlucky group that 
got bad cards from us in November and December, follow the directions 
from here:

        http://www.ettus.com/flash

If you bought the card somewhere else, there is nothing we can do.  You 
could try another one, or buy a known working one from us.  We sell our 
SD cards at a loss to encourage you to buy them from us and avoid trouble.

We currently have very good luck with Kingston 2GB SD and 2 GB micro SD 
cards, and that is what we ship now.  Patriot 2GB micro SD cards seem to 
be good, but recent Patriot 2GB full-size SD cards do not.  Purple 
Sandisk 2GB cards are good, but they are also the most commonly 
counterfeited -- because of their good reputation they command a premium 
in the market.  Older and smaller cards are more likely to work.

=========================================================================

So why would some SD cards work and some not?

We do not 100% understand the mechanisms behind this.  It could be 
actual problems with the cards, or it could be in the design we use to 
read the cards.  SD cards have many supported access methods and we use 
one of them, SPI.  You can see the design which we put in the CPLD on 
the USRP2 to read from the card in the FPGA repo under usrp2/boot_cpld 
and usrp2/opencores/spi_boot.  CPLDs are very simple devices, and it is 
not possible to handle all possible error or retry responses which a 
card might give in response to a request, and that would make it fail to 
boot.  SD cards are normally read by a microprocessor which can handle 
these cases due to increased resources.

Cards which don't work with USRP2s usually work in cameras, but not 
always.  I just walked into a camera store yesterday and bought an SD 
card for my camera, and it doesn't work.  It works in other cameras, but 
not mine.  I don't have a USRP2 with me to test it, but the point is 
that not all SD cards are created equal, as Bunnie Huang and others have 
found:

        http://www.bunniestudios.com/blog/?p=918

One thing that was left out of the above analysis is the fact that these 
cards are made at many different factories, from parts from different 
manufacturers.  They are also commonly counterfeited or rebadged. 
Sometimes a card with some bad flash will be sold as one half as large. 
 Often is there is no way to tell the difference between 2 very 
different cards by looking at the packaging.

The SD card business is very complex.  We are unable to buy them from 
the manufacturers because we don't buy 100,000 or more per month.  We 
are unable to buy them from primary distributors because we don't buy 
10,000 or more per month.  Instead, we have to buy them from retailers 
who are willing to sell them to us wholesale.

We have found a reputable consistent company to get cards from.  But 
even then, there can be problems.  We shipped hundreds of blue Patriot 2 
GB cards with no problems.  Then all of a sudden we got a shipment with 
half bad (for our definition of bad) cards.  There is no way to look at 
the card and see if it is bad or good, but the bad ones have a different 
chip inside.