SD - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 213

FAT, short for File Allocation Table, has been used since the early days of PCs.
It has undergone several changes over the years. The original FAT specii cation,
FAT8, is no longer in use. FAT16 uses 16 bits to dei ne sector entries (a method
of storing i le information) and is limited to 2 gigabyte partitions. FAT32 was
released after this, and storage space was theoretically increased to 2 terabytes;
although in practice, few systems used it beyond 32 gigabytes. Newer systems
use the exFAT i lesystem, a new but incompatible i lesystem that allows huge
storage capacity; in theory, up to 64 zettabytes. For comparison, in 2013, the
entire World Wide Web was estimated at 4 zettabytes.
FAT32 has been surpassed technically by several i lesystems, including exFAT
and NTFS, but still remains in use for its simplicity. NTFS adds several interesting
features such as journaling, linking, and quotas; features that are not required
by a digital camera. The code required to interact with a FAT32 i lesystem is
extremely small, making it ideal for embedded systems.
Speed
There is also another factor to consider when choosing SD cards: their speed.
The SD Speed Class Rating is a simple way of understanding the minimum
speed of a card. Visible by either a letter C with a number inside, it shows the
number of guaranteed megabytes-per-second transfer speed. A Class 2 card
(A C with the number 2 inside) guarantees that the write transfer speed will
not drop below 2 megabytes per second. A Class 10 will not drop below 10
megabytes per second. The newer speed category is shown by the letter U, and
to date, two categories exist. UHS-1 (a U with the number 1 inside) guarantees
read/write performance of 10 megabytes a second, and UHS-3 (a U with the
number 3 inside) is guaranteed for 30 megabytes read/write per second. Please
note that these i gures are stated only as a minimum; some Class 2 cards are
more than capable of being branded as a Class-6 or higher but have not gone
through certii cation.
Using SD Cards with Arduino
Arduinos cannot natively use SD cards; they need a shield or a breakout board
to provide an SD slot. Fortunately, several shields exist with SD capacity. Most
Ethernet and wireless shields provide micro-SD slots, and numerous vendors
provide datalogging shields —a shield with a micro-SD slot and space to add your
own sensor components, as shown in Figure 12-4.