Hardware Reference
In-Depth Information
SD Card Slot
Flip over your Pi and you will see the SD card slot at the rear end of the Pi. If you're using a full-length SD card, watch
out because they poke out the back of the Pi pretty far. The SD card slot is labeled as an SD/MMC/SDIO card slot.
Exactly what does that all mean, because it sounds pretty useful on paper? Well, it's not.
What Is SD?
SD as you most likely know by now stands for “Secure Digital” and it is a standard held by the SD Association. As you
would expect, there are also standards for all the SD cards; the main one you need to pay attention to is the “speed
class” rating. The speed class rating starts at 2 and works its way up to 10 (unfortunately, it does not go up to 11). All
speeds of cards should work fine, although I recommend you stick with speed 6 as that type of card will provide the
best mix of speed and compatibility.
Speed class 10 may work but be sure to check the Raspberry Pi hardware compatibility list as there are lots of
reports of the speed class 10 cards not working correctly. You can find an updated list of what SD cards have been
tested at the following site:
You will also want to stay away from any cards with a “UHS speed class” as they are not compatible at all with the
Raspberry Pi. UHS cards use a different timing source for access. The SD card slot on the Raspberry Pi is unable to
provide the correct timing for you to be able to use UHS SD cards. Even Micro SD cards with adapters can be known to
cause issues as well. Do yourself a favor and pick up a quality speed class 6 nonmicro SD card.
You may also see a reference to SDHC and SDXC: they are SD High Capacity and SD eXtended Capacity and both
will work on the Pi without any issues. SDXC will be at 32 GB and greater in size. SDXC cards come preformatted with
the exFAT filesystem. If you format the card in your Raspberry Pi, it may no longer work in other SDXC devices like
digital movie cameras and digital picture cameras.
What Is MMC?
The next set of letters to learn is MMC. MMC is another standard and stands for Multi Media Card. Nowadays the SD
standard supersedes the MMC standard. I would not expect that you would find many pure MMC cards for sale new
today. Your SD card may well be MMC-compatible, and the Raspberry Pi will read and write to your old MMC cards
as well. MMC cards will be slower and potentially of smaller capacity: while they will work fine on the Raspberry Pi,
I would recommend you buy a newer SD card.
What Is SDIO?
Lastly, we have SDIO. You may have guessed that the IO part of the name stands for input/output. SDIO cards have
some form of input and output; the most common are WiFi and Bluetooth. This means that when you insert an
SDIO card into a device it will be seen as a WiFi device or a Bluetooth device if the host device and operating system
support that. You will come across two types of SDIO cards: pure IO cards and mixed-storage IO cards. Given that
the Raspberry Pi must boot from the first partition on the SD card, this makes the pure IO SD cards useless because
your Pi won't be able to boot anything. The mixed-storage cards will work as long as the operating system has drivers
for the IO part of the card. SDIO cards are not all that popular and will cost a lot more than adding a USB WiFi or
Bluetooth dongle to your Raspberry Pi.
I would suggest you avoid the SDIO cards completely: not only will they cost more but there is a good chance that
Linux won't support it.
